The community composition of native and exotic species on the artificial
oyster reefs at the Oesterdam in the Eastern Scheldt.
A monitoring survey of artificial oyster reefs at the Oesterdam in the Eastern Scheldt in combination with a behavioral experiment on competition for food and shelter between the exotic crab Hemigrapsus takanoi and native crab Carcinus maenas in a laboratory environment.
Author: S.M.Hutting
Place of publication: Vlissingen
Date: 20- 03 - 2015
Version: Final version
Publishing organization: Hogeschool Zeeland University of Applied Sciences Commissioned by: Delta Academy, Research group Building with Living Nature
The community composition of native and exotic species on the artificial
oyster reefs at the Oesterdam in the Eastern Scheldt.
A monitoring survey of artificial oyster reefs at the Oesterdam in the Eastern Scheldt in combination with a behavioral experiment on competition for food and shelter between the exotic crab Hemigrapsus takanoi and native crab Carcinus maenas in a laboratory environment.
Author: S.M. Hutting
Student number: 00047811
Date: 20- 03 - 2015
place of publication: Vlissingen
Version: Final version
Study: Bachelor of Water management
Thesis supervisor: A. Verkruysse Company supervisor: A. van den Brink
Publishing organization: HZ University of Applied Sciences
Commissioned by: Delta Academy, Research group Building with Living Nature
Summary
At the Oesterdam four artificial oyster reefs were built to stabilize the sediment of the sand nourishment. These provide a new hard substrate in a previously soft substrate environment, thereby creating a new habitat. This allows species that were previously excluded from the area to now occur there. If the artificial reefs have a positive effect for one or more exotic species, it may cause problems for the native species.
The aim of this research project is to investigate the biodiversity on the artificial oyster reefs to get an indication of the ratio between the native and exotic species on this new habitat. As an indicator for the effect of the oyster reefs on the community composition the two crab species Hemigrapsus takanoi (exotic species) and Carcinus maenas (native species) were studied. Van den Brink et al.
(2012) hypothesized that C. maenas was being outnumbered by H. takanoi due to adult H. takanoi out-competing juvenile C. maenas of smaller or similar size to adult H. takanoi for shelter.
To get a better insight on the effect both species have on each other a behavioral experiment was set up to be able to see the competition for food and shelter between juvenile C. maenas and adult H.
takanoi.
The ratio of exotic and native species seems to be close to that of the natural reef. The species richness on both artificial reefs is higher compared to the natural reef. This is expected to decrease as the species find a balance by outcompeting each other resulting in a lower species richness and a higher evenness like the natural reef. Some exotic species pose a threat. Maybe these populations can be slowed down to give other species the change to conquer the niche hopefully keeping the invasive species at bay.
It is likely considering the results of this study that juvenile C. maenas are outcompeted for food and shelter by H. takanoi of a similar size. This will in turn cause a problem for the population as a whole since less crabs were able to reach maturity ultimately preventing them to reproduce.
If in the future more reefs will be built it is recommended to look into ways to improve the design in order to help some species like giving the reef a more natural shape and add some open spaces in between to create patches.
Preface
Being my final thesis this research was the last piece of the puzzle on the eve of a very big event in my life; the graduation for my bachelor’s degree in Water Management or as we sometimes jokingly called it: ditchology. I have always been and will always be drawn to water and the secrets it holds underneath the surface or a rock at the shore. I have had some great rocks in the waves as the Dutch like to say it who I will thank at the end of this report. For now I am excited to bring this phase to a close looking back on times in which I have learnt a great deal of things and had lots of doing it. The crabs in this research left me the message that something seemingly small can still surprise you. Be curious, be brave and face (and preferably pinch) your fears.
Contents
Summary ... 3
Preface ... 3
Figures and tables... 5
Contact ... 6
1. Introduction ... 7
1.1 Research species ... 8
Hemigrapsus takanoi ... 9
Carcinus maenas ... 10
1.2 Previous experiments ... 10
2. Problem definition and objectives ... 12
2.1 research questions ... 13
3. Method ... 14
3.1 Monitoring survey oyster reefs ... 14
Sampling area ... 17
3.2 Behavioral experiment ... 18
3.2.1 Collection of H. takanoi and C. maenas ... 18
3.2.2 Set-up ... 19
3.2.3 Competition monitoring ... 20
4. Results ... 21
4.1 Biodiversity ... 22
4.2 Crab populations ... 26
4.3 Behavioral experiment ... 29
5. Discussion ... 33
6. Conclusions ... 37
7. Recommendations ... 37
7.1 recommendations on the methodology ... 37
7.2 recommendations for further research ... 38
References ... 38
Acknowledgements ... 39
5
Figures and tables
FIGURE 1ARTIFICIAL OYSTER REEF NEAR THE OESTERDAM ... 7
FIGURE 2MALE HEMIGRAPSUS TAKANOI.SOURCE:(VAN BRAGT,2015) ... 9
FIGURE 3CARCINUS MAENAS.SOURCE:A.M.ARIAS ... 10
FIGURE 4 AVERAGE DENSITY OF CARCINUS MAENAS AND HEMIGRAPSUS TAKANOI FROM 1990-2010 IN THE EASTERN SCHELDT. .... 13
FIGURE 5.LOCATION OF THE OESTERDAM COMPARED TO THE HZUNIVERSITY OF APPLIED SCIENCES. ... 14
FIGURE 5 LOCATION NATURAL ARTIFICIAL OYSTER REEFS, GABIONS AND NATURAL OYSTER REEFS. ... 15
FIGURE 6SCHEMATIC DRAWING OF THE MONITORED REEFS. ... 15
