Species richness and abundance of decapod crustaceans associated with a Maltese Posidonia oceanica (L.) Delile meadow

Abstract

The decapods associated with a Posidonia oceanica meadow in the Malta-Comino Channel were sampled from depths of 6m, 11m, 16m and 21m, using a suction sampler and a hand-held net, between August 1993 and September 1994. A total of 4,607 individuals belonging to 41 species were collected. The hermit crab Cestopagurus timidus had the highest abundance in the suction samples whilst the shrimp Hippolyte inermis had the highest abundance in the net samples. While most of the decapods were microhabitat-specific and occurred either in the rhizome layer/matte surf ace or in the leaf stratum, some hermit crabs were recorded from both microhabitats. Overall, the rhizome layer/matte surface supported a higher species richness than the leaf stratum. The decapods were grouped into three ecological categories on the basis of their occurrence in the P. oceanica meadow: (i) species which appeared to be permanent inhabitants of the meadow; (ii) species which used the shallow (6m) part of the meadow as a nursery only; (iii) species which occurred in the meadow only at specific times of the year; and (iv) casual species which are not usually known to occur in P. oceanica meadows. The results of this study showed that decapod species richness was more influenced by the structural complexity of P. oceanica 'matte' than by shoot density. However, the possibility that abundance decreased with a decrease in shoot density was not excluded. The application of Cluster Analysis and Multidimensional Scaling to the data obtained from the suction samples showed that the decapod assemblage could be divided into three sub-assemblages, namely those occurring at 6m, at 11m to 16m, and at 21m. It was concluded that overall, the decapod species richness and abundance were influenced by a complex set of environmental factors, mainly: the P. oceanica 'matte' structure, water movement, seasonal and climatic factors, as well as by behavioural processes. Most of the decapods were not characteristic of the P. oceanica biotope and were presumed to have moved into the meadow from adjacent habitats. This showed that rather than supporting a characteristic assemblage, P. oceanica meadows may support faunal assemblages whose nature is determined, amongst other things, by the types of adjacent habitats present. The suction sampler collected more than twice the number of species collected by the hand-held net. Furthermore, no brachyurans were collected with the hand-held net. When compared with the suction sampler, the hand-held net was not reliable as a quantitative sampler for the decapods because the high P. oceanica shoot density hindered the collection of the species living in the rhizome layer/matte surface and in the leaf stratum. It appears that the suction sampler is the best available method for the quantitative sampling of the whole decapod assemblage associated with P. oceanica meadows. A number of decapods were recorded for the first time from the Maltese Islands. These were: the shrimps Processa ? edulis edulis, Thoralus cranchii and Hippolyte inermis; the hermit crabs Pagurus cuanensis and Anapagurus cf. chiracanthus; and the crabs Xantho pilipes, Pilumnus aestuarii, Eurynome ? spinosa and Inachus cf. phalangium.