The present study aims to examine the age class composition of wild Leptodius exaratus populations, using lipofuscin as an age marker. Stereometrical quantification of lipofuscin granules on histological sections of nervous tissue from wild crabs was conducted using a fluorescence microscope. The highest concentration of lipofuscin was observed in the olfactory lobe cell mass of the brain. Five groups could be distinguished within the normal distribution of lipofuscin concentration values and the mean values for the groups increased linearly. This suggests that lipoftiscin accumulation rate was constant between the groups. Based on the size of the specimens in the group with lowest lipofuscin concentration values (hence group 1), it is surmised that they were born in the previous reproductive season. These results suggest that the lipofuscin groups 1-5 might correspond to the 1-5+ aged groups in wild population of L. exaratus. Examination of the lipofliscin accumulation, number of moltings and growth of captive-reared crabs during a period of one year supported the age class estimation of wild samples.

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Occurrence of planktonic decapod larvae (Crustacea) in covering water at a station on the tidal-flat off Obitsu River estuary, eastern coast of the Tokyo Bay, central Japan, was investigated during 16-17th August of 2010. Samples were obtained by vertical hauls of a Marutoku net (mouth diameter 45 cm, mesh size 330 μm) from the bottom to the surface. Nineteen taxa of decapod larvae, including 8 taxa identified to species level, were occurred. These decapod larvae consisted of the species that benthic populations are distributed around the Obitsu River estuary, such as salt-marsh crab Chiromantes dehaani, salt-marsh ghostshrimp Laomedia astacina, sand-flat ghostshrimp Nihonotrypaea japonica, and sand-flat crabs Scopimera globosa and Philyra pisum. The abundance of each taxa excluding P. pisum was very low during the daytime (1-6 inds. m^<-3>), but high in the nighttime (1-470 inds. m^<-3>), largely reflected by change of abundance of N. japonica (maximum 470 inds. m^<-3> at 22 : 00). The abundance of P. pisum was positively synchronized with water level (3-36 inds. m^<-3>). Most larvae collected in this survey are in first zoeal stage that should be newly hatched from adult. A small number of megalopae and juveniles (N. japonica and unknown Brachyura) occurred mainly during the nighttime suggesting revolving larvae with diel migration between water column and seabottom.

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