FIGURE 7 LOOSE SHELLS NEXT TO ONE OF THE MONITORED REEFS. ... 16
FIGURE 8 NATURAL OYSTER REEFS DURING LOW TIDE ... 16
FIGURE 9 ONE OF THE ARTIFICIAL OYSTER REEFS NEAR THE OESTERDAM ... 17
FIGURE 10EXAMPLE OF DIVISION OF SAMPLING SECTIONS ON AN ARTIFICIAL OYSTER REEF.THE NUMBERS REPRESENT THE THREE METER SECTIONS ON THE REEF.THE LETTERS A AND B REPRESENT THE MORE AND LESS SHELTERED SIDES OF THE REEF. ... 17
FIGURE 11 SATELLITE IMAGE OF THE LOCATIONS FOR THE COLLECTION OF CARCINUS MAENAS AND HEMIGRAPSUS TAKANOI ... 19
FIGURE 12 PICTURE OF THE EXPERIMENTAL SET-UP.ALL 9 AQUARIUMS ARE SHOWN WITH ONE CAMERA FOR EVERY 3 AQUARIUMS. . 20
FIGURE 13 A CRAB WAS CONSIDERED TO USE THE SHELTER IF AT LEAST HIS LEGS ON ONE SIDE WERE UNDER THE SHELL. ... 20
FIGURE 14.OVERVIEW OF THE DIFFERENT SPECIES FOUND ON THE ARTIFICIAL REEFS OVER THE THREE SAMPLE DATES COMBINED.ALGAE SPECIES ARE SHOWN SEPARATELY BECAUSE THEY WERE ESTIMATED FOR PERCENT COVERAGE INSTEAD OF ABUNDANCE CLASS. . 22
FIGURE 15.OVERVIEW OF THE DIFFERENT SPECIES FOUND ON THE NATURAL OYSTER REEF.ALGAE AND OTHER SPECIES WHICH WERE ESTIMATED ON PERCENT COVERAGE INSTEAD OF ABUNDANCE CLASS ARE SHOWN SEPARATELY. ... 23
FIGURE 16.SPECIES RICHNESS PER REEF ON SAMPLE DAY 1 ... 24
FIGURE 17.THE TOTAL ABUNDANCE PER REEF ON SAMPLE DAY 1... 24
FIGURE 18.EVENNESS AND SHANNON-WIENER DIVERSITY INDEX VALUES ON THE DIFFERENT REEFS ON DAY 1 ... 25
FIGURE 19.THE EVENNESS AND SHANNON WIENER DIVERSITY INDEX VALUES PER REEF OVER TIME ... 25
FIGURE 20.RATIO BETWEEN NATIVE AND EXOTIC SPECIES ON EACH REEF ON THE DIFFERENT SAMPLE DAYS. ... 26
FIGURE 21COLLECTED CRAB SPECIES OVER TIME ON THE TWO ARTIFICIAL OYSTER REEFS. ... 27
FIGURE 22.PERCENTAGE OF TOTAL SUCCESSFUL ENCOUNTERS FOR H. TAKANOI AND C. MAENAS ... 31
FIGURE 23.TOTAL NUMBER OF SHELTER USE BY H. TAKANOI AND C. MAENAS WITH AND WITHOUT COMPETITION PRESENT.THERE WAS A SIGNIFICANT DIFFERENCE BETWEEN SPECIES (P=0,002) BUT NO SIGNIFICANT DIFFERENCE FOR EACH SPECIES WHEN COMPETITION WAS PRESENT ... 32
FIGURE 24.SHELTER USE OF BOTH C. MAENAS AND H. TAKANOI IN THE PRESENCE OF A COMPETITOR. ... 32
TABLE 1 SAMPLE DATES AND TIMES WITH CORRESPONDING LOW TIDE LEVELS IN NAP. ... 16
TABLE 2 ABUNDANCE CODES FOR MOTILE SPECIES ... 18
A TOTAL OF NINE AQUARIUMS WERE USED SIMULTANEOUSLY.A PICTURE OF THE SET-UP IS SHOWN IN FIGURE 14.TABLE 4 SHOWS AN OVERVIEW OF THE DIFFERENT METHODS PER TREATMENT. DURING AN EXPERIMENT EITHER THE SHELTER OR THE FOOD TREATMENT WAS USED FOR ALL NINE AQUARIUMS.TABLE 3 OVERVIEW OF ALL TREATMENT AND THE ACCOMPANYING METHODS ... 19
TABLE 4.DIFFERENCE IN FEEDING BEHAVIOR OF H. TAKANOI AND C. MAENAS IN RELATION TO COMPETITION.THE P-VALUE IS THE RESULT OF A CHI² TEST ... 30
TABLE 5.DIFFERENCE IN FEEDING BEHAVIOR BETWEEN H. TAKANOI AND C. MAENAS.THE P-VALUE IS THE RESULT OF A CHI² TEST ... 30
TABLE 6.OUTCOME OF ENCOUNTERS BETWEEN H. TAKANOI AND C. MAENAS.SUCCESSFUL ATTACKS ARE WHEN A CRAB SUCCESSFULLY DISPLACED THE OTHER FROM THE FOOD.SUCCESSFUL DEFENSE IS WHEN A CRAB MANAGES TO KEEP THE FOOD WHEN ATTACKED. ... 31
6
Contact
HZ University of Applied Sciences Delta Academy, Research group Aquaculture:
Address: Edisonweg 4 Zip code: 4382 NW City: Vlissingen Country: Netherlands
Contact: Anneke van den Brink Phone:
E-mail: a.van.den.brink@hz.nl Internet: www.hz.nl
The delta academy is a part of the HZ University of Applied Sciences, which focuses on delta areas. It hosts three studies: Water management, Delta management and civil engineering and 4 research groups: aquaculture, building with living nature, water technology and water safety& area development. The research group Building with Living Nature looks into possibilities to use living nature for sustainable development in and around coastal defense.
7
1. Introduction
This report is the result of a final thesis for the Water Management bachelor program at the HZ University of Applied Sciences. The research was commissioned by the research group Building with Nature which is part of the Applied Research Centre Delta Academy at the HZ University of Applied Sciences in Vlissingen, The Netherlands.
The research group Building with Living Nature investigates possibilities to use living nature for sustainable development in and around coastal defense. This project is part of the Oesterdam project which is aiming to protect the Dutch coast and enhance the biology of the intertidal areas in the Eastern Scheldt at the same time.
The Eastern Scheldt is one of the biggest nature reserves in the Netherlands and is an important habitat for many native species due to the intertidal areas. Since the construction of the storm surge barrier the dynamics in the Eastern Scheldt changed considerably. Although there is still a free flow of water when the barrier is open, the amount of water going into the area has been reduced by 30%. (De Jong, et al., 1988). Because of this the gullies, which were already there, are now too large for the amount of water coming in. This results in the problem that the force of the water is not strong enough to deposit sand onto the sand flats which are an important part of the ecosystem. At the same time wind and outflowing water will take sand away from the sand flats. These sand flats normally fall completely dry during low tide which is an important factor in the survival of many species in the area. Since they are becoming smaller, less area is falling dry every year endangering the ecosystem as a whole. If this problem would be ignored the surface area of the tidal flats will be halved by 2050. (Van Zanten & Adriaanse , 2008)
One of the projects trying to solve this problem is the sand nourishment. In 2012, 400.000 m³ of sand was deposited next to the Oesterdam. This sand will slowly spread out over time onto the tidal flats.
