There are at least two different North American species of Chaetopterus. C. pergamentaceus is found on the Atlantic Coast from Florida at least to Cape Cod; it is more common in warmer than in cooler water. C. variopedatus is found on the Pacific Coast, at least from California to Washington; it too is more common in warmer than in cooler water. We have not examined specimens from the Gulf of Mexico, but suspect that they, too, will be C. pergamentaceus. Despite the similar anatomy that has made early researchers proclaim them to be a single species (e.g. Enders, 1909), reciprocal cross-fertilizations between these specimens cannot be accomplished in the laboratory (Eckberg and Anderson, 1985; also pers. comm. of W.R. Jeffery); accordingly we consider them to be separate species.
Throughout the century, the Woods Hole Chaetopterus has been referred to by both species names, so there is considerable confusion in the literature. Enders (1909) mentioned both names but considered all North American specimens to be a single species - variopedatus. Hartman (1945) made the same decision for North Carolina specimens. Nomenclature has tended, unfortunately, to follow disciplinary lines. Following Lillie (1902) and Costello and Henley (1971), most papers on gametes and development refer to pergamentaceus; papers dealing with morphology and regeneration have used the name variopedatus. Rather than relying on species names in publications, readers should check collection locations.
Although the Atlantic and Pacific specimens are different species, we have performed many studies on both species with identical results, so this difference should not influence the decision of which species to study.