Bet-hedging dispersal strategy of a specialist marine herbivore: a settlement dimorphism among sibling larvae of Alderia modesta

Dispersal polymorphisms are known from many terrestrial organisms that inha bit fluctuating environments, but they are not well-recognized among marine invertebrates. An unusual form of variation was found in the settlement be havior of lecithotrophic larvae of the mollusc Alderia modesta (Loven, 1844 ) that markedly altered the dispersal potential of sibling larvae. Most clu tches contained, in variable proportions, larvae that spontaneously metamor phosed in the egg mass or within 2 d of hatching, and larvae that delayed m etamorphosis until encountering the obligate adult food, the yellow-green a lga Vaucheria longicaulis. The proportion of larvae undergoing spontaneous metamorphosis within a clutch was highly variable, ranging from 0 to 90 %, and was unrelated to adult size. The proportion of intracapsular metamorpho sis was also variable, ranging from 0 to 32%. The percentage of spontaneous metamorphosis per clutch was phenotypically plastic: when adults were star ved, there was a significant tendency for each successive clutch to have a lower percentage of spontaneous metamorphosis, thus increasing the dispersa l potential of the offspring. The decrease in spontaneous metamorphosis was significant after only 24 h of starvation. There was no significant increa se in the cumulative level of spontaneous metamorphosis after the 2nd day p ost-hatching, but larval mortality increased as a linear function after the 5th day. Most larvae that did not spontaneously settle in the first 2 d de layed metamorphosis until they were exposed to the adult host alga, V; long icaulis. When 2 d old larvae were exposed to 17 species of macroalgae and s ediment from the adult habitat, only V. longicaulis induced significant met amorphosis (93.3 +/- 6.7 %); all other species of algae and field sediment induced no metamorphosis or low levels (0-9 %) that were not significantly higher than sea water controls. There was no difference in the settlement s pecificity of 2 and 9 d old larvae when tested against 2 different algae, n or in their ability to complete metamorphosis. The high initial percentage of spontaneous metamorphosis reduces the dispersal potential of some larvae in each clutch relative to their siblings that delay metamorphosis until s timulated by V. longicaulis; this settlement dimorphism represents a novel bet-hedging strategy among marine invertebrates.