One of our recent interns, Andre Wax, completed and presented a research study on a topic of his choosing! Read about Andre’s work below.
Trapezia Bidentata, also known as the Rusty Guard Crab, are the local species of guard crabs found on Pocillopora corals near Quepos – Manuel Antonio region of the Eastern Tropical Pacific in Costa Rica. They are a species of smaller guard crab, maxing out at around 2.5 cm in length, and provide protection against smaller corallivores. The Pocillopora corals that they inhabit are species of branching corals, which provide shelter for the guard crabs. T. Bidentata will also stimulate the coral to release mucus by massaging the coral with their legs and collect the mucus the coral excretes and will consume this as their food source.
This study aims to discover if there is a relationship between the health of Pocillopora corals in the Quepos – Manuel Antonio area and the number of Trapezia Bidentata crabs found living in the coral. A null and alternative hypothesis were generated, where the null stated that there would be no relationship found while the alternative hypothesis stated that the number of T. Bidentata crabs found on the corals would depend on the health of the coral. If the alternative was true, it could be found that on healthier corals there are more guard crabs. Furthermore, larger corals likely have more crabs due to there being more space available.
The depth, length, number of T. Bidentata present, and coral health were recorded. Coral health was recorded using a scale from zero to four, representing dead and healthy respectively. Initially, depth was compared with the number of crabs and the length of the coral, and no correlation was found (Figure 1.1 and 1.2).
When coral length and number of crabs were compared with each other a strong correlation was found. The trendline generated from this data allowed for the data to be analyzed for the corals that had more crabs than expected (data points above the trendline) and corals that had less crabs than expected (data points below the trendline). Samples that had more crabs than expected had health ratings of 3.5 or 4 most frequently, with a few data points with a health rating of 3 and only one data point that was below a health rating of 3 (see figure 1.3).
When coral length and number of crabs were compared with each other a strong correlation was found. The trendline generated from this data allowed for the data to be analyzed for the corals that had more crabs than expected (data points above the trendline) and corals that had less crabs than expected (data points below the trendline). Samples that had more crabs than expected had health ratings of 3.5 or 4 most frequently, with a few data points with a health rating of 3 and only one data point that was below a health rating of 3 (see figure 1.3). Corals with less crabs than expected were generally corals that we unhealthy. When the frequency of finding different numbers of crabs on the corals was compared, it was revealed that as the corals get healthier, crabs would be found in higher numbers more often than compared to the corals that are unhealthy, and high numbers of crabs are only found on corals that are the healthiest (see figure 2.1).
Finally, based analysis of the data, the corals were separated into two categories, healthy corals (3-4 health rating), and unhealthy corals (0-2.5). Box and whisker plots were utilized to remove outliers and ensure normal distribution before conducting a two-variable t-test assuming unequal variances (see figure 3.1 and 3.2). This test was selected due to the possibility of unequal standard deviation among data sets and due to the large sample size. Using α = 0.05, the two hypotheses were H0 (T. Bidentata number is different when comparing health of Pocillopora corals): p ≥ 0.05 and HA (T. Bidentata number is different when comparing health of Pocillopora corals): p<0.05. When conducted, a p-value of 4.175*10-8 was obtained. 4.175*10-8 < α = 0.05 so the null hypothesis is rejected.
Analysis of the data reveals that T. Bidentata were found in larger quantities on the healthier corals. The coral length vs. crab number data reveals that very few unhealthy corals can have more crabs than expected, and this is supported by the t-test which resulted in an incredibly low p-value, suggesting a very strong significant difference between the number of T. Bidentata on healthy vs. unhealthy corals.
One possible reason for this could be that unhealthy corals are unable to sustain higher numbers of T. Bidentata. Since dead coral could not be able to produce mucus for the guard crabs to feed on, it would not be possible for guard crabs to survive on the coral.
The feeding strategy of crabs could also better prepare the immune system of the corals for future stressors. When corals release mucus, it is part of their immune response to stressors in their environment.
There are several reasons that this could be occurring. Since guard crabs feed on the mucus from the corals, unhealthy corals would supply guard crabs with less food so a lower number of guard crabs would be found. Another possibility is that a higher number of crabs would allow the coral to be more resistant to stressors in the environment. Corals with higher numbers of crabs would have more crabs to protect against corallivores. This would keep the corals healthier because predator attacks would be less successful in consuming the coral. The feeding strategy of Trapezia crabs could also keep corals healthier. When they feed, they stimulate the immune response of corals causing them to release mucus. Keeping the immune response stimulated could keep the coral more prepared for stressors that would cause this immune response.
Guard crabs such as T. Bidentata can be used in the future as an effective indicator species. Due to the strong relationship the number of guard crabs found on Pocillopora corals can be used to quantify the health of the corals. The benefits that this can provide is another way to determine coral health besides visual examination.
When fragmenting corals, it is important to ensure that guard crabs can remain on the corals. Guard crabs need to be on corals to survive, so if they are removed from corals during the fragmenting process, it is essential to reattach them to a fragment and attempt to keep the crabs on the fragments through the out-planting process. When fragmenting, cutting slowly allows the crabs time to move out of the way of the blade.
Trapezia Bidentata crabs play an important role in the reefs in the Quepos – Manuel Antonio area. With invasive species such as the Crown of Thorns seastar entering Costa Rica, the protection guard crabs provide is going to be essential in ensuring the survival of the reefs. Further research on this subject could include examining the coral health and guard crabs in a controlled setting, examining the size and number of crabs found depending on the shape and structure of corals, and comparing the health of corals and the number of guard crabs in coral nurseries.