The free swimming larvae of many marine invertebrates actively respond to light. Light cues can be used to regulate position in the water column and to facilitate encountering sites suitable for metamorphosis. We examined the ontogeny of larval phototaxis and the ontogeny of metamorphic competency in larvae from three congeneric species of bryozoans. Larvae of Bugula neritina 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 photonegative with time. Temporally coincident with this change to negative phototaxis is an increase in the competency of larvae to initiate metamorphosis. 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 positive to negative using 10-5 M bath- applied 5-hydroxytryptamine (5HT) in larvae of B. neritina that were swimming for 1 h. Larvae that were photopositive and 1-h-old did not metamorphose at levels significantly different from larvae that were 1-h-old and treated with 5HT (i.e., young, photonegative larvae). Additionally, photopositive larvae which were swimming for 4 h initiated metamorphosis at rates nearly identical to photonegative larvae of the same age. Our data document that in larvae of B. neritina the changes in sign of phototaxis and levels of metamorphic competency are independent developmental events that occur in temporal coincidence. The concurrent timing of these two pathways may have been synchronized through selective processes resulting in a tight coupling between arrival at potentially suitable sites for metamorphosis and ability to respond to metamorphic cues.



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