Purple Shore Crab Behavior

Background

La Jolla tide pools are home to many purple shore crabs, formally known as Hemigrapsus Nudus. Previous research done on H. nudus have noted that it is common for this species to engage in adaptive sheltering behavior. In natural intertidal settings, these crabs were often observed either sheltering under rock formations or occasionally foraging on exposed surfaces (Greenway et. al, 794). This dual behavioral strategy is what spurred my project to investigate the ethological conditions that may drive H. nudus to spend time hiding in comparison to other adaptive behaviors. Moreover, it appears that hiding spaces are of particular importance to these crabs. In a study conducted by Jacoby with the goal of creating a clearly defined ethogram in the H. nudus, during field studies they were frequently found in crevices and interactions mainly took place near a shelter, in some cases defending their chosen location (Jacoby, 533). In this paper, shelter and hiding locales are demonstrated as a limited and defendable resource, emphasizing the ecological importance of hiding behavior. With this in mind it becomes apparent that hiding can be more than simply passive concealment, but rather an active defense. It’s important to note that despite the literature on the value and prominence of hiding behavior, these species also show voluntary preferences to remain out of water (Greenway et. al, 792). Although it seems the consensus for emersion behavior is variable upon experimental conditions, this observed preference to remain out of water may indicate that the drive to engage in hiding is adaptive and functional advantageous.

For my observational experiment, I chose to study the hiding behavior of the H. nudus at both high vs low tide. By measuring this behavior and comparing it against contextual predictors (tide level), findings may offer an insight to the functional significance of hiding in the context of environmental conditions. Whether or not there are differences can lead to certain inferences explaining these behaviors such as predation avoidance, mate attraction, and other important drives.

Hypothesis

I hypothesized that purple shore crabs spend more time hiding during low tide than high tide.

Methods

Data was collected on the purple shore crabs (Hemigrapsus Nudus) located at the La Jolla tide pools in San Diego, California. I went for a total of 5 days where I conducted 6 rounds of observations per day; 3 trials during low tide and 3 trials during high tide. I consulted the US Harbors website to obtain information about the tides. For each trial, I observed crabs in the same 3 different tide pool formations, one for each trial and crab. In order to assess the crabs hiding behavior I conducted focal sampling recorded as ad libitum during both high and low tide conditions. To begin my observation, I first located a crab not currently in hiding. I observed this crab until it underwent hiding behavior, described by near or completely blocking their entire body from view by moving behind an object such as rocks, seaweed, anemones, or other natural marine structures. Once the crab fully positioned itself in hiding, I began my timer, recording the duration that the crab was in hiding, and stopping once the crab moved locations, revealing itself once again.

Results

This study was conducted to evaluate if purple shore crabs exhibited different hiding behaviors, particularly duration spent hiding, during low and high tide conditions. I conducted an independent sample T-test to examine the relationship between the time (in seconds) the crabs were in hiding and tide condition. The statistical analysis did not show evidence for a relationship between time hiding and tide conditions. Contrary to my hypothesis there is a slight difference in the time spent hiding with crabs exhibiting longer hiding times on average during high tide (M = 1049, SD = 1062) compared to low tide (M = 837, SD = 888). However the results show that there was no significant difference in hiding time between the two tide conditions, t(28) = 0.595, p = 0.278. Despite this nonsignificant result, the Shapiro-Wilk test of normality was significant (W = 0.839, p < .001), suggesting that the assumption of normality was violated and indicating a non-normal distribution.

Figure 1

Conclusion

My findings do not provide evidence for a relationship between tide condition and the duration of hiding within the purple shore crabs located at the La Jolla Tide pools and therefore failing to reject the null hypothesis. In fact, as the results demonstrate, my observed population on average spent more time hiding in high tide conditions, the opposite of what I originally hypothesized (time spent hiding would be greater in low tide). Although we cannot draw conclusions from these results as they are not significant, some possible explanations for why this might be the case circles back to the work of Greenway et. al (1996) where they recorded emersion behavior. Their results are indicative of a preference to be out of water and therefore out of hiding. One possibility may be that low tide conditions make it more accessible for crabs to engage in preferential emersion rather than in hiding whereas high tide conditions may increase their effectiveness to conceal.

Future Directions

Alterations I would implement to improve my study include the utilization of a camera and performing observations during different times of the year. Having a camera that can be submerged under water to record the purple shore crabs during trials may yield more accurate time recordings and help to combat frothy water, obstructing the observer's view from the crab subject. Additionally, performing trials during different seasons where the tide conditions or the behavior of the H. nudus may vary, can yield more accurate observations and therefore conclusions. Subsequent research should examine the potential predictive conditions of emersion and immersion behavior. Examining these behaviors may give better insight to the relationship between the H. nudus and its environmental conditions, highlighting prospective survival mechanisms.

Ethogram 

References

Greenaway, P., Morris, S., McMahon, B. R., Farrelly, C. A., & Gallagher, K. L. (1996).

Air Breathing by the Purple Shore Crab, Hemigrapsus nudus (Dana). I. Morphology, Behaviour, and Respiratory Gas Exchange. Physiological Zoology, 69(4), 785–805. http://www.jstor.org/stable/30164230

Jacoby, C. A. (1981). Behavior of the Purple Shore Crab Hemigrapsus nudus Dana, 1851.

Journal of Crustacean Biology, 1(4), 531–544. https://doi.org/10.2307/1548130