A QUANTITATIVE STUDY OF FEEDING BY THE HYDROID TUBULARIA LARYNX ELLISAND SOLANDER, 1786

The natural diet and feeding rate of the benthic hydrozoan The The nat ural diet and feeding rate of the benthic hydrozoan Tubularia larynx E llis & Solander, 1786, were studied by analyzing the enteron contents of fifty hydranths collected approximately every 6h (April 1991) and 4 h (September 1993) over two 24h periods. Crustacean eggs, nauplii and copepodids formed a greater proportion of the diet than of the plankto n. Copepods and cladocerans formed a smaller proportion of the diet th an of the plankton even though they comprised 40% of the identifiable food items. The mean number of prey items per hydranth was closely rel ated to the density of zooplankton, which increased approximately 10 f old at night when more than 90% of the diel prey biomass was captured. The mean number and mean size of prey were positively correlated with hydranth size (width), However, there was an inverse relationship bet ween prey biomass and hydranth biomass, which included some pedicel, s ince larger hydranths have longer hydrocauli beneath them. The percent age of hydranths containing prey varied between 48 and 100%. The prey frequency distribution differed from the Poisson distribution, with mo re hydranths having fewer and more prey items than predicted. These da ta may indicate an innate rhythm of feeding-digestion-egestion. The ma ximum time taken for items to be digested was estimated to be 5h. It w as calculated that each T. larynx hydranth consumed a mean of 36 prey items day(-1) (in September 1993). The data from this and other studie s indicate that the contribution of hydroids to the transfer of energy from the plankton to the benthos may be much higher than suggested by hydroid biomass values alone.