Planktonic Larval Stage Research Articles

Multipurpose Gills: Effect of Larval Brooding on the Feeding Physiology of Freshwater Unionid Mussels

During reproduction, the lateral (outer) demibranchs of the unionid mussel Pyganodon cataracta function in brooding females as marsupia in addition to serving in gas exchange, feeding, and ion transport. Recent studies indicate that glochidial brooding reduces clearance rates and particle retention efficiencies, but the opaque shell prevents direct observations of suspension feeding structures and makes it difficult to identify the underlying causes of the changes in feeding dynamics. In this study, video endoscopic techniques were used to describe and compare, in vivo, the feeding structures and dynamics of brooding and non-brooding females. Although circulation within the mantle cavity was slightly altered by the enlarged lateral (gravid) gills of brooding females, both medial and lateral gills continued to retain and process particles. During brooding, circulation through medial gills was maintained by the construction of secondary water tubes near the medial and lateral ends of the brood chambers. In vivo monitoring of particles retained by the frontal surface of the gill indicated that transport rates for particles processed by gravid gills of brooders were significantly slower than on lateral gills of non-brooders or on medial gills. Similarly, gravid gills were less efficient at retaining small particles (<6 ptm) than medial or non-gravid lateral gills. These findings are consistent with the hypothesis that observed reductions in particle clearance rates and retention efficiencies in brooding female mussels are the result of functional changes in the ciliature and flow dynamics of the marsupial gills. Moreover, similar mechanisms mediating particle capture and processing on medial demibranchs appear to be unaffected by the presence of developing glochidia in the water tubes of the lateral gills. Additional key words: glochidia, endoscopy, suspension feeding, filtration Though the gills of suspension feeding bivalves are typically identified as structures for feeding, gas exchange, and ion transport, the ctenidia of many marine and freshwater species also incubate larvae during reproductive periods. Brooding mechanisms vary considerably among species. Most freshwater bivalves, including members of the Corbiculidae and Unionidae, lack planktonic larval stages and incubate developing embryos within internal spaces between gill lamellae (Ortmann 1911; Britton & Morton 1982; Kat 1984; Mackie 1984). In other freshwater and marine bivalve molluscs, developing young are attached to pallial structures or restricted to specialized brood masses, papillae, or sacs (Heard 1977; Bartlett 1979; Richardson 1979; Mackie 1984; Kabat 1985). Conversely, some species, including oysters of the genus Ostrea, brood within the infrabranchial cavity but the young are not confined or physically attached to the mother and move about freely in the inter-demibranchial spaces (Chaparro et al. 1993). Although numerous studies have examined the mechanics of the bivalve pump and filter (for review see J0rgensen 1990), the effect of larval incubation on the gills' customary roles as feeding and respiratory structures has been ignored, except for the observation of Walne (1972) that clearance rates of the oyster Ostrea edulis are lower during brooding periods. In the Unionidae, modifications of ctenidia for brooding, including the location and arrangement of larvae (glochidia) within brood chambers, the degree of swelling of the lamellae, and the duration of larval incubation, vary among species and often serve as important taxonomic characters (Ortmann 1911; Heard & Guckert 1971). In most species, the water tubes of either the lateral (outer) demibranchs or all four demibranchs serve as ovisacs. In others, the marsupium is restricted to a portion of the lateral gills, often forming bulges or sulci along the ventral aspect of the demiThis content downloaded from 207.46.13.21 on Tue, 27 Sep 2016 04:23:58 UTC All use subject to http://about.jstor.org/terms

VARIATION IN GENOTYPE FREQUENCIES DURING THE LIFE HISTORY OF THE BIVALVE, DREISSENA POLYMORPHA.

’Address for correspondence. larva. Gametes are shed into the water where fertilization occurs, and larvae remain in the plankton for approximately 10 d before settling (Walz 1978; Lewandowski 1982b). The Great Lakes population appears to be derived from a single founding group (Griffiths et al. 1991); thus, migration from other, genetically distinct populations can be discounted as an explanation for any genetic differences observed among life history stages. Furthermore, D. polymorpha is the only mollusk in most of the Great Lakes with a planktonic larval stage thus, problems associated with identification of larvae are avoided. We examined allozyme frequencies throughout the life history of D. polymorpha in the Great Lakes in order to directly compare the genetic structure of the planktonic larvae with that of benthic stages.

