About the Cover
Cover
Sabellids, or feather-duster worms, are a diverse group of polychaete annelids common in many benthic marine habitats. Each adult sabellid secretes and inhabits an organic tube, from which it extends a crown of tentacles used for both suspension feeding and respiration. The image on the cover shows three individuals of a sabellid common in shallow waters of the northeastern Pacific, Schizobranchia insignis; its generic name reflects its distinctive, dichotomously branching tentacles (which, because of their respiratory function, are sometimes called branchiae). In life, the crown of the largest individual shown here is about 4 cm in diameter.
Like the planktonic larvae of other sabellids, those of Schizobranchia insignis do not ingest particulate food; instead, development to metamorphosis is fueled by lipids, proteins, and carbohydrates stored in the eggs. Phylogenetic evidence suggests that the nonfeeding mode of development common to sabellids is a specific example of a frequent evolutionary transition among marine invertebrates-the loss of the requirement for particulate food during the larval stage. In most known cases, this transition in larval nutrition is soon followed by dramatic reduction or outright loss of the larval structures involved in feeding. But as reported by Bruno Pernet in this issue of The Biological Bulletin (p. 295), the nonfeeding larvae of S. insignis (and some other sabellids) possess ciliary bands that resemble, in form and behavior, bands that are used by the feeding larvae of closely related annelids to capture food particles (see Figures 3 and 4, pp. 300 and 301). In fact, larvae of S. insignis can use these vestigial feeding structures to capture food particles and transport them to the mouth; but because the mouth is not connected to the midgut, these particles cannot be ingested. A functional digestive system does not develop in these larvae until well after metamorphosis.
Why the larvae of some sabellids retain functional particle capture systems despite loss of both the need for food and the ability to ingest it is an interesting puzzle that remains unresolved. More generally, however, these observations suggest new, testable hypotheses about the developmental processes underlying the evolutionary loss of larval feeding in annelids and other animals with embryos that undergo spiral cleavage.
The image on the cover was photographed by Bruno Pernet (Friday Harbor Laboratories, University of Washington). The cover was designed by Beth Liles (Marine Biological Laboratory, Woods Hole, Massachusetts).
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Copyright © 2004 by the Marine Biological Laboratory.
