Reproductive Isolation in Two Forms of the Serpulid Polychaete, Spirobranchus Polycerus (Schmarda) in Barbados

Two morphotypes (described as varieties by ten Hove, 1970) of the tropical serpulid Spirobranchus polycerus are known in Barbados. One is a simultaneous hermaphrodite, has seven opercular horns and occurs singly or in small groups on live coral, particularly the hydrozoan Millepora complanata, on fringing and patch reefs. The other is gonochoric and highly gregarious, has two horns on the operculum and forms compact masses at mean low tide level on undercut boulders and cliffs on exposed coasts. The two forms are considered to belong to the same species largely because opercula with an intermediate number of horns were found on 57 worms in a 1,075-specimen collection from the Caribbean islands (ten Hove, 1970). None of the intermediates came from Barbados. Ten cross-breeding experiments were carried out attempting to fertilize eggs of the 2-horned form with sperm from the 7-horned form. Mean percent of cleaving eggs in the cross was 4.2. For self-fertilizing eggs of the 7-horned form it was 98.8; for eggs of the 2-horned form fertilized by sperm from males of the same form it was 97.4. The percent of eggs cleaving in the cross did not differ significantly from the percent cleaving among experimentally unfertilized eggs of the 2-horned form. The relative number of swimming larvae found in each situation on the following day supported the absence of any significant fertilization in the cross. The two forms of Spirobranchus polycerus in Barbados may, therefore, deserve consideration as separate species. The situation is, however, complicated by many examples in the collection studied by ten Hove of both forms, collected, in the Dutch Antilles, from substrates (beachrock, mangrove, pilings) on which they are not found in Barbados, by evidence for opercula with an intermediate number of horns and by the absence of information on tidal level and sexuality. More information is needed on the situation in other Caribbean islands before the specific status of the 2- and 7-horned forms can be adequately assessed.