Reference: Biol. Bull. 165: 221-240. (August, 1983) \SCIDI\N-PROCHLORON SYMBIOSIS: THE ROLE OF LARVAL PHOTOADAPTATIONS IN MIDDAY LARVAL RELEASE AND SETTLEMENT RICHARD RANDOLPH OLSON Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138 ABSTRACT Colonies of the algal-ascidian symbiosis Didemnum molle at Lizard Island, Aus-tralia, release more than 95% of their larvae daily between 1 1:00 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 col-onies. 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 fj-E m" 2 s" 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. INTRODUCTION The availability of suitable habitats for the settlement of larvae of sessile marine invertebrates is known to vary spatially (Grosberg, 1981; Palmer and Strathmann, 1981; Sebens, 1981; Keough and Downes, 1982) as well as temporally (Grosberg, 1982). Although considerable research has been conducted on factors that induce larvae to settle (Meadows and Campbell, 1972), very little is known about the ecological significance of the time of day that larvae are released. Many species of sessile invertebrates have larvae that swim for less than an hour before settling [e.g., some ascidians (Crisp and Ghobashy, 1971), bryozoans (Ryland, 1974), and corals (Lewis, 1974)]. Such a short time between larval release and settlement potentially enables the parent to control the time of day its larvae will settle. Colonial ascidians are commonly members of fouling communities in temperate waters (Millar, 1971) and cryptic communities on coral reefs (Jackson, 1977). Eigh-teen species of one family (Didemnidae) possess symbiotic unicellular algae (Kott, 1980). These species are found only in the tropics and commonly occur, not in cryptic communities, but in fully sunlit areas on coral reefs. Although considerable research has been conducted on the symbiotic algae, there is little known about the ecology of the animals or their larvae. Of the eighteen species of ascidian-algal associations, two species possess algae of the genus Synechocystis (Lafargue and Duclaux, 1979; Olson, 1980), a cyanophyte Received 17 February 1983; accepted 25 May 1983. 221
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