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’ species have nonplanktotrophic larvae, and organisms living in less prdictable environments often have planktotrophic larvae, some of the most opportunistic marine species have nonplanktotrophic larvae. Nonetheless, planktotrophic development seems most suited for exploitation of patchy but widespread habitats.



(b) Latitude. At shelf depths, planktotrophy is predominant in the tropics, and decreases sharply at high latitudes.



(c) Depth. Incidence of planktotrophy decreases with depth across the continental shelf, at least in some taxa. Beyond the shelf, many deep-sea organisms are nonplanktotrophic (e.g. most bivalves, peracarid crustaceans), but planktotrophic development appears to be present in other groups (prosobranch gastropods, ophiuroids, and bivalves inhabiting transient habitats such as sunken wood and hydrothermal vents).



These trends in developmental types will be accompanied by trends in evolutionary rates and patterns as outlined above. The study of larval ecology by paleobiologists will yield insights into the processes that gave rise to ancient evolutionary and biogeographic patterns, and will permit the development and testing of hypotheses on the origins of the patterns observed in modern seas.

Larval ecology of marine benthic invertebrates: paleobiological implications

Polyploid fish and shellfish: Production, biology and applications to aquaculture for performance improvement and genetic containment

The stratigraphy of shell concentrations.

. Bivalve classification has suffered in the past from the crossed-purpose discussions among paleontologists and neontologists, and many have based their proposals on single character systems. More recently, molecular biologists have investigated bivalve relationships by using only gene sequence data, ignoring paleontological and neontological data. In the present study we have compiled morphological and anatomical data with mostly new molecular evidence to provide a more stable and robust phylogenetic estimate for bivalve molluscs. The data here compiled consist of a morphological data set of 183 characters, and a molecular data set from 3 loci: 2 nuclear ribosomal genes (18S rRNA and 28S rRNA), and 1 mitochondrial coding gene (cytochrome c oxidase subunit I), totaling ∼3 Kb of sequence data for 76 molluscs (62 bivalves and 14 outgroup taxa). The data have been analyzed separately and in combination by using the direct optimization method of Wheeler (1996), and they have been evaluated under 12 analytical schemes. The combined analysis supports the monophyly of bivalves, paraphyly of protobranchiate bivalves, and monophyly of Autolamellibranchiata, Pteriomorphia, Heteroconchia, Palaeoheterodonta, and Heterodonta s.l., which includes the monophyletic taxon Anomalodesmata. These analyses strongly support the conclusion that Anomalodesmata should not receive a class status, and that the heterodont orders Myoida and Veneroida are not monophyletic. Among the most stable results of the analysis are the monophyly of Palaeoheterodonta, grouping the extant trigoniids with the freshwater unionids, and the sister-group relationship of the heterodont families Astartidae and Carditidae, which together constitute the sister taxon to the remaining heterodont bivalves. Internal relationships of the main bivalve groups are discussed on the basis of node support and clade stability.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1744-7410.2002.tb00132.x

On bivalve phylogeny: a high‐level analysis of the Bivalvia (Mollusca) based on combined morphology and DNA sequence data

The importance of seagrass beds as a habitat for fishery species

Polyploidy induced in the American oyster, Crassostrea virginica , with cytochalasin B.

Growth of American oysters increased by polyploidy induced by blocking meiosis I but not meiosis II

The effects of sediment type on growth rate and shell allometry in the soft shelled clam Mya arenaria L

Triploid soft-shell clams (Mya arenaria) were produced by inhibiting polar body extrusion with cytochalasin B immediately after fertilization. Diploid and triploid clams grown in suspended tray culture were examined histologically during the reproductive season of their second year. Most diploids had matured by the end of May and displayed normal gametogenic development in every individual. Triploids did not mature and most had undeveloped gonads. The few triploid females which developed some oocytes by the end of May had abnormal maturation. Triploids were 77% female as judged by the presence of oocytes; another 16% were female-like and may have been intersexes; the sex of 6% could not be identified because there was no sexual differentiation. Sex determination in this species, based on the sex ratio in triploids, best fits the model of an X:autosome balance mechanism as exemplified in some insect species.

Herbert Hidu

Papers

Polyploidy induced in the American oyster, Crassostrea virginica , with cytochalasin B.

Growth of American oysters increased by polyploidy induced by blocking meiosis I but not meiosis II

The effects of sediment type on growth rate and shell allometry in the soft shelled clam Mya arenaria L

Abnormal gametogenesis and sex ratio in triploid soft-shell clams (mya arenaria)

Induced triploidy in the soft-shell clamCytogenetic and allozymic confirmation