Natural diet and grazing rate of the temperate sponge Dysidea avara (Demospongiae, Dendroceratida) throughout an annual cycle

Abstract

Sponges are one of the major invertebrate groups inhabiting hard-bottom communities worldwide.In this study, we measured in situ rates of grazing on DOC (dissolved organic carbon), POC (particulate organic carbon), and pico-, nano-and microplankton for the common temperate sponge Dysidea avara throughout a yearly cycle.The natural diet of the species was h~ghly heterogeneous and included procaryotes (heterotrophc bacteria, Prochlorococcus sp., Synechococcus sp.) and eucaryotes (protozoa, phytoplankton, and ciliates) ranging in size from 0.5 * 0.3 (heterotrophic bacteria) to 70 k 0.3 pm (pennate diatoms).Procaryotic cell clearance rates were higher than those for the other groups, suggesting a higher grazing efficiency upon these prey types.Specific clearance rates showed a pattern of decrease with sponge size increase, although they did not vary with prey concentration or with temperature.Overall, procaryotes contributed 74 k 14 % of the total ingested carbon, pico-and nanoeucaryotes contributed 11 i 3"&, and phytoplankton contnbuted 11 i IOU:>.Therefore, Dysidea avara obtained 85% of its ingested carbon from the fract~on smaller than 5 pm and 15% from the fractlon larger than 5 pm.Howcver, the partial contributions of the different groups varied seasonally, following the planktonic composition of the water column Dur~ng winter, phytoplankton was an important component of the total uptake [26°<)), whereas durlng the rest of the year it contnbuted less than 7 % of the total uptake.The capacity of this sponge to feed on a broad size range of prey allowed it to maintain rather constant food uptake throughout the year.These results show the importance of particle type (size) for selective uptake in sponges, as well as the relevance of phytoplankton in the sponge diet.This trophic plasticity may represent an advantage for the species because it attenuates the effects of seasonal fluctuations in the planktonic community.This plasticity in trophic ecology may be one of the main factors contributing to the worldwide abundance and distribution of sponges despite large spatial and temporal variations in food sources.