Morphology and biology of a new Rhynchospio species (Annelida: Spionidae) from the South China Sea, Vietnam, with the review of Rhynchospio taxa

Abstract

Adults of a new spionid polychaete, Rhynchospio nhatrangi, inhabit sandy tubes on muddy sand intertidal flats in an estuary of Nha Trang Bay, southern Vietnam. The worms are up to 8 mm long and 0.5 mm wide for 55 chaetigers. They are unique among spionids in the changes in the arrangement of hooks during ontogenesis: first development of tridentate hooks in neuropodia from chaetiger 10, later loss of hooks in chaetiger 10, and replacement of tridentate hooks by heavy unidentate hooks in neuropodia of chaetigers 11–14. Adults are simultaneous hermaphrodites having sperm in chaetigers 11–14 and oocytes from chaetiger 15 to 23–33. Spermatozoa are introsperm, about 300 µm long. Oocytes are about 120 µm in diameter, with a thin and smooth envelope. Fertilization and early larval development occur in a hatchery formed by elongated dorsal capillaries on the posterior chaetigers. Larvae escape from the hatchery probably when they have developed four chaetigers and then continue development in seawater, feeding on the plankton. When larvae have grown to 14–15 chaetigers, they likely undergo gradual metamorphosis and settle on the bottom. Metameric nuchal organs, glandular pouches in neuropodia, metanephridial segmental organs, hermaphroditism, sperm, and early larval morphology are described here for the first time for Rhynchospio species. An identification key is provided to eight currently recognized Rhynchospio species.

Keywords:

• Brooding
• hermaphroditism
• larval development
• reproduction
• Rhynchospio nhatrangi

Acknowledgements

My sincere thanks are due to Temir A. Britayev for inviting me to participate in the expedition to southern Vietnam in 2006 and for commenting on the early draft of this paper, and to Andreas Bick for valuable comments on the manuscript after submission to the journal. Financial support was provided by the Russian–Vietnamese Tropical Research and Technological Center (Project ECOLAN E‐3.2.1 in 2006), the Russian Foundation for Basic Research (RFBR Project 06‐04‐49508), the RFBR and National Science Council of Taiwan (NSC) through a Cooperative Programme (Joint Research Project RFBR 05‐04‐90589—NSC RP05B11), and by the Far East Branch of the Russian Academy of Sciences (FEB RAS Project 06‐III‐A‐06‐169).