Population Dynamics and Behavioral Ecology of Halimeda slug Elysia pusilla (Bergh, 1872)

Abstract

Elysia pusilla (Bergh, 1872) is a sacoglossan sea slug that feeds on calcified green algae, Halimeda spp. It can incorporate and maintain chloroplasts from its algal food in its digestive glands, exhibiting kleptoplasty. The slug distributes in tropical Indo-Pacific. Evolution, symbiotic relationship, and kleptoplastic ability of sea slug have been well documented. However, less is known about their temporal variation in natural populations including the relationship between abundance of slugs and its algal hosts. Therefore, in this study, population dynamics of E. pusilla, the relationship between abundance of E. pusilla and its algal hosts, Halimeda macroloba, and behavioral ecology of E. pusilla were investigated at Lidee Island, Satun province. The results show that there was temporal variation in density of H. macroloba and E. pusilla egg masses but there was no variation in density of E. pusilla individuals because the number of the slugs was low. The analysis suggested that density and total surface area of H. macroloba which is the habitat availability might determine the occurrence of the slugs and the egg masses. There was a higher occasion to find the slugs in dense patches of the algae in which it related to surface area of the algal host. In this study, the age of algae was categorized into 4 stages, stage 1 is the new recruitment and stage 4 is mature plant. The highest number of slugs, egg masses and grazing marks were found on stage 4 which has the largest surface area comparing to the other stages. The highest number of egg masses and segments with grazing marks were found on the terminal segments. This part of the thalli has high concentration of secondary metabolites. These secondary metabolites reported to be utilized by slugs to deter their predators. In addition, terminal segments have thin calcification which helped the slugs graze easier. Moreover, the upper segments including terminal segments have larger surface area than the basal segments, reflecting the larger area of habitat use for the slugs. It might be another reason of a high occurrence of egg masses and grazing marks. Therefore, the amount of surface area which is related to the availability of habitat might be the important factor of habitat selection in E. pusilla.