SPATIAL DISTRIBUTION AND
ABUNDANCE OF INVERTEBRATE LARVAE IN THE BLANES SUBMARINE CANYON
AND ADJACENT OPEN SLOPE ( NORTH -
WESTERN M EDITERRANEAN S EA )
Clare Devine
Supervisors:
Joan B. Company
Institut de Ciències del Mar (ICM-CSIC)
Eva Ramirez-LLodra
Institut de Ciències del Mar (ICM-CSIC)
Eize Stamhuis
Faculty of Mathematics and Natural Sciences (RuG)
October 24th 2013
ABSTRACT
Knowledge of larval ecology in marine science is poor due to difficulties in sampling and
identification. Larvae from non-crustacean invertebrates were collected from the area of the Blanes submarine canyon and the northern and southern adjacent open slope in the North-western Mediterranean Sea. Sampling took place from the 26th to 29th of September 2011 using a Bongo planktonic net with a mesh size of 500 µm. The larvae were identified to Phylum level and to lower taxonomic levels where possible. Larvae were found to belong to seven phyla; Mollusca,
Echinodermata, Cnidaria, Annelida, Phoronida, Sipuncula and Bryozoa. The Mollusca dominated, making up 82.8% of the total larvae densities, followed by the Echinodermata at 9.9%. The larvae found did not correspond to adult populations observed in studies of deep-water fauna from the Blanes canyon and adjacent slope, suggesting some species may have a coastal origin. Spatial distribution of larval densities appeared unrelated to atmospheric and water mass characteristics.
Currents seemed to have a strong influence on larval distribution, with higher abundances directly above the central canyon, where eddies are trapped for longer periods compared to adjacent open slope, and along the eastern canyon wall where the current has a highly variable direction. This study provides a pioneer overview of which Phyla are represented in this region and how they are spatially distributed. Further developments in larval ecology will aid identification and help better understand the underlying causes of the distribution patterns observed.
CONTENTS
1. INTRODUCTION ... 1
2. MATERIALS AND METHODS ... 2
2.1. Sampling area ... 2
2.2. Sampling methodology ... 3
2.3. Sorting and identification of larvae... 4
3. RESULTS ... 4
3.1. Larval densities and distribution by Phylum ... 4
3.2. Larval identifications to lower taxonomical levels ... 11
3.2.1. Mollusca ... 11
3.2.2. Echinodermata ... 12
3.2.3. Annelida ... 16
3.2.4. Cnidaria ... 16
3.3. Atmospheric and surface water characteristics in the study area ... 17
3.4. Water column characteristics ... 17
4. DISCUSSION ... 19
REFERENCES ... 22
ACKNOWLEDGEMENTS ... 24
APPENDICES ... 25
1 1. INTRODUCTION
For organisms with multiple life stages, the larval phase is often overlooked and there is instead a strong research focus on the adult organisms. This effect has been enhanced in the marine environment, where the complexities of sampling, not to mention replicating reproduction under laboratory conditions, combine with the difficulties caused by the small size and fragility of larval organisms (Pfeiffer-Herbert et al. 2007, Metaxas & Saunders 2009). The lifestyle and morphology of adults and larvae of the same species can vary greatly, particularly in the marine environment where the majority of benthic organisms produce planktonic larvae with metamorphosis between each larval stage. Hence, the distribution, maintenance and colonisation potential of adult populations can be indirectly influenced by processes such as predation or dispersal by currents that affect their larval stage (Ellien et al. 2004). When these environmental factors are taken into consideration, it is important that more focus be given to larval ecology.
