Hybridization and introgression have been considered important aspects in the speciation process and evolution of reproductive barriers. Although most hybrid zones of animals have been consideredas ‘tension zones, ’ maintained by dispersal/selection balance of hybrids, many of the hybrid zones of freshwater fishes may lead to hybrid swarm. On the other hand, some of the hybrid zones of diadromous, brackishwater and maritime fishes may, infact, be ‘tensionzones’ and widespread introgression of genes. These differences may be dependent upondispersal abilities, because migration from ‘pure’ populations to a hybrid zone (and vise versa) can be expected in widely dispersible species, but not in drainage-restricted freshwater fishes. Although many of the examples of fish hybrid zones may have originated fbllowing secondary contact of geographically-isolated populations, reinfbrcement of reproductive barriers have not appeared. In particular, hybrid swarms of freshwater fishes have often emerged as hybrid-origin species. In some cases in brackishwater species, mtDNA trees have suggested long term hybridization, there being no evidence for reinfbrcement. One of the reasons fbr the continuing existence of hybrid zones (and genetic differences among populations) may be environmental selective pressures in ecotones, because ‘tensionzones’ of fishes originally fbrmed in a transitional zone between different environments.
Freshwater fish in rice fields near the Chikuma River, Nagano Prefecture, were investigated by visual census and net sampling. The rice fields were classified into three types. Type 1-terraced and supplied with water from an upper pond; type 2-supplied with water from drainage ditches; and type 3-supplied and drained by separate irrigation ditches. Rhinogobius sp.OR (sensu Kawanabe and Mizuno, 1989), Misgurnus anguillicaudatus and Pseudorasbora parva were abundant in type 1 fields, whereas Tribolodon hakonensis, Carassius spp., M. anguillicaudatus and Gnathopogon elongatus elongatus were recorded in type 2 fields. Only M. anguillicaudatus was found in type 3 fields. The fish abundance and diversity did not differ significantly between type 1 and 2, but was extremely poor in type 3. The recent rearrangement of rice fields from types 1 and 2 to type 3 evidently reduced fish abundance and diversity.
Fish fauna of the rivers in the Shima Region, Mie Prefecture, central Japan, was surveyed between August 1997 and March 1998. A total of 4, 206 individuals of 37 species/subspecies belonging to 14 families were collected from 17 stations (15 rivers or streams) by casting net, hand net, or small set net. Of these, in addition, Cyprinus carpio were observed at 5 stations, 18 species were new additions to the river fish fauna of the region. Generally, river fish fauna of the region did not show large seasonal fluctuations, although migration and rarity of some species resulted in apparent seasonal fluctuations at some stations. Fishes collected or observed during the survey comprised genuine freshwater (18 spp.), diadromous (12 spp.) and peripheral freshwater (8 spp.) species, the overall displaying intermediate between those in the Ise-Bay Region (characterized by plentiful genuine freshwater species) and those in the Kumano Region (fewer genuine freshwater but abundant peripheral freshwater species). Rivers in the Shima Region have remained relatively suitable environments from the point of view of natural habitats with the existence of five species classified as ‘endangered’, ‘vulnerable’ or ‘near threatened’.
A single large specimen (MUFS 18880, 739 mm in standard length) of polynemid fish, Eleutheronema tetradactylum (Shaw, 1804), collected by a large set net off the mouth of Azuma River, Fukaura, Aomori, northern Japan, represents the first reliable record from Japan and northernmost record of the species. Furthermore, a lateral line squamation character on the caudal fin membrane is herein described as newly-recognized diagnostic character for the species. Eleutheronema tetradactylum is characterized as follows: four pectoral filaments; lower jaw lips restricted to posterior one-third of lower jaw, vomer with tooth plates on either side; lateral line on caudal fin membrane divided into three lines.
The three species of amphidromous gobies (Rhinogobius sp.CB, CO and LD) allopatric in the Aizu River, Wakayama Prefecture, Japan were examined for their clutch size and egg size. Clutch size was the largest in R. sp.CO and the smallest in R. sp.CB, whereas egg size was in the reverse order. Larger clutch size in R. sp.CO and LD than R. sp.CB was partly explained by the difference in body size, because the formar two species have a larger body size at maturation than the latter. The differences in reproductive characteristics may be a result of the different survival rates of larvae during migration to the sea, because their spawning ground distributions are different from one another along the stream.
Mating behavior of a freshwater goby Eleotris oxycephala was observed 1-day after the injection of human chorionic gonatropin. Such behavior began with a female visiting a nesting male, which there upon courted the former. During mating, the nuptial coloration in both sexes changed, becoming dark-brown and light-brown in the female and male, respectively. Both sexes showed erection of the genital papilla, that in the female spawning eggs in a single layer on the substratum, and in the male releasing sperm on the eggs. The number of eggs laid by a single female was estimated as being from 35, 000 to 190, 000.