Specific free fatty acids induce larval settlement and metamorphosis of the reef-building tube worm Phragmatopoma califomica (Fewkes)

Abstract

Planktonic larvae of the Californian reef-building sabellariid polychaete worm, Phragmatopoma califomica, settle and metamorphose in the presence of specific free fatty acids (FFAs) isolated from the sand matrix of adult tubes. Thirty-seven commercially-available FFAs of increasing acyl chain length and of variable unsaturation were assayed for their capacity to induce larval metamorphosis. At 100 μg FFA/g sand, settlement and metamorphosis peaked in response to palmitoleic acid (16:1; 24.5 % metamorphosis versus 0.8% metamorphosis in control assays), linolenic acid (18:3, 24.9% metamorphosis), eicosapentaenoic acid (20:5; 16.9% metamorphosis) and docosahexaenoic acid (22:6; 8.7% metamorphosis), and decreased as the acyl chain length or the number of double bonds was changed. Palmitelaidic acid, the trans isomer of palmitoleic acid, was ineffective at inducing a larval response. Assays of the monoglycerides and methyl esters of palmitoleic and linoleic acids, cis-9-hexadecen-1-ol, cis-9,12-octadecadien-1-ol, the corresponding acetates and cis-9,12-octadecadiene resulted in little or no larval metamorphosis. Larval response is dependent on (1)the presence of at least one cis double bond in the molecule, (2) conservation of molecular shape with increasing acyl chain length by addition of cis double bonds, and (3) the presence of a free carboxyl group. This specificity resembles that found in studies of insect chemoreception and suggests that receptors may similarly mediate larval response in this marine invertebrate.