The deep-sea is the largest biome on Earth, and yet is one of the least studied and understood (Ramirez-Llodra et al. 2010b). Its physical characteristics make it a unique ecosystem. Photosynthetic light is unable to penetrate deeper than 200 m, so the majority of the deep sea is heterotrophic and reliant on nutrients falling from the surface (Gage 2003). This results in strong food limitation, and has also allowed areas with higher concentrations of nutrients, including submarine canyons, seamounts and hydrothermal vents, to become hotspots for biomass and, in some cases, biodiversity. Pressure is an issue that must be overcome, increasing by one atmosphere with every 10 m depth. Other physical traits of the deep sea are fairly uniform globally and with changing depth; temperature decreases rapidly and is typically ~2°C; salinity is ~35‰; and dissolved oxygen is near saturation at 5-6 ml l-1 (Ramirez-Llodra et al. 2010b).
One region in which these characteristics differ is the Mediterranean Sea, the largest of the peripheral deep seas. The hot, dry climate and the fact that exchange between the Mediterranean and the Atlantic is limited by the shallow Strait of Gibraltar result in a higher temperature and salinity, 14 °C and 39‰ respectively (Sardà et al. 2004). The continental shelf of the Mediterranean Sea features a high number of submarine canyons, with a provisional count standing at 348 (Würtz 2012; Company et al. 2012). These are found throughout the Mediterranean but in especially high numbers in the western basin.
Submarine canyons increase habitat heterogeneity, affect current circulation, and act as sediment traps, as organic matter is transported from the continental shelf to the deep basin (McClain & Barry 2010; Sardà et al. 2009b). Because of these particular characteristics, submarine canyons can become biodiversity hotspots, where biomass and abundance can both increase up to 15 times when compared to the equivalent depth outside of the canyon (Würtz 2012). The same currents that cause downward transportation can also act to concentrate plankton, including planktonic larvae, within the canyons (Vetter et al. 2010). The increased diversity of adult organisms and the high concentration of zooplankton make submarine canyons an ideal location for larval studies.
The aim of this study was to investigate the distribution and abundance of non-crustacean invertebrate larvae within a submarine canyon, specifically the Blanes canyon in the North-west Mediterranean Sea (Company et al. 2012). This canyon is highly studied due to its importance in the fishery of the red shrimp Aristeus antennatus. Although the biodiversity and biological processes of the deep-sea megafauna in the Catalan margin, including the Blanes canyon, have been studied intensively in the last decade (Sardà et al. 1994a, Sardà et al. 1997, Company et al. 2004, Sardà et al.
2004, Company et al. 2008, Sardà et al. 2009a, Ramirez-Llodra et al. 2010a, Fernandez-Arcaya et al.
2012, Tecchio et al. in press), very few studies have been conducted on the early stages of organisms
2 ((Ramirez-Llodra et al. 2010a; Fernandez-Arcaya et al. 2012), and only one on the larval phase of decapod crustaceans (Boné, 2012). As well as giving an initial idea of which non-crustacean invertebrate larvae are represented in the plankton over the Blanes canyon, the findings have been discussed in relation to environmental conditions in the area.
2. MATERIALS AND METHODS 2.1. Sampling area
The samples used in this study were collected from the Blanes submarine canyon situated between 41.0 - 41.8 °N and 2.7 – 3.2 °E, off the coast of Catalonia in the North-west Mediterranean Sea. It is one of many submarine canyons on the narrow continental shelf of the Catalan margin. The shelf itself breaks at 150 m depth and slopes down to 2000 m depth (Zuniga et al. 2009a). The start of the Blanes canyon is 4 km offshore at a depth of 60 m, with a connection to the Tordera River on land.
The top of the canyon is V-shaped in cross section, widening to a U-shaped cross section with the shift from high erosion to high sediment deposition with increasing depth (Flexas et al. 2008) (Figure 1). At its deepest point the canyon is also at its widest, 20 km across (Zuniga et al. 2009a). The west canyon wall is sharp and abrupt, contrasting with the smooth east wall.
The canyon intersects the Northern Current which flows in a south-westerly direction along the coast from the Gulf of Lions off the coast of France (Granata et al. 1999). The influence of the Blanes canyon on this current results in downwelling, an anticyclonic rotation upstream from the canyon, and cyclonic rotation above the canyon itself. Between 400 and 800 m depth, the Blanes canyon is heavily trawled by the Aristeus antennatus shrimp fishery (Granata et al. 1999; Sardà et al., 2009b).
