Molecular phylogeny of a circum-global, diverse gastropod superfamily (Cerithioidea: Mollusca: Caenogastropoda): pushing the deepest phylogenetic limits of mitochondrial LSU rDNA sequences

The Cerithioidea is a very diverse group of gastropods with ca. 14 extant families and more than 200 genera occupying, and often dominating, marine, estuarine, and freshwater habitats. While the composition of Cerithioidea is now better understood due to recent anatomical and ultrastructural studies, the phylogenetic relationships among families remain chaotic. Morphology-based studies have provided conflicting views of relationships among families. We generated a phylogeny of cerithioideans based on mitochondrial large subunit rRNA and flanking tRNA gene sequences (total aligned data set 1873 bp). Nucleotide evidence and the presence of a unique pair of tRNA genes (i.e., threonine + glycine) between valine-mtLSU and the mtSSU rRNA gene support conclusions based on ultrastructural data that Vermetidae and Campanilidae are not Cerithioidea, certain anatomical similarities being due to convergent evolution. The molecular phylogeny shows support for the monophyly of the marine families Cerithiidae [corrected], Turritellidae, Batillariidae, Potamididae, and Scaliolidae as currently recognized. The phylogenetic data reveal that freshwater taxa evolved on three separate occasions; however, all three recognized freshwater families (Pleuroceridae, Melanopsidae, and Thiaridae) are polyphyletic. Mitochondrial rDNA sequences provide valuable data for testing the monophyly of cerithioidean [corrected] families and relationships within families, but fail to provide strong evidence for resolving relationships among families. It appears that the deepest phylogenetic limits for resolving caenogastropod relationships is less than about 245--241 mya, based on estimates of divergence derived from the fossil record.     

Early Miocene (Ottnangian) Mollusca of the Western Paratethys—ontogenetic strategies and palaeo-environments

Early ontogenetic shells of 25 species of brackish water and freshwater molluscs from the Ottnangian (Lower Miocene) Oncophora Beds (Lower Bavaria, South Germany) and Kirchberg Formation (Upper Bavaria, South Germany) are described for the first time. Taxonomic implications are discussed. The investigated bivalves (Cardiidae and Dreissenidae) were characterised by an indirect development with inclusion of a planktonic veliger stage. Among the gastropods only three species of the genus CtyrokiaSchlickum, 1965 were characterised by veliger larvae, all the other gastropod species were direct developers, which hatched as crawling young. The species Agapilia schlickumi nov. sp. (Neritidae), Nematurella pseudozilchi nov. sp. and Nematurella strauchi nov. sp. (Hydrobiidae) are introduced. Our study revealed the co-occurrence of 10 molluscan species in the Oncophora Basin of Lower Bavaria and the Kirchberg Basin of Upper Bavaria and thus indicates similar oligohaline to mesohaline coastal swamp milieus for both depositional environments. The presence of planktonic larval development in gastropods indicates a connection to the open sea.     

Changes in the timing of mantle formation and larval life history traits in linguliform and craniiform brachiopods

The distribution of embryonic and larval mantles is documented in linguliform and craniiform brachiopods. Criteria are presented for identifying these mantle types. The mantle type is related to planktotrophic and lecithotrophic larval life history patterns. In the Linguliformea and Craniiformea, all Lower Palaeozoic families with adequate preservation had larval mantles, indicating the presence of a planktotrophic larva. Heterochronic changes in the time of mantle origin, from the larval to the embryonic stage of development, has occurred several times. In the Lingulidae this change appears to have taken place at about the time the family originated in the Devonian and has been retained in extant genera. The family Discinidae has also retained a planktotrophic larval stage from the Lower Palaeozic to the present. The extant genus Crania in the Craniidae has a short-lived lecithotrophic larva that lacks a mantle. Through the Lower Jurassic, this family had planktotrophic larvae with a larval shell. During the Upper Jurassic, genera with a lecithotrophic larva that lacked a larval shell began to appear; however, the last genera in this family with a planktotrophic larva and a larval shell did not become extinct until the Tertiary.     

Larval muscle contraction fails to produce torsion in a trochoidean gastropod.

