Research articles on the molecular mechanisms of fish visual adaptation are reviewed, in addition to present studies on the visual pigments of ayu (Plecoglossus altivelis). To adapt to various photic environments, fishes use three main strategies of molecular mechanism: (1) changing the ratio of Al/A2 retinal in the chromophore; (2) mutation of opsin genes resulting in changes to the amino acid sequence of opsin protein; and (3) changing the expression patterns of opsin genes. Amphidromous fishes change the retinal ratio according to their habitat and adjust the absorption maximum (λmax) of rodopsin to environmental light. The same phenomenon has been observed in landlocked ayu in Lake Biwa. Various fish species inhabiting deep seas or lakes, at depth that very little blue light can reach, have rodopsin with λmax of about 480 nm. Some fishes including African cichlids, eels and ayu, utilize different sets of visual systems, combining cone opsins so as to adapt to diverse habitats. Fishes have apparently evolved a visual system with high plasticity, being visually adapted to various environments by combining such molecular mechanisms. Fish receive light not only via visual cells in the retina, but also via non-visual cells in both the retina and extraocular organs. In addition to visual opsins, non-visual photoreceptive opsins of fishes are also reviewed.
Black bass populations in Japan were examined for haplotypes of the mitochondrial DNA control region. A total of 16 haplotypes were found from specimens representing 47 Japanese populations and five in North America. Ten haplotypes were of largemouth bass, and three each of Florida bass and smallmouth bass. Three clades of largemouth bass haplotypes were identified by the maximum parsimony method, the major clade comprising seven haplotypes including those found in Iowa, Minnesota and Ontario (USA). It ispossible that the haplotypes of this clade in Japan originated from the introduction of fish from Minnesota and Pennsylvania in 1972. One of the remaining clades, comprising a single haplotype and found throughout Japan, may have originated from the introduction of cultured fish from California in 1925. All of the seven largemouth bass haplotypes found in Japan were found in Lake Yamanaka, such haplotype richness reflecting active stockings of largemouth bass from other Japanese freshwaters. Florida bass haplotypes were found only in Lake Biwa, indicating that the haplotypes could function as indicators of future invasion of black bass into other waters with the active transplantation of other commercially valuable fishes from the lake.
To clarify the early life period distribution and migration pattern of Ryukyu-ayu (Plecoglossus altivelis ryukyuensis), investigations were conducted around Sumiyo and Yakeuchi Bays, Amamioshima Island, southern Japan, from November-2001 to May-2002. Larvaeand juveniles (from ca. 10 mm SL; 10 dayold) were present in brackish water inlets and near coastal shorelines, their persistence and frequency in brackish waters being longer and greater, respectively, than in seawater areas (recorded for only a few days in latter). Therefore, it was concluded that brackish waters were more important as nursery grounds for larvae and juveniles of Ryukyu-ayu than seawater areas.
To clarify the early life history of pale chub, Zacco platypus, and the mechanism for avoiding egg predation, the spawning behaviour, egg and larval distribution in spawning redds, and emergence time of larvae from the redds were investigated in the Saho River, Osaka Prefecture, Japan. In pair spawning, the male applied lateral pressure to the female, and stirred up the river bed with its anal and caudal fins when eggs were released. The female occasionally rotated in the redd at the end of spawning behaviour. Some satellite males and sexunknown individuals occasionally dashed into the spawning redd. Single females averaged 1026.4 mature eggs, spawning 94.5 eggs during each of 10.9 spawning acts per day. The number of eggs released during a single spawning act and buried successfully in the spawning redd following pair spawning was greater than that following spawning with surrounding individuals. More eggs were also found in the spawning redd after the female rotated. Eggs were buried 0-14 cm deep in spawning redds, those resulting from pair spawning being deeper than those from spawning with surrounding individuals. Larvae emerged around midnight from the spawning redd 7-8 days after spawning, emergence continuing for 3-5 days. Females were consid-ered to reduce the risk of egg cannibalism by spawning many times per day, mid-night emergence of larvae also being important for avoiding predation.
The geographical distributions of mitochondrial DNA (mtDNA) haplo-types in non-indigenous populations of a freshwater goby, Tridentiger brevispinis, were investigated. Although 26 mtDNA haplotypes were obtained from 168 individuals (including a closely-related species, T. obscurus) representing 36 indigenous populations, only two haplotypes (I-Al and III-B1) were found in 59 individuals of T.brevispinis from 12 non-indigenous populations. Many of the latter had the I-Al haplotype, thepopulations being located near indigenous populations having the same haplotype. A non-indigenous population in Lake Biwa had the III-B1 haplotype, the lake being close to the natural distribution area of that haplo-type. Thus, the non-indigenous populations of T. brevispinis may have become es-tablished following artificial transplantations from nearby populations. The III-B 1 haplotype, however, was also scattered throughout geographically-distant, non-in-digenous populations, its dispersal possibly having been a consequence of trans-plantation of Ayu (Plecoglossus altivelis), from Lake Biwa, accompanied by T. brevispinis. However, the primary cause of the expansion of non-indigenous goby populations retains unclear.
Color sense in the nibbler (Girella punctata) was investigated by means of physiological and behavioral observations. S-potentials of the horizontal retinal cells in response to visible light (wave lengths from 430 to 697 nm) were recorded. We found that all the horizontal cells (270 cells) recorded showed L-type response, suggesting colorblindness. However, behavioral observations, including color discrimination training followed by transposition tests, clearly demonstrated that the species has color sense. This is the first report on color sense in a marine fish conserning both physiological and behavioral aspects. The results showed that S-potentials should be carefully considered, preferably in association with behavioral tests, before color sense determinations are made.
A single specimen (URM-P 21436, 91.9 mm in standard length) of an apogonid fish, Apogon rhodopterus Bleeker, 1852, collected at Amitori Bay (24°19'N, 123°42'E), Iriomote Island, Okinawa Prefecture, Japan, was found in the fish collections of the University of the Ryukyus. It represents the first reliable specimen-based record of A. rhodopterus from Japan and northernmost record of the species. The specimen is described and a new Japanese name “Futasuji-akahire-ishimochi” proposed.
A cobitid fish Lefua echigonia is a small threatened freshwater fish. Examination of white linear abdominal marks on specimens for the purpose of in-dividual identification, disclosed the following: 1) marks existed on all specimens collected (n=476); 2) specimens collected on the same day (mean=23) each had unique marking patterns; 3) the shape of the marks on 12 specimensdid not change during 359-474 day periods under rearing conditions; 4) in a photograph-matching test, all test-takers could identify 20 individuals from 50 pictures includ-ing 1-4 views of each individual. It was considered that the marks of L. echigonia can be used for individual identification