Planktonic Larval Stage Research Articles

Limits to gene flow in the slipper limpet Crepidula fornicata as revealed by microsatellite data and a larval dispersal model

Crepidula fornicata, one of the most invasive marine sessile invertebrates in Europe, has traits commonly associated with a weak population genetic structure (high population connectivity): wide environmental tolerance, high fecundity, and a long planktonic larval stage (21 d). In order to determine the scale at which effective dispersal occurs in this potentially highly dispersive species, we designed a hierarchical sampling protocol and analyzed genetic variation at 4 microsatellite loci in 17 populations from 6 French bays situated in the English Channel and at the entrance of the Eng- lish Channel (Iroise Sea). A strong regional pattern with populations clustered into 3 main groups (Iroise Sea, western and eastern English Channel) was revealed, as was significant isolation by dis- tance along the Channel coasts. Conversely, a high degree of genetic connectivity was observed within the study bays. These estimates of effective dispersal were in agreement with outputs of a 2D larval-dispersal model based on local hydrodynamic conditions, suggesting that connectivity between populations of C. fornicata is influenced by hydrodynamic feature. This study suggests the existence of barriers to gene flow of C. fornicata over moderate distances (ca. 100 km).

An alpha2-macroglobulin-like protein is the cue to gregarious settlement of the barnacle Balanus amphitrite.

Many benthic marine invertebrates, like barnacles, have a planktonic larval stage whose primary purpose is dispersal. How these species colonize suitable substrata is fundamental to understanding their evolution, population biology, and wider community dynamics. Unlike larval dispersal, settlement occurs on a relatively small spatial scale and involves larval behavior in response to physical and chemical characteristics of the substratum. Biogenic chemical cues have been implicated in this process. Their identification, however, has proven challenging, no more so than for the chemical basis of barnacle gregariousness, which was first described >50 years ago. We now report that a biological cue to gregarious settlement, the settlement-inducing protein complex (SIPC), of the major fouling barnacle Balanus amphitrite is a previously undescribed glycoprotein. The SIPC shares a 30% sequence homology with the thioester-containing family of proteins that includes the alpha(2)-macroglobulins. The cDNA (5.2 kb) of the SIPC encodes a protein precursor comprising 1,547 aa with a 17-residue signal peptide region. A number of structural characteristics and the absence of a thioester bond in the SIPC suggest that this molecule is a previously undescribed protein that may have evolved by duplication from an ancestral alpha(2)-macroglobulin gene. Although the SIPC is regarded as an adult cue that is recognized by the cyprid at settlement, it is also expressed in the juvenile and in larvae, where it may function in larva-larva settlement interactions.

Skeletal characters for identification of juvenile Ophiactis resiliens and Amphiura constricta (Echinodermata): cryptic ophiuroids in coralline turf habitat

Skeletal development of Ophiactis resiliens and Amphiura constricta was examined to determine characters that could be used to identify the juveniles of these species, which co-occur in algal turf habitat. Juvenile O. resiliens recruit to turf after a planktonic larval stage while A. constricta recruit through release from brooding adults. Skeletal development in O. resiliens and A. constricta is typical of most ophiuroids with diagnostic adult characters taking some time to appear. The main characters that can be used to distinguish the two species from an early stage are the number and arrangement of the oral papillae and the presence of disc spines. In O. resiliens diagnostic adult skeletal characters developed at 1.5 mm disc diameter. In A. constricta most identifying adult characters were evident at 700 μm disc diameter. Identification of juvenile O. resiliens is assisted by the presence of disc spines and thorny arm spines, features that disappear as the juveniles grow. Juvenile A. constricta did not develop disc spines or thorns on the arm spines.

Genetic differentiation in relation to marine landscape in a broadcast‐spawning bivalve mollusc (Placopecten magellanicus)

