Deposit Feeding and Sediment: II. Decomposition of Fecal Pellets of Holothuria tubulosa (Holothurioida, Echinodermata)

Abstract. The effect of defecation and decomposition of fecal pellets egested by the deposit feeding Holothuria tubulosa Gmelin on the microbial productivity of sediments was investigated in microcosm experiments. H. tubulosa specimens were collected from a 5m deep seagrass meadow in the nearshorc waters of the Mediterranean Sea off Lacco Ameno (Island of Ischia, Gulf of Naples, Italy). Experiments were performed under different temperature regimes. Bacterial production in the incubation media and within fecal pellets increased with increasing temperature, ranging from 0.8 to 1.65 μg C 1^‐1 h^‐1 at 14 ^oC and up to 16.5 μg C ‐1^‐1 h^‐1 (27 ^oC) in the incubation media. The corresponding values within fecal pellets ranged between 3.6 and 8.5 μg C g^‐l (fecal matter dry wt) h^‐1 (14^oC) and 30.3μg C g^‐1 (fecal matter dry wt) h^‐1 (27^oC). Incubation experiments on the effect of feces on natural sediments indicated that fecal pellet production by H. tubulosa enhanced sediment bacterial production by about 30 %. Two independent methods were used to estimate leaching of dissolved carbohydrates from fecal pellets. One method was based on the increase in total dissolved carbohydrates (TDCHO) in the incubation medium of the HgCl₂‐treated jars, the other on the decrease in total particulate carbohydrates (TPCHO) of the fecal matter. For the experiment performed at 14 ^oC, we calculated a TDCHO‐leaching rate of 26.4 % d^‐1 based on the increase in the HgCl₂‐treated jar, while 20 % d^‐1 were obtained from the decline in the TPCHO fraction of the fecal matter.