The ecology and functional morphology ofTrigonothracia jinxingae(Bivalvia: Anomalodesmata: Thracioidea) from Xiamen, China

All anomalodesmatans are ‘rare’ butTrigonothrucia jinxingaeis relatively common in Xiamen Harbour, Fujian Province, China. This is because the species has a life span of approximately one year and is a simultaneous hermaphrodite, probably with either a short or absent planktonic larval stage. That is, success results from rapid maturation, self‐fertilization, direct development and within‐habitat recruitment over an extended period in early summer.Trigonothracia jinxingaeis interesting in another way, however. The Thraciidae is the Mesozoic stem group of the Thracioidea which also contains the more modern (Caenozoic) Laternulidae and Periplomatidae. Features of the anatomy ofT. jinxingae, such as the method of hydraulically moving the foot by the pumping of blood into a capacious pallial haemocoel, and the structure of the stomach, are reminiscent of the earliest (Palaeozoic) anomalodesmat́ans, i.e. the Pholadomyoidea, represented today byPholadomya candida.The thraciidAsthenothaerussp. (Pelseneer, 1911) even has, likeP. cundida, an opisthopodium on its visceral mass.P. candida, however, fed on sub‐surface deposits using its foot.T. jinxingaeis also a deposit feeder, but on surface deposits using the inhalant siphon.Modern periplomatids resemble thraciids in their separate siphons, but both representatives of this family and the Laternulidae are suspension feeders with extensive sorting areas on the wall of the stomach to process such material. The Thraciidae thus form a link between the oldest, pedal feeding, pholadomyoidean anomalodesmatan and the most advanced, suspension feeding, laternulids and periplomatids.

Genetic Differentiation and Dispersal among Populations of Paratya australiensis (Atyidae) in Rainforest Streams in Southeast Queensland, Australia

We used measures of genetic differentiation at seven allozyme loci to estimate the degree of movement of a stream invertebrate within and between catchments. Populations of Paratya australiensis, a common atyid shrimp, were sampled from three rainforest streams from each of two subcatchments in two adjacent drainage systems in southeast Queensland. Marked genetic differentiation was observed at all loci, suggesting extremely limited movement on a small spatial scale. This result is surprising given the widespread geographic distribution of P. australiensis in Australia, its great abundance in headwater streams in southeast Queensland, and the presence of a planktonic larval stage. Large differences in genetic structure occurred between streams, in some instances between those in the same subcatchment. Even larger genetic differences were seen between samples from streams in different subcatchments, with alternative alleles being fixed at different sites. The greatest differences were not, however, between...

Dispersal Dynamics of the Bivalve Gemma Gemma in a Patchy Environment

The purpose of this study was to analyze the dispersal dynamics of the ovoviviparous bivalve Gemma gemma (hereafter referred to as Gemma) in an environment disturbed by the pit—digging activities of horseshoe crabs, Limulus polyphemus. Gemma broods its young and has no planktonic larval stage, so all dispersal is the result of juvenile and adult movement. Animal movement was measured using natural crab pits, hand—dug simulated crab pits, and cylindrical bottom traps in the intertidal zone at Tom's Cove, Virginia, USA. This study demonstrated that horseshoe crabs create localized patches with reduced densities of Gemma, that all sizes and ages of Gemma quickly disperse into these low density patches, and that the mechanism of dispersal is passive bedload and suspended load transport. Freshly excavated natural pits had significantly lower Gemma densities than did undisturbed background sediment, but there were no significant differences in total density of other species, number of species, and species diversity (H'). Equitability (J') was greater in pits than in controls because of the reduced abundance of Gemma, the numerically dominant species. Newly dug simulated crab pits also had significantly lower Gemma densities than controls and returned to control levels by the next day. Density recovery trajectories for individually marked pits showed consistent responses in summer and fall, but not in winter when low Gemma abundance resulted in greater variability among pits. Significant positive correlations between the volume of sediment and the number of Gemma collected per bottom trap support the hypothesis that Gemma dispersal is a passive transport phenomenon. Assuming no active, density—dependent movement, the product of the Gemma density frequency distribution in undisturbed background sediment and the frequency distribution of sediment volume collected per trap created a predicted Gemma frequency distribution in traps that matched the actual distribution. Absolute dispersal rates and relative dispersal rates (absolute dispersal rate divided by background density in undisturbed sediment) into pits and traps were greater in summer than winter. Dispersal rate results suggest that increased horseshoe crab disturbance in summer may cause an increase in Gemma transport. Because Gemma individuals are dispersed by hydrodynamic action, it was expected that small, young individuals would be most easily transported in the bedload. There was, however, little evidence that movement into pits and traps was size— or age—selective. Most recent benthic dispersal research has focused on the large—scale movement and settlement patterns of invertebrate larvae. The results from this study illustrate that dispersal of bottom—dwelling juveniles and adults plays an important role in regulating the local distribution and abundance of Gemma. Previous workers have shown that young Gemma live in dense aggregations and that growth and fecundity are reduced at such high densities, leading to population crashes. This study demonstrated a mechanism by which Gemma disperses into low—density patches where intraspecific competition may be mitigated, possibly resulting in enhanced individual reproductive success and population fitness.

