Light as an ecological factor in the dispersal and settlement of larvae of marine bottom invertebrates

Abstract

Roughly 90,000 species of marine bottom invertebrates reproduce by pelagic larvae, thus spending the most sensitive period of their life under the influence of light. Data on their responses to light have here been brought together for larvae of 141 species. In their early pelagic stages. 82 % of these species respond positively to light and migrate to the surface layers, 12 % seem to be indifferent to light. and 6 % respond negatively to light and continue to do so during their whole pelagic life. In their later and oldest pelagic stages most of the initially photopositive larvae will turn over to photonegativity, seeking down to deeper lying bottoms. but larvae of many intertidal species will remain photopositive till they stop swimming, thus sticking to the surface layers where they can get into contact with intertidal localities. Larvae of such groups which already in their pelagic phase live in symbiosis with zooxanthellae, do not seem to have another pattern of light-response than larvae of such groups in which zooxanthellae are lacking. Strong light intensities, increased temperatures and reduced salinity will all stimulate photopositive larvae to become photonegative. This fact helps to explain why even young photopositive larvae will in most cases concentrate somewhat deeper than the very surface of the sea and why the number of intertidal species in brackish areas is so small. Even such larvae of many (not all) intertidal species which are photopositive up to the moment they stop swimming seem to tum photonegative during metamorphosis and settlement, thus preferring shaded areas and dark surfaces for attachment. If they settle at night, when any substratum looks dark and :tshaded4:, they might find themselves exposed to direct sunlight next morning. which has proved injurious to many species. Experiments have shown that in most cases total darkness will prevent or postpone settlement, and that light is needed for such larvae when making their photonegative schoicee for a final place to settle. Dark surfaces contrasting to the surrounding medium and causing a general shading will attract most metamorphosing larvae of intertidal species. Since several larvae respond most sensitively to light after having been kept some time in darkness. the most favourable period for settling of shortlived intertidal larvae seems to be just about sunrise, and many species with an ultrashort pelagic larval stage are known to release their larvae in the early morning. From the moment the larvae stop swimming. several factors other than light may influence their settling behaviour, above all the chemical attraction from earlier settled populations of their own species.