The goal is to protect the Oesterdam from incoming waves and at the same time create a buffer for the tidal flats. (Linkit, 2011)
At the Oesterdam four artificial oyster reefs were built to stabilize the sediment. Over time these reefs will hopefully develop into natural reefs. The artificial reefs provide a new hard substrate into a soft substrate environment, thereby creating a new habitat. The addition of a new habitat might encourage the growth and dispersal of some of the many exotic species in the Eastern Scheldt. Of all the exotic species in the Netherlands, 55 % are found in Eastern Scheldt and 14 % of all exotic species in the Netherlands are found only in Eastern Scheldt. (Wolff, 2005)
Figure 1 Artificial oyster reef near the Oesterdam
8 The most important reason for the arrival of these exotic species is aquaculture. By accidental escapes or transfer to the ecosystem by open cage systems.
Another vector for the introduction of exotic species is hull fouling and ballast water associated with the high volume of shipping in these waters. This is the reason why shipping regulations have been tightened over the years. Natural processes such as global warming also play a role into the invasion of exotic species. Species who normally could not live in this climate are now able to survive. (Wolff, 2005)
There are many risks involved with exotic species. 10 % of total invaders in an aquatic system have a high impact (Ricciardi & Kipp 2008). They can change ecosystem functions like hydrology and nutrient cycles and might pose health risks by acting as vector for diseases. Clavero & Garcia-berthou (2005) state that exotic species are major cause of extinction because of competition, niche displacement, hybridization and predation.
Sometimes these effects can be used as an advantage by using the species an ecosystem engineer.
Ecosystem engineers are species that can create, modify and maintain habitats. Ecosystem engineers can influence the distribution and abundance of large numbers of plants and animals, by causing physical changes in biotic and abiotic materials that, directly or indirectly, modulate the availability of resources to other species. (Jones, et al., 1994). In the case of the artificial oyster reefs the exotic C.
Gigas is used to build the reefs which protect the coastline by reducing the waves.
The aim of this research project is to investigate the biodiversity on the artificial oyster reefs to get an indication of the ratio between the native and exotic species on this new habitat. As an indicator for the effect of the oyster reefs on the community composition, two crab species were chosen.
Crabs are a motile and competitive species, making them a good indicator of short term changes in an environment as they can easily enter or exit an area according to its suitability. For this reason the populations of both Hemigrapsus takanoi (exotic species) and Carcinus maenas (native species) were studied. To get a better insight on the effect both species have on each other a behavioral
experiment was set up to be able to see the competition for food and shelter between juvenile C.
maenas and adult H. takanoi according to the hypothesis suggested in van den Brink (2012) (see ‘1.2 previous experiments’ ).
1.1 Research species
Two species of crabs were investigated during this study. Hemigrapsus takanoi which is native to Japan and Carcinus maenas which is native to the Netherlands. Both being successful invaders in multiple places around the world they have proven to be resilient species (Klassen & Locke , 2007) (Dauvin, et al., 2009) (Noël, et al., 1997). In the Eastern Scheldt, among other places, they occur together.
9 Hemigrapsus takanoi
Figure 2 Male Hemigrapsus takanoi. Source: (Van Bragt, 2015)
Closely related to Hemigrapsus penicillatus, this species was only recently recognized as a separate species (Asakura & Watanabe, 2005). They are usually found under hard structures in intertidal areas. They are generally in more wave-sheltered areas than H. penicillatus, although they are also often found living in the same areas. Their native distribution ranges in the north/west Pacific Ocean around Taiwan, Korea, Japan and China. (Asakura & Watanabe, 2005)
It is believed that these crabs were introduced into Europe during the early 1990’s by the transport of Asian oyster or hull fouling or ballast water. Since then they have spread out along the European Coast at a high rate (100 km per year). They are able to spread that fast because their young reproduction age starting at 10 months old (7 mm CW) and because of their planktonic larvae (Noël et. al. 1997) which they may have up to six times a year (Dumoulin , 2004).
Their carapace is square shaped and adults can get up to 28 mm carapace width (Noël, et al., 1997).
Males have larger chelae compared to females with a patch setae on them. They vary on color from grayish to greenish or brownish (Asakura & Watanabe, 2005). Their breeding season ranges from spring until autumn. Females are mature at a size of 7mm CW which can be reached in 10 months depending on the temperature. They can have up to 6 broods per year (Dumoulin , 2004)
10 Carcinus maenas
Figure 3 Carcinus maenas. Source: A.M. Arias
Carcinus maenas or the green crab is native to the Atlantic, Baltic, and North Sea coasts of Europe from Norway to Mauritania (Groholz & Ruiz, 1996). But ranked by (Lowe, et al., 2000) as one of the world’s 100 worst invasive alien species they have spread out across the world. (Klassen & Locke , 2007)
They occur in a variety of habitats including both hard and soft substrates (Groholz & Ruiz, 1996).
They are known to migrate into deeper water when the water gets colder during autumn. (Audet , et al., 2008)
Reaching up to 10 cm carapace width they can live up to 4-7 years (Klassen & Locke , 2007). Females get mature at a minimum CW of 36 mm. Mating finds its peak in July and they can bear eggs from March and April, but also during November and December. Usually they will release their larvae during July and August. (Broekhuysen, 1936)
1.2 Previous experiments
The following papers were used as the base for both the reason and methods for this project. The first two research project took place at the same general area providing more information about it.
The first report is about natural reefs in the Eastern Scheldt and the second one about the
populations of C. maenas and H. takanoi. The second paper is the lead paper for the reason to start this research providing a hypothesis which can be investigated further. The last paper is about some behavioral experiments between C. maenas and Hemigrapsus sanguineus which is closely related to H. takanoi. This paper was used as the most important source for the methodology used in this project.
11 Fritz (2013)
This study focused on the ecological difference between dense oyster reefs, patchy oyster reefs and tidal flats at Viane and Sint-Annaland to give recommendations for the design of artificial oyster reefs. The research focused mainly on the organic material of the sediment, amount of biomass, population densities. It was found that the dense oyster reefs showed the best ecological values although the results were not significant. The method in this study was limited to only 3 species:
Arenicola marina, Littorina littirea and Pygospio elegans. This research was taken into account to get some insight on the natural oyster reefs studied around the same area, but the method for the current research project were more elaborate.