Mitochondrial DNA variation and population structure in the spiny lobster Panulirus argus

Adult spiny lobsters (Panulirus argus) were collected from nine locations throughout the tropical and subtropical northwest Atlantic Ocean and examined for mitochondrial DNA (mtDNA) variation. 187 different mtDNA haplotypes were observed among the 259 lobsters sampled. Haplotype diversity was calulated to be 0.986 and mean nucleotide sequence-diversity was estimated to be 1.44%; both of these values are among the highest reported values for a marine species. Analysis of molecular variance (AMOVA) and phenetic clustering both failed to reveal any evidence of genetic structure within and among populations of P. argus. The present data are consistent with high levels of gene flow among populations of P. argus resulting from an extended planktonic larval stage and strong prevailing ocean currents.

Evidence of antibacterial and lysozyme-like activity in different planktonic larval stages of Paracentrotus lividus

Larval lysate (four-armed stage) exerts a lysozyme-like and antibacterial activity directed against Vibrio alginolyticus and other bacterial strains. This lysozyme activity is dependent on the pH and ionic strength of the reacting medium and sample. The other larval stages (six-armed, eight-armed and a stage approaching metamorphosis) possess protection against bacteria as well. The highest antibacterial activity is present in the six-armed stage. These antibacterial compounds avoid exploitation by bacteria. Paracentrotus lividus larvae used in our experiments were laboratory-reared from specimens collected in 1993 in Porto Cesareo (Lecce, Italy).

Larval dispersal via entrainment into hydrothermal vent plumes

One of the most intriguing ecological questions remaining unanswered about hydrothermal vents is how vent organisms disperse and persist. Because vent species are generally endemic and their habitat is patchy and ephemeral on time scales as short as decades, they must disperse frequently, presumably in a planktonic larval stage. We suggest that dispersal occurs not only in near‐bottom currents but also several hundred meters above the seafloor at the level of the laterally spreading hydrothermal plumes. Using a standard buoyant plume model and observed larval abundances near hydrothermal vents at 9°50′N along the East Pacific Rise, we estimate a mean vertical flux of approximately 100 vent larvae/h at a single black smoker. Larval abundances were extremely variable near vents, resulting in a range in estimated fluxes of at least an order of magnitude. The suitability of the plume model for these calculations was determined by releasing dyes (fluorescein and rhodamine) as larval mimics into a black smoker plume. The plume model predicted dye fluxes in the plume adequately, given the short averaging times of our measurements and the difficulty of sampling the plume centerline. Our calculations of substantial numbers of vent larvae entrained into the plume support the idea that transport in the lateral plume is an important mechanism of dispersal. Because vertical shear in flows above vents can cause larval dispersal trajectories in the plume to deviate considerably from those along the seafloor, larvae in the plume may have access to habitats that are unreachable by larvae in near‐bottom flows.

Genetic population structure in two tropical sponge-dwelling shrimps that differ in dispersal potential

The spatial context in which “host races” of parasitic animals originate is a central issue in the controversial theory of sympatric speciation. Sponge-dwelling shrimps in the genus Synalpheus provide a good system for evaluating the possibility of resource-associated divergence in sympatry. I used allozyme electrophoresis to assess the genetic population structure of two Caribbean Synalpheus species sampled in 1988 to 1990 at a hierarchy of spatial scales. S. brooksi Coutiere is a host-generalist, using several sponge species in an area, and develops directly, with no planktonic larval stage. G-tests and estimates of F ST revealed highly structured populations in this species, with significant differentiation among samples from individual reefs within a region, and strong divergence among regions (Panama, Belize, Florida). Moreover, samples of S. brooksi taken from the two sponges Spheciospongia vesparium (Lamarck) and Agelas clathrodes (Schmidt) in Panama, and separated by ≤3 km, showed significant differentiation at both of the loci that were polymorphic in these populations. Genetic distances between these host-associated populations averaged >60% greater than distances between samples from the same host species and were comparable to, or greater tha, those for some inter-regional comparisons. These genetic data corroborate a previous finding of demographic differences between the same populations. The second species, S. pectiniger Coutiere, occurs only in Spheciospongia vesparium, and produces swimming larvae. Although allele frequencies in this species differed significantly among the three regions, S. pectiniger showed no differentiation within regions, and significantly lower differentiation (F ST) among regions than its direct-developing congener. These data suggest that genetic population structure in these two commensal crustaceans is related to dispersal potential, and that restricted dispersal may allow the divergence of host-associated populations on a local scale.