Marine bivalves are sessile or sedentary as adults but have planktonic larvae which can potentially disperse over large distances. Consequently larval transport is expected to play a prominent role in facilitating gene flow and determining population structure. The sea scallop (Placopecten magellanicus) is a dioecious species with high fecundity, broadcast spawning and a c. 30-day planktonic larval stage, yet it forms discrete populations or 'beds' which have significantly different dynamics and characteristics. We analysed variation at six microsatellite loci in 12 locations throughout the geographic range of the species from Newfoundland, Canada, to New Jersey, USA. Significant differentiation was present and the maximum pairwise theta value, between one of the Newfoundland samples in the north and a sample from the southern portion of the range, was high at 0.061. Other proximate pairs of samples had no detectable genetic differentiation. Mantel tests indicated a significant isolation by distance, but only when one of the populations was excluded. A landscape genetic approach was used to detect areas of low gene flow using a joint analysis of spatial and genetic information. The two major putative barriers inferred by Monmonier's algorithm were then used to define regions for an analysis of molecular variance (amova). That analysis showed a significant but low percentage (1.2%) of the variation to be partitioned among regions, negligible variation among populations within regions, and the majority of the variance distributed between individuals within populations. Prominent currents were concordant with the demarcation of the regions, while a novel approach of using particle tracking software to mimic scallop larval dispersal was employed to interpret within-region genetic patterns.

Effects of the 26 December 2004 tsunami on littorinid molluscs near Phuket, Thailand

A major effort is being undertaken to understand the effects of the December 2004 Indian Ocean tsunami on the marine environment. The emphasis has been on coral reefs, which are subtidal and offshore of the areas that were most heavily affected by the tsunami. The greatest losses of human life and destruction of human infrastructure occurred in the intertidal and supratidal regions where the waves had the greatest destructive force after they broke. We seek to determine the effects of the tsunami in the upper intertidal region, where the effects might be greatest, by using littorinid molluscs as an indicator group. An extensive 2002 survey of the distribution of littorinid molluscs at 50 sites in mangroves and on rocky shores in Thailand was recently published. Six of the study sites were on the Andaman Sea coast in the area of Phuket, Thailand, one of the regions most heavily hit by the tsunami. These sites (Fig. 1) were resurveyed between 13 and 18 April 2005, 3.5 months after the tsunami, to determine qualitatively what damage had occurred. Thirteen species of littorinids were recorded in the initial survey at the six sites; only seven species were recollected after the tsunami. The tsunami was not uniform, and depending on local topography affected the shorelines in different ways. The study sites included both rocky shores and mangrove areas (Table 1). Rocky shores at Kalim Beach and Nang Thong Beach were directly hit by the tsunami. While littorinid populations decreased considerably, the rocks themselves were not affected. At Nang Thong Beach the adjacent sandy shoreline was extensively modified (Figs 2, 3). The rocky shore of Mai ngarm Bay, Surin Island, has scattered mangroves (Rhizophora apiculata and R. mucronata ). The shore was in the lee of the tsunami and there were no visible effects. A site at an extensive mangrove at Laem Mai Kaew was heavily hit by the tsunami and was unrecognizable after the event; all littorinids at the site had disappeared. It should be noted that the site was at the seaward fringe of the mangrove, which was affected by the tsunami. The trees rapidly dissipated the force of the tsunami and there were no obvious effects shoreward of the mangrove fringe. Mangroves at Pak Meng Beach and Ta la Beach were largely unaffected by the tsunami. Even where littorinid species were present after the tsunami, densities appeared to have decreased. Even in areas where there were no apparent effects of the tsunami on rocks or mangroves, some of the littorinids were washed off the rocks or trees. Population reductions were likely to have been caused by the tsunami. However, it has been three years since the initial surveys were undertaken at the sites. As population densities are naturally variable, the reductions could have been due to another cause. For example, substantial natural declines in the population of Nodilittorina unifasciata (Gray, 1826) at Waterman Bay, Western Australia occurred over a two-year period. The fact that only seven of the 13 littorinid species originally found at the six study sites were present after the tsunami is not a cause for concern. The pattern found in the littorinids, as in the reefs, is of different effects in different areas. It is likely that more detailed studies would show that there are populations of the species remaining in the area that can recolonize areas affected by the tsunami. All of the species are widespread. The species with the smallest known distribution, Littoraria bengalensis, was described in 2001, and is known from the west coast of Thailand and also from India. All of the other species have much broader ranges. As far as is known, all of the littorinid species occurring on the west coast of Thailand have a planktonic larval stage, which will allow the ready dispersal of larvae into affected areas during the next spawning season. In summary, the immediate effects of the tsunami on littorinids in the Phuket area have been patchy, and it is likely there will be no major permanent changes to the distribution of species in the area.

Dispersal and divergence across the greatest ocean region: Do larvae matter?

