Larval settlement and metamorphosis of two gregarious sabellariid polychaetes:Sabellaria alveolatacompared withPhragmatopoma californica

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Abstract
Two sabellariid polychaetes,Sabellaria alveolatafrom European waters andPhragmatopoma californicafrom 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 forP. californica.In the present study, larval behaviour ofS. alveolataandP. californicawas 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 ofS. alveolatawith organic solvents diminished its capacity to induce metamorphosis pi conspecific larvae, but this capacity was not transferred to the extracts, as was the case forP. californica. The substance responsible for the enhanced metamorphosis ofS. alveolataon conspecific tube sand remains unknown. The free fatty acid (FFA) inducers of larval metamorphosis ofP. californicaeither inhibited, or had no effect on, metamorphosis ofS. alveolata. Both species responded abnormally upon exposure to unnaturally high concentrations of certain (particularly polyenoic) FFAs. Abnormal larval responses ofS. alveolata, however, did not incorporate behavioural components of normal metamorphosis, as were observed forP. californica. FFAs were isolated from the natural tube sand ofS. alveolataat less than one-tenth the concentration found in the natural tube sand ofP. 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 ofP. californica.

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