Figure 1: The location of the Blanes submarine canyon (extracted from Sardà et al., 2009a)
3 2.2. Sampling methodology
The samples used in this study were collected during the oceanographic cruise OAMMS-04, part of the OAMMS project “Observation, Analysis and Modelling of the Mediterranean Sea” (CTM2008- 03983 to Nixon Bahamon). This project had the main objective of consolidating a system of observation of the Mediterranean Sea through the real-time measurement of a number of meteorological and oceanographic magnitudes with a moored buoy, complemented by periodic visits with a research vessel to the mooring site and assimilation of the observations obtained in order to carry out the systematic forecasting of sea conditions in the NW Mediterranean Sea (http://www.oceans.cat/OAMMS).
The OAMMS-04 cruise was conducted between the 26th and 29th of September 2011, aboard the R/V
“García del Cid”. Pelagic sampling was conducted at 19 stations over the Blanes canyon and surrounding area (Figure 2). This sampling was carried out using a Bongo net towed obliquely, at depths down to 300 m in the case of this study. The Bongo net used had a mouth diameter of 60 cm and a mesh size of 300 μm. Following collection, the samples were collected from the cod end of the bongo net and preserved in formaline.
In addition to the planktonic sampling 34 CTD (conductivity, temperature, and depth) profiles were performed at a range of stations in the area (Figure 3). Station D1 was both a Bongo trawl site and the location of a permanent buoy (D1, OOCS “Operational Observatory of the Catalan Sea”).
Figure 2. Bathymetric map of Blanes canyon showing station locations from the OAMMS cruise.
4 Figure 3. Location of CTD stations and bongo samples from the OAMMS cruise. Grey dots represent the CTD stations; red drops represent the bongos sampling locations.
2.3. Sorting and identification of larvae
In the laboratory, the larvae were rinsed in salt water to remove the formaline prior to sorting. All work was carried out under a fume hood to avoid personal hazard. Samples from station B13 and B18 were not included. In the case of B13, this was due to a high number of adult Euphausia which impeded correct sorting. The sample from B18 contained a large amount of sediment and benthic organisms, suggesting that there was contact with the bottom during sampling.
Non-crustacean invertebrate larvae were sorted from the samples and identified to the lowest level possible. Crustacean larvae had been the focus of a previous study (Boné, 2012). Larval identification was done following Young et al. 2002, Smith 1977, and Larink & Westheide 2011. Advice from experts on the various phyla was also sought, namely Dr Daniel Martin, Dr Xavier Turón and Dr Iosune Uriz (CEAB-CSIC, Spain), as well as Dr Craig Young (Uni. Oregon, USA). Once the larvae were identified, their taxonomic classification was checked on the WoRMs database (World Register of Marine Species, http://www.marinespecies.org/).
The depth from which the samples were collected and the volume of water filtered during each sampling was used to calculate the larval abundance in individuals/m2.
3. RESULTS
3.1. Larval densities and distribution by Phylum
A total of 5618 invertebrate (non-crustacean) larvae specimens were found from seven Phyla (Table 1). Of these, the most dominant phylum was the Mollusca, accounting for 82.8% of all the larvae found (Figure 4). The next most abundant was the Echinodermata, making up 9.9% of the total. Of the other phyla found, each accounted for less than 2% of the total, with the exception of larvae that
5 could not be classified. Difficulties with identifying certain individuals meant that for many larvae phylum was the lowest possible level of identification. However, for Mollusca, Echinodermata, Cnidaria and Annelida, lower taxonomic levels could be reached.
Table 1: Number of individual larvae (N), abundance (ind/m2), and percentage of total larvae found in each phylum.
PHYLUM N
DENSITY
(ind/m2) %
Mollusca 4671 2861 82.8
Echinodermata 536 341 9.9
Cnidaria 108 66 1.9
Annelida 61 37 1.1
Phoronida 18 12 0.4
Sipuncula 12 8 0.2
Bryozoa 10 7 0.2
Unknown 202 124 3.6
Figure 4. Total abundance of invertebrate larvae across all stations, by phylum.
