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

Abstract

The life history of the cumacean Nippoleucon hinumensis was investigated in the Seto Inland Sea of Japan from August 1998 to February 2002. The population produced 1 generation per year and recruitment of offspring occurred in April. The post-marsupial period consisted of 8 instars in males and 9 (plus generally 2 repeated instars) in females. Post-marsupial development up to the 4th instar (2nd juvenile stage) was characterized by periodic molting at intervals of 10 d, slow growth rate, and suppression of sexual dimorphism. Thereafter, the juveniles entered a long-term summer diapause from mid-May to early December, during which the ambient seawater temperature ranged from 15 to 28°C. Ninety-nine % of the population during the diapause were at the 2nd juvenile stage, and 1% were at the 1st juvenile stage. In December, the cumaceans resumed growth, characterized by a fortnightly molt cycle, until the females began to incubate the first brood in late February. Each molting was highly synchronized among individuals and phased with the lunar cycles. After the end of the summer diapause, sexual dimorphism became apparent, with ovigerous females becoming about 1.2 times larger than adult males. After release of manca larvae in early April, most females incubated a second brood in late April, and then rapidly disappeared due to mortality probably caused by high water temperature. Clutch size was 57.9 ± 9.4 for the first brood and 42.4 ± 7.3 for the second.

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

“…The specimens described in the present paper and in Akiyama & Yamamoto (2004) were sorted from the same sediment samples. Because of the scarcity of specimens of the non-diapausing subpopulation, sediment samples were gathered twice or thrice a week and at least 3 sediment samples were obtained on each sampling day from December 2000 to April 2001.…”

“…Fecundity and intramarsupial stages were assessed by direct observation of the individuals removed from the marsupium. The intramarsupial stage was identical with that in Akiyama & Yamamoto (2004). The postmarsupial stages were determined on the basis of the shape of the 4th and 5th pereopods and the uropod in males, and the shape of the pereopod, uropod and oostegite plus body size in females.…”

“…To investigate the molt cycle of non-diapausing individuals, 15 to 20 sediment samples were collected at intervals of about 10 d from 23 February to 1 May 2001. For details of collection and treatment of the specimens see Akiyama & Yamamoto (2004).…”

“…In an earlier paper (Akiyama & Yamamoto 2004, this volume), we reported that a population of Nippoleucon hinumensis (Gamô) (Crustacea: Cumacea) in the Seto Inland Sea of Japan undergoes a long-term summer diapause from May to November. Individuals born in early April begin fasting in the 2nd juvenile stage (4th instar) in mid-May, and remain in the same instar to the end of November.…”

“…(Gamô, 1967) (Crustacea: Cumacea: Leuconidae) collected in the central region of the Seto Inland Sea of Japan (off Ushimado, Okayama Prefecture; 34°36' N, 134°09' E). Details of the sampling site and water temperature near the site have been described in Akiyama & Yamamoto (2004).…”