Difference in predation deterrence behavior and evisceration frequency between two holothurian species: Holothuria mexicana and Euapta lappa.
Driving the high productivity characteristic of coral reefs, holothurians play an integral role in maintaining the reef ecosystem: mixing substrates, recycling detritus, nutrients, consuming sediment, and increasing oxygenation. Two of the most common holothurian species are the diurnal Holothuria mexicana (aspidochirote) and nocturnal Euapta lappa (apodaceates). These sediment feeding organisms, although of different species, have adapted similar strategies to deter predators and thereby sustain high productivity. Holothuria mexicana and Euapta lappa incapacitate predators using evisceration (expulsion of internal organs), leading to entanglement and confusion. The objective of this study was to determine species more adept at evisceration. Differences in evisceration frequency between Holothuria mexicana and Euapta lappa were assessed. Additional predation deterrence strategies, as well as the effect of volume on reaction rates were also taken into account. Specimens were submerged in cold (~ 8° C) seawater to attain maximum contraction length. Volumetric measurements were then taken before and after placement under uniform mass (1.12 x 104 g). Repeated observations were conducted in 20 minute intervals to assess types of predation deterrence reaction as well as volumetric changes in both species. The relationship between volume and predation reaction rate was analyzed using regression correlation. Regression equations correlating biometric characteristics for both species were determined. While Holothuria mexicana showed a higher percentage of evisceration than Euapta lappa did, the latter exhibited a wider variety of predation deterrence strategies. There was not a strong correlation between volume and reaction rate in both Holothuria mexicana and Euapta lappa. However, it should be noted Euapta lappa trials were capped at 180 minutes to maximize sample size over the five day period.