The highest total larval abundance was found at station B6 (Figure 5). This was one of the deeper stations with a bottom depth of 1360 m. This was also the site with the highest relative abundance of Mollusca (Figure 6). The dominance of molluscs in the samples resulted in a strong similarity between patterns of Mollusca abundance and total larval abundance per station. The eastern canyon wall and central canyon also had a high larval abundance. The shallower waters had higher abundance of invertebrate larvae on the western side of the canyon. This same trend can be observed in the distribution of Mollusca larvae (Figure 7a).
Annelida 1.1%
Bryozoa 0.2%
Cnidaria 1.9%
Echinodermata 9.9%
Mollusca 82.8%
Phoronida 0.4%
Sipuncula 0.2%
Unknown 3.6%
Other 3,8%
6 Figure 5. Total larval density (ind/m2).
The larvae of other phyla do not follow the same pattern of distribution so closely. Echinodermata were most abundant at stations B4, B8 and B10 (Figure 7b), all of which have an intermediate density of Mollusca. Conversely, station B6, which supports the highest abundance of Mollusca larvae, has one of the lowest abundances of Echinodermata. Echinodermata abundance was at its lowest in the central canyon as well as at B19, the shallowest station. The relative abundance of Echinodermata was noticeably high at stations B8, B9, and B10, and was highest at D1 where it made up 28% of the total larvae (Figure 6). These stations are all in the shallower waters of the upper canyon. The high percentage of Echinodermata larvae at D1 is also linked to the fact that this station had the lowest abundance of the dominant phylum, Mollusca. Overall, the density-distribution trend for the Echinodermata larvae was a higher abundance in the upper canyon and east face, and lower abundance above the central part of the canyon.
7
Figure 6. Relative abundance of larval invertebrate phyla per station (Echino.: Echinodermata)
8
Figure 6 (continued). Relative abundance of larval invertebrate phyla per station. Echino.: Echinodermata.
9 The next most abundant group overall were the Cnidaria, represented by their planula larvae and making up 1.9% of the total (Figure 4). Cnidaria larvae were found with a fairly even distribution across all stations (Figure 7c). They showed slightly lower abundance in the southwest part of the canyon, at stations B15 and B16, and were completely absent from the similarly located but deeper B14.
The Annelida larvae showed more variation in their distribution. Stations D1, B12 and B15 had the highest abundance (Figure 7d). Though geographically isolated from one another, these stations were all located at a similar depth of around 400 m.
The other phyla that could be identified were the Bryozoa, Sipuncula, and Phoronida, as well as some larvae that could not be identified at all but were still included in the total count. These three phyla had very low abundance overall and were absent from most stations. The Bryozoa were found only on the eastern face of the canyon, with the addition of station Bo-17 (Figure 7e). The Sipuncula had its highest abundance at station B2, the site with the lowest total larval abundance (Figure 7f).
The Phoronida was most abundant at B14, where they made up almost 5% of the larval composition at a density of 8.9 individuals m-2 (Figure 7g).
10
a) b)
c) d)
e) f)
g) h)
Figure 7. Density by phylum (individuals / m2). a) Mollusca; b) Echinodermata; c) Cnidaria; d) Annelida; e) Bryozoa; f) Sipuncula; g) Phoronida, and h) Unidentified larvae. Note that density scale varies between phyla.
11 3.2. Larval identifications to lower taxonomical levels
3.2.1. Mollusca
The majority of the Mollusca were gastropods in the veliger stage (Figure 8). Of these, approximately one third were of the clade Pteropoda, which is comprised of the clades Thecosomata and Gymnosomata (Table 2). The other gastropod veligers had the spiral form typical of this phylum. A few Bivalvia specimens were also identified, though only present in the sample B19, and some Cephalopoda juveniles from B9, B16 and B17, all on the west face of the canyon.
