Ontogenies of Phototactic Behavior and Metamorphic Competence in Larvae of Three Species of Bugula (Bryozoa)

Abstract

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 bathapplied 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. Additional key words: bryozoans, phototaxis Many benthic marine invertebrates have as part of their life cycle a planktonic larval stage. Successful completion of the life cycle requires return to a suitable benthic habitat. Larvae utilize a variety of physical and biological cues to facilitate location of sites favorable for subsequent adult life (e.g., Pawlik 1992; Young 1995). Light is one such physical signal. Light is a ubiquitous vector in relatively shallow waters that exhibits both spatial and temporal variation. The temporal variation can be highly predictable (sunrise and sunset) or highly variable (changes in cloud cover). Responses to light are one potentially useful means of regulating vertical orientation and position in the water column (e.g., Thorson 1964; Clarke 1970; Cronin & Forward 1979; Sulkin 1984; Forward 1988; Barile et al. 1994; Young 1995). Additionally, in the case of meroplanktonic larvae, phototactic responses can condu tribute to delivery of larvae to benthic sites suitable for metamorphosis (e.g., McDougall 1943; Ryland 1960; Crisp 1974; Young & Chia 1982; Olson 1985; Dirnberger 1993). Thorson's (1964) seminal work was the first detailed comparative study of the ontogeny of phototaxis across several phyla of benthic marine invertebrates. He observed that 82% of 141 species from 11 phyla had early-stage larvae that were initially photopositive. Of these, 76% became photonegative before the conclusion of larval life. Thorson suggested that earlystage positive phototaxis increases the chances that larvae will be transported into the water column and hence away from conspecific adults and benthic predators. In the water column, larvae can be passively dispersed by currents, and feeding larvae can have access to more abundant populations of phytoplankton. Larvae that change to photonegative with time will then likely move to benthic sites, which is essential for completion of the life cycle. ies tactic ior t rphic tence e i s la ( a)