Ecology ofHydractinia echinata(Fleming) (Hydroidea, Athecata) I. Feeding biology

Abstract

The feeding biology of Hydractinia echinata is described based on individuals collected on the Knähaken Reef in the Øresund. Data are derived from analyses of stomach contents and from feeding experiments, including measurements of the length of the period in which the individual particles of food remain in the stomachs of the hydranths. Finally, also the relative value of the various food items was investigated by observations on the rate of growth of the Hydractinia. The growth of the colony of Hydractinia is exponential, at least for the first 24 days of its life. Within this period a linear proportion is maintained between the size of the colony and the amount of food eaten by the colony. The energy used by Hydractinia for growth during the month of March is gained for about 59 ‰ from Nematoda, for about 18 ‰ from Ophiura albida and for about 15 ‰ from Harpacticoidea. A medium-sized colony catches about 47 Nematoda, about 6 Harpacticoidea and about 18 arm-joints of Ophiura albida every 24 hours (March). Hydractinia consumes an unknown amount of eggs of Pagurus bernhardus. It is concluded that Hydractinia is not specialized in feeding in the way that it is selective in its choice of food. It is the availability of the food items which determines the composition of the food. However, it must be stressed that Hydractinia almost exclusively gathers its food from the bottom, not from the free water.