Figure 8. Mollusca larvae. a-e) Gastropoda; f) Cephalopoda; g) Bivalvia; h-j) Pterapoda.
a) b) c)
d) f)
h) j)
e)
g) i)
12 Table 2: Number, density, and classification of the different larvae found in all the bongo samples
PHYLUM CLASS ORDER FAMILY SPECIES LARVAL
STAGE
Number Density (ind/m2)
Annelida 55 33
Polychaeta Sabellida Oweniidae 6 4
Spionida Poecilochaetidae 11 7
Spionida Unid. 13 8
Unid. 25 15
Bryozoa 10 7
Cnidaria 114 70
Hydrozoa Siphonophorae 24 15
Unid. 90 55
Echinodermata 536 341
Ophiuroidae Unid. Ophiopluteus 451 288
Unid. Postlarvae 21 13
Unid.
In
metamorphosis 8 5
Ophiurida Ophiotrichidae
Ophiothrix
fragilis Postlarvae 54 34
Unid. Pluteus 2 1
Mollusca 4671 2861
Gastropoda 3095 1870
Pteropoda
(clade) 1569 988
Bivalvia 3 2
Cephalopoda 4 3
Phoronida 18 12
Sipuncula 12 8
Unidentified 202 124
Total 5618 3456
3.2.2. Echinodermata
All but two specimens of Echinodermata found were Ophiuroidea (brittle stars). The ophiuroid larvae were present in three stages of development: ophiopluteus, post-larvae, and in metamorphosis between these two stages (Figures 9 and 10). One post-larval type could be identified as the species Ophiothrix fragilis, a shallow-water species. The other post-larvae appeared to be Ophiura albida. Two stations, B5 and B15, contained only the ophiopluteus stage (Appendix 3).
At the stations with the highest echinoderm abundance (B4, B8 and B10) the echinoderm larvae were comprised of over 85% ophiopluteus. Three stations contained a higher percentage of post- larvae than ophiopluteus; these were B7, B12, and B19. The highest percentage of larvae in metamorphosis was on the western face, in stations B16 and B17.
13 Figure 9. The relative abundance of Ophiuroidea larval types by station.
85%
14%
Total
1%Ophiopluteus Postlarvae In metamorphosis
98%
D1
2%Ophiopluteus Postlarvae
75%
25%
B2
Ophiopluteus Postlarvae
90%
7% 3%
B3
Ophiopluteus Postlarvae In metamorphosis
97%
B4
3%Ophiopluteus Postlarvae
100%
B5
Ophiopluteus
50%
50%
B6
Ophiopluteus Postlarvae
42%
58%
B7
Ophiopluteus Postlarvae
86%
13% 1%
B8
Ophiopluteus Postlarvae In metamorphosis
75%
20%
B9
5%Ophiopluteus Postlarvae In metamorphosis
14
Figure 9 (continued). The relative abundance of Ophiuroidea larval types by station.
91%
B10
9%Ophiopluteus Postlarvae
81%
19%
B11
Ophiopluteus Postlarvae
47%
53%
B12
Ophiopluteus Postlarvae
88%
12%
B14
Ophiopluteus Postlarvae
100%
B15
Ophiopluteus
65%
27%
B16
8%Ophiopluteus Postlarvae In metamorphosis
77%
15%
B17
8%Ophiopluteus Postlarvae In metamorphosis
40%
60%
B19
Ophiopluteus Postlarvae
15 Figure 10. Ophiuroidea larvae; a) Ophiothrix fragilis post-larva; b) post-larva, thought to be Ophiura albida; c) ophiopluteus (species unknown); d) larva in metamorphosis. Spines from pluteus are still visible (species unknown).
a) b)
c)
d)
16 3.2.3. Annelida
Identifications to lower taxonomical levels than Phylum were also possible for the Annelida specimens. All of the Annelida were in the class Polychaeta. Of these, two orders were identified, the Sabellida for which Oweniidae were found, and Spionida, which contained several unidentified specimens as well as individuals of the family Poecilochaetidae (Table 2; Figure 11).
