ASCIDIAN-PROCHLORONSYMBIOSIS: THE ROLE OF LARVAL PHOTOADAPTATIONS IN MIDDAY LARVAL RELEASE AND SETTLEMENT

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Abstract
Colonies of the algal-ascidian symbiosis Didemnum molle at Lizard Island, Australia, release more than 95% of their larvae daily between 11:00 and 14:00 with a peak around 12:30, shortly after meridian passage of the sun. In shallow-water habitats, larvae are photoadapted to lower light environments than are adult colonies. Unlike adult colonies, larvae lack spicules and brown pigmentation in their tunic. They also have a lower chlorophyll a/b ratio than do their parent colonies. In the field, larvae seek a light intensity of approximately 100 µ m-2s-1 and settle preferentially on dark or shaded substrata. Settled larvae that were transplanted into full sunlight perished after 4 days. Larvae observed in the field swam for less than 10 minutes before settling. When denied a shaded substrate, larvae swam for up to 1.5 hours and eventually settled in full sunlight (an unsuitable habitat). Larvae in total darkness swam for at least 2 hours before settling. The larval photoadaptations, settlement behavior, and mortality of D. molle juveniles in full sunlight suggest that the release of larvae at midday, when sunlight is greatest, enables larvae to search for settlement sites when conditions are most severe, minimizing the chance they will settle in unsuitable habitats.