OCCURRENCE OF PELAGIC LARVAE IN SPIO SETOSA VERRILL, l873 (POLYCHAETA: SPIONIDAE)

Abstract

1.Spio setosa exhibits poecilogony, spawning once in the spring, resulting in benthic larvae; again in the fall, with pelagic larvae.2. Pelagic larvae from 4- to 22-setigers were collected between mid-October and mid-February in Great Harbor, Woods Hole, Massachusetts. Larvae were reared in laboratory culture through metamorphosis and juvenile stages.3. The morphology of pelagic larvae paralleled generally that of benthic larvae with the following exceptions: pelagic larvae from 6- to 13-setigers were smaller than comparable benthic larvae; pelagic larvae had gastrotrochs on segments 3, 5, 7, 9, 12, 14, rarely on 15; a rigid pattern of black pigment spots was present on pelagic larvae; pelagic larvae were planktotrophic; metamorphosis occurred at 18- to 20-setigers.4. In response to lack of substratum, most Spio larvae metamorphosed, forming tubes of mucus. Some larvae did not metamorphose for periods up to two months, either increasing in size and number of segments, or just increasing in size.5. Larvae survived and metamorphosed in 50%, 75%, and 100% sea water. In dilute sea water, additional incomplete gastrotrochal bands appeared on segments, 4, 6, 10, 13, and 15 through 19.6. Juvenile worms 6 to 30 mm. long reappeared in the night plankton during May and early June.