Figure 11. Annelid larvae: a) Owenia; b&c) Spionidae; d) Poecilochaetidae; e&f) Unidentified polychaetes.
3.2.4. Cnidaria
Of the Cnidaria only Siphonophorae could be positively identified (Figure 12). For this group, it is likely that the count underestimates the actual number, as there were many broken fragments that could not be included in the count. In addition to the Siphonophorae, two unidentified planula types were found.
Figure 12. Cnidaria larvae: a) Unidentified Cnidaria planula; b) Siphonophora planula.
a) b) c)
d) e) f)
b)
a)
17 Larvae representing the Bryozoa, Phoronida and Sipuncula could also be identified, but only to Phylum level (Figure 13).
Figure 13. Other represented phyla. a) Bryozoa cyphonaute; b) Phoronida; c) Sipuncula.
3.3. Atmospheric and surface water characteristics in the study area
Data collected from the permanent buoy at station D1 in the canyon mouth showed high atmospheric conditions of 1022-1025 mbar during the week of the cruise. The air temperature ranged between 19 and 24°C and relative humidity was between 50 and 80%. The wind had a southwest direction and a speed of 0 to 5 m/s. The surface current was 10 to 50 cm/s, a moderate speed that in this area usually occurs during storms.
During the time of the cruise, the mean current was found to decrease with depth. The current had a southerly direction down to a depth of 80 m, where the direction changed to the east (Table 3).
Table 3: Current velocity and direction at station D1 (data obtained from permanent oceanographic buoy OOCS, located in the mouth of the Blanes submarine canyon).
Water depth (m) Velocity (cm/s) Direction
7 14 181° S
25 12.2 182° S
50 9.7 186° S
80 5.9 106° E
3.4. Water column characteristics
The CTD stations from which water column data was collected do not correspond directly with the locations of the Bongo samples of this study (Figure 3). However, the oceanographic data was consistent across the CTD sampled area. With this fact and knowledge of the physical oceanographic dynamics of the study area, it was assumed that the water conditions were comparable throughout the whole area (Flexas et al. 2008).
For all CTD stations, temperature and fluorescence were profiled to a depth of 150 m. The samples all showed a similar pattern with no one sample standing out from the others. The only measured
18 variable without a homogenous distribution was salinity, resulting from the mixing of continental fresh water with the sea water in the canyon.
The observed temperature profile was typical for the end of summer in the Mediterranean Sea. The thermocline was located at 30 to 50 m depth (Figure 14). Chlorophyll fluorescence was low, although it reached 1.5 µg/l at some stations. The Deep Chlorophyll Maximum could be seen at 40 m depth.
Below this depth, the fluorescence detected was related to dead phytoplankton.
Figure 14. Environmental data from CTD stations: a) Temperature; b) fluorescence chlorophyll; c) dissolved oxygen; d) density; e) salinity.
19 4. DISCUSSION
Previous studies (Ramirez-Llodra et al. 2010a; Tecchio et al., in press) have sampled the adult populations of megafauna in the Blanes submarine canyon and adjacent slope. All the Mollusca found in these studies were cephalopods, with no gastropods sampled. This contrasts with the present findings in the larval community, where gastropods dominated at all sites. One explanation could be that the above-mentioned studies, which focused on megabenthic organisms and sampled using commercial bottom otter trawls, did not capture infaunal species such as burrowing gastropods. Nevertheless, this may not be the only explanation, as trawling studies using the same sampling gear on the nearby La Merenguera canyon found a variety of gastropod species were present (Ramirez-Llodra et al. 2008). An alternate explanation is that these gastropod larvae originated not from the canyon population, but from shallower coastal area gastropods. As it was not possible to identify to species level the sampled larvae, it is impossible to be certain of their source location.
