Long-term study on a brachyuran crab community (Crustacea: Decapoda) inhabiting a boulder shore in the Eastern Mediterranean: relative abundance

Abstract

A crab community inhabiting a cobble shore was studied from samples taken on 41 collecting trips in six individual years over a long period of 16 years (1984–1999). The aim of this long-term study was to find whether oscillations in abundance can be distinguished from decline in numbers. Quantitative changes taking place in the relative abundance of the four most common species during that period are described. Thus, 352 Pachygrapsus transversus, 233 P. marmora- tus, 48 Eriphia verrucosa and 155 Xantho poressa specimens were collected throughout the study. Some oscillations were observed in the relative abundance of both Pachygrapsus species and in E. verrucosa. In X. poressa a marked decline was noticeable after 1986. The sex ratio was male-biased in most cases except in P. marmoratus and in E. verrucosa in August. Since apparently no major change took place in the habitat itself, the main conclusion of this study is that the fluctuations in relative abundance of all species may be due to fluctuations in the sex ratio that occur naturally. Thus, the fact that during five months no E. verrucosa females were found is sufficient to effect such fluctuations in their population numbers. On the other hand, the decline in Xantho populations after 1986 could have its cause in the zoea larvae that were somehow affected, with the consequence that fewer megalops settled successfully in that specific habitat.