Development and Distribution of the Early Life History Stages of the Mesopelagic Fish Tactostoma macropus (Stomiidae) in the Transitional Waters of the North Pacific

Abstract

The early life history stages of the mesopelagic fish, Tactostoma macropus (Stomiidae), were described from eggs to juveniles based on specimens collected in the subarctic and transitional waters of the North Pacific. The eggs are pelagic and may be distinguished from those of other species that inhabit subarctic and transitional waters of the North Pacific by the following characters: 1) egg large with a shell diameter of 1.38-1.55mm, 2) wide perivitelline space occupying 40-47% of shell diameter, 3) smooth transparent shell, 4) lack of secondary inner membrane, 5) segmented yolk, and 6) single oil globule. Larvae are slender, ca. 4mm notochord length (NL) in size at hatching and attain a large size; metamorphosis begins at ca. 40mm standard length (SL) and is complete at ca. 50mm SL. The larval pectoral fin degenerates during metamorphosis and is absent thereafter. The pigment pattern of larvae is unique, as is the sequence of formation and loss. Almost all body pigment disappears during metamorphosis.
Distribution and occurrence patterns of eggs and larvae were analyzed in relation to the temperature, salinity fields and zooplankton abundance, mainly based on specimens collected on California Cooperative Fisheries Investigations cruises (1949-1984) in the California Current region. Although samples were limited in the western North Pacific, the distributional pattern off Japan was similar to that in the eastern Pacific. Eggs and larvae occur in the warmer shallow layer (ca. 14-18°C) above the seasonal thermocline in the subarctic and transitional waters with peak abundance in the summer.