Life history of Nippoleucon hinumensis (Crustacea: Cumacea: Leuconidae) in Seto Inland Sea of Japan. II. Non-diapausing subpopulation

Abstract

The great majority of Nippoleucon hinumensis (Gamô) in the Seto Inland Sea of Japan enters a long-term summer diapause from May to November. We identified a minor subpopulation of N. hinumensis (0.7% of the total population) that became adults in a short period without diapause. Small-sized ovigerous females (15% shorter than the ordinary-sized ovigerous females) appeared from January to early February, a month earlier than the appearance of the ordinary-sized ovigerous females that produce diapausing offspring. These precociously ovigerous females released offspring from early February to early March. The offspring developed from 1st manca larvae to 2nd manca larvae, 1st juvenile stage and 2nd juveniles stage in that order, and in mid-April they reached the adult stage (5th instar) without diapause. In the non-diapausing subpopulation, sexual dimorphism was apparent from the 1st manca instar and the sex ratio (males:females) was 2 to 4:1. The molts, occurring at intervals of ca. 10 d, were not highly synchronous among individuals. The non-diapausing females mostly disappeared in April without producing offspring, but many non-diapausing males survived until the ordinary-sized ovigerous females started incubation of the second brood in late April. Possibly, the non-diapausing subpopulation is maintained through mating of non-diapausing males with ordinary-sized ovigerous females. The non-diapausing mode of life in N. hinumensis may represent the ancestral life mode before this species acquired a summer diapause mechanism.KEY WORDS: Crustacea · Cumacea · Leuconidae · Life history · DiapauseResale or republication not permitted without written consent of the publisher

Life history of Nippoleucon hinumensis (Crustacea: Cumacea: Leuconidae) in Seto Inland Sea of Japan. I. Summer diapause and molt cycle

“…Although males became markedly less abundant than females after incubation of the first brood, all females could nevertheless incubate a second brood. The females may retain spermatozoa required for the second brood in their body for the second brood, but another possibility is proposed in Akiyama & Yamamoto (2004). In the present study, adult individuals, especially preparatory females, showed a patchy distribu-tion on the seabed.…”

“…The duration from emergence to entering the diapause was too short for larvae derived from the second brood to have grown fully and reached the 2nd juvenile instar stage. The small subpopulation of individuals derived from the second brood may not affect the total population of N. hinumensis, but there is a possibility (Akiyama & Yamamoto 2004) that they play a pivotal role in maintaining a subpopulation of individuals without a diapause during their life span (nondiapausing individuals).…”

“…The present paper deals with the great majority of the N. hinumensis population which shows a summer diapause unique among crustaceans. In a succeeding paper (Akiyama & Yamamoto 2004, this volume), we describe the small population of the N. hinumensis population with a non-diapausing mode.…”