Van den Brink et al. (2012)
This research investigated the populations of the three crab species: C. maenas, H. takanoi and H.
sanguineus. This was done by monitoring the benthic communities in four Dutch delta waters and doing a snapshot survey in the Eastern Scheldt tidal bay. These waters were dominated by C.
maenas but their numbers declined in the past two decades. In 1999 the two exotic Hemigrapsus species were introduced to these waters and started to invade the habitat of C. maenas. Although they were not the reason for the initial decline for C. maenas they have taken advantage of it. H.
takanoi now dominates the hard substrates and both exotic species are equally abundant on soft substrates compared to C. maenas. On hard substrate where juvenile C. maenas normally would find shelter they now appeared to be excluded from it by H. takanoi. This increases the chance of
desiccation during low tide or in general the chance of getting preyed upon and thereby increasing their mortality.However on soft substrates the C. maenas population, were there are fewer H.
takanoi, the populations seems to be maintained by crabs that survive and reproduce.
Jensen et al. 2002
This research compares the competitive behavior of Carcinus maenas against Hemigrapsus sanguineus on the east coast and against Hemigrapsus oregonensis on the west coast of North America. By both field sampling and laboratory experiments, competition for space and food were tested. In competition for food C. maenas was dominant over H. oregonensis, while H. sanguineus were very dominant over C. maenas. By sampling rocks and sand in the habitat of C. maenas large differences in habitat use were recorded between when Hemigrapsus were present or not. If Hemigrapsus species were present only 20 % of juvenile C. maenas were found under rocks while more than 97 % of C. maenas were found under rocks in a habitat without H. sanguineus. Both Hemigrapsus species were also found to be dominant over C. maenas in competition for shelter in laboratory trials. It is likely the habitat use and by that the distribution of C. maenas is affected.
12
2. Problem definition and objectives
At the Oesterdam four artificial oyster reefs were built to stabilize the sediment of the sand nourishment. Over time these reefs will hopefully develop into natural reefs. The artificial reefs provide a new hard substrate in a previously soft substrate environment, thereby creating a new habitat, which allows species that were previously excluded from the area to now occur there. If the artificial reefs have a positive effect for one or more exotic species, it may cause problems for the native species.
Every established exotic species has its effect on the community composition in the Eastern Scheldt.
Some more than others but, as described in chapter 1, there can be a risk for the native species in the area. These risks include direct danger like predation, but there can also be competition for resources like food, shelter or light.
Habitat changes also affects the community composition. A large change for the Eastern Scheldt was the construction of the storm surge barrier which changed the dynamics of the entire system (De Kluijver & Leewis, 1994) (De Jong, et al., 1988). However, also smaller changes like the artificial oyster reefs may have an effect (see chapter 1). It is important to know the effect of the artificial reefs on the community composition to have a complete insight on the total effect on the
biodiversity. More reefs may be built in the future and if the current reefs cause problems for some species, the right actions can be advised to prevent further damage. An example may be found between some species of crabs found on the oyster reefs.
Crabs are motile and competitive species, making them a good indicator of short term changes in an environment as they can easily enter or exit an area according to its suitability. In the Dutch delta Carcinus maenas has dominated the native crab population in the past. Around the turn of the century Hemigrapsus takanoi was unintentionally introduced into the Dutch delta via shellfish transport for aquaculture purposes. While the number of Carcinus maenas had declined over the past 20 years the decline already started before the introduction of Hemigrapsus takanoi but C.
maenas is now clearly outnumbered by H. takanoi (figure 2). It is clear the decrease in the C. maenas population was not started by H. takanoi but it is possible that H. takanoi is taking advantage of these decreasing numbers and so contributing to the decline of C. maenas (van den Brink et al.
2012).
Van den Brink et al. (2012) hypothesized that this outnumbering of C. maenas by H. takanoi was due to adult H. takanoi out-competing juvenile C. maenas of smaller or similar size to adult H. takanoi for shelter. Although the adult size of both species are very different (C. maenas grows up to 10 cm CW compared to 2.5 cm CW for H. takanoi) it is possible H. takanoi competes with juvenile C. maenas and maybe even preys on them. This may make it much harder for the juvenile C. maenas to reach maturity and reproduce (Van den Brink et al. 2012). While Van den Brink et al. (2012) did not test this hypothesis, they opened an opportunity to investigate the competition between C. maenas and H.
takanoi in a laboratory environment. This can give important information on their behavior in a natural environment.
13
Figure 4 average density of Carcinus maenas and Hemigrapsus takanoi from 1990-2010 in the Eastern Scheldt.
By collecting all crab species on the oyster reefs and conducting a behavioral experiment on the two species above, insight can be obtained into the relationship between these two species as an example of the interactions between exotic and native species on the oyster reefs.
2.1 research questions
To document the community composition on the artificial oyster reefs, the following key and sub- questions are asked.
Key question:
What is the community composition regarding native and exotic species on the artificial oyster reefs at the Oesterdam in the Eastern Scheldt?
Subquestions:
- What is the community composition of native and exotic species on natural and artificial oyster reefs at the Oesterdam in the Eastern Scheldt?
- What are the size, gender and ratio of the populations of Hemigrapsus takanoi and Carcinus maenas on artificial oyster reefs in the Eastern Scheldt?
- What competition for food can be observed between similarly sized H. takanoi and C. maenas?
- What competition for shelter can be observed between similarly sized H. takanoi and C. maenas?
- What inferences can be made about the laboratory observed competition between the crabs and their competition in the field?
14
3. Method
The method for this project is divided into two parts. The first part is a monitoring survey on the artificial oyster reefs near the Oesterdam (fig. 5) to assess the development of the biodiversity on these reefs.
Figure 5. Location of the Oesterdam compared to the HZ University of Applied Sciences.
As an indicator for the effect of the oyster reefs on the community composition, two crab species were chosen; Hemigrapsus takanoi (exotic species) and Carcinus maenas (native species). All
collected crabs were identified to species level, counted and their size and gender were noted during the survey. This gives an overview of the population density and distribution of both species.
To gain a better insight on the effect both species have on each other a behavioral experiment were set up to document the competition for food and shelter between Hemigrapsus takanoi and Carcinus maenas.
3.1 Monitoring survey oyster reefs
Both artificial and natural oyster reefs were monitored. The natural reefs are found close to the artificial oyster reefs located near the Oesterdam (fig. 1).
15
Figure 6 location natural artificial oyster reefs, gabions and natural oyster reefs.
There are two sets of two reefs located near the Oesterdam. Because of the limited time due to the tides, two out of the four reefs were chosen to be monitored. One reef out of each set was chosen to
be monitored. The first one (number 1 in fig. 6) was chosen because some of the shells have been washed out of the reefs and are have loosely collected next to the reef outside of the wire mesh (fig. 7). This makes it easier to collect crabs from between the shells. The other reef (number 2 in fig. 6) was selected because it is the largest reef and because it is the most outside and thereby exposed reef out of the upper pair.
Since that reef has no loose shells a single gabion filled with empty oyster shells was attached to that reef two months before the first sampling event. This gabion was opened during sampling by removing the cable-ties. The gabion were inspected and the organisms present were recorded and all crabs were collected.
