The diet of the potamid crab Geothelphusa dehaani was studied by examining the stomach contents of middle- and large-sized crabs (carapace width mostly >10 mm) in four seasons. Crabs were collected from a small mountain stream in Fukuoka City, Japan. Diet analysis was performed using the frequency of occurrence and percentage point methods. Geothelphusa dehaani was found to be omnivorous, with its feeding preference varying widely among individuals. However, the presence in the stomach of large amounts of tissues from various vascular plant organs was high for all seasons and size classes. The presence of moss and filamentous algae was relatively rare. The predominant animal tissue was from insects, G. dehaani preferring aquatic insects, especially caddisfly larvae (Trichoptera). Tissues of G. dehaani were also frequently found, suggesting the high prevalence of cannibalism of young crabs just after their release from maternal care. Spiders and sand were rarely found. Some fishes and snails were found, the former more common in large-sized male crabs. Feeding preferences and the postulated ecological niche were compared between the coexisting crabs G. dehaani and Eriocheir japonica: the former is an omnivore and a predator of small live animals, while the latter is an omnivore and scavenger that mainly eats detritus.
Metabolic adaptation to 7-d sustained anoxia, and the activation of anaerobic metabolism by a series of O2 concentrations during a 12-h period, were examined in the ark shell Scapharcakagoshimensis. Laboratory experiments and biochemical analyses were conducted under summer conditions. The pattern of metabolic adaptation to prolonged anoxia was clearly biphasic, similar to that in many other bivalve species. The first, transition stage lasted 12 h and was characterized by sharp decreases in adenylate energy charge, aspartate consumption, and accumulation of succinate and alanine. The second, stationary stage was characterized by glycogen depletion and propionate accumulation. In a series of dissolved oxygen (DO) conditions, the onset of anaerobic metabolism (as indicated by a decrease in fermentative substrate, aspartate; and increase in the end-products, succinate and alanine) was detected only under hypoxic conditions (<2 mg O2 L-1) and was accompanied by depressed physiological performance, as measured by clearance rate and ammonium excretion rate. However, under milder hypoxia (1–2 mg O2 L-1), a significant clearance rate was still observed, suggesting that both aerobic and anaerobic metabolisms were contributing toward the ATP yield. Lower DO (<0.61 mg O2 L-1) resulted in an increased rate of anaerobic metabolism and inactivated aerobic ATP production due to impaired filter feeding.
Seasonal occurrence and longitudinal distribution patterns of freshwater shrimp were investigated in two rivers in western Japan. Four species of the family Atyidae and two species of Palaemonidae were observed. Water temperature and life cycle patterns of these shrimps affected their seasonal occurrence. The amphidromous shrimp species Caridina leucosticta, Caridina serratirostris, and Macrobrachium nipponense were distributed within a brackish estuary and the lower reaches of rivers, while the landlocked shrimp species Paratya improvisa, Neocaridina denticulata, and Palaemon paucidens were found in the lower and middle reaches. The biomass of all shrimp in each sampling area measured between December and March ranged from 0.49 to 34.72 gm-1 (wet weight in grams per meter of river bank) in the Isazu River with relatively dense riverbank vegetation, and from 0.06 to 1.22 g m-1 in the Yura River with less riparian vegetation. Environmental factors such as stream gradient, distance to saltwater intrusion, structure of riverbank habitat, and the life history of each species were important factors in determining their longitudinal distribution.
Seasonal changes in abundance of four neritic Acartia species (A. hudsonica, A. omorii, A. longiremis and A. steueri), including identifiable copepodid stages, were investigated in the inner reaches of Okkirai Bay, Sanriku, northern Japan, to elucidate their seasonal succession patterns. Samples were collected monthly at intervals from August 2007 to July 2009 by vertical hauls of a NORPAC net of 100 μm mesh aperture. For identification of morphologically allied A. omorii and A. hudsonica, the dimensional differences between them were statistically analyzed for the stages of C4 to C6. The dominant species were A. longiremis in the colder season and A. steueri in the warmer season. A. longiremis and A. omorii appeared from early spring (February or March) to summer with numerical peaks in April. These April peaks were considered to result from immigration from outside the bay with intrusions of Oyashio Current water. A. steueri increased during the summer with a peak in September, then decreased until December or January, and disappeared for two or three months from April, when they were probably only present as diapausing eggs. A. hudsonica occurred from early spring to mid-summer as in A. omorii but with higher abundances in summer than in spring, though the seasonal abundances varied somewhat between years. These results suggest that temperature, determining reproductive activity (and production of diapausing eggs in A. steueri), and intrusions of Oyashio water are important environmental factors determining seasonal succession of acartiid copepods in the bay.
A simple reliable method using multiplex PCR with species-specific primer sets was developed for identification of three Pacific Hediste species ( Polychaeta: Nereididae), which are morphologically indistinguishable in sexually immature specimens. They commonly inhabit estuaries in eastern Asia (H. atoka and H. diadroma) or the western coast of North America (H. limnicola). The species-specific oligonucleotide primers were designed based on interspecific polymorphism of mitochondrial 16S rRNA genes. The three Hediste species were distinguishable according to their electrophoretic patterns showing different molecular sizes of the species-specific PCR Products. This method is superior, both in speed and lower cost, to the PCR-RFLP method we proposed previously.
More than 50 specimens of the megamouth shark Megachasma pelagios have been reported so far, but biological observations on the species are still limited. We examined a female megamouth specimen that was captured by the bonito purse seine fishery in the Kuroshio Extension in July 2007 and donated to the Okinawa Churaumi Aquarium. The specimen was an immature female, 3,667 mm in total length and 361 kg in weight. The stomach contents measured 2,200 mL excluding the portion that was lost from the specimen before measurement. Detailed examination of a small part (14.6 mL) of the stomach contents revealed undamaged worm-like organisms, 10 partially damaged euphausiids and abundant fragmented pieces of euphausiids. The worm-like organisms were probably parasites such as tapeworms. The partially damaged euphausiids were identified as Euphausia pacifica except for one individual of Nematoscelis difficilis. Based on the number of fragmented right mandibles, which were less damaged than the other fragmented appendages, the total number of euphausiids in the stomach contents is estimated to be at least 18,000 individuals. The high abundance of euphausiids in the stomach contents suggests that the present specimen has fed on a swarm of E. pacifica in an oceanic area.