REPRODUCTION AND EARLY DEVELOPMENT OF THE SPIONID POLYCHAETE, SCOLECOLEPIDES VIRIDIS (VERRILL)

¹ Institute of Oceanography, Dalhousie University, Halifax, Nova Scotia, and the Laboratory, Citadel Hill, Plymouth, England

1. A redescription of the external morphology of Scolecolepides viridis (Verrill) is given.

2. From data obtained by regular sampling of a population at Lawrencetown Nova Scotia, it was found that the worm has a short breeding period in late March and early April.

3. The onset of gamete formation coincides with a fall in the temperature following the summer maximum. Gametes occur in the coelom along the greater part of the animal's length and are shed through mixonephridia. Spawning is due to rising temperatures in the spring. In addition some spawning may result from sudden changes in salinity during a tidal cycle.

4. The thick-walled eggs containing membrane vesicles and the short-headed sperm are probably shed whilst the worms are still in their burrows. Larval development is entirely planktonic.

5. In laboratory cultures maintained at 10° C. and 30% photopositive trochophores begin active swimming in 3-4days, and the 3-setiger stage is reached in 10 days. The larvae start to metamorphose at the 9-to l0-setiger stage, but metamorphosis can be postponed until the 13-setiger stage if the substratum is not suitable. The fully-metamorphosed l0-setiger worm is 750 µ long and 40 days old.

6. The rate at which development proceeds in laboratory cultures is dependent on temperature and salinity. At 20° C. metamorphosis is reached in approximately half the time taken at 10° C. At 2° C. no larvae develop beyond the 3-to 4-setiger stage. The rate of development is similar in salinities varying from l0 to 30. At 5, however, growth ceases.

7. An attempt is made to relate the distribution of the worm along the east coast of North America to temperature.