Yoshikazu Nagata

Distribution of the freshwater fishes of Japan: an historical overview

Japanese freshwater fishes, including lampreys, comprise 15 orders, 35 families, and 96 genera, with 211 species and subspecies. Most belong to the families Cyprinidae (29% of species and subspecies), Gobiidae (21%), Salmonidae (10%), and Cobitidae (8%). Cyprinids and cobitids presumably originated from east Asia, gobiids from southeast Asia, and cottids and salmonids from the north Pacific. Japanese freshwater fishes include 88 endemic species and subspecies, of which three have been extirpated. Fishes introduced into natural rivers and lakes for inland commercial fisheries and sport fishing, and by accident, include many exotic species, of which 23 now inhabit natural freshwaters. These often have destroyed the local fish fauna by predation, and caused genetic pollution by hybridization with local strains. Destruction of freshwater environments by land development also poses a threat to Japanese freshwater fish communities. In addition Japanese freshwater systems have been markedly altered by development of rice paddy fields which have caused some species to decline but others to flourish, and changed the distribution patterns of fishes between upstream and downstream areas. To conserve endangered species and declining communities of Japanese freshwater fishes, we need to clarify the characteristics of their original habitats and the effects of developing paddy fields, from both the ecological and historical points of view.

Genetic Introgression by the Rose Bitterling, Rhodeus ocellatus ocellatus, into the Japanese Rose Bitterling, R. o. kurumeus (Teleostei: Cyprinidae)

Abstract The Japanese rose bitterling, Rhodeus ocellatus kurumeus, is an endangered cyprinid species. The main reason of drastic diminution of R. o. kurumeus has been suggested to be due to hybridization between this subspecies and R. o. ocellatus introduced from China. Both RFLP analysis of mitochondrial DNA and RAPD-PCR analysis of genomic DNA were performed to elucidate the genetic introgression of R. o. ocellatus into R. o. kurumeus. The two subspecies were distinguished in the D-loop region of mtDNA by six restriction endonucleases. Rhodeus o. kurumeus had eleven subspecies-specific RAPD markers and R. o. ocellatus had two. Except some R. o. kurumeus populations, most of the populations of R. ocellatus in Japan were hybrids, equipped with subspecies-specific RAPD markers for the two subspecies, respectively. The genetic constitution of these markers in hybrids, however, greatly differed among populations. The R. o. ocellatus mtDNA was predominantly observed in hybrid populations, except two populations with mtDNAs of the two subspecies. Judging from the genetic dominance of morphological and physiological characters of R. o. ocellatus against R. o. kurumeus, hybrids probably have the same ecological dominance as R. o. ocellatus against R. o. kurumeus. Therefore, it is considered that R. o. kurumeus not only has its genetic property spoiled by hybridization with R. o. ocellatus, but also is expelled by R. o. ocellatus and hybrids. The replacement of mtDNA and genomes of R. o. kurumeus with those of R. o. ocellatus in hybridization might be accelerated by the backcross between hybrids and R. o. ocellatus.

Sexual size dimorphism, growth and maturity of the Japanese fluvial sculpin,Cottus pollux (large egg type), in the Inabe River, Mie prefecture, central Japan

The population structure of the Japanese fluvial sculpin,Cottus pollux (large egg type), in the upper reaches of the Inabe River, Mie Prefecture, central Japan, was investigated by a mark-and-recapture method from July 1989 to January 1991. Breeding of the species occurred from mid February to early May, peaking from mid February to late March. The mean size of mature males observed in March 1990 was significantly larger than that of females, showing apparent sexual size dimorphism. Data analysis of the growth of 1658 marked individuals revealed that the species matured at 2 years of age in both sexes. Whereas 1 year old males reached ca. 50–70 mm SL, females were less than 50 mm SL at the same age, size dimorphism already being apparent. Immature males exhibited higher growth rates than females during their first and second years, some of the former outstripping mature males of the preceding year class in total length. After attaining sexual maturity, both males and females grew mainly from July to December, with no significant differences in mean growth rate between them. Sexual size dimorphism of the species seems to be attributable to different growth rates between the sexes during their immature stage.

Genetic diversity in the Japanese rosy bitterling, Rhodeus ocellatus kurumeus (Cyprinidae)

Abstract Comparison of meristic characters (pored lateral line scales, vertebrae, and fin rays), and PCR-RFLP analysis in the D-loop and ND1 regions of mitochondrial DNA were performed to estimate the genetic diversity in local populations of the Japanese rosy bitterling, Rhodeus ocellatus kurumeus. In terms of meristic characters, the Fukuoka population was the largest in both range and variance of the number of pored lateral line scales and vertebrae (abdominal and caudal), and Osaka was the second, whereas the Kagawa population showed the smallest range and variance in these characters. In PCR-RFLP analysis, 11 haplotypes (3 in Fukuoka, 2 in Okayama, 2 in Kagawa, and 4 in Osaka) were observed, and nucleotide sequence divergence (NSD) was approximately two times larger in ND1 (mean, 0.61%) than in D-loop (mean, 0.31%). In the neighbor-joining (NJ) tree, based upon the NSD value in ND1, haplotypes were arranged into four clades, which corresponded to the locality of each haplotype. The Fukuoka population was conspicuously apart from the other populations (mean, 0.90% in NSD), but the remaining three showed a similar genetic distance with each other (mean, 0.48%–0.52% in NSD). In haplotype diversity of mtDNA, half the stations in Osaka and all in Kagawa were monomorphic. Especially, two haplotypes endemic to Kagawa were randomly distributed, irrespective of drainages. Rhodeus o. kurumeus in Fukuoka inhabits small rivers and creeks (open water systems), while that in Kagawa and Osaka lives in small ponds (closed water systems). Taking the information of morphology, mtDNA, and habitat into consideration, the low genetic diversity in Kagawa and Osaka populations of R. o. kurumeus is thought to be mainly the result of the isolation of their habitat.

