Gaetice depressus (Crustacea, Varunidae): Species profile and its role in organic carbon and nitrogen flow

Gaetice depressus is one of the most dominant macrozoobenthos species in boulder shores of intertidal coastal ecosystems in Japan. As recorded in previous studies, this species is also considered as having high density and biomass. Consequently, it is thought to be one of the more important species in the organic matter flow of boulder shores, especially through the food web. In this study, some taxonomic problems related to G. depressus were tackled and the autoecology and ecological processes in the intertidal ecosystem of G. depressus, such as organic matter flow, were investigated. Furthermore, in order to clarify the taxonomy description, resolve inconsistencies in the scientific name, and learn about the life history, a literature review was conducted. Seasonal changes in density, morphology pattern and population structure were determined based on the data obtained in Ebisu Island, Japan. Then, the role of G. depressus was determined by estimating the intake and emittance fluxes of organic carbon and nitrogen through ingestion and egestion process in the boulder shores of Ebisu Island. A feeding rate experiment was also conducted in order to estimate the intake flux by using the catch-release-recapture method. Meanwhile, to estimate the emittance flux, a defecation rate experiment was conducted by catching some individuals of G. depressus, and then incubating them in the laboratory. The feeding rate measured by the speed of diet consumption of G. depressus was about 12.6 mg ind^-1 h^-1. Considering the average density, the intake flux through the feeding process could be estimated as 25.2 mgC m^-2 h^-1 and 2.6 mgN m^-2 h^-1. On the other hand, G. depressus egested fecal pellet at the rate of 5.4 mg ind^-1 h^-1. The average emittance flux through the fecal pellet egesting process is estimated at 5.6 mgC m^-2 h^-1 and 0.7 mgN m^-2 h^-1. Therefore, it can be estimated that about 25% of organic matter from diet is egested as fecal pellet, which means that about 75% of the intake flux of organic carbon and nitrogen is used for the total assimilation of G. depressus. Intake flux was also considered as affecting the high dynamism of primary producer consumption. The total population of G. depressus is estimated to consume about 18.4% of primary producer in average throughout the year. Therefore, the turnover time of primary producer by consumption of G. depressus was about five days.