Thursday, May 20, 2010

Trochophore larva of the polychaete Sabellaria

Sabellaria cementarium is a polychaete worm that lives in hard tubes constructed of sand held together with a glue-like secretion. The adult worm can be up to 7 cm long and lives in clusters subtidally (Kozloff, 1974). A few adult worms were collected by Richard Emlet and George von Dassow from the dredge (about 150 ft deep, a couple of miles south of Cape Arago, OR). Luckily, two of the worms spawned, when Paul Dunn, our TA, cracked their tubes open with forceps. One of them was a male, and another one — a female! So we were able to fertilize the eggs and start a culture.

These photos are of 11-day old trochophore larvae. The first one shows the ciliated band, called the prototroch, which encircles the larva just anterior to the mouth. The long bristles are called setae (or chaetae) and are characteristic of both the larvae and adults of polychaete worms. The setae serve as defense against planktonic predators (Pennington & Chia, 1984). Fanning out the setae (second picture), the larva can nearly double its diameter (140μm without the setae, and 250μm with setae spread out).

In the third photo you can also see the two reddish eyespots anterior to the prototroch. This trochophore will continue adding new segments, each segment bearing more setae. Once it has more than three setigers (segments with setae) it will find a suitable place to settle and build its sand tube. In some areas species of Sabellariaform extensive reefs, because their larvae prefer to settle on the tubes of adult worms of their species.

Kozloff, E.N. 1974. Seashore Life of the northern Pacific Coast; an illustrated guide to northern California, Oregon, Washington, and British Columbia. U of Washington P: Seattle.

Pennington, J. T., & Chia, F.-S. (1984). Morphological and Behavioral Defenses of Trochophore Larvae of Sabellaria cementarium (Polychaeta) against Four Planktonic Predators. Biological Bulletin. 167 (1), 168-175.

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