Genetic markers distinguishing between the two subspecies of the rosy bitterling, Rhodeus ocellatus (Cyprinidae)

Eleven populations of the rosy bitterling,Rhodeus ocellatus, from different localities in Japan, were genetically compared at 16 protein-coding loci using starch-gel electrophoresis. Two loci,Ldh-2 andPgdh, were demonstrated as diagnostic markers for the identification of two subspecies;R. ocellatus kurumeus, which is native to Japan, andR. ocellatus ocellatus, which was introduced from China. The two subspecies were distinguished by the complete substitution of different alleles between them. Population ofR. ocellatus kurumeus occurring in Yao City, Osaka, and in Kanzaki, Saga Prefecture were genetically closely related to each other (genetic distance: D=0.056) but distantly so toR. ocellatus ocellatus from Saitama Prefecture (D=0.202 or 0.265). Electrophoretic analyses also elucidated the existence of hybrid populations of the two subspecies. The populations ofR. ocellatus kurumeus in Yao City had lower genetic variability and a lower incidence of white coloration on the ventral fins than populations of the same in Saga Prefecture. The present study strongly implies that the introduction of the foreign freshwater fishes with subspecific differentiation, into the original range of indigenous subspecies, should be averted not to bring the genetic pollution.

Diversity in interspecific interactions between a nest-associating species, Pungtungia herzi, and multiple host species

Nest association in fishes affects the reproductive success of the host to varying degrees, from complete failure of the host’s reproduction to fitness advantages. Such varying impacts of nest association are considered to result from both the associate’s behavior and the characteristics of the host’s reproduction. To investigate the relationship between the impact of nest associate and host/associate ecology as well as evolutionary strategies involving nest association, we compared reproductive strategies and success between an associate East Asian minnow, Pungtungia herzi, and two host fishes (a goby, Odontobutis obscura, and a catfish, Pseudobagrus nudiceps) that have different reproductive ecologies. The associate frequently spawned around the days on which both hosts spawned, and gained a fitness advantage through continuous egg protection. Spawning by the associate had a negative effect on the survival of O. obscura eggs. Male O. obscura tended to abandon their nest when it was parasitized during the pre-spawning period. This is likely a tactic to alleviate the costs of brood parasitism. In contrast, survival of P. nudiceps eggs was consistently high in all nests, regardless of the intensity of associate’s spawning, and P. nudiceps young fed on cyprinid offspring (most probably the associate’s eggs/young) in the nest. Both P. nudiceps and P. herzi populations likely gain fitness benefits from their relationship. When comparing these two hosts with another known host, the freshwater perch Coreoperca kawamebari, the observed differences in the impact of nest association to the associate species likely correspond to differences in the spatial reproductive resources used by the respective hosts.

Flexibility of reproductive tactics and their consequences in the brood parasitic fish Pungtungia herzi (Teleostei: Cyprinidae).

Although previous studies have claimed that Pungtungia herzi (Cyprinidae) is an obligate brood parasite, in this study a significant proportion of P. herzi egg masses was found in crevices formed by stones and rocks, which was not guarded by hosts. Egg masses were also found in almost all nests of a goby Odontobutis obscura, suggesting that P. herzi has two alternative reproductive tactics: brood parasitic spawning and non-parasitic crevice spawning, the latter of which may occur when a shortage of reproductive resources (i.e. host nests) exists. The hatching rates of parasitic egg masses were significantly higher than those of non-parasitic egg masses, and the hatching rate of P. herzi eggs exhibited a strong positive correlation with continuous egg guarding by a reproductive O. obscura male. These results indicate that brood parasitism effectively improves reproductive success in P. herzi. The alternative reproductive tactic of this species suggests that brood parasitism has evolved directly from crevice spawning via the sharing of spawning habitat with host species.

Timing of juvenile emergence from host mussels in the Japanese rosy bitterling, Rhodeus ocellatus kurumeus

The diel rhythm of juvenile emergence from host mussels in the Japanese rosy bitterling, Rhodeus ocellatus kurumeus, was investigated in a small pond in Osaka prefecture, Japan. The emergence of juvenile bitterling from host mussels occurred principally at night, with a peak in the early part of the night. The pattern of juvenile bitterling emergence was discussed in the context of an adaptive response to diurnal predation.

Exploitation of the eggs of nest associates by the host fish Pseudobagrus nudiceps

The Japanese minnow Pungtungia herzi deposits its eggs in the nests of other fish species, including the bagrid catfish Pseudobagrus nudiceps. The minnow generally spawns on days around the spawning day of the host species, probably to ensure egg care by the host. However, Pu. herzi sometimes spawns in the nest during the larvae/juvenile-guarding period of Ps. nudiceps, and in such cases, host young are known to feed on the eggs of their associates. We conducted fieldwork in the Umemoto River, central Japan, to clarify the details of the utilization of Pu. herzi eggs by Ps. nudiceps and its effects on the early life history of the latter. Host young fed on almost all of the Pu. herzi eggs available to them, stayed in nests with more of the associate’s eggs available for longer periods, and grew to larger sizes. Since a larger body size at independence is expected to improve the subsequent survival of the young, associate spawning should provide benefit to the host young. We revealed that nesting host males also exploited Pu. herzi eggs. Considering the unusual timing and low survival rate of Pu. herzi eggs, their spawning during the latter half of the host nesting period possibly suggests that the host has developed specific behavior to induce such associate spawning.