Three species of the callianassid genus Nihonotrypaea occur in the Ariake Sound estuarine system, southern Japan; they consist of two tidal-flat species (N. harmandi; N. japonica) and one boulder-beach species (N. petalura), with maximum population densities of 1440, 343, and 12 ind m−2, respectively. Nihonotrypaea harmandi and N. petalura are distributed along the coastline from the outermost part of the sound to the open sea, while N. japonica occurs in the middle part of the sound. Nihonotrypaea japonica has an extended reproductive period from late winter to autumn, while those of the other species are from late spring or summer to autumn. Interspecific comparisons were made for recently laid egg size (as volume) and clutch size (as number of eggs per female). Only in N. japonica was a seasonal egg size variation observed, being significantly larger in winter to spring (mean=0.106 mm3) than in summer (0.080 mm3). By contrast, clutch size was significantly smaller in winter to spring, resulting in nearly the same clutch volume per female (product of the mean egg volume and clutch size) between the seasons. Among the three species, the egg size was ordered as N. japonica (overall mean volume through the seasons=0.092 mm3)>>N. petalura (0.057 mm3)>N. harmandi (0.054 mm3). The clutch size was ordered as N. harmandi>N. petalura≈N. japonica. The clutch volume was ordered as N. japonica≈N. harmandi>N. petalura. The smallest clutch volume value for N. petalura female showed an opposite trend to the relative size of the major cheliped found in a previous study.
The discoloration of yellowtail dark muscle begins at the boundary part between dark muscle and ordinary muscle, and progresses toward the body surface side. To gain a deeper understanding of this phenomenon, we examined the processes of glycolysis and lipid oxidation in the two types of muscle. The glycolysis analyses indicated that an increase of lactic acid and decline of pH occurred at the boundary between dark muscle and ordinary muscle. An in vitro experiment using dark muscle extract showed that the decline of pH promoted the oxidation of myoglobin. An in vitro experiment using a mixture of extracted lipid and myoglobin from the dark muscle showed that the oxidation of phospholipid promoted the oxidation of myoglobin. Thus, we conclude that the discoloration, caused by the decline of pH at the boundary between dark muscle and ordinary muscle, progresses toward the body surface with oxidation of the phospholipid.
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