Reproductive Swarming of Sympatric Nereidid Polychaetes in an Estuary of the Omuta-gawa River in Kyushu, Japan, with Special Reference to Simultaneous Swarming of Two Hediste Species

Abstract

Habitat differences and spatial and temporal separation in reproductive swarming among sympatric nereidid polychaetes were examined in an estuary of the Omuta-gawa River, Kyushu, Japan by annual periodical sampling from December 2003 to January 2005. Benthic adults of Tylorrhynchus osawai and Hediste diadroma occupied mainly the upper reaches of the estuary, whereas those of H. japonica usually inhabited the middle reaches, though their distributions overlapped. Reproductive swarming of mature adults occurred in the estuary just after high tide at night during spring tides in four nereidids: H. japonica (in the middle and lower reaches from late December to late February), H. diadroma (throughout the whole estuary from middle December to late April), T. osawai (in the middle reaches and another estuary from late October to late December), and Nectoneanthes oxypoda sensu Imajima, 1972 (in the lower reaches in late April and early May). This result shows that temporal separation of reproductive swarming may act as a reproductive isolation mechanism among these nereidids, except for H. japonica and H. diadroma. Simultaneous swarming and mass-spawning of the two Hediste species were commonly observed in the middle and lower reaches from late December to early February, suggesting the absence of a pre-spawning barrier to reproductive isolation between them. We found no difference in spawning behavior between H. japonica and H. diadroma. Males of both species seemed to participate in swarming earlier than females.

Life History Annelida: Polychaeta in an Estuary of the Omuta-Gawa River, Kyushu, Japan

“…The reproductive period of this species in the Omuta-gawa River is from middle December to late April, when reproductive swarming of mature adults occurs 14 (Hanafiah et al 2006). This reproductive period is much longer than that of H.…”

“…The reproductive period of this species is from late December to late February, when reproductive swarming of mature adults occurs (Izuka, 1908;Sato and Nakashima, 2003;Hanafiah et al 2006.…”

“…Spawning, fertilization and early development in H. japonica and H. diadroma occur in water column after reproductive swarming of mature adults (Kagawa, 1955;Sato & Tsuchiya, 1987;Sato, 1999;Hanafiah et al 2006. On the other hand, in H. atoka, spawning, fertilization and early development occur within (or around) a burrow without reproductive swarming (Sato & Tsuchiya, 1987;Sato, 1999).…”

“…The life span of H. atoka seemed to usually be about half a year, yielding two generations per year (Kikuchi, 1998) (Sato and Nakashima 2003;Hanafiah et al 2006.…”

Anthropogenic decline of the peculiar fauna of estuarine mudflats in Japan

“…10). Maximum biomass of a population of Hediste japonica was recorded as much as about 1 kg m Ϫ2 in wet weight in an estuary of the Omutagawa River in Ariake Bay (Hanafiah et al 2006). Such large biomass of macrobenthic invertebrates on estuarine mudflats seems to support the lives of carnivores such as birds and fishes through food chains in estuarine ecosystem.…”