Reproductive Behavior and Salinity Favorable for Early Development in Two Types of the Brackish-water Polychaete Neanthes japonica (lzuka)

Abstract

SATO, MASANORI (Department of Biology, Faculty of Science, Kagoshima University) and MAKOTO TSUCHIYA (Department of Biology, University of the Ryukyus). 1987. Reproductive behavior and salinity favorable for early development in two types of the brackish-water polychaete Neanthes japonica (Izuka). Benthos Research (Bull. Jap. Assoc. Benthology), 31 : 29-42.
Neanthes japonica (a brackish-water polychaete) can be distinguished into two types (the small- and the large-egg types) by the characteristics of egg morphology and developmental pattern (Sato and Tsuchiya, in press). The spawning behavior and the effect of salinity on the early development were studied in the two types. Reproductive swarming was observed in 6 demes of the small-egg type. The swarming worms were carried toward the sea on the ebb after the night high tide, during the spring tides in winter or early spring. It appears that they spawn around the river-mouth, and that they are semelparous. The mature worms of the large-egg type were collected from the sediment in 3 demes in spring, summer and/or autumn. The post-spawners were found within the sediment together with the immature and mature worms, but reproductive swarming was not observed in the case of worms of the large-egg type. The salinity isotonic to the ooplasm of the small-egg type was 27-30, while that of the large-egg type was about 15. The optimal salinity for the early development of the small-egg type was 24-34, while that of the large-egg type was 9-21. These results suggest that the spawning and the early development of the small-egg type occur following the reproductive swarming in water which has a higher salinity, while those of the large-egg type occur in situ in water including a lower-salinity condition.