Population density and spatial arrangement of two holothurian species in a coral reef system: Is clumping behavior an anti-predatory strategy?

A first attempt to estimate population densities of Holothuria atra and H. pardalis in relation to the spatial arrangement of individuals was made around Clipperton atoll. Seven sites of 120 m2 each with four transects were examined, counting the individuals per holothurian group: in total 1711 of which 1199 H. atra and 512 H. pardalis. The Morisita index of aggregation was applied to identify the spatial arrangement of individuals: random, uniform or clumped. Holothuria atra was found all around the atoll and densities varied between 0.03 to 4.78 ind m-2. It showed a clumped distribution on the eastern side of the atoll, and random and uniform patterns on the western side. Its broad distribution was attributed to its being a 'habitat generalist species', its tolerance to high temperatures, and its ability reproduce both sexually and asexually. Holothuria pardalis was found in only one site with 4.27 ind m-2 and a clumped distribution. The correlation of the Morisita index and density values of both species revealed that high densities were associated with a high degree of aggregation. This result led us to hypothesize that the clumped distribution of holothurians might provide protection against predators. Other ecological factors causing aggregations are discussed.