PALATABILITY AND CHEMICAL DEFENSE OF MARINE INVERTEBRATE LARVAE

Risk of larval mortality is a critical component of models and debates concerning the ecology and evolution of the differing reproductive ch aracteristics exhibited by marine invertebrates. In these discussions, predation often is assumed to be a major source of larval mortality. Despite limited empirical support, most marine larvae are thought to b e palatable and broadly susceptible to generalist predators. Previous studies of larval-planktivore interactions have focused primarily on l arvae that typically feed, grow, and develop for weeks to months in th e plankton. Such planktotrophic species commonly produce large numbers of small larvae that disperse over vast distances. In contrast, the n onfeeding lecithotrophic larvae from sessile invertebrates that brood are often large and conspicuous, lack morphological defenses, and have limited dispersal distances because they typically are competent to s ettle minutes to hours after spawning. Interactions between lecithotro phic larvae and consumers are not well studied. This has limited the a bility of previous authors to test broad generalities about marine lar vae. We show that brooded larvae of Caribbean sponges (11 species) and gorgonians (three species) as well as brooded larvae of temperate hyd roids (two species) and a bryozoan are unpalatable to co-occurring fis hes. In contrast, brooded larvae of temperate ascidians (three species ), a temperate sponge, and Caribbean hard corals (three species) are r eadily consumed by fishes, as are larvae from four of six species of s ynchronous broadcast-spawning gorgonians from the Florida Keys. Freque ncies of survivorship for larvae attacked and rejected by fishes were high and statistically indistinguishable from frequencies for unattack ed control larvae. Frequency of metamorphosis (when it occurred) of re jected larvae never differed significantly from that of unattacked con trol larvae. Assays testing for larval vs. adult chemical defenses for five species with distasteful larvae showed that larvae of all five s pecies were chemically distasteful to fishes whereas only three of fiv e adult extracts deterred fish feeding. A comparison of larval palatab ility among chemically rich taxa showed that brooded larvae were signi ficantly more likely to be unpalatable (86% of the species tested) tha n larvae of broadcasters (33%), and that palatable larvae were rarely released during the day (23%) while unpalatable larvae usually were (8 9%). Additionally, the frequency of bright coloration was high (60%) f or unpalatable larvae and low (0%) for palatable larvae, suggesting th at unpalatable larvae often may be aposematically colored. Results of this broad survey cast doubt on the widely accepted notion that virtua lly all marine larvae are suitable prey for most generalized planktivo res. Among species that do not chemically or physically protect larvae against fishes, selection appears to favor the release of larvae at n ight, or the production of smaller more numerous offspring that grow a nd develop at sea as a way of escaping consumer-rich benthic habitats. Because distasteful larvae are not similarly constrained, distasteful species should exhibit reproductive and larval characteristics select ed more by the fitness-related consequences of larval development mode and dispersal distance than by the necessity of avoiding benthic pred ators. Production of large larvae and retention of offspring in parent al habitats that have proved to be suitable for growth and reproductio n have both been proposed as advantageous, but these advantages often were assumed to be offset by losses due to increased larval apparency to fishes. This assumed trade-off is not mandatory because larvae can be defended chemically. Distasteful larvae tend to be conspicuous, loc alized dispersers that can co-occur with benthic fishes, and yet not b e consumed.