Timing of larval release by two Caribbean demosponges

The tlmlng of larval release has ~mportant implications for a larva's potential vulnerability to predators and environmental stresses. For larvae with planktonic pe r~ods of only minutes to hours, timing of larval release also determines the light levels that larvae experience, which can strongly influence their settlement behavior We placed larval traps over colonies of 2 common Caribbean sponges and found that both species released larvae during dayhght hours. Niphates digitalis generally releases its larvae between 07:OO and 11.00 h, while Callyspongia vag~nalis releases 11s larvae Idter in the day. Of 17 captured N. digitalis larvae released O.i to 1 m above the substrate. 13 remained near the bottom, and 8 of these 13 contacted the substrate within 10 min. For shortlived larvae, daytime release and their photonegative behavlor at settlcnicnt should drive them to settle in shaded microhab~ta ts that protect juveniles from predators and soldr 11I' radiation. Larvae of N dlgitalls and C. vaglnalis are distasteful to potential predators; thus they can remaln near the bottom with fishes and yet not be consumed. These data support a g e n c ~ d l CO-occurrence of daytime spawnlng of conspicuous, shor t l~\cd larvae and larval distastefulness that previously had been noted only for tropical ascidians. K E Y WORDS: Sponges Larvae . Tlm~ng of larval release Larval d~spersal Larval palatability Predation by fishes is thought to have strongly influenced the evolution of larval morphological characteristics and behaviors among benthic invertebrates, as well as adu1.t spawning patterns (reviewed by Strathmann 1985, Young & Chia 1989, Rumrill 1990, Morgan 1995a) Nighttime spawning is one means to diminish the morlality of vulnerable eggs and larvae by allowing them to disperse away from areas of high fish abundance under the cover of darkness. Nocturnal spawning is common among many groups of marine invertebrates, such as crustaceans (Morgan 1995b) and scleractinian corals (Babcock et al. 1986). In contrast, colonial ascidians release their larvae during daylight hours (reviewed by Svane &Young 1989), and these larvae typically disperse only a short distance before settling (van Duyl et al. 1981. Grosberg & Quinn 1986, Olson & McPherson 1987, Davis & Butler 1989, Stoner 1990, Bingham & Young 1991). The risk of predation for conspicuous, day-active ascidian larvae is low because they commonly are distasteful to fishes and can survive repeated 'sampling' attacks by fishes (Young & Bingham 1987, Lindquist et al. 1992). Daytime spawning and settlement allows larvae of colonial ascidians to choose settlement sites when strong photic cues and their photonegative behavior at settlement (Svane & Young 1989) drive them toward shaded microhabitats such as crevices and overhangs. Settlement in such sites diminishes juvenile mortality from large mobile predators, exposure to solar UV radiation, and other physical stresses (Keough & Downes 1982, Olson 1983, Young & Chia 1984) Lindquist & Hay (1996) found that the conspicuous larvae of many Caribbean reef sponges, like those of tropical colonial ascidians, are distasteful to m a l l predatory reef fishes. Thus, sponges might also be capable of exploiting the advantages of daytime larval release and settlement. Several lab-based studies have found (1) that brooded sponge larvae are released during daylight hours (Amano 1986, 1988, Lindquist & Hay 19961, and (2) that sponge larvae are competent to settle within minutes to hours of their release from the adult (Ilan & Loya 1988, 1990, Woollacott 1993, Meroz & llan 1995, Lindquist pers. obs.). Like larvae of colonial ascidians (van Duyl et al. 1981, Olson 1983), in situ swimming periods of sponge larvae may be considerably shorter than their swimming durations under lab-