Morphology and reproductive biology of a new hermaphroditic Rhynchospio (Annelida: Spionidae) species brooding larvae on the parent’s dorsum

Abstract

Rhynchospio glandulosa sp. nov. inhabits silty tubes on muddy sand intertidally and in shallow water in the Yellow Sea, Sea of Japan (East Sea), and the Sea of Okhotsk (Korea, China, and Russia). Adults are up to 9 mm long, 1 mm wide for 62 chaetigers, with characteristic groups of glandular cells in the bases of branchiae from chaetiger 7 to chaetigers 14–23. Individuals with 29–40 chaetigers have mature spermatozoa in chaetigers 11–12 or 11–13, while larger worms are simultaneous hermaphrodites with sperm in chaetigers 11–14 and oocytes from chaetiger 16 to chaetigers 22–42. The oocytes are about 120 µm in diameter, with thin and smooth envelopes. The spermatozoa have a small subspherical acrosome about 1 µm long, a tusk-like nucleus 3–4 µm long, small midpiece with subspherical mitochondria, and a flagellum 45–47 µm long. Early larvae develop on the parent’s dorsum from chaetigers 21–27 to chaetigers 30–36, where they are loosely held by flat branchiae and long dorsal capillaries. After utilizing internal egg yolk, the 4-chaetiger larvae 250–280 µm long escape from the parent and continue its development in sea water, feeding on the plankton. About 2 weeks after hatching, the 11-chaetiger larvae 550–570 µm long get ready for settlement and metamorphosis. Rhynchospio glandulosa sp. nov. is one of the seven Rhynchospio species (12 species described so far) known as simultaneous hermaphrodites and one of the three species known to brood their larvae on the parent’s dorsum. The arrangement of male segments and sperm morphology are suggested as important taxonomic characters for the identification of Rhynchospio species. Feeding mechanism of the planktotrophic Spionidae larvae is briefly discussed.