Gonadal maturation is dependent on body size in the sea urchin, Echinometra tsumajiroi

Summary

This study examined gonadal maturation in the sea urchin, Echinometra tsumajiroi, in relationship to body size. In the breeding season, E. tsumajiroi were collected and divided into six groups based on the length of the major axis (LM) as a measure of body size: Group 1 (6–10 mm in LM), Group 2 (11–15 mm in LM), Group 3 (16–20 mm in LM), Group 4 (21–25 mm in LM), Group 5 (26–30 mm in LM), and Group 6 (> 31 mm in LM). Although gonads were present in individuals larger than 8 mm in LM, the germ cells were undifferentiated in Group 1. Gonadal sex differentiation was first seen in Group 2: a few young oocytes appeared at the periphery of the ovary, and both spermatogonia and spermatocytes were also observed at the periphery of the testis. As size increased from Group 3 to Group 4, more mature germ cells such as ripe ova or spermatozoa were observed in ovaries and testes. In Group 5, the ovaries and testes were filled with a large number of mature eggs and spermatozoa, respectively. Spawning could be induced in this group by the injection of 0.5 M KCl into the coelomic cavity. In contrast, gonads of some of the biggest individuals, belonging to Group 6, appeared to be spent due to spawning, because few eggs were present in the ovary and relict spermatozoa were present in the testis. These results suggest that reproductive activity in the sea urchin, E. tsumajiroi, depends on body size.

Key words:

• Sea urchin
• gonadal maturation
• body-size
• gametogenesis
• histological studies