Figure 7 Schematic drawing of the monitored reefs.
2
1
16 To be able to compare the results obtained from the artificial oyster reefs, a natural oyster reef (fig.
4) were also monitored using the same method except instead of dividing it into section like the artificial reefs, the natural reef it is treated as one section due to its smaller size.
Figure 9 natural oyster reefs during low tide
All reefs were checked three times with one month in between (table 1).
Table 1 sample dates and times with corresponding low tide levels in NAP.
Date Time at low tide Low tide lvl NAP
30-09-2014 13.35 -134
28-10-2014 11.45 -138
25-11-2014 10.55 -144
Figure 8 loose shells next to one of the monitored reefs.
17 The artificial oyster reefs consist of empty oyster shells caged by mesh and attached to the sediment (fig. 4). These cages make quantitative sampling impractical as access to the oysters is very limited.
Therefore a systematic qualitative sampling method were applied to determine species richness along with subjective estimates on quantity.
Sampling area
Along the length of the oyster reefs three, 3 m wide sections were marked on both the more exposed and more sheltered sides. Each section was labeled so that sampling can be repeated per section as illustrated in Figure 5.
Figure 11 Example of division of sampling sections on an artificial oyster reef. The numbers represent the three meter sections on the reef. The letters A and B represent the more and less sheltered sides of the reef.
Each section was visually inspected for organisms with particular focus on periwinkles, crabs and algae.
Using a 0.25 m² quadrate haphazardly placed in each section of the reef, all species were recorded and given an abundancy class (table 2). For algae and colonial ascidians such as Didemnum sp. the percent coverage were estimated. The natural oyster reef was treated as one section.
All organisms within a quadrate were identified with the help of a field guide. This information was written on pre-made field sheets (appendix A). If a species could not be identified on site, a sample was placed in a 500 ml plastic container and taken to the HZ University of Applied Sciences for further identification. All crab species observed within the quadrate, were collected by hand and put into a bucket. For all crab species the carapace width (CW) is measured with calipers to the closest mm. Their gender were noted as well. Besides the crabs within the quadrates, more crabs were
3m
Figure 10 one of the artificial oyster reefs near the Oesterdam
18 collected by hand on other parts of the reefs. This was done to get a larger sample size for the
population comparison. One person would continuously collect crabs as long as it took to gather all the results from the quadrates.
Table 2 abundance codes for motile species
Code Estimate of Abundance
1 1-5
2 5-10
3 10-20
4 20-30
5 30-40
6 40+
3.2 Behavioral experiment
To gain a better insight on the effect exotic and native species can have on each other a behavioral experiment were set up to be able to see the competition for food and shelter between the two crab species Hemigrapsus takanoi (exotic species) and Carcinus maenas (native species).
All treatments will done in triplicate and the entire experiment was repeated 5 times for a total of 15 replicates. New crabs were used for each experiment. Before the crabs were used in the experiment they were put into holding tanks in the same room as where the experiment will take place. The holding tanks were filled with a layer of water from the Eastern Scheldt, deep enough for the crabs to be completely emerged. An air pump and some rocks will also be provided to maintain the oxygen level in the water and provide shelter. The crabs were fed every other day with some dead mussels.
Each species was held in a separate holding tank.
For each treatment a control with a single individual crab was used as a reference for the behavior without any competition.
3.2.1 Collection of H. takanoi and C. maenas Location
Both species of crabs were collected from the Eastern Scheldt. H. takanoi were collected from a dike near the village of Tholen. Carcinus maenas were collected from a dike at the Goese Sas (fig 8). They were collected at two different locations because C. maenas was more abundant at the Goese Sas and therefore easier to collect. Two people were needed to collect them so one person can turn over the rocks and the other can collect as many crabs as possible.
19
Figure 12 satellite image of the locations for the collection of Carcinus maenas and Hemigrapsus takanoi
3.2.2 Set-up
A total of nine aquariums were used simultaneously. A picture of the set-up is shown in figure 14. Table 4 shows an overview of the different methods per treatment. During an experiment either the shelter or the food treatment was used for all nine aquariums. Table 3 overview of all treatment and the accompanying methods
Treatment Shelter Food Control
C. maenas
Control H. takanoi Basic
set-up
3 aquariums < 0.5 cm sand Air stone
3 aquariums 0.5 cm sand
3 aquariums 0.5 cm sand
3 aquariums 0.5 cm sand
Duration Acclimate for 20 min Runs for 48 h
Starved/acclimated for 48 h
Runs for 13h
Runs as long as experiment
Runs as long as experiment Crabs One of each species
with equal CW (±5mm)
One of each species with equal CW (±5mm)
Single individual Single individual
Monitor method
Check position at start, low tide, high tide and end
Videotaped Same as treatment Same as treatment Attributes Bivalve of twice CW in
length
Attach mussel on plate in center
20 3.2.3 Competition monitoring
Food experiment
The videos made during this experiment, lasting 13 hours per replicate, were analyzed for:
- First to find the food - Total time spent feeding
- Number of times feeding without interruption - Number of times feeding in total
- number of times feeding was unsuccessful
- number of times a crab successfully displaced the other from the food
Feeding was unsuccessful if a crab was prohibited to feed by the other crab. Meaning the other crab had successfully defended the food.
Shelter experiment
For the shelter experiment the tides were simulated twice within 48 hours by slowly draining the water with plastic tubes. Half a mussel shell was provided as shelter. The position of the crabs was noted at the start of the experiment, during low tide, high tide and at the end of the experiment.
A crab is considered to use the shelter if at least all his legs on one side are covered by the shell (fig. 15), because this would enable him to retreat quickly if necessary
Data analysis
A metafile in excel was the collection place for all data collected from both the field experiment as well as the behavioral study. These results were statistically analyzed in Excel.
Figure 14 a crab was considered to use the shelter if at least his legs on one side were under the shell.
Figure 13 picture of the experimental set-up. All 9 aquariums are shown with one camera for every 3 aquariums.
21
4. Results
In this section the results of the monitoring survey and experiments are shown. The results are divided into three parts: biodiversity, crab populations and the behavioral experiment.
All tests for statistical significance were done by using a CHI² test because the data sets were either too small or not normally distributed. A p-value <0,05 was considered significant.
The biodiversity was assessed with the help of the Shannon-Wiener diversity index combined with the richness and evenness.
