Ontogenies of phototactic behavior and metamorphic competence in larvae ofthree species of Bugula (Bryozoa)

The free swimming larvae of many marine invertebrates actively respond to l ight. Light cues can be used to regulate position in the water column and t o facilitate encountering sites suitable for metamorphosis. We examined the ontogeny of larval phototaxis and the ontogeny of metamorphic competency i n larvae from three congeneric species of bryozoans. Larvae of Bugula nerit ina are positively phototactic on emergence from the brood chamber, whereas larvae of B. simplex and B. stolonifera appear initially photoneutral when populations of larvae are examined. Larvae of all three species become pho tonegative with time. Temporally coincident with this change to negative ph ototaxis is an increase in the competency of larvae to initiate metamorphos is. This observation suggests that these events are either physiologically linked or co-occurring, but independent developmental processes. We tested these hypotheses by artificially changing the sign of phototaxis from posit ive to negative using 10(-5) M bath-applied 5-hydroxytryptamine (5HT) in la rvae of B. neritina that were swimming for 1 h. Larvae that were photoposit ive and l-h-old did not metamorphose at levels significantly different from larvae that were l-h-old and treated with 5HT (i.e., young, photonegative larvae). Additionally, photopositive larvae which were swimming for 4 h ini tiated metamorphosis at rates nearly identical to photonegative larvae of t he same age. Our data document that in larvae of B. neritina the changes in sign of phototaxis and levels of metamorphic competency are independent de velopmental events that occur in temporal coincidence. The concurrent timin g of these two pathways may have been synchronized through selective proces ses resulting in a tight coupling between arrival at potentially suitable s ites for metamorphosis and ability to respond to metamorphic cues.