The morphological diversity within a species can obscure the correct identification

Critical points of various diagnostic characters and a paucity of information relating to the geographical distribution of several marine species can hinder real species delimitation, particularly if they are supposed to be cosmopolitan. Such constraints characterize many amphipod species and are mainly due to the variation in morphological characters during growth. Specifically, the benthic filter-feeding corophiid Cheiriphotis mediterranea Myers, 1983 displays different shapes for the male gnathopod 2 as it grows. This variation has hitherto never been described but an extensive sampling has provided us with the opportunity of studying it in detail. More than six thousand individuals, belonging to C. mediterranea, were collected along the Israeli coast during a five-year soft-bottom monitoring survey, comprising 282 sites at a depth between 4 and 37 m. One of the objectives of this study was to integrate the original description of C. mediterranea with the identification of definite six morphotypes, which alternate during growth. A mtDNA Cytochrome c oxidase subunit I sequence was obtained, representing the first molecular datum for the genus Cheiriphotis. The results offer the possibility of including variable morphological characters and molecular sequences in the identification process. Throughout the survey, C. mediterranea was confirmed to be one of the most abundant five species in the study area, in particular in Haifa Bay, where the environmental conditions are stressful. Barely-known abundance and distribution notes relating to this species are reported in this paper; a putative role of bioindicator is also suggested