Shannon-Wiener diversity Index(H). The Shannon-Wiener Diversity index calculates how well the samples represent the community and therefore increases when the richness and evenness increase:
-
Where Pi = Number of individuals of species i/total number of samples∑ = sum
Evenness (E). Evenness is a measure of the homogeneity of abundances in a sample or a community:
E = H / ln( S ) Where H= Shannon-Wiener index and S= species Richness
- Richness (S): total number of species in the community - Species richness:the number of species in a community.
22
4.1 Biodiversity
Because of the mesh structure of the reefs it was difficult to monitor the artificial reefs. And oyster reefs in general can be very dense structured. Therefore it is only possible to observe the biodiversity on the surface. This might prevent some species from being found. In general it could be observed that
biodiversity of the first artificial oyster reef mainly consisted of Littorinidae and Cirripedia. It appeared to have lower diversity than the second artificial reef.
When looking at the natural reef the exotic colonial ascidian Didemnum sp. occurred in higher densities than on the artificial reefs.
Figure 15. Overview of the different species found on the artificial reefs over the three sample dates combined. Algae species are shown separately because they were estimated for percent coverage instead of abundance class.
A B
C D
23 Although observations indicate that higher taxonomic levels presented in figures 16 and 17 are of a single species, care must be taken in the interpretation due to the comparison of different taxonomic levels. For ease of explanation the lowest taxonomic level used for each group were hereafter referred to as ‘species’.
When the species of all three measuring days are combined it can be observed that artificial Reef 2 had a higher species richness with 27 species total (fig. 16 Band D) compared to artificial Reef 1 with 17 species total (fig. 16 A and C). Algae species are shown separately because they were estimated for percent coverage instead of abundance class.
The natural oyster reef had a lower species richness, with only 10 species in total (fig. 17 A and B), than both artificial reefs (17 on reef 1 and 27 on reef 2). However the graphs of the artificial reefs (fig 16 A, B, C and D) consist of the results of all three monitoring dates combined and the graphs of the natural reef consist of results from the first day only. Therefore figure 16 and 17 should not be compared. However, when, the species richness on only day 1 in considered, there is a simlar general outcome. where both artificial reefs still have a higher species richness compared to the natural reef on day 1 with 13 and 19 species in total for Reef 1 and 2 respectively compared to 10 species total on the natural reef.(fig. 16).
The total abundance for Reef 1 is more than twice as high as for the Reef 2 (fig. 17). Reef one has fewer different species (figure 16) but more individuals per species on average (figure 17). This is
Figure 16. Overview of the different species found on the natural oyster reef.
Algae and other species which were estimated on percent coverage instead of abundance class are shown separately.
A
B
24 mainly due to the high amount of Littorinidae (Figure 15A) on the first reef which were occasionally counted as more than a hundred individuals in a single quadrant.
Figure 17.Species richness per reef on sample day 1
Figure 18. The total abundance per reef on sample day 1
The evenness on the natural reef was highest out of all reefs meaning the species are more equally divided over the community. However the Shannon-Wiener Index was highest on the second artificial reef due to the higher species richness (fig. 18).
25
Figure 19. Evenness and Shannon-Wiener Diversity Index values on the different reefs on day 1
If the Shannon Index is shown over time for the two artificial reefs (fig. 19) it is consistently higher for Reef 2. Day one has an almost equal evenness, but still a higher Shannon Index because the species richness was consistently higher on Reef 2 compared to reef one. For day two and three the difference is more extreme between the reefs due to the higher evenness for the second reef on those days.
Figure 20. The evenness and Shannon Wiener diversity index values per reef over time
Out of all species the exotic species on Reef 1 take up close to a quarter (26 % average) of the total species where as it is just over a third (36% average) for Reef 2 which even has 44 % of exotic species in total on day 3. Of the organisms on the natural reef, 30 %, were exotic species (figure 20).
26
Figure 21. Ratio between native and exotic species on each reef on the different sample days.
4.2 Crab populations
As part of the monitoring survey all crab species were counted and their size and gender were noted.
This gives an overview of the population density and distribution of the species on the artificial reefs.
Combined with the behavioral experiments this can give an indication of the effect exotic species may have on the community composition.
In general only Hemigrapsus takanoi and Carcinus maenas were found, however on day 2, four Porcellana platycheles individuals were found on Reef 1 (figure 21). The total number of crabs cannot be compared between the reefs because of the use of a different collection method (chapter 3.1).
27 When the results of both reefs are combined it can be observed for all three sampling days that individuals of both H. takanoi and C. maenas of the same size are present on the reefs (fig. 22 A, B and C) Where on day 1 there are still some individuals of C. maenas that are larger than the largest found H. takanoi (fig. 22 A) they are none left on day 3 (fig. 22 C).
Figure 22 Collected crab species over time on the two artificial oyster reefs.
28
A B
C
Figure 22 total frequency distribution of the size of different crab species found on bothartificial oyster reefs combined. The Carapace width (CW) was measured to the closest mm.
Figure A shows the frequency distribution of the CW on day one, figure B the frequency distribution of the CW on day 2 and figure C the frequency distribution of the CW on day three.
29 4.3 Behavioral experiment
Observations made while studying the crabs during the experiments are that C. maenas comes across as being more assertive. They were the first to move if something happened either being interested or trying to escape or fight when being handled. Instead of sitting in a corner hiding like H.takanoi, they appeared to be much more active, even trying to climb onto the air stone or tube which was connected to it. One individual even managed to escape this way. This did not happen during an experiment and therefore did not influence the results. H. takanoi acted more docile and were easier to handle. However when in the presence of C. maenas they seemed more dominant sometimes chasing them, perhaps as territorial behavior. C. maenas reacted submissive in these cases by running away even before H. takanoi was close. The only moments were C. maenas could be observed actively aggressive towards H. takanoi was when they wanted to take over the food during the food competition experiment. This was however not often compared with H. takanoi. These findings are pure interpretations of personal observations. In the tables below the statistical significance of these findings are shown.
All tests for statistical significance were done by using a CHI² test because the data sets were either too small or not normally distributed. A p-value <0,05 was considered significant
Competition for food
First the difference in feeding behavior with and without competition was investigated by comparing the behavior of the crabs in the competition experiment with crabs from their own species without competition. Then the difference between the two species was investigated by comparing the two different crab species in the competition experiment. The results are discussed per criterion.
The presence of a competitor did not make any significant difference for the behavior of which crab was first to eat meaning the crabs behaved the same either with, or without a crab from the other species present (table 4). C. maenas was significantly the first to eat (table 5).
Besides that competition did affect the number of times eaten, the total time eaten and the average eating time. In general the crabs without a competitor ate less often, but for a longer amount of time on average and in total (table 4).
Between the species C. maenas was significantly the first to eat and both species ate more often but less time on average and in total (table 5).