The one species that could be positively identified was the echinoderm Ophiothrix fragilis. This ophiuroid species was also absent from the adult populations in the canyons sampled in previous studies. In fact, the Echinodermata were poorly represented and the Ophiuroidea were not present at all (Ramirez-Llodra et al. 2010a). Ophiothrix fragilis is a coastal species found in shallow waters throughout Europe. The presence of this species in the larvae collected from above the Blanes submarine canyon provides further evidence that some of these planktonic larvae have coastal origins. The canyon acts as a funnel for currents off the continental shelf, allowing transport of particulate material from the shelf to the open sea, and so it can be assumed that coastal larvae can also be transported in this way (Zuniga et al. 2009b) and potentially be lost for recruitment to their adult habitat.
Passive movement by ocean currents is a main driving force behind larval distribution. This can often result in an uneven distribution (Maynou et al. 2008). The currents around the Blanes submarine canyon typically enter up the canyon from the north-eastern side and the water moves west with increasing depth as it moves down the canyon away from the coast (Figure 15) (Flexas et al. 2008).
Along the sharp western wall of the Blanes canyon, the current is unidirectional and favours downwards transportation of particulate material (Zuniga et al. 2009a). This may explain why fewer larvae were found on this side of the canyon, for example at stations B9, B14 and B17, compared with the eastern wall whose smooth surface creates a flow with highly variable direction that may favour larval retention.
Directly above the Blanes canyon, there is cyclonic circulation resulting from the narrow shape of the canyon (Flexas et al. 2008). This circulation type has been shown to act as larval retention zones by trapping larvae in eddies, often leading to strong settlement and recruitment in these areas (Landeira et al. 2010). Stations B5, B6 and B7, directly over the canyon, had some of the highest larval abundances and this may be due to the presence of eddies in this area.
Distribution patchiness can be increased further by differences in larval origin and planktonic duration (Ellien et al. 2004). As previously discussed, some of the larvae found have originated from the coast; however, as submarine canyons can be hotspots of biodiversity, it is likely that some of the larvae will have their source in the deep canyon waters. The site and timing of spawning can affect distribution due to the currents in different areas varying in strength and direction. Planktonic duration is also important as this is strongly linked to potential distribution distance (Metaxas &
Saunders 2009). In addition, larvae that are in the plankton for longer periods are more likely to become trapped in larval retention zones due to their extended exposure to currents. Many species
20 also show ontogenic migration, moving deeper with increasing age, and this can expose them to different hydrodynamic forces (Ellien et al. 2004; Sardà et al. 1994b; Company & Sarda 1997). These factors can be used to explain not only the overall variability in distribution, but also the differences between Phyla, which are likely to have different spawning patterns and larval durations.
Figure 15. Current velocity vectors over the submarine canyon at 200 m depth. Summer sampling from 22 – 25 June, fall sampling from 2 – 5 November. Extracted from Flexas et al. 2008.
Oceanographic data collected from the CTD casts showed that fluorescence, chlorophyll, temperature, density and dissolved oxygen were fairly constant across stations, especially at the depths at which the samples were collected. Although the location of these stations does not correspond directly to the location of the Bongo samples, the consistency across the CTD stations suggests that that these factors are similar across the entire Blanes canyon. As such, it is unlikely that these environmental factors played a role in the observed larval distribution. The only environmental factor that varied strongly between stations was salinity, due to the influx of fresh water from the coast. If this was affecting the larval distribution, there should be a trend with increasing distance from the coastline and this was not the case.
The difficulties with identifying the samples to lower taxonomic levels, together with the scarce information on the life-history traits of the organisms such as spawning sites and times and larval duration, limited our ability to reach solid conclusions as to the cause of larval densities and distribution. Environmental factors such as temperature and chlorophyll concentration seem unrelated. The main patterns observed seem to be related to currents in the area. Higher abundances of larvae occur in the centre of the canyon, where eddies form particle retention zones, as well as along the eastern wall of the canyon, where the current moves in various directions due to the bathymetry of the area, again favouring larval retention. It is clear that some larvae, such as
21 Ophiothrix fragilis, originated from the coastal area rather than the populations living on the canyon seafloor and this difference may have affected distribution patterns.
