Environmental Cues and the Phasing of Larval Release in the Bryozoan Celleporella hyalina (L.)

Celleporella hyalina broods lecithotrophic larvae that are pelagic for a few hours after release. Colonies of this bryozoan were placed in chambers suspended in the sea. Pre-filtered, ambient water, continuously pumped through the chambers, was delivered into sieves on a fractionator, partitioning the output into hourly samples. Larvae retained by the sieves were counted under a low-power microscope. Larval release occurred almost entirely during daylight, beginning shortly after sunrise when irradiance had reached about 1 μE m-2 s-1 (6 x 1017 photons), and peaked some 1.5 h later. When subjected to continuous darkness, larval release continued irregularly at a very low level, both in a chamber placed near the seabed and in another at constant depth near the water surface. Hydrostatic pressure varying tidally, therefore, did not influence larval release. After continuous darkness, both natural light and artificial illumination in reversed photoperiod immediately induced larval release at normal levels. Light is therefore the primary inducing factor for larval release. Tidal oscillation has a secondary effect on larval release by controlling ambient light intensity. Thus, the daily pattern of larval release at variable depth changed between unimodality and bimodality, according to the amplitude and timing of low tide in relation to photoperiod. Small peaks of larval release persisted under continuous illumination, suggesting an endogenous rhythm, although this was not evident in continuous darkness. Such an endogenous rhythm might ensure that larvae are released early in the morning, leaving sufficient time for them to encounter suitable substratum and settle during daylight. This would enable the larvae, then in their photonegative phase, to detect shaded microhabitats.