For marine, benthic animals, duration of planktonic larval stages is expected to correlate with dispersal ability, and thus species ranges, at least where planktonic dispersal is necessary to reach habitats. Yet past analyses of larval duration and species ranges across the insular Pacific show at most a weak correlation. So, do larvae matter in determining species ranges in such an island setting? We analyze an extensive dataset on cowries and find, again, that estimated larval duration does not correlate with species ranges. Several factors can obscure a real correlation, however, including estimation error, intraspecific variation, other factors affecting dispersal, poor taxonomy, and remote endemics. We show that taking these into consideration greatly improves correlation. Further evidence for the importance of larval duration comes from diversity and speciation patterns. Diversity of poor dispersers drops more rapidly eastward across the Pacific and leads to taxonomic differences in community composition across the basin. Geographic scale of differentiation is strongly influenced by larval duration and leads to the most rapid diversification at intermediate dispersal capacities. A major lesson from the phylogenetically corrected cowrie dataset is that without accurate and appropriate taxonomy, clear and important distributional and diversity patterns can become obscured. Inappropriate taxonomic scale can also obscure macroecological patterns: cowrie tribes/subfamilies show substantial variation in the steepness of their diversity cline across the Pacific and in their proportional local abundance, showing the importance of ecological traits in controlling distributions. In contrast such variation was not evident in a study focused at the family level in corals and fishes.

Comparison of Tehuelche octopus ( Octopus tehuelchus) abundance between an open-access fishing ground and a marine protected area: Evidence from a direct development species

Marine protected areas (MPAs) have been established during the last few decades for conservation and management purposes. Most studies showing a positive effect of MPAs on abundance or size have focused on species exhibiting a planktonic larval stage. In this study, the impact of an MPA on the local abundance of a direct development species, the Tehuelche octopus Octopus tehuelchus, is evaluated. Octopus abundance (catch per unit of effort) was compared between a traditional fishing ground (El Fuerte) and an MPA in northern Patagonia (Argentina), during one spawning/recruitment period (August–October; pre-fishing) and during the following fishing season (January–April). Abundance in the MPA was twice as high as in El Fuerte during the fishing season, but not during the pre-fishing season. Females were more abundant than males at both sites. Contrary to expectations, mean octopus weight was lower in the MPA than in El Fuerte, which can be explained by higher abundance of recruits. In fact, the ratio recruit/spawner was three times higher in the MPA than in El Fuerte. Removal of brooding females during the brooding season in El Fuerte may affect the survival of the embryos, since females provide parental care. More information on the wide diversity of life history of marine species is needed since the current literature on MPAs is strongly biased towards species exhibiting planktonic stages.

The occurrence of the cold-water coral Lophelia pertusa (Scleractinia) on oil and gas platforms in the North Sea: Colony growth, recruitment and environmental controls on distribution

This study reports a newly established sub-population of Lophelia pertusa, the dominant reef-framework forming coral species in the north-east Atlantic, on oil and gas platforms in the northern North Sea. L. pertusa was positively identified on 13 of 14 platforms examined using existing oil and gas industry visual inspections. Two platforms were inspected in more detail to examine depth and colony size distributions. We recorded 947 colonies occurring between 59 and 132 m depth that coincides with cold Atlantic water at depths below the summer thermocline in the northern North Sea. We suggest that these colonies provide evidence for a planktonic larval stage of L. pertusa with recruits initially originating from populations in the north-east Atlantic and now self recruiting to the platforms. Size class distribution showed a continuous range of size classes, but with few outlying large colonies. The break between the largest colonies and the rest of the population is considered as the point when colonies began self recruiting to the platforms, resulting in greater colonization success. We present the first documented in situ colony growth rate estimate (26 ± 5 mm yr −1) for L. pertusa based on 15 colonies from the Tern Alpha platform with evidence for yearly recruitment events starting the year the platform was installed. Evidence of contamination from drill muds and cuttings was observed on the Heather platform but appeared limited to regions close to drilling discharge points, where colonies experience partial as well as whole colony mortality.

Image analysis techniques: A tool for the identification of bivalve larvae?

Despite the importance of the planktonic larval stage in intertidal bivalves, our understanding of this stage is still insufficient. A major obstacle in the quantification of planktonic larval distributions is the identification of sampled larvae. Identification is difficult due to the uniform morphology of many larval species. We evaluated the morphology of bivalve larvae reared in our laboratory ( Crassostrea gigas, Cerastoderma edule, Macoma balthica, Mytilus edulis) and literature data on larvae from the 1960s, using image analysis techniques. We used this dataset to compile species-specific dimensions (length–width of the larval shell) and shape parameters (contour of the larval shell). The first method yielded different slopes when length and width were plotted against each other, but regression lines overlapped, which rendered the technique impractical for field identification. Multidimensional scaling of larval shape within one species showed shape development of the larvae during ontogeny. Linear discriminant analysis did not produce results when the whole data set was used. But discriminant analysis on larger individuals (length > 150 μm) was relatively successful for species of which sufficient individuals were available. The identity of up to 74% of the large larvae could be predicted correctly.

