The morphology of the miniature gobies Pandaka trimaculata and P. lidwilli on Okinawa Island was described and their habitats were investigated. Postflexion larvae, juveniles and adults of the two species could usually be identified by body depth, in addition to the pigment pattern on the first dorsal fin of adults and juveniles, formerly the only known distinguishing characteristics. Postflexion larvae could also be identified by the arrangement of ventral melanophores on the tail. Postflexion larvae of the two species (6－7 mm in standard lengthSL), collected by small seine net from the Teima Stream estuary at high tide and the surf zone of a beach near the estuary, from April to November, 1999, had almost full fin ray complements, there being no earlier stage larvae present. The occurrence of the two Pandaka species was investigated at 21 sampling stations along the Teima Stream from February to March and in July, 2006. Fishes at various developmental stages (6－14 mm SL), from postflexion larvae to adults, were collected from stations set in shallow shores, and in small creeks and tide pools within the estuary. Therefore, it is suggested that the larvae arrived at the beach and estuary after spending most of their larval stages at sea, subsequently settling into adult habitats in the estuary where they became pigmented and completed their adult transformation. Pandaka trimaculata occurred in various habitats, being broadly distributed from the middle to upper reaches of the estuary. By contrast, P. lidwilli was restricted to stations in the middle reaches of the estuary, the preferred habitat always consisting of soft muddy substrate close to mangroves.
The influence of highly concentrated suspended solids on Ayu (Plecoglossus altivelis altivelis) survival was examined experimentally. Ayu were initially exposed to particles of eight different sizes, ranging in concentration from 1,500 to 15,940 mg/l, and the survival rate over 24 hours recorded. Subsequently, Ayu were exposed to particles of two different sizes, ranging in concentration from 560 to 20,750 mg/l, the mass and particle size distribution of particles adhered to the gills being noted. The particle size distribution of suspended solids differed from that of adherent material on the gills. A comparison of the mass of particles adhered to the gills of surviving and dead fish indicated that fatal adherence of particles occurred in a short time. The diameter of particles adhering to the gills was related to the space between two adjoining filaments. The use of particle sizes ranging from 19 to 54m m to correct suspended solid concentrations enabled the determination of 90% survival levels of Ayu under different materials and suspended solid concentrations. The results indicated that survival states of adult Ayu in highly turbid rivers can be predicted by corrected suspended solid concentrations, which can be determined for any range of particle size.
Examination of the morphological and genetic features of spined loach (Cobitis sp.) collected from the Oyodo River system, southern Kyushu Island, Japan, indicated that they represented a new taxon. The specimens were clearly distinguished from other known Japanese Cobitis species by their mitochondrial DNA sequences. In addition, the shape of the adult male lamina circularis differed to those of the C. sp. ‘yamato’ complex and C. biwae.
A cluster analysis of freshwater fish fauna of 30 rivers on 13 islands in the Seto Inland Sea and 17 short rivers on the mainland of Hiroshima Prefecture revealed two island groups, based on fish species composition: eastern islands group (8 islands) and western islands group (5 islands). The fish fauna of the western islands group showed a greater resemblance to that of the mainland rivers than shown by the eastern islands group, indicating differing geographical relationships between the island groups and between the latter and the mainland.
Examination of Biwa salmon (Oncorhynchus masou subsp.) in four rivers discharging into Lake Biwa, Japan, demonstrated the absence of sexual size dimorphism (SSD). Previous studies having reported the direction or extent of SSD as being likely attributable to the relative strength of sexual selection (favouring larger adult males) versus natural selection favouring larger adult females (e.g., fecundity selection) in masu salmon populations, the lack of SSD in this case suggests a balance between those two factors.
Two specimens of the robust assfish, Bassozetus robustus Smith and Radcliffe, 1913 (84.1 and 341.9 mm in standard length, SL), collected from Suruga Bay and the continental slope of the Ryukyu Trench, respectively, represent the first records of the species from Japanese waters. Bassozetus robustus is distinguished from all congeners by the following combination of characters: dorsal fin rays 112－130, anal fin rays 92－103, pectoral fin rays 24－28, precaudal vertebrae 13-16, long gill rakers on first gill arch 11-16, oblique scale lows between anus and dorsal fin 25－36, pelvic fin length 16.5－20.5% SL, body depth at anal fin origin 10.0－19.0% SL, a well-developed basibranchial tooth patch and inner surface of sagittal otolith with an ostial channel. The new Japanese name “Ishi-fukumenitachiuo” is proposed for the species.
Seasonal occurrence patterns of Mola sunfishes (Mola spp. A and B) in waters off the Sanriku region, eastern Japan were examined with particular focus on sea surface temperatures (SST) during 2002－2008. The two species differed from each other in both seasonal occurrence pattern and body size. SSTs during the occurrence of Mola sp. A (16.8－25.6°C) were higher than those during the occurrence of Mola sp. B (11.5－25.6°C). Although sex-ratio differences were not correlated with SSTs during the occurrence of Mola sp. B., body size and SST were negatively correlated for Mola sp. B. Thus, the occurrence patterns of Mola sunfishes around the Japanese coast may involve not only species-level characteristics but also intraspecific growth-stage differences, probably representing differences in water temperature preference.
Genetic disturbances in wild populations of Medaka (Oryzias latipes) have resulted from the introduction of populations originating from the commercial orange-red strain (himedaka). The “himedaka” phenotype, caused by a defect in melanin deposition in skin cells, is conferred by a mutated recessive allele (b allele) on the slc45a2 locus (linkage group 12). To examine genetic introgression from “himedaka” to wild Medaka populations, a new DNA marker (b-marker) was constructed that detects fish with the b allele according to a length polymorphism of the promoter region between the wildtype (B) and b alleles. Among 169 fish from 45 wild populations in the Yamato River, Nara Prefecture, all specimens with the orange-red phenotype (from 7 populations) had a homozygous b/b genotype. Although most of the wildtype fish had a homozygous B/B genotype, 12 wildtype fish from 9 populations (3 of which consisted of fish with the orange-red phenotype) had the heterozygous genotype (B/b). These results indicated a high level of cryptic genetic introgression from “himedaka” to wild Medaka populations.
Freshwater mussel (family Unionidae) utilization for oviposition by Sarcocheilichthys variegatus variegatus was investigated in a drainage ditch and the Harai River. Twenty five freshwater mussels in Pronodularia japanensis (4% of 616 individuals examined) in a drainage ditch connected to the Kushida River hosted S. v. variegatus eggs, compared with none of two other freshwater mussel species, Inversidens brandti and Unio douglasiae douglasiae. In the Harai River, eight freshwater mussels (3 I. brandti, 2 Obovalis omiensis, 1 Lanceolaria grayana and 1 Anodonta spp.) (2.7% of 290 individuals examined) hosted S. v. variegatus eggs, compared with none of P. japanensis and U. d. douglasiae. Among the available freshwater mussel species, U. d. douglasiae may be only one selected against for oviposition by S. v. variegates.