30
Table 4. Difference in feeding behavior of H. takanoi and C. maenas in relation to competition. The p-value is the result of a CHI² test
Behavioral difference with competition
p-value Significant Meaning
First to eat 0,72 No competition made no significant difference on which species was first to eat
Number of times eaten 4,35E-07 Yes both species ate significantly more often when there was competition
Total time eaten 0,00 Yes both species significantly ate shorter in total when there was competition
Average time eating time
3,27E-83 Yes both species had a significantly shorter average feeding time when there was competition
Table 5. Difference in feeding behavior between H. takanoi and C. maenas. The p-value is the result of a CHI² test
Behavioral difference between species
p-value Significant Explanation
First to eat 2,98E-20 Yes C. maenas was signicantly more often the first to eat
Number of times eaten 2,87E-06 Yes C. meanas has significanlty eaten more often Total time eaten 0,00 Yes C. maenas spent significantly less time
feeding in total Average time eating
time
1,00E-95 Yes C. maenas maenas spent significantly less time feeding on average
After the general feeding behavior the outcome of the encounters between the two crab-species were investigated. A crab was successful if he either displaced the other crab from the food, which was called a ‘successful attack’ or if he managed to keep the food and thus successfully defended it from the other called a ‘successful defense’.
31
Table 6. Outcome of encounters between H. takanoi and C. maenas. Successful attacks are when a crab successfully displaced the other from the food. Successful defense is when a crab manages to keep the food when attacked.
Confrontation results
p-value significant Explanation Total successful
encounters
1,83E-14 Yes H. takanoi had significantly more succesfull encounters in total
Successful attacks 1,53E-07 Yes H. takanoi does significantly more succesful attacks
Successful defense 1,25E-08 Yes H. takanoi is significantly more succesful when defending
The confrontation results show H.takanoi was significantly more succesfull in both attack (p=1,53E - 07) and defense (p=1,25E -08) at the moment of a confrontation (table 6). When the total successful encounters are put into percentages per species H.takanoi had a successful outcome for 88% of the time and C. maenas only 12% of the time ( fig. 24).
Figure 23. Percentage of total successful encounters for H. takanoi and C. maenas
12%
88%
Total successful encounters per species
C. maenas H. takanoi
32 Competition for shelter
A crab was considered to use the shelter if at least all his legs on one side are covered by the shell, because this would enable him to retreat quickly if necessary.
In general it seemed H. takanoi had a better tactic to get under the shell because the C. maenas often struggled not to flip the shell over.
Both species showed no significant difference (p=0,18) for when competition was present or not. Meaning competition had no effect on the amount of times shelter was used. There was however a significant difference (p= 0,02) between the behavior comparing the two species. Meaning H. takanoi used the shelter significantly more than C. maenas. When put in percentages C. maenas used the shelter 25% and H.
takanoi 75% out of the total times the shelter was used.
Figure 24. Total number of shelter use by H. takanoi and C. maenas with and without competition present. There was a significant difference between species (p=0,002) but no significant difference for each species when competition was present
Figure 25. Shelter use of both C. maenas and H.
takanoi in the presence of a competitor.
33
5. Discussion
Biodiversity
Both artificial reefs have a higher species richness compared to the natural reef and the total abundance for Reef 1 is more than twice as high as for the Reef 2. This was mainly due to the high amount of Littorinidae on the first reef which were occasionally counted as more than a hundred individuals in a single quadrant.
The higher species richness on the artificial reefs might be because of their considerably larger size compared to the natural reef. This provides a larger and denser structure and thereby probably a calmer shelter area because it is able to reduce the strength of the water flow more. (Fritz, 2013) It is also possible that the conditions at the location of the natural reef limit the amount of species for example turbidity or water depth. Even small changes on the specific location can determine the survival of some species. (Thomsen, et al., 2007)
The evenness on the natural reef was highest out of all reefs. The species were more equally divided over the community compared to the other reefs. However the Shannon-Wiener Index was highest on the second artificial reef due to the higher species richness.
In this case the Shannon-Wiener index is the most important tool to assess the biodiversity on the reefs because it takes multiple things into account. If the number of species is low they also experience less competition. This might result into a reef with a higher evenness, but if that is combined with a low species richness the index will still be low. It is clear that artificial reef 2 shows the highest biodiversity. This might be due to several characteristics. To start off it is the biggest reef out of all reefs found in the area. As said before this makes it able to change the dynamics of the system like flow velocity more helping species to settle. Besides that the reef is placed in a different position more in line with the flow direction compared to Reef 1. Also it has a more dense structure than artificial reef 1 because Reef 1 has the loose shells which are washed from one side to the next depending on the tides. All these things combined provide a more stable habitat helping species to settle. (Fritz, 2013) (Harwell, et al., 2011) Finally as said about the natural reef it might just be
because it is at a better location in general when comparing factors as flow velocity, light or nutrients (Thomsen, et al., 2007). Besides these characteristics it can also be the case that there are more species on the artificial reefs because they have been there for a shorter amount of time. The species may not have had time to outcompete each other yet which still gives some species a shot at being there at the moment. This situating is sometimes called the ‘boom and bust’ strategy. (Harwell, et al., 2011)
If the Shannon Index is shown over time for the two artificial reefs it is consistently higher for Reef 2.
Day one has an almost equal evenness, but still a higher Shannon Index because the species richness was consistently higher on Reef 2 compared to reef one. For day 2 and 3 the difference is more extreme between the reefs due to the higher evenness for the second reef on those days.
Out of all species the exotic species on Reef 1 take up close to a quarter (26 % average) of the total species where as it is just over a third (36% average) for Reef 2. Of the organisms on the natural reef,
34 30 % were exotic species. If the total number of exotic species is compared per day between reefs, Reef 2 has more than twice as many exotic species compared to the other reefs. Every exotic species found on the natural reef and Reef 1 also occurs on Reef 2. This can be explained because the total species richness of reef 2 is much higher in general. Even though the species richness differed greatly between reefs, the percentages are not that far apart and appear to behave the same.
A remarkable difference however, was the amount of the colonial ascidian Didemnum sp. present at the natural reef covering 53% of the reef on average compared to a maximum of 15% on reef 2. This type or organism is known to be able to grow rapidly overgrowing almost all other kinds of sessile organisms, suffocating them. This makes it an invasive species with a high impact on the ecosystem.
(Gittenberger, et al., 2010)
Another species which should be mentioned is the Japanese Oyster drill, Ocinebrellus inornatus. As the name already suggests it feeds on oysters, but also mussels. This makes it a threat to the oyster reefs and the aquaculture in the Eastern Scheldt. Only one individual was found which suggests they have not truly establishes themselves on the reefs yet, but the find itself is proof that they are getting to this part of the Eastern Scheldt. It is unclear on when they were introduced, but they did start to become more abundant after 2007 (Lützen, et al., 2012).