The causes and effects of ontogenetic torsion in gastropods have been debated intensely for more than a century (1-19). Occurring rapidly and very early in development, torsion figures prominently in shaping both the larval and adult body plans. We show that mechanical explanations of the ontogenetic event that invoke contraction of larval retractor muscles are inadequate to explain the observed consequences in some gastropods. The classic mechanical explanation of Crofts (4, 5) and subsequent refinements of her explanation have been based on species with rigid larval shell properties (18, 19) that cannot be extrapolated to all gastropods. We present visual evidence of the lack of rigidity of the uncalcified larval shell in a basal trochid gastropod, Margarites pupillus (Gould), and provide photographic confirmation of our prediction that larval retractor muscle contraction is insufficient to produce more than local deformation or dimpling at the site of muscle insertion. These findings do not refute muscular contraction as a primary cause of ontogenetic torsion in gastropods that calcify their larval shells prior to the onset of torsion, nor do they refute the monophyly of torsion. They do, however, suggest that torsion may be a loosely constrained developmental process with multiple pathways to the more constrained end result (20, 21).     

Biotic interactions revealed by macroborings in arctic bivalve molluscs

The presence of organisms whose bodies have low preservation potential may be deduced by searching for the traces produced by them. The addition of predatory gastropods and soft-bodied epizoans to Quaternary marine faunas dominated by bivalves was facilitated by an examination of borings in bivalve shells. Borings attributed to predatory gastropods (ichnogenus Oichnus ) were observed in shells of Astarte spp., Hiatella arctica and Macoma calcarea. Astarte, Hiatella and Macoma were preyed upon in preference to other members of a diverse suspension-feeding bivalve community. Borings attributed to epizoans (ichnogenus Cautostrepsis ) were observed in bivalve shells (Astarte spp. Hiatella arctica ), calcareous algae and limestone clasts. Biotic interactions revealed by trace fossils are employed, for the first time, to reconstruct the trophic structure of arctic Quaternary marine benthic faunas. ▭ Arctic molluscs, palaeoecology, Oichnus, Caulostrepsis.     

Larval ecology and morphology in fossil gastropods

The shell of marine gastropods conserves and reflects early ontogeny, including embryonic and larval stages, to a high degree when compared with other marine invertebrates. Planktotrophic larval development is indicated by a small embryonic shell (size is also related to systematic placement) with little yolk followed by a multiwhorled shell formed by a free‐swimming veliger larva. Basal gastropod clades (e.g. Vetigastropoda) lack planktotrophic larval development. The great majority of Late Palaeozoic and Mesozoic ‘derived’ marine gastropods (Neritimorpha, Caenogastropoda and Heterobranchia) with known protoconch had planktotrophic larval development. Dimensions of internal moulds of protoconchs suggest that planktotrophic larval development was largely absent in the Cambrian and evolved at the Cambrian–Ordovician transition, mainly due to increasing benthic predation. The evolution of planktotrophic larval development offered advantages and opportunities such as more effective dispersal, enhanced gene flow between populations and prevention of inbreeding. Early gastropod larval shells were openly coiled and weakly sculptured. During the Mid‐ and Late Palaeozoic, modern tightly coiled larval shells (commonly with strong sculpture) evolved due to increasing predation pressure in the plankton. The presence of numerous Late Palaeozoic and Triassic gastropod species with planktotrophic larval development suggests sufficient primary production although direct evidence for phytoplankton is scarce in this period. Contrary to previous suggestions, it seems unlikely that the end‐Permian mass extinction selected against species with planktotrophic larval development. The molluscan classes with highest species diversity (Gastropoda and Bivalvia) are those which may have planktotrophic larval development. Extremely high diversity in such groups as Caenogastropoda or eulamellibranch bivalves is the result of high phylogenetic activity and is associated with the presence of planktotrophic veliger larvae in many members of these groups, although causality has not been shown yet. A new gastropod species and genus, Anachronistella peterwagneri, is described from the Late Triassic Cassian Formation; it is the first known Triassic gastropod with an openly coiled larval shell.     

Larval development of hyolithids

The larval development of hyolithids is inferred to have been closely similar to that of primitive gastropods. A trochophore-like larva produced a swollen initial conch and a convex operculum. The initial conch was either subsphaerical and smooth or fusiform with a pointed apex and growth lines. This is taken to indicate an initial development within egg covers or free-living, respectively. A pelagic veliger phase followed. preceded by hatching in the first case. Veliger shells are similar to adult ones. Metamorphosis (transformation of veliger into benthic adult animal) in both hyolithids and gastropods was accompanied by an increase in mortality.     

