Journal of The School of Marine Science and Technology,Vol.7 No.1
Occurrence, gonad morphology and maturity of Japanese seahorse
Hippocampus mohnikei in Matsushima Bay, Japan.
Yu OTSUKA, Hiroyasu SUZUKI and Izumi AKAGAWA
Abstract
The number of individuals/hour (CPUE) of Hippocampus mohnikei sampled in Matsushima Bay, Miyagi Prefecture, Japan was 4.6-5.3 in May to July 2006, and rapidly increased to 8.4 in August, before subsequently declining. It is suggested that the population emigrated from the Bay, because the eelgrass bed which it inhabited disappeared seasonally from September. Immature individuals, collected from September to November, were considered to have been born in the same year. No significant sexual differences in SL were found in any month
(Females, 62.9±6.7mm, n ＝ 46；Males, 66.2±8.9mm, n ＝ 29). In contrast, the sex ratio was significantly biased
towards females in June and July(Females/(Females ＋ Males)；June,0.789；July,0.813).Histological observations
showed both ovary and testis to comprise a pair of cystiform organs suspended by gonad mesenteriolum that
extended to the parietal dorsal side. The ovary had a unique feature common in Syngnathidae in that the ovarian
lamellae had appearance of a rolled sheet. Oogonia were observed in the base of each ovary, and oocytes being
distributed in growth stage order towards the tip of the lamellae.Each testis was covered by a thickened epithelium,and contained a circular seminal lobule.Spermatogonia were distributed outside the seminal lobule,which contained seminal cells of all stages. Females collected at the end of May contained mature oocytes, the ratio of latter increasing in June and peaking at the end of August.All adult males collected from May to November had sperm,indicating that the breeding season of H.mohnikei in Matsushima Bay was from May to September in 2006.