Settlement-competency Period of Planulae and Genetic Differentiation of the Scleractinian Coral Acropora digitifera

Broadcast-spawning corals expel eggs and sperm, and the fertilized eggs develop into planulae in the water column. As these sessile corals generally disperse during the planktonic larval stage, their larval characteristics (e.g., survival and settlement rates) are thought to be important for their dispersal. Although some studies of coral larval dispersal have focused on the maximum time that larvae can remain viable and settle, the relevance of this maximum settlement competency period for long distance dispersal remains unclear. To examine the relationship between competency periods and genetic differentiation, we performed laboratory experiments to investigate settlement rates of planulae and determine the degree of genetic differentiation in Acropora digitifera in the Ryukyu Archipelago, southern Japan. In addition, we compared our findings to published data for A. tenuis, which was studied using our methods. Our results indicated that the maximum settlement competency period was lower in A. digitifera planulae (54 days) than in A. tenuis (69 days) planulae. The mean survival rates at 45 days and 59 days after spawning were less than 10%. Furthermore, percentages of planulae that remained viable and settle at 30 days after spawning (survival rate x settlement rates at 30 days) were approximately 18% and 25% in A. digitifera and A. tenuis, respectively. By contrast, gene flow (N(e)m: number of migrations per generation) was significantly higher in A. digitifera (7.8 to 41.4) than in A. tenuis (3.1 to 22.5). These results indicate that the settlement competency period and survival rates are unlikely to be robust predictors of gene flow. Overall, we detected significant genetic differentiation between Kerama and Okinawa in A. digitifera. As direct observation of planula dispersal between Kerama and Okinawa has been reported, we concluded that genetic mixing is not complete, but that some localized planulae may disperse from Kerama to Okinawa via a specific current depending on reef or locality.

Limited nucleotide divergence over large spatial scales in the asterinid sea star Patiriella exigua

DNA sequence data were collected from two segments of the mitochondrial genome for four populations of the sea star Patiriella exigua, a potentially hermaphroditic species with occasional self-fertilisation, and with a benthic, apparently non-dispersive larval stage. The populations, covering nearly all the range of this species in Australia, included samples from the Yorke Peninsula (South Australia), Tasmania (Woodbridge) and two from the Sydney region, New South Wales (Gordons Bay, Edwards Beach). Six haplotypes were observed in the Yorke Peninsula sample, three in Woodbridge, nine in Edwards Beach and nine in Gordons Bay. The evolutionary relationships between the samples included in this study are close, with average Tamura-Nei nucleotide distances between populations being at most 0.006. Migration between the NSW, Tasmanian and South Australian regions is, however, extremely rare. All haplotypes are restricted to single samples except for four found in both Edwards Beach and Gordons Bay. Monophyly of the haplotypes in Yorke Peninsula and Woodbridge suggests that much of their evolution has occurred locally or regionally and that migration has been low for long periods. This pattern strongly contrasts with the absence of lineage assortment in Echinodermata with extensive planktonic larval stages.

Rapid recovery of genetic diversity of dogwhelk (Nucella lapillus L.) populations after local extinction and recolonization contradicts predictions from life‐history characteristics

The dogwhelk Nucella lapillus is a predatory marine gastropod populating North Atlantic rocky shores. As with many other gastropod species, N. lapillus was affected by tributyltin (TBT) pollution during the 1970s and 1980s, when local populations became extinct. After a partial ban on TBT in the United Kingdom in 1987, vacant sites have been recolonized. N. lapillus lacks a planktonic larval stage and is therefore expected to have limited dispersal ability. Relatively fast recolonization of some sites, however, contradicts this assumption. We compared levels of genetic diversity and genetic structuring between recolonized sites and sites that showed continuous population at three localities across the British Isles. No significant genetic effects of extinction/recolonization events were observed in SW Scotland and NE England. In SW England we observed a decrease in genetic diversity and an increase in genetic structure in recolonized populations. This last result could be an artefact, however, due to the superposition of other local factors influencing the genetic structuring of dogwhelk populations. We conclude that recolonization of vacant sites was accomplished by a relatively high number of individuals originating from several source populations (the 'migrant-pool' model of recolonization), implying that movements are more widespread than expected on the basis of development mode alone. Comparison with published data on genetic structure of marine organisms with contrasted larval dispersal supports this hypothesis. Our results also stress the importance of local factors (geographical or ecological) in determining genetic structure of dogwhelk populations.