Descriptions of some planktonic larvae of the Calappidae (Crustacea: Decapoda; Brachyura)

Some unidentified planktonic larval stages have been assigned to the Calappidae using Rice's key of Brachyura families. Zoea stage I from the eastern region of the Algerian coast (western Mediterranean Sea) has been attributed, according to Manning and Holthuis (1980), to Calappa granulata (L.), the only recorded representative of the family from the Mediterranean Sea. Zoeal stages 1-lV from the Jordanian coast of the Gulf of Aqaba, Northern Red Sea, have been reported to the subfamily Matutinae. Comparisons of larvae of Calappa and of Hepatus and then, of larvae of Hepatus and of Matuta , show that Hepatus larvae are more closely related to those of Calappa , while those of Matuta are clearly distinct. It s suggested to place the genus Hepatus with the Calappinae. Guinot (1966. 1978) placed some genera, including Hepatus . in a 'unité parthenoxystomienne, lea Aethrinae'. Larval development is unknown for Aethra, Actaeomorpha. Hepatella and Osachila . However, comparison of Hepatus larvae with those of Parthenopidae shows that larval characters, particularly the setation of the endopod of the first maxilliped, are not conclusive for supporting such a regrouping. Moreover, the Calappidae have a combination of primitive and advanced features which suggest that their grouping in the Oxystomata seems more likely to be an adaptation to a similar way of life. The morphotypal taxon Oxystornata has been definitively rejected by Guinot (1978).

Studies on the larval rearing of giant spider crab, Macrocheira kaempferi-III. Influence of Temperature on Survival and Growth of Larvae of the Giant Spider Crab Macrocheira kaempferi(Crustacea, Decapoda, Majidae).

The influence of temperature on the survival and growth of larvae of Macrocheira kaempferi was investigated in the laboratory. Artemia nauplii were given as prey. Survival temperature ranges for zoea and megalopa were 12 to 23°C and 11 to 18°C, respectively. The duration of each stage increased exponentially at lower temperature. In the survival range of temperature, survival time in starvation decreased with the increase in temperature for the larvae in each stage. Molting interval and survival time in starvation were longest for megalopa, followed in order by the 2nd zoea and 1st zoea in that order. The optimum rearing temperature for all the larval stages was 15 to 18°C. The duration of the planktonic larval stage could range from one to three months. The larvae could therefore live in depths shallower than 300m.

POPULATION GENETIC CONSEQUENCES OF DEVELOPMENTAL EVOLUTION IN SEA URCHINS (GENUS HELIOCIDARIS).

Within the sea urchin genus Heliocidaris, changes in early embryonic and larval development have resulted in dramatic differences in the length of time larvae spend in the plankton before settling. The larvae of one species, H. tuberculata, spend several weeks feeding in the plankton before settling and metamorphosing into juveniles. The other species, H. erythrogramma, has modified this extended planktonic larval stage and develops into a juvenile within 3-4 days after fertilization. We used restriction site polymorphisms in mitochondrial DNA to examine the population genetic consequences of these developmental changes. Ten restriction enzymes were used to assay the mitochondrial genome of 29 individuals from 2 localities for H. tuberculata and 62 individuals from 5 localities for H. erythrogramma. Within H. tuberculata, 11 mitochondrial genotypes were identified. A GST analysis showed high levels of genetic exchange between populations separated by 1,000 kilometers of open ocean. In contrast, in H. erythrogramma, 13 mitochondrial genotypes differing by up to 2.33% were geographically partitioned over spatial scales ranging from 800 to 3,400 kilometers. Between distant localities, there was complete mitochondrial lineage sorting and large sequence divergence between resulting clades. Over much smaller spatial scales (< 1,000 km), genetic differentiation was due to the differential sorting of very similar genