MEPS 175:201-213 (1998)  -  doi:10.3354/meps175201

ABSTRACT: The natural diet and prey capture rate of the temperate ascidian Halocynthia papillosa were studied in the field in a Western Mediterranean population. In situ grazing rates on dissolved organic carbon, detrital particulate organic carbon, procaryotes, pico- and nanoeucaryotes, phytoplankton and ciliates were examined through a year cycle. The natural diet of the species included detrital organic matter, heterotrophic bacteria, Prochlorococcus sp., Synechococcus sp. (cyanobacteria type), protozoa and phytoplankton with a mean size range from 0.6 ± 0.3 (heterotrophic bacteria) to 70 ± 22 µm (pennate diatoms). Specific clearance rates varied seasonally and exhibited a pattern of increase with temperature increase in which temperature explained 55% of the variance in clearance rate throughout the year. Annual variation in prey concentration did not affect specific clearance rate. Ingestion rate of the species showed a marked seasonal pattern which was different for the 2 main food sources: detrital and live particles. Thus, while the highest ingestion of detrital particles was in spring, the highest values of live particle ingestion occurred during summer and fall. Overall, an H. papillosa specimen of mean size 0.25 g AFDW (ash-free dry weight) was estimated to ingest an annual mean of 1305 ± 496 µg C g AFDW^-1 h^-1 and 84 ± 16 µg N g AFDW^-1 h^-1. Carbon from detrital origin accounted for 92 ± 2% of the total ingested carbon, while ingestion of live carbon accounted for 8 ± 2%. However, the seasonal variation of ingested nitrogen from live particles explained 91% of the gonadal development variance for the year, suggesting that live particles are likely to be of more significance in the diet of the species than particles from detrital origin. Feeding rates are discussed in relation to seasonal changes in food availability and production of the species (i.e. growth and reproduction). The results suggest that factors other than food availability determine the seasonal dynamics of the species.

KEY WORDS: Diet · Feeding behavior · Prey capture · Ascidians · Mediterranean