Palaeobiogeography of the Ordovician trilobite Prosopiscus, with a new species from western New South Wales

Prosopiscus is particularly important in Ordovician palaeobiogeography because of its wide geographic distribution in Gondwana and peri-Gondwanan regions. It appears to have been confined to low palaeolatitudes, representing a characteristic member of the warm water eastern Gondwanan shelf faunas. Trends in the distribution of the Ordovician genus can be observed due to its long stratigraphic range. Prosopiscus was restricted to, and may have originated in, Australia during the late Early Ordovician (Bendigonian-Chewtonian). By the Middle Ordovician (Darriwilian), Prosopiscus had dispersed to other parts of Gondwana and peri-Gondwana, including the North and South China blocks, Tarim, central Himalayas, and the Argentine Precordillera (South America). Possible explanations for the distribution of Prosopiscus are that: (1) there were no oceanic barriers preventing dispersal of trilobites between different regions of Gondwana, thus permitting uninhibited migration over vast distances; (2) Prosopiscus was not restricted to a specific biofacies; (3) a major eustatic transgression during the early Darriwilian may have facilitated the dispersal of Prosopiscus in allowing further development and expansion of marine environments; and (4) a prolonged planktonic larval stage may have permitted wide dispersal. Prosopiscus lauriei sp. nov. is described from the late Early Ordovician (Bendigonian-Chewtonian) Tabita Formation at Mount Arrowsmith, northwestern New South Wales, Australia. The new species is closely related to P. praecox, from the Nora Formation, Georgina Basin, central Australia, and to P. magicus from northwest China.

Redescription of Opsipedon Gracilis Heer (Crustacea, Tanaidacea) from the Middle Jurassic of Northern Switzerland, and the Palaeoenvironmental Significance of Tanaidaceans

Opsipedon gracilis Heer, 1865, is the oldest valid name for a fossil tanaidacean crustacean. The type material, which was originally described as coming from the Lower Jurassic ‘Insect marls’ (Insektenmergel, Hettangian), is actually of Middle Jurassic age (Opalinum Clay, Aalenian). A redescription of this species is given, and its position within the suborder Apseudomorpha is established. A new model is proposed which relates the occurrence of fossil tanaids with palaeo‐oxygenation, both long‐term average values and intensity of (seasonal) fluctuations. Many tanaidaceans occur in oxygen‐depleted benthic environments, but they cannot tolerate anoxia for more than a few days. Therefore, tanaidaceans are completely wiped out in sedimentary basins that experienced anoxia. After such an event, recolonization proceeds very slowly because tanaidaceans do not possess a planktonic larval stage, and their rate of dispersal as juveniles and adults is exceedingly low. Therefore, fossil tanaidaceans have great potential for aiding distinction between more or less stable dysoxic environments, and environments with recurrent bottom‐water anoxia (poikilaerobic biofacies). In the latter case, no fossil tanaidaceans occur.

Larval development and metamorphic transformation of the feeding system in the kleptoparasitic snailTrichotropis cancellata(Mollusca, Caenogastropoda)

Trichotropis cancellata Hinds, 1849 has a planktonic larval stage that feeds on microalgae and a benthic stage that feeds both by ctenidial suspension feeding and by stealing food ("kleptoparasitism") from several species of suspension-feeding, tube-dwelling polychaete worms. We used scanning electron microscopy, histological sections, and observations on live animals to document the sequence and timing of morphogenetic events during larval and metamorphic development of T. cancellata. These data were compared with other accounts of gastropod development to test for differences in the timing of developmental events among feeding larvae of two major gastropod clades: the caeno gastropods and heterobranchs. In T. cancellata, as in feeding larvae of previously studied caenogastropods, components of the post-metamorphic body plan differentiate at an earlier stage of larval development (relative to times of hatching and ability to undergo metamorphosis) than in feeding heterobranch larvae. Metamorphosis of T. cancellata was induced by polychaete hosts of this snail's kleptoparasitic benthic stage, and young juveniles of T. cancellata could steal food from these polychaetes within a day after snail metamorphosis began. Rapid onset of kleptoparasitic feeding following metamorphosis of T. cancellata was permitted by development of a specialized feeding structure, the pseudo proboscis, during the larval stage. This novel embellishment of larval development was likely preceded during evolution by selective larval induction by polychaete hosts.