There is still much to be learnt about larval composition, distribution and ecology of deep-sea species in general, and in particular of the non-crustacean community of the Blanes canyon and adjacent slope. This study provides a pioneer overview of which Phyla are represented in this region and how they are distributed. Further developments in larval ecology will aid identification and help understand better the underlying causes of the distribution patterns observed.
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24 ACKNOWLEDGEMENTS
I would like to thank all the people who supported me throughout this research experience. Without them this thesis would not be possible.
To Eva Ramirez-Llodra and Joan B. Company who provided me with this opportunity, providing guidance and support throughout and helping me with all my questions with their kindness and infectious passion for the subject.
To Alexandra Boné for helping me from the very beginning, no matter how big or small the question, and for all the guidance with the lab work and beyond.
To Daniel Martin, Xavier Turon and Iosune Uriz, of CEAB-CSIC (Spain), and Craig Young of the University of Oregon (USA) for their invaluable help with larval identification, and for their enthusiasm which was highly encouraging.
To Eize Stamhuis for providing support from my home university and for allowing me to take up this opportunity in the first place.
And finally to all my family and friends for their emotional support through this experience.
25 APPENDICES
Appendix 1. Number of larvae found per phylum at each station.
Annelida Bryozoa Cnidaria Echinodermata Mollusca Phoronida Sipuncula Unknown Total
D1 8 0 14 51 98 0 0 12 183
B2 0 1 3 12 74 3 5 5 103
B3 3 2 3 29 268 0 1 11 317
B4 0 0 4 71 420 0 0 10 505
B5 2 0 7 15 492 0 0 9 525
B6 1 0 12 8 696 0 1 10 728
B7 5 1 7 12 327 1 0 6 359
B8 3 0 7 87 362 0 0 9 468
B9 1 0 6 44 158 0 0 10 219
B10 2 3 5 68 208 0 0 12 298
B11 2 2 6 21 102 1 1 8 143
B12 11 0 9 15 192 0 0 26 253
B14 6 0 0 26 208 13 1 10 264
B15 12 0 2 21 285 0 0 15 335
B16 0 0 3 38 208 0 2 12 263
B17 3 1 11 13 173 0 0 22 223
B19 2 0 9 5 400 0 1 15 432
Total 61 10 108 536 4671 18 12 202 5618
Appendix 2. Abundance of larvae found per phylum at each station (ind/m2).
Annelida Bryozoa Cnidaria Echinodermata Mollusca Phoronida Sipuncula Unknown Total
D1 4.6 0 8.0 29.0 55.8 0 0 6.8 104.1
B2 0 0.7 2.0 8.1 49.8 2.0 3.4 3.4 69.3
B3 1.9 1.3 1.9 18.6 171.6 0 0.6 7.0 202.9
B4 0 0 2.3 41.1 243.2 0 0 5.8 292.4
B5 1.2 0 4.2 9.1 297.5 0 0 5.4 317.5
B6 0.6 0 7.4 5.0 431.0 0 0.6 6.2 450.9
B7 2.9 0.6 4.1 7.1 192.7 0.6 0 3.5 211.5
B8 1.9 0 4.3 53.9 224.3 0 0 5.6 290
B9 0.7 0 4.1 29.8 106.9 0 0 6.8 148.1
B10 1.6 2.4 3.9 53.7 164.2 0 0 9.5 235.2
B11 1.4 1.4 4.2 14.6 70.9 0.7 0.7 5.6 99.4
B12 5.7 0 4.7 7.8 99.4 0 0 13.5 130.9
B14 4.1 0 0 17.9 143.0 8.9 0.7 6.9 181.4
B15 7.4 0 1.2 13.0 176.6 0 0 9.3 207.6
B16 0 0 1.7 21.7 119.0 0 1.1 6.9 150.4
B17 2.0 0.7 7.2 8.5 113.1 0 0 14.4 145.8
B19 1.0 0 4.6 2.5 202.5 0 0.5 7.6 218.7
