Larval Ecology of Marine Benthic Invertebrates: Paleobiological Implications

Abstract

Summary 1. Modes of larval development play important roles in the ecology, biogeography, and evolution of marine benthic organisms. Studies of the larval ecology of fossil organisms can contribute greatly to our understanding of such roles by allowing us to race effects on evolutionary time scales. 2. Modes of development can be inferred for well preserved molluscan fossils because the size of the initial larval shell (Protoconch I in gastropods, Prodissoconch I in bivalves) reflects egg size. Other morphological criteria are also available, and a comparative approach based on related taxa with known development may be the most reliable method. By combining larval and adult traits, it is possible to recognize modes of larval development in at least some fossil bryozoans, brachiopods, and echinoderms as well. (a) Planktotrophic larvae arise from small eggs, are released in enormous numbers with little parental investment per offspring, and suffer tremendous mortality during and shortly after a planktic existence. These larvae feed on the plankton during development, and are commonly capable of a prolonged free‐swimming existence, and thus wide dispersal. (b) Nonplanktotrophic larvae (which include both planktic lecithotrophic forms and ‘direct developers’) generally arise from large eggs, with relatively few young produced per parent. Relative to planktotrophic larvae, nonplanktotrophic larvae generally receive greater parental investment per larva, and larval mortality is generally lower. These larvae rely on yolk for nutrition during development, and planktic durations are generally much briefer than for species with planktotrophic larvae, so that dispersal capability is considerably less. Energetic investment per egg is generally higher than in planktotrophs, but as there are lower fecundities as well it is difficult to generalize about the total energetic cost of one mode of reproduction against the other. 3. Owing to the high dispersal capability of planktotrophic larvae, it has been suggested that species with such larvae will be geographically widespread, geologically long‐ranging, and exhibit low speciation and extinction rates. Species with nonplanktotrophic larvae will tend to be geographically more restricted, geologically short‐ranging, and exhibit high speciation and extinction rates (again, as a consequence of their characteristically low larval dispersal capabilities). 4. Recognition of differential dispersal capabilities can play a role in paleobiogeo‐graphic analyses. Concurrent study of the distribution of groups with contrasting modes of development will permit testing of hypotheses concerning timing, magnitudes and frequencies of migration and vicariance events. 5. Larval types are not randomly distributed in the oceans, but relationships with other aspects of the organisms' biology and habitats are very complex. Mode of development varies with: (a) Ecology. A simple r–––K model of adaptive strategies is clearly insufficient to explain the observed relationships: while many ‘equilibrium’ sp...

Natural and anthropogenic dispersal mechanisms in the marine environment: a study using cheilostome Bryozoa

“…The likely evolutionary consequences of species-level dispersal traits (for larvae, but are also applicable to any method of frequent dispersal) conferring large geographic ranges have been well documented; bu¡ering against local environmental disturbances a¡orded to species with greater dispersal capabilities (and larger geographic ranges) results in such species having longer geological durations than species with restricted dispersal (Scheltema 1978;Hansen 1978Hansen , 1980Jablonski 1986; reviewed by Jablonski & Lutz 1983). Because of the very short (geological) time that fouling has been (and will remain?)…”

“…Evidence from the fossil record indicates that species that are geographically widespread are generally longer-lived than those with restricted geographic ranges (Jackson 1974;Shuto 1974;Scheltema 1978;Hansen 1978Hansen , 1980Jablonski 1991Jablonski , 1995Jablonski & Raup 1995). Furthermore, the amount and frequency of genetic exchange between distant populations is thought to be an important determinant of both the tempo and mode of speciation (Shuto 1974;Scheltema 1978;Jablonski 1986; reviewed by Jablonski & Lutz 1983). The geographic range of a species, therefore, is likely to be a major factor in both the maintenance and generation of species diversity.…”

High gene flow due to pelagic larval dispersal among South Pacific archipelagos in two amphidromous gastropods (Neritomorpha: Neritidae)

“…The pelagic larva has only been lost from the life history a few times among the extant freshwater Neritids, and only once in an island species (Holthuis, 1995), even though pelagic larvae have been lost multiple times in other families of marine invertebrates (Duda and Palumbi, 1999;Hart, 2000). Given their unstable habitat, this makes sense, as the evolution of adaptations for larval retention in a freshwater species would lead to its restriction to individual islands and a heightened risk of species extinction (Hansen, 1978;Jablonski and Lutz, 1983).…”

Hydrological features above a Southern Ocean seamount inhibit larval dispersal and promote speciation: evidence from the bathyal mytilid Dacrydium alleni sp. nov. (Mytilidae: Bivalvia)

“…Shell morphology and dentition were compared with the exhaustive catalogue of Dacrydium species descriptions compiled by Allen (1998) and the review of deep sea Atlantic species by Salas and Gofas (1997). Prodissoconch measurements were compared to the criteria for distinguishing different reproductive modes in bivalves in general (Jablonski and Lutz 1983) and in the genus Dacrydium (Salas and Gofas 1997).…”