Natural diet and feeding habits of the crabs Liocarcinus puber and L. holsatus (Decapoda, Brachyura, Portunidae)

Abstract

foregut volumes of locarcinuspuber and L. holsatus were related to body size as y = 0 . 0 3 3 8 e ~ ~an~d~ yx = 0.0313e00598xr espectively, where y = foregut volume (m]) and X = carapace width (mm). The amount of food remaining in the foregut of these crabs decreased exponenhally with time so that for both species the foregut was half empty about 5 h after feeding and almost empty about 20 h later (T = 13.5 f 1.5"C). However, shell fragments and algae were still found after 36 and 72 h, respectively. Crustaceans (mainly crabs and barnacles) were the most important food group of juvenile L. puber, and brown algae (Laminaria and Fucus spp.) and bivalve n~olluscs (esp Myhlus edulis) were the next 2 important groups. Algae were found predonunantly In crabs >25 mm CW. The main food group of adult L. puber was brown algae (Lamlnaria and Fucus spp ), occurring in about 80 O/O of the foreguts examined and constitutmg over 40 % of the total volunle of diet (the highest ever recorded for portunid crabs). The next 2 major groups in their diet were crustaceans (anomuran crabs) and molluscs (M. edulis). Results strongly suggest that L. puber feeds selectively on algae and can also digest them. Crustaceans (esp. juvenile Crangon spp.), molluscs (esp. Spisula elliptica) and fish (probably juvenile gobies and pleuronectids) were the dominant food groups of L. holsatus For both species diet was largely dependent upon local availability although, in the case of L. puber feeding on algae, other factors such as palatability, calorificlnutritive value and/or moult stage of the crab may have been important. Algae were eaten mainly between late spnng and early autumn by L. puber in the early intermoult (B) stage. Despite the preponderance of brown algae in the natural diet of L. puber, in the laboratory these crabs preferred animal matenal. Activity patterns suggest that L. puber feeds only when submerged by the tide especially at night while L. holsatus seems to do so mainly during lncomng and outgoing tides irrespective of the tlme of day. Most empty foreguts were found during wlnter (particularly in gravid female and parasitised crabs) although in summer a high proportion of recently moulted crabs also had empty foreguts. Calcareous matenal was sometimes found in soft crabs. This and other related studies suggest that despite the diversity in their diet and feeding habits portunid crabs are oppotunistic omnivores a preference for animal food and with predatory tendencies