Does the development of respiratory regulation always accompany the transition from pelagic larvae to benthic fossorial postlarvae in the Norway lobster Nephrops norvegicus (L.)?

The ability to regulate O 2 uptake during exposure to acutely declining P O 2 s co-occurred with metamorphosis from a planktonic to a benthic existence in the Norway lobster Nephrops norvegicus when cultured under normoxic conditions in the laboratory. The onset of this regulation appears to be related to the presence of a hypoxia-related hyperventilation of extrabranchial exchange surfaces (telson/uropods) by the pleopods and a shift from a low to a high Hc O 2 affinity. This co-occured with a dramatic calcification of the exoskeleton at metamorphosis, which will restrict gas exchange over the general body surface. Evidence marshalled here points to a shift from a pleopod-based, hypoxia-related, hyperventilation of the uropods/telson as gas exchange surfaces to, in later postlarval stages, a scaphognathite-based response, where the gills are the main areas for gas exchange. This is the first time that experimental evidence has been put forward for the use of uropods/telson by larval lobsters as extrabranchial gas exchange surfaces. Pre-exposure of larval and postlarval N. norvegicus to reduced P O 2 , both in the laboratory and in the field, resulted in the “adult” pattern of M O 2 regulation present during exposure to declining P O 2 being established before metamorphosis. Bringing the ontogeny of this regulation forward in the development itinerary was to some extent a result of a shift from a low to a high Hc O 2 affinity well before metamorphosis and an increase in the magnitude of the hypoxia-related hyperventilatory response in the planktonic larval stages. Thus, the development of respiratory regulation does not always accompany the transition from pelagic larvae to benthic fossorial larvae in N. norvegicus as its onset can be influenced prevailing O 2 conditions, both in the laboratory and the field.

A review of Asian Hediste species complex (Nereididae, Polychaeta) with descriptions of two new species and a redescription of Hediste japonica (Izuka, 1908)

Three Asian species of the genus Hediste (Nereididae, Polychaeta, Annelida), including two new species, are reviewed. Hediste japonica (Izuka, 1908) was redescribed based on recently discovered type material and on additional specimens newly collected from the Ariake Sea in Japan and the west coast of Korea. Two new species, H. diadroma and H. atoka, were described based on specimens collected from Japan. These three species are morphologically similar, but distinguishable by some differences in parapodial and chaetal morphology, at least at the sexually mature stage. The presence of homogomph falcigers and the absence of heterogomph spinigers are unique characteristics of H. japonica among this genus. Inconspicuous epitokous metamorphosis occurred as a slight enlargement of the eyes and parapodial ligules in mature adults of H. diadroma sp. nov. and H. japonica in combination with their reproductive swarming and pelagic larval development. The epitoke-specific sesquigomph spinigers were added in mature adults of H. diadroma sp. nov., while the number of ordinary homogomph spinigers increased in some adults of H. japonica. Epitokous metamorphosis in the Hediste species was compared with that in other nereidid genera. No epitokous metamorphosis was observed in H. atoka sp. nov., corresponding to its reproduction without swarming and to its development without a planktonic larval stage. Significant differences in paragnath numbers in the maxillary ring of the proboscis were found among the three species, suggesting different feeding habits. In Japan, the present distribution of H. japonica is restricted to muddy tidal flats in the inner part of the Ariake Sea, probably due to recent human impact, while the other two species are widespread.

Resting stage in benthic foraminiferal propagules: a key feature for dispersal? Evidence from two shallow-water species