Hemigrapsus takanoi was the only exotic crab species. It is believed these crabs were introduced into Europe during the early 1990’s by the transport of Asian oyster or hull fouling or ballast water. Since then they have spread out along the European Coast at a high rate (100 km a year). They are able to spread that fast because their young reproduction age and because of their planktonic larvae (Noël et. al. 1997) which they may have up to six times a year. (Dumoulin 2004)
Around the turn of the century Hemigrapsus takanoi was unintentionally introduced into the Dutch delta via shellfish transport for aquaculture purposes. While the number of Carcinus maenas had declined over the past 20 years the decline already started before the introduction of Hemigrapsus takanoi but C. maenas is now clearly outnumbered by H. takanoi. The decrease in the C. maenas population was not started by H. takanoi but it is possible that H. takanoi is taking advantage of these decreasing numbers and so contributing to the decline of C. maenas (Van den Brink, 2012).
Van den Brink et al. (2012) hypothesized that this outnumbering of C. maenas by H. takanoi was due to adult H. takanoi out-competing juvenile C. maenas smaller or similar size to adult H. takanoi for shelter.
As part of the monitoring survey all crab species were counted and their size and gender were noted.
This gives an overview of the population density and distribution of the species on the artificial reefs.
Combined with the behavioral experiments this can give an indication of the effect exotic species may have on the community composition.
In general only Hemigrapsus takanoi and Carcinus maenas were found, however on day 2, four Porcellana platycheles individuals were found on Reef 1. The total number of crabs cannot be compared between the reefs because of the use of a different collection method (chapter 3.1).
35 When the results of both reefs are combined it can be observed for all three sampling days that individuals of both H. takanoi and C. maenas of the same size are present on the reefs. The fact that there are crabs of both species within the same size range on the reef supports the hypothesis that the H. takanoi is competing with juvenile C. maenas.
Where on day 1 there are still some individuals of C. maenas that are larger than the largest found H.
takanoi they are none left on day 3. This is explained by the fact that larger C. maenas move to deeper water when it gets colder (Audet , et al., 2008).
Behavior experiments on the competition for food and shelter can give more insight to support the hypothesis even further.
Behavioral experiments
During the behavior experiment for food. The presence of a competitor did not make any significant difference (p= 0, 72) for the behavior of which crab was first to eat meaning the crabs behaved the same either with, or without a crab from the other species present.
Besides that competition did affect the number of times eaten, the total time eaten and the average eating time. In general the crabs without a competitor ate less often, but for a longer amount of time on average and in total.
Between the species C. maenas was significantly the first to eat (p=3E -20) and both species ate more often but less time on average and in total.
C. maenas seemed to be better at getting the mussel open to feed on it. All mussels were drilled to Anker them and to trigger an extra feeding response and the crabs were always trying to open that hole further. The slimmer chelae of C. maenas managed to chip of pieces easier than H. takanoi did.
All these results were expected and are similar to the results from (Jensen , et al., 2002) who did similar behavioral experiment between C. maenas and H. sanguineus and C.maenas and H.
oregonensis. This also accounts for the confrontation results.
The interaction results show H.takanoi was significantly more succesful in both attack (p=1,53E -07) and defense (p=1,25E -08) at the moment of a confrontation (table 6). When the total successful encounters are put into percentages per species H.takanoi had a successful outcome for 88% of the time and C. maenas only 12% of the time. Although H. sanguineus gets larger than the closely related H. takanoi their behavior is very similar. The results found by (Jensen , et al., 2002) showed an even more extreme difference with 95% successful encounters for H. sanguineus compared to 5%
for C. maenas.
It was observed that H. takanoi often acted as a ‘bully’ meaning they would displace C. maenas from the food but afterwards hardly paid attention to the food. This could be named an act of territorial display.
The shelter experiment consisted of a more straight forward measurement than the behavioral experiment. For the shelter experiment the tides were simulated twice within 48. Half a mussel shell
36 was provided as shelter. Instead of videotaping it, the position of the crabs was noted at the start of the experiment (high tide), during the first low tide, the second high tide and at the second low tide.
A crab was considered to use the shelter if at least all his legs on one side are covered by the shell, because this would enable him to retreat quickly if necessary.
In general it seemed H. takanoi had a better tactic to get under the shell because the C. maenas often struggled not to flip the shell over. This happened because C. maenas grabbed the edge of the shell with its chelae instead of just using it as a lever like H. takanoi did.
Both species showed no significant difference (p=0,18) for when competition was present or not.
Meaning competition had no effect on the amount of times shelter was used. There was however a significant difference (p= 0,02) between the behavior comparing the two species. Meaning H. takanoi used the shelter significantly more than C. maenas. When put in percentages C. maenas used the shelter 25% and H. takanoi 75% out of the total times the shelter was used. This is again similar to the results of C. maenas (6,6%) against H. sanguineus (93,3%) from (Jensen , et al., 2002).
These results cannot be compared to the natural behavior of the crab species in the wild because the crabs in this setting were forced to interact due to the confined space. Normally they would have the ability to flee or take the food with them to an easier to defend spot. However it does shed some light on the interaction between an exotic and native species. H. takanoi was more successful in both experiments. Even though they interacted in a different in an experimental set up, we have seen both species occur on the reefs within the same size range and are therefore competing for both food and shelter. This means H. takanoi is very likely to negatively influence the population of C maenas.
It can be expected the artificial reefs will develop into natural reefs if more oyster spat settles onto the already existing shells. One of the conditions however is that the oyster shells are packed dense enough to prevent the shells from shifting too much as seen on artificial reef 1. The biodiversity on reef 2 may decrease eventually when some species outcompete each other. Besides that both Didemnum sp. and O. inornatus do pose a risk for the reefs and aquaculture in the area. The reefs provide the perfect habitat for them and they hardly have any competition from the other species.
This risk should be taken seriously and must be taken into account when building more reefs.
Also it could be wise to change the shape of the reef. Suggested by (Fritz, 2013) was to give the reefs a more natural rounded shape. Besides being more appealing this shape helps to curve the incoming water flow making it easier for the shells to stay in place and for the organisms on the edge to stay put while still benefitting from the water flow on the outside of the reef. Even deliberately add open spaces in between the structure creating some larger patches can be recommended. Some species prefer to live on the edge and this increases their chances. These can also act as refuge for some species of fish (Harwell, et al., 2011).