Inferring larval type in fossil bryozoans

Pachut, J.F. & Fisherkeller, P. 2010: Inferring larval type in fossil bryozoans. Lethaia, Vol. 43, pp. 396–410. Larval type in extinct organisms might be recognizable because larvae of living marine invertebrates are approximately of the same size as the initial post‐larval organism. Two larval types typically occur. Planktotrophic larvae feed on other members of the plankton, potentially prolonging their larval existence and producing broad geographic distributions. Conversely, lecithotrophic larvae feed on yolk supplied by the fertilized egg, often settle quickly after release, and display more restricted distributions. However, some lecithotrophic bryozoans undergo embryonic fission forming multiple, small, polyembryonic larvae. The relationship between post‐larval size and larval type was evaluated in bryozoans by comparing the size of the ancestrula, the founding individual of a colony, to the sizes of extant planktotrophic, lecithotrophic and polyembryonic lecithotrophic larvae and ancestrulae. The sizes of larvae and ancestrulae in extant lecithotrophic and planktotrophic cheilostome (gymnolaemate) species are statistically the same. They are, however, statistically larger than the polyembryonic larvae of extant cyclostomes (stenolaemates). In turn, the sizes of cyclostome larvae are indistinguishable from the ancestrulae of extant and fossil cyclostomes, the ancestrulae of other fossil stenolaemate species measured from the literature, and the ancestrulae of three of four genera from North American Cincinnatian strata. Ancestrulae of a fourth genus, Dekayia, are the same size as cyclostome ancestrulae but are statistically smaller than the ancestrulae of other stenolaemates. With few exceptions, stenolaemates have statistically smaller larvae and ancestrulae than both lecithotrophic and planktotrophic cheilostomes. We infer that the sizes of fossil ancestrulae permit the discrimination of taxa that had polyembryonic lecithotrophic larvae from those possessing other larval types. This inference is strengthened, in several cases, by the co‐occurrence of brood chambers (gynozooecia) and restricted palaeobiogeographic distributions. The presence of cyclostomes in Early Ordovician strata suggests that polyembryony may have been acquired during the initial radiation of Class Stenolaemata. Polyembryony appears to be a monophyletic trait, but confirmation requires the demonstration that species of several stenolaemate suborders lacking skeletally expressed brood chambers possessed polyembryonic larvae. □Ancestrulae, evolution, fossil bryozoans, gynozooecia, larvae.     

The problem of similarity: analysis of repeated patterns of microsculpture on gastropod larval shells

Abstract. The problem of similarity is one of explaining the causes of striking resemblances between patterns and architectural themes that recur in organisms at various scales of biological organization. Classical explanations that have considered only the alternatives of homology and analogy overlook similarities of form that are primarily a consequence of fabrication, conveying little information about evolutionary relationships or functional role. When viewed at successively higher magnifications and when mapped onto a phylogeny, patterns of delicate cancellate microsculpture and granular microprotuberances on the surfaces of larval shells of marine gastropods fail to meet the predictions of exclusively historical or exclusively functional explanations, but are shown to be rich in fabricational information. Similar patterns suggest that early biomineralization of the initial organic shell is under weaker biological control than the processes that modulate assembly of the multi-layered, hierarchically-organized composite materials of the adult shell. Some patterns suggest remote biomineralization, without direct influence of living tissue. Scanning electron microscopy of larval shell features reveals previously undetected variation on basic themes that may have implications for the traditional disciplines of systematics, functional morphology, and fabricational morphology. The integration of the approaches of the traditional divisions of biology is required for full explanation of similarity and to generate a unified set of principles for the analysis of form in living and fossil organisms.     

Morphogenesis and phenotypic divergence in two developmental morphs of Streblospio benedicti (Annelida,Spionidae)

Abstract. The morphology of marine invertebrate larvae is strongly correlated with egg size and larval feeding mode. Planktotrophic larvae typically have suites of morphological traits that support a planktonic, feeding life style, while lecithotrophic larvae often have larger, yolkier bodies, and in some cases, a reduced expression of larval traits. Poecilogonous species provide interesting cases for the analysis of early morphogenesis, as two morphs of larvae are produced by a single species. We compared morphogenesis in planktotrophic and lecithotrophic morphs of the poecilogonous annelid Streblospio benedicti from the trochophore stage through metamorphosis, using observations of individuals that were observed alive, with scanning electron microscopy, or in serial sections. Offspring of alternate developmental morphs of this species are well known to have divergent morphologies in terms of size, yolk content, and the presence of larval bristles. We found that some phenotypic differences between morphs occur as traits that are present in only one morph (e.g., larval bristles, bacillary cells on the prostomium and pygidium), but that much of the phenotypic divergence is based on heterochronic changes in the differentiation of shared traits (e.g., gut and coelom). Tissue and organ development are compared in both morphs in terms of their structure and ontogenetic change throughout early development and metamorphosis.     

