a1 Scripps Institution of Oceanography; A-008, University of California, San Diego, La Jolla, California 92093, U.S.A.
Two sabellariid polychaetes, Sabellaria alveolata from European waters and Phragmatopoma californica from the west coast of North America, are known from previous work to have larvae that settle and metamorphose preferentially on the cemented sand tubes of conspecific adults. The naturally occurring inducers of larval metamorphosis were recently isolated and identified for P. californica. In the present study, larval behaviour of S. alveolata and P. californica was compared in reciprocal laboratory settlement assays. For both species, metamorphosis occurred to a greater extent on conspecific tube sand than on control sand or on heterospecific tube sand. Extraction of the tube sand of S. alveolata with organic solvents diminished its capacity to induce metamorphosis pi conspecific larvae, but this capacity was not transferred to the extracts, as was the case for P. californica. The substance responsible for the enhanced metamorphosis of S. alveolata on conspecific tube sand remains unknown. The free fatty acid (FFA) inducers of larval metamorphosis of P. californica either inhibited, or had no effect on, metamorphosis of S. alveolata. Both species responded abnormally upon exposure to unnaturally high concentrations of certain (particularly polyenoic) FFAs. Abnormal larval responses of S. alveolata, however, did not incorporate behavioural components of normal metamorphosis, as were observed for P. californica. FFAs were isolated from the natural tube sand of S. alveolata at less than one-tenth the concentration found in the natural tube sand of P. californica. The differences between the two species provide further evidence that a very specific mechanism is responsible for the perception of FFAs by the larvae of P. californica.