4. Results
4.2 Crab populations
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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.
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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).
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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’.
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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.
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