Chemical induction of larval settlement and metamorphosis in the reef-building tube worm Phragmatopoma californica (Sabellariidae: Polychaeta)

Abstract

Naturally-occurring lipophilic inducers of larval settlement and metamorphosis wer isolated and identified for Phragmatopoma californica, a gregarious tube worm from southern California. Organic solvent extraction of the sand/organic cement matrix of tubes diminished the inducing capacity of the tube matrix. The inducing capacity was restricted to a single, highly active, HPLC-purified fraction of the organic solvent extract. Chemical analysis of this fraction revealed a mixture of free fatty acids (FFAs), dominated by eicosapentaenoic acid (20:5, ∼20%), palmitic acid (16:0, ∼14%) and palmitoleic acid (16:1, ∼12%). In assays of the nine FFAs that each contributed 3% or more to the active fraction, only 16:1, 18:2, 20:4 and 20:5 induced larval settlement and metamorphosis, while the others were ineffective. The larval response was contact-dependent, highly specific, and concentration-dependent, with a significant response to 16:1 and 20:4 at as low as 10 μg FFA spread onto 1 g of sand (surface area ⋟36 cm2). Active FFAs were extracted at approximately 14 μg g-1 sand from the tube matrix, although the levels encountered by larvae in nature are believed to be higher.