Origin of the Japan Sea as Viewed from the Evolution and Distribution of Marine Fauna Part I

Abstract

In Part I of the present study, some facts and principles in the evolution and distribution of marine animals, mostly fishes, in the Japan Sea and its neighboring waters in the North Pacific, which are to be taken into account when we consider the problem of the origin and development of the Japan Sea from a zoogeographical standpoint, were enumerated and their significance was discussed. Major points of the conclusion are summarized as follows. 1. Fishes of the families Stichaeidae (prickle-backs) and Agonidae (poachers) are best represented in the Japan Sea of all the North Pacific regions ; not only they show the largest number of occurring genera and species as well as the greatest degree of endemism, but also they contain many primitive forms in this marginal sea, These families are also distinguished in the fish fauna of the Japan Sea because of their having been differentiated in the most diverse directions, suggesting an occurrence of something like the so-called adaptive radiation within this sea basin. It may be concluded that the marine fishes which intruded into and colonized the Japan Sea basin for the first time since the latter had been filled with sea water and since any marked dilution of the environmental medium had no longer taken place were possibly the ancestral forms of these teleostean families. 2. Certain subfamilies of Stichaeidae and Agonidae are well represented along the western coast of North America, too. These counterparts of the families on the opposite side of the temperate North Pacific, which are composed largely of quite different genera, are considered to be emigrants from the Japan Sea at a moderately early date. Essentially the same thing can be said also of the subfamily Pleuronectinae (flounders). 3. Warm-temperate cottid fishes or sculpins of the subfamilies such as Jordaniinae, Oligocottinae, Pseudobleniinae, Ricuzenniinae, Radulinae, etc. are also well differentiated on both sides of the temperate North Pacific but quite missing in the more northerly waters. Of these subfamilies, the most primitive ones are limited solely to the American side, whence a phylogenetic emigration toward the Asiatic side must have taken place at an ever earlier date ; this is because the warmtemperate cottid fauna on either side is composed of different subfamilies, respectively. On the other hand, specialized forms of sculpins belonging to the subfamilies such as Myoxocephalinae, Triglopinae, Gymnocanthinae, Icelinae (particularly the genus Ice/us), etc. are remarkably well differentiated in the OkhotskoBering region. A similar conclusion may be drawn regarding the Hexagrammidae (greenlings), which have primitive representatives along the west coast of North America and in a lesser degree around Japan, with some specialized forms being found thriving in the more northerly waters. 4. The subfamily Sebastinae or rockfishes are very poor in the Okhotsko-Bering region but excellently differentiated on both Asiatic and North American sides of the temperate North Pacific ; the rockfish fauna on either side is entirely different from each other as regards the species composition, but all except for a few species belong to a single large genus Sebastes. This may suggest that their separation into the Asiatic and the North American population is of a recent date. The marked poverty of the rockfish fauna in the Japan Sea as compared with that on the Pacific coast of Japan is another interesting fact. 5. The Zoarcidae (eelpouts), cold-water Cottidae and Psychrolutidae (soft and tadpole sculpins), Cyclopteridae (lumpsuckers) and Liparidae (snailfishes) are well represented in the deep waters of the Japan Sea. The majority of these species, however, indicate a close affinity with the species of respective families in the Okhotsk Sea, the sea supposed to have been a center of speciation of these phylogenetically quite young fish groups. These fishes, well adapted

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