Abstract. Understanding dispersal mechanisms in benthic formainifera has wide implications for explaining their geological and biological distribution in space and time, including evolutionary and genetic trends. Because they lack a planktonic larval stage (although a few have a meroplanktonic life stage), their mode of dispersal in the marine environment has been subject to much speculation, and a range of alternative mechanisms have been suggested (for a review, see Alve (1999) and further discussion in Hayward et al. (1999)).In the present note, we suggest a mechanism which is more efficient and probably more widespread than most of those previously proposed. We hypothesize that many species produce ‘propagules’ (small juveniles, perhaps just the proloculus) released in large numbers following reproduction that may enter a resting stage. Propagules can be widely dispersed by normal physical processes (currents, sediment transport) and ultimately settle over a range of habitats and conditions. Growth and reproduction commence only in those individuals and species that reach a suitable environment. We have two lines of evidence from completely different shallow-water environments (Norwegian fjord and the Florida Keys, USA) that strongly support our hypothesis.ObservationsSurface sediments from a box core were collected at 60 m water depth in the inner part of Oslofjord 10 September 1999. Some of the sediment was processed immediately after collection, and the remaining bulk sample was transferred to a transparent container with ambient seawater, sealed, and placed on a window ledge until 30 March 2000. Those sub-samples processed immediately after collection contained no live . . .

ASSORTATIVE FERTILIZATION AND SELECTION AT LARVAL STAGE IN THE MUSSELS MYTILUS EDULIS AND M. GALLOPROVINCIALIS

Assortative mating (prezygotic isolation) and reduced hybrid fitness (postzygotic isolation) are typically invoked to explain the stability of hybrid zones. In the tension zone model, these factors work in opposition to migration, which promotes genetic homogeneity. Many marine animals migrate over long distances through a planktonic larval stage. Therefore, strong reproductive isolation is needed to maintain stable marine hybrid zones. However, surprisingly little is known about mating preferences and hybrid fitness in marine organisms. Smooth-shelled mussels (Mytilus spp.) form a well-known species complex, with hybridization over extensive areas such as the contact zone of M. edulis and M. galloprovincialis around European Atlantic coasts. This paper reports direct experimental evidence of assortative fertilization, hybrid larval inviability, and early heterosis for growth rate in M. edulis and M. galloprovincialis. Four crosses between pure M. edulis and M. galloprovincialis were analyzed with a new polymerase-chain-reaction-based diagnostic marker. Gamete competition between taxa was allowed in two out of the four crosses. Genotype frequencies observed at an early stage (36 h after fertilization) unambiguously revealed assortative fertilization when gamete competition was allowed. A significant reduction in hybrid viability was subsequently observed during the larval stage. At the same stage an antagonistic effect, heterosis, was observed on growth rate. However, even if heterosis is observed in the F1, it is expected to vanish in subsequent hybrid generations. Although specialization for different habitats and asynchronous spawning have been mentioned as factors contributing to the maintenance of the blue mussel hybrid zone in Europe, we argue that assortative fertilization and reduced hybrid fitness are important factors that also contribute to the stabilization of this zone. These results emphasize that multiple factors may act concomitantly in a barrier to gene flow, especially in complex life cycles. Furthermore, they show that assortative mating through gamete preference, as already demonstrated for sea urchins, may play a role in speciation processes taking place in the sea.Corresponding Editor: R. Burton

Foundations of gregariousness: a dispersal polymorphism among the planktonic larvae of a marine invertebrate.

Theory predicts that selection should favor genotypes that can vary their tendency to disperse in habitats that are spatially or temporally variable or those that remain near their carrying capacity. Although many marine habitats appear to fit these criteria, confirmed examples of dispersal polymorphism among marine invertebrates are exceedingly rare. Competent larvae of the gregarious tubeworm, Hydroides dianthus, settle specifically in response to living conspecific worms, but a small proportion of each spawn settle nonspecifically on uninhabited substrata concurrently with their gregarious siblings. Here, using a parental half-sib analysis, we show that the proportion of a spawn settling in response to uninhabited biofilm is highly heritable. When estimated as a continuous trait based on a one-way ANOVA, heritability is estimated to be 0.83 +/- 0.31. When founder production was analyzed as a threshold trait, heritability was estimated to be 0.68 +/- 0.10 based on the breeding design experiment and 0.65 +/- 0.09 based on the artificial selection experiments. Realized heritability based on the selection experiments was considerably lower, however (0.17 per generation and 0.02 cumulative). Artificial selection was ineffectual at sequentially increasing the proportion of founder larvae among inbred family lines, but after three generations of selection, the proportion of larvae settling in response to biofilm was significantly higher among inbred lines than among the field-collected parents. The obligate planktonic larval stage common among so many marine invertebrates is thought to preclude the evolution of dispersal polymorphisms in these animals. Theoretical expectations of variable dispersal may instead be realized through individual behavioral differences resulting in differential transport or settlement preference, but this possibility remains largely unexplored among marine invertebrates.