Diet, food preference and digestive efficiency in intertidal isopods inhabiting macroalgae

Abstract

Diet, food preference and digestive efficiency were investigated for four species of intertidal isopods inhabiting macroalgae on shores of northern Spain: Dynamene bidentata, D. magnitorata, Cymodoce truncata and C. emarginata. Diet in the field was consistent with the patterns of distribution of the isopods. D. bidentata has a broad distribution, recruits fairly continuously and can be collected on the macroalgae throughout the year. Its diet consists mostly of large perennial macroalgae, a resource usually available in most places at most times of the year. Seasonal variation in diet was low. D. magnitorata and Cymodoce spp. have a more restricted distribution than D. bidentata and recruitment takes place only during 2 months. D. magnitorata consumed large amounts of ephemeral filamentous algae while the most common items in Cymodoce spp. were sponges (newly settled specimens) and detritus. The amount of perennial macroalgae consumed by D. magnitorata and Cymodoce spp. was smaller than that by D. bidentata. Composition of the diet exhibited strong seasonal variations in D. magnitorata and C. truncata. In D. magnitorata, ephemeral algae formed the bulk of the diet during late spring and early summer. Differences in the composition of the diet appeared to be related to some extent to differences in the structure of the mouthparts. Food preference experiments were consistent with diets in the field. Items consumed in higher percentage in the field were preferred by the animals in laboratory experiments. Differences in the consumption of different algae between species of isopod were also maintained in the laboratory. Preference in D. bidentata appeared to be independent of relative abundance of algae provided, while preference in D. magnitorata was strongly dependent on the relative abundance of algae in the experimental chambers. This result supports the different response observed between the two species in the field to changes in the availability of potential resources. D. magnitorata exhibited greater digestive efficiency when it was fed with the algae for which the isopod showed strong preference (the epiphyte Sphacelaria fusca). Thus in this species, composition of the diet appeared to be related to food quality. In contrast, while D. bidentata in laboratory experiments preferred large macroalgae to ephemeral species, it exhibited greater digestive efficiency when it was offered S. fusca than when fed with Cystoseira. This result suggests that factors other than food quality might be important in food selection by D. bidentata, particularly availability of the resource in time and space.