Spermatophores of thalassoid gastropods (Paludomidae) in Lake Tanganyika, East Africa, with a survey of their occurrence in Cerithioidea: functional and phylogenetic implications

Abstract. With few exceptions, spermatozoa-encapsulating packages in molluscs are known mostly from cephalopods and pulmonate gastropods. Among non-stylommatophoran gastropods, the marine Cerithioidea are second only to the Neritimorpha in the number of species that posess a spermatophore, but they have only rarely been found in freshwater taxa of this superfamily. We describe and compare here the sperm packages of 11 paludomid cerithioideans as part of an ongoing study on the evolution and systematics of the thalassoid (i.e., "marine-like") endemic species flock from Lake Tanganyika. Stereomicroscopic and SEM examination revealed unexpected complexity in shape and structure of spermatophores within paludomids. In addition, we present a survey of other Cerithioidea, which revealed that spermatophores are in general structurally simple and confirmed their presence in 12 marine species (five families) and 15 limnic species (four families), including those of 10 thalassoid species for which spermatophores are described herein. Based on histological sections of the male genital tract, we hypothesize that spermatophores are formed anteriorly, wholly or in part, by the so-called spermatophore-forming organ, and which is considered a synapomorphy of the Paludomidae. In addition, we briefly discuss functional aspects inferred from morphological study of the spermatophore-forming organ, with possible implications for spermatozoan transfer and fertilization. Finally, we place the features of the spermatophores and the spermatophore-forming organ in a phylogenetic framework of cerithioidean and paludomid systematics, which suggests that the spermatophore-forming organ is a synapomorphy of Paludomidae, that a bifurcate spermatophore structure is plesiomorphic, and that the evolution of structurally complex, spiny spermatophores has occurred independently in disparate lineages within the thalassoid species flock.     

Sinistral snail shells in the sea: developmental causes and consequences

The paucity of sinistral (left-coiling) relative to dextral (right-coiling) species of gastropods in the marine realm is an enigma. In Conus , one of the most diverse marine animal genera, sinistral shell coiling has evolved as a species-wide character only once. Fossils of this species, Conus adversarius , are found in Upper Pliocene and lowermost Pleistocene deposits in the southeastern USA. Conus adversarius had nonplanktonic larval development; this may have been a critical factor for the early establishment of the species, as well as sinistral marine species in other clades. Notably, most specimens of aberrantly sinistral modern Conus are derived from typically dextral species that have nonplanktonic development. If C. adversarius was reproductively isolated from dextral conspecifics, then this species may provide an example of nearly instantaneous sympatric speciation in the fossil record. Furthermore, the common and widespread – while geologically short-lived – fossil shells of C. adversarius show large amounts of variability in form and this variation may be related, at least in part, to a pleiotropic effect associated with the reversed coiling direction of this species.     

Phylogeny, speciation and species turnover. The case of the Mediterranean gastropods of genus Cyclope Risso, 1826

The genus Cyclope Risso, 1826 (family Nassariidae) has appeared in the fossil record since the Pliocene. Although it is still found today, the teleoconch morphology has never undergone modification, despite the fact that the protoconch morphologies of fossils (multispiral) and living forms (paucispiral) are different. They vary in their embryological and larval development and, hence, are two different species: C. migliorinii (Bevilacqua, 1928), the fossil species, and C. neritea (Linnaeus, 1758), the living species. We discuss the morphologic modifications in the evolution of this genus: the speciation that leads to its appearance and the speciation driving the Pliocene species to the living one. The order and the direction of these changes are based on phylogenetic analysis. No intermediate forms have been found showing a gradual morphological change that could have been worked by natural selection. Our analysis takes as the origin of the morphological novelties the genetic modifications in the ontogenetic processes which resulted in rapid and important phenotypic changes. Both speciation processes are sympatric cladogenetic. The changes that determine the appearance of the genus affect only the teleoconch, not the larval development. The modifications that lead from one species to the other, within the genus Cycope, affect the larval development exclusively. This points to a certain disconnection between the development of the embryo-larval phase and the young-adult formation, such that evolutionary processes could have occurred independently in different ontogenetic stages. The influence of larval ecology in relation to extinction of the ancestor and persistence of the derived species is also analysed. We hypothesize that climatic fluctuations may have affected the planktonic larvae of the fossil species, driving it to extinction. The living species, developing without the planktonic phase, would have resisted these climatic changes. We consider that the mechanisms described as drivers of the evolution of this genus can be of more general validity in prosobranch gastropods.     

Larval Planktotrophy—A Primitive Trait in the Bilateria?

The concept of Gösta Jägersten of a primary biphasic metazoan life-cycle, consisting of a planktotrophic larva and a benthic adult, forms the basis for several theories on metazoan phylogeny. In this paper the assumed planktotrophic life-style of the larva is critically analyzed and reconsidered. It is shown, in particular for the Mollusca, that a biphasic life-cycle with a lecithotrophic larva is probably the plesiomorphic condition. Character distribution and structural data suggest a parallel evolution of the downstream collecting system used in planktotrophic larvae or filter-feeding adults of gastropods, bivalves and other spiralian or aschelminth taxa. In the basic metazoans (Parazoa, Placozoa, coelenterates) direct or lecithotrophic development dominates by far. For the acoelomate (Platyhelminthes, Gnathostomulida) and pseudocoelomate taxa direct development is probably the plesiomorphic condition. The structural similarities of the upstream collecting system in tentaculate and deuterostome phyla may also be explained by parallel events of heterochrony out of an ancestor with adult filter-feeding. The main conclusion of this survey is that larval planktotrophy is likely to be secondary and not a plesiomorphic condition among the Bilateria. Accordingly, theories which are based on the assumed plesiomorphy of larval planktotrophy of the Bilateria, need careful reevaluation.     

Developmental dimorphism: consequences for larval behavior and dispersal potential in a marine gastropod

Specific effects of alternative developmental programs on swimming and settlement behavior for marine larvae have not been identified experimentally. A major impediment to this research has been the rarity of species with variable development. Here, we compared traits related to movement and habitat selection for different ontogenetic stages of long-lived, feeding larvae (planktotrophic) and short-lived, nonfeeding larvae (lecithotrophic) of the herbivorous gastropod Alderia modesta. Newly hatched planktotrophic larvae swam in meandering paths with equal rates of upward and downward movement. As planktotrophic larvae developed towards competence (physiological ability to metamorphose), their swimming paths became straighter, faster, and increasingly directed towards the bottom, traits shared by newly hatched lecithotrophic larvae. Despite differing in developmental history, competent planktotrophic (32-d-old) and lecithotrophic larvae (competent upon hatching) exhibited qualitatively similar swimming behaviors and substrate specificity. However, lecithotrophic larvae moved downward at twice the speed of competent planktotrophic larvae, potentially producing a 5-fold higher rate of contact with the bottom in natural flows. Competent larvae swam downwards rather than passively sinking, even though sinking rates were faster than swimming speeds; active swimming may allow larvae to keep the velum extended, permitting rapid response to chemical settlement cues and promoting successful habitat colonization. Differences between larvae of the two development modes may reflect fine-tuning by selection of traits important for dispersal and settlement into patchy adult habitats.     

Shell and hinge morphology of juvenile Limopsis (Bivalvia: Arcoida) - implications for limopsid evolution

Early ontogenetic shells of Limopsis angusta, L. aurita, L. cristata, L. friedbergi and L. minuta are described in detail and their morphogenetic traits discussed in a phylogenetic context. Prodissoconch length is found to range from 170-370 µm discounting the so-called prodissoconch 2 stage of authors. Reports of such a stage refer to the early postlarval 'interdissoconch'. Prodissoconch sizes are indicative of lecithotrophy but not of brooding. Limopsid-like prodissoconchs are rather common among pteriomorphs and at present of little use for phylogenetic interpretations, therefore. The early postlarval hinge is characterized by two tooth generations: an early one, (G1), representing the postlarval continuation of provincular teeth, and a late one, (G2), representing the independent adult dentition. This is a plesiomorphic trait for Pteriomorphia. The postlarval ligament is continuous with the larval resilium; it is neither co-functional with nor is it substituted by the typical duplivincular ligament as in other arcoids. Occasionally developed ridges, gutters or multiplication of ligament sublayers remain restricted to the primordial resilifer. It is concluded that the limopsid adult ligament represents a fusion of adult sublayer repetition within a retained larval resilium, a phenomenon best described as a heterochronic process. Comparisons with homologous characters of other arcoids support the view that Limopsidae evolved from parallelodontid arcoids and that they gave rise to Philobryidae but not to Glycymerididae.     

Decline in diversity of early Palaeozoic loosely coiled gastropod protoconchs

Quantitative data on molluscan larval conch fossil assemblages of ages ranging from the Ordovician (Argentina and the Baltic region), through Silurian (Austria), Devonian (Poland) to Carboniferous (Texas) supplement knowledge of early planktonic gastropods communities transformations. They show that larval shells of the bilaterally symmetrical bellerophontids and dextrally coiled gastropods with a hook-like straight apical portion of the first whorl initially dominated. Their relative frequency, as well as that of the sinistrally coiled ‘paragastropods’, diminished during the Ordovician and Silurian to virtually disappear in the Late Devonian and Early Carboniferous. Already during the Ordovician, diversity of larvae with gently loosely coiled first whorl increased, to be replaced then with more and more tightly coiled forms. Both the aperture constrictions and mortality peaks, probably connected with hatching and metamorphosis, indicate that the Ordovician protoconchs with hook-like first coil represent both the stage of an embryo developing within the egg envelope and a planktonic larva. The similarity of the straight apex to larval conchs of hyoliths and advanced thecosome pteropods is superficial, as these were not homologous stages in early development.     

Ontogenetic torsion in two basal gastropods occurs without shell attachments for larval retractor muscles

Results of this study on two species of vetigastropods contradict the long-standing hypothesis, originally proposed by Garstang (1929), that the larval retractor muscles power the morphogenetic movement of ontogenetic torsion in all basal gastropods. In the trochid Calliostoma ligatum and the keyhole limpet Diodora aspera, the main and accessory larval retractor muscles failed to establish attachments onto the protoconch (larval shell) when the antibiotics streptomycin sulfate and penicillin G were added to cultures soon after fertilization. Defects in protoconch mineralization were also observed. Despite these abnormalities, developing larvae of these species accomplished complete or almost complete ontogenetic torsion, a process in which the head and foot rotate by 180 degrees relative to the protoconch and visceral mass. Analysis by using phalloidin-fluorophore conjugate and transmission electron microscopy showed that myofilaments differentiated within myocytes of the larval retractor muscles and adherens-like junctions formed between muscle and mantle epithelial cells in both normal and abnormal larvae. However, in abnormal larvae, apical microvilli of mantle cells that were connected to the base of the larval retractor muscles failed to associate with an extracellular matrix that normally anchors the microvilli to the mineralized protoconch. If morphogenesis among extant, basal gastropods preserves the original developmental alteration that created gastropod torsion, as proposed by Garstang (1929), then the alteration involved something other than the larval retractor muscles. Alternatively, the developmental process of torsion has evolved subsequent to its origin in at least some basal gastropod clades so that the original alteration is no longer preserved in these clades.     

Biogeographic and bathymetric ranges of Atlantic deep-sea echinoderms and ascidians: the role of larval dispersal

Dispersal plays an important role in the establishment and maintenance of biodiversity and, for most deep-sea benthic marine invertebrates, it occurs mainly during the larval stages. Therefore, the mode of reproduction (and thus dispersal ability) will affect greatly the biogeographic and bathymetric distributions of deep-sea organisms. We tested the hypothesis that, for bathyal and abyssal echinoderms and ascidians of the Atlantic Ocean, species with planktotrophic larval development have broader biogeographic and bathymetric ranges than species with lecithotrophic development. In comparing two groups with lecithotrophic development, we found that ascidians, which probably have a shorter larval period and therefore less dispersal potential, were present in fewer geographic regions than elasipod holothurians, which are likely to have longer larval periods. For asteroids and echinoids, both the geographic and bathymetric ranges were greater for lecithotrophic than for planktotrophic species. For these two classes, the relationships of egg diameter with geographic and bathymetric range were either linearly increasing or non-monotonic. We conclude that lecithotrophic development does not necessarily constrain dispersal in the deep sea, probably because species with planktotrophic development may be confined to regions of high detrital input from the sea surface. Our data suggest that more information is necessary on lengths of larval period for different species to accurately assess dispersal in the deep sea.     

Larval ecology of prosobranch gastropods and its bearing on biogeography and paleontology

The larval ecology of prosobranch gastropods, which ranges from long pelagic plankto-trophic life histories to directly developing lecithotrophic ones, has a strong influence on larval dispersion. Because of low migrative ability in the post-larval stage, gastropod distribution must depend fundamentally on larval dispersion. It is probable that the modes of larval dispersion relate importantly to the mechanism by which prosobranch populations become isolated, and consequently to their modes of speciation. This idea is applicable for analysis of fossil material, for both biological and oceanographical information.