Ecological distribution of pycnogonids on the Catalan coast

Abstract

Thirty-three samples, collected from seven stations between Cadaqués and Casas d'Alcanar on the Catalan coast, yielded 870 specimens of 20 species and subspecies of Pycnogonida for a study of shallow water pycnogonocoenosis. Achelia echinata and Ammothella longipes are the numerically dominant species. Amm. uniunguiculata, Callipallene emaciata spectrum and Tanystylum orbiculare are, in order, the next most abundant forms, although occurring in low numbers. Nymphon gracile, Ach. langi, T. communis, Nymphonella tapetis, Ascorhynchus casteRi, Call. ph. phantoma, Call. brevi-rostris producta, Call. e. emaciata, Anoplodactylus petiolatus, An. angulatus and An. virescens are rare forms, each comprising less than 1% of communities. Communities occurring on Halopteris scoparia and Posidonia oceanica were compared for species diversity (Shannon-Weaver index) and statistical testing revealed a higher diversity on the more stable, latter substratum. Some substrata influence community composition, but some pycnogonids do not find the substratum limiting, probably reflecting wide feeding preferences. There appear to be several reproductive cycles in the year, although individual populations remain in their biotopes and do not undergo reproductive migrations. It is not clear what are the durations of reproductive cycles, nor whether single populations undergo more than one reproductive cycle in a year. The sites of larval development remain undiscovered and the patterns of grouping of adults and juveniles remain to be elucidated.