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Revision of the cottid genus Gymnocanthus, with a description of their osteology Wilson, Donald Edward

Abstract

Gymnocanthus is a cottid genus containing six species which inhabit the North Pacific, Arctic, and North Atlantic Oceans. The genus is characterized by edentulous palatines and prevomer, granulations on the nape, scales restricted to axillary prickles, and an elongate, multicusped pre-opercular spine. Six species (Gymnocanthus detrisus, galeatus, herzensteini, intermedius, pistilliger, and tricuspis) are recognized. These are identified by their different meristic characters, interorbital width, cleithral spine development, presence or absence of subpectoral dermal pistillae in males, and other features. No subspecies are recognized. Synonymies, descriptions, colour, maximum size, etymology, and range are given for each species. The key, used in conjunction with diagrams showing several diagnostic characters is useful for identifying specimens to at least 60 mm. Though some morphometric characters (head length, interorbital width and others) are significantly different between some species because of their great variability they are seldom taxonomically useful. Superimposed on this variability is strongly developed sexual dimorphism, males possessing significantly longer dorsal and pelvic fins than females. Other sexually dimorphic characters include the roughening of the inner edge of the pectoral rays, the brighter colours, and the penis in males of all species, and the axillary pistils in males of Gymnocanthus pistilliger and G. intermedius. Though its systematic position is obscure, Gymnocanthus probably arose in the Aleutian Islands, dispersing westward by planktonic larvae, and eastward by migration along the shallow Aleutian shelf. Gymnocanthus evolved in two directions, one forming a group of three smaller species (G. tricuspis, pistilliger, and intermedius) showing great sexual dimorphism and low meristic characters, the other, a group of larger species (G. galeatus, detrisus, and herzensteini) with lesser developed dimorphism and high meristic characters. Possible origins for the two lines are discussed. For lack of comparative information, the systematic position of Gymnocanthus within the family Cottidae is not examined in the present paper. To facilitate future intergeneric comparative studies, the osteology and cephalic lateral line system is described for the genus. No profound osteological differences between species exist; superficial differences between species reflect those diagnostic specific differences ( i.e. interorbital width, head length) previously noted. Pore number and distribution in the cephalic lateral line system tend to differ between species, but are too variable to be of taxonomic use. Breeding seasons are inferred from presence of young and from gonad maturity. G. herzensteini is a winter breeder, G. galeatus and G. pistilliger breed in spring, while G. tricuspis breeds in late summer. Data are not available for G. detrisus and G. intermedius. In three species examined (G. tricuspis, galeatus, and pistilliger) fish live to four years or more. Females grow more quickly than males, especially after two years. G. tricuspis has a wider salinity range and lower temperature tolerance than have the other species, permitting it to survive in Arctic waters. Ecological separation of the remaining species is not so clear, though G. pistilliger and G. intermedius seem to inhabit shallower water as adults than do G. galeatus and G. herzenstein.

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