BEHAVIOR AND RESPONSE OF DEEP-SEA BENTHIC FORAMINIFERA TO FRESHLY SUPPLIED ORGANIC MATTER: A LABORATORY FEEDING EXPERIMENT IN MICROCOSM ENVIRONMENTS

Abstract

The behavior of deep-sea benthic foraminifera in response to seasonal inputs of food material was examined through feeding experiments. The experiments using sediments from Sagami Bay (1425 m water depth) containing living foraminifera were carried out using microcosms (aquaria) with thin-walled glass sides. Three different nutrient levels were tested in order to observe how benthic foraminifera reacted to deposited organic matter. Observations of both behavior and microhabitat segregation with respect to the sediment-water interface were conducted using a sideways-mounted binocular microscope. Benthic foraminifera were vertically distributed according to three types of microhabitat segregation patterns, comparable to those of natural populations in Sagami Bay: shallow infaunal, intermediate infaunal and deep infaunal. After addition of food into the aquaria, many foraminifera migrated upwards to a shallower part of the sediment and some foraminifera ingested food. Shallow and intermediate infaunal species reacted faster to newly deposited food materials than deep infaunal species. Among deep infaunal species, Globobulimina affinis reacted very slowly to added food, whereas Chilostomella ovoidea did not respond at all. Reactions to newly deposited organic matter observed in this experiment vary from one microhabitat to another. These different food preferences are an important factor for understanding foraminiferal microhabitats and the degradation of phytodetritus. The size distribution of foraminiferal tests after the experiment was completed suggests that some species reproduced during the experimental run, although there was no significant difference in total numbers of foraminifera between fed and unfed aquaria. Our results indicate that some benthic foraminifera may reproduce in response to organic matter arriving before the spring bloom.