The early morphological development, seasonal and spatial occurrence patterns, and food habits of a seahorse, Hippocampus mohnikei, in offshore waters of Tokyo Bay, central Japan, were studied on the basis of 206 juvenile and young specimens (6.0-65.3 mm TL) collected between August 1995 and January 1999. All the specimens were collected within the period from May to January, inclusive, each year. In the least developed specimen (6.0 mm TL), the number of dorsal, anal, and pectoral fin rays had attained the adult complement, whereas the minute caudal fin, consisting of two rays, was present in juveniles of 6.0-26.4 mm TL. Hippocampus mohnikei greater than or equal to 35 mm TL, being larger than settlement size (ca. 30 mm TL), had very low gut fullness index values (GFI = 0, >70% of specimens), whereas those of 15-29 mm. TL had higher values (GFI = 2-4, >80% of specimens). In addition, larger individuals selectively fed on larger planktonic animals (species of Brachyura), which occurred naturally at low densities, although smaller food items, such as Oithona davisae and Penilia avirostris, occurred abundantly, being consumed by smaller H. mohnikei individuals (15-34 mm TL). These results indicated that food availability for H. mohnikei in offshore waters of Tokyo Bay is significant for individuals larger than settlement size. because their food preference would shift from smaller food items to larger food items, which would be scarce in their environments.
