Horizontal Transfer of Matter by a Cave-Dwelling Mysid

Abstract. The role of a cave-dwelling mysid population as a matter carrier was evaluated in a cave of the Medes Is. (NW Mediterranean) during 1988–89. Hemimysis spehmcola (Ledoyer , 1963) is a gregarious mysid whose swarms migrate daily from the inner end of the cave - where they remain during the day–to the exterior where they feed during the night. Fecal pellet composition, pellet egestion and pellet decomposition were measured in order to evaluate the transfer of matter by mysids. Based on the strong daily behavioural rhythms of these mysids. special importance was attached to the sampling frequency (every 2 h). In order to assess seasonal variability, four daily cycles were evaluated within a year. In situ incubations were carried out to determine changes on the egestion rates, morphology, weight and composition (AA. C: N ratios) of pellets in the course of a day. The number of pellets deposited inside the cave was estimated using containers distributed along the cave bottom by SCUBA divers. Fecal pellets showed an amorphous composition, whereby diatom frustules, dinoflagellate loricae and coccolithoporids were very scarce. About 25% of the body weight were estimated to be egested daily as fecal pellets, suggesting a detritivorous feeding habit. The population oscillated seasonally between 1 and 12 millions. Individuals egested between 1.6 and 3.5 pellets a day into the cave, each averaging from 9.4 to 11.9 μg DW, 0.5 to 1.0 μg C and 0.046 to 0.27 μg N. Therefore, the population carried daily about 20 407 g DW POM. 2–21 g C and 0.5 2.7 g N from outside to inside the cave. Pellets decomposed very quickly; between 20 to 50% of both C and N were released from pellets in less than 2 h after egestion. Oxidation of pellets theoretically consumes an amount of oxygen which agrees with the high BOD values previously reported for this cave. Marine caves are generally viewed as strictly oligotrophic systems; dense mysid populations, however, could strongly modify the trophic relationships in marine caves.