From 25 populations of Pseudorasbora in Japan, polymorphism at 22 allozyme loci indicated that the level of genetic differentiation between Pseudorasbora parva and Pseudorasbora pumila (D = 0·421–0·517) was greater than that between the two subspecies of P. pumila, P. pumila pumila and P. pumila subsp. (D = 0·164), consistent with morphological differences. While P. parva displayed genetic variation (H = 0·003–0·100) similar to other freshwater fishes, P. pumila pumila and P. pumila subsp. populations showed no genetic variation. In five of the 15 populations collected from the contact zone between P. parva and P. pumila pumila, hybrids were detected by allozyme analyses. All hybrids were presumed to represent the F1 generation, because they were heterozygous at all 12 loci diagnostic between P. parva and P. pumila pumila. Although four populations were characterized by high frequencies of F1 hybrids (c. 40%), only one of the parental species was observed in each case. The results indicated that the two species have hybridized easily under natural conditions, but cannot coexist in the long‐term. It is suggested that continued invasion of P. parva would hasten the extinction of P. pumila pumila.

No abstract

Marine environments often allow interbreeding of individuals over the species range, and analyses using neutral molecular markers may lose extant genetic boundaries laid between geographic majorities. Ayu Plecoglossus altivelis has a typical amphidromous life history, migrating between rivers and the sea. In order to clarify reproductive elements of the species, migrants from 64 rivers and streams sampled over a wide latitudinal range were examined for their vertebral number (VN) and dorsal pterygiophore number (DPN) as morphological markers to estimate the temperature history. The irregular variability in VN without a geographic cline suggests that the ascending schools of fish are composed of conspecifics sharing incubation temperature or hatching site. The intersample difference in DPN indicates that larvae and juveniles of ayu spend their marine life inside a water body with a distinct temperature. Site tenacity during the marine stage may be helpful to enhance the opportunity to return to the river where they hatched. Each assemblage of fish ascending to rivers and streams is deemed to mostly represent a reproductive element, and therefore, each acts as an evolutionarily significant unit within a metapopulational structure.KEY WORDS: amphidromy, dorsal pterygiophore number, metapopulation, Plecoglossus altivelis, vertebral number, water temperature.

Pseudorasbora parva, a species native to western Japan, has been accidentally introduced into eastern Japan, where P. pumila is indigenous. We investigated inter-and intraspecific, male-male competition between P. parva and P. pumila for acquisition of spawning substrates in an experimental setting. Within each species, males of larger standard length and heavier body weight were more successful in acquiring a substrate. Males of the two species competed, but the outcome was determined primarily by body weight. This interspecific, size-dependent, male-male competition might be an important factor in the species replacement of P. pumila by P. parva.

We investigated the occurrence pattern of the pond-living endangered cyprinid, Pseudorasbora pumila, and also compared its habitat characteristics with those of the congeneric invasive species, Pseudorasbora parva, in the contact zone. Comparison of 16 environmental variables among the P. pumila habitats, P. parva habitats, and unoccupied ponds revealed that conductivity was a common limitation factor of distribution of both species. We found that emergent vegetation occupancy along the pond bank was the most important factor determining P. pumila occurrence and that ponds with steep banks may have a low probability of containing P. parva. We constructed a logistic regression model to predict the establishment risk of P. parva in ponds occupied by P. pumila. The model demonstrated that more than half of the ponds exhibited a high establishment risk of P. parva. Principal component analysis using six parameters selected from stepwise logistic regression analysis revealed that seven unoccupied ponds had the potential to sustain P. pumila, suggesting that our study site is capable of supporting more P. pumila populations and expanding the current range.