These large tentacles are thought to function as locomotory suspension organs (Wilson 1982).While observing the larva under a compound microscope I noticed that it would contract and expand the tentacles and move around under the cover slip. The chaetae found on the larva may also aid in defense against predators. The larva’s tentacles have also been hypothesized by Wilson (1982) to assist in the capture of prey. Lebour (1922) and Smidt (1951) observed bivalve veliger larvae in the guts of larval Magelona. During metamorphosis, the larval tentacles are replaced by proportionally smaller adult tentacles.
Lebour MV. 1922. The food of plankton organisms. Journal of the Marine Biological Association of the United Kingdom. 12: 644-677.
Smidt ELB. 1951. Animal production in the Danish Waddensea. Meddelelser Kommission fra Danmarks Fiskeri- og Havundersogelser. 11 (6): 151.
Wilson DP. 1982. The larval development of three species of Magelona (Polychaeta) from localities near Plymouth. Journal of the Marine Biological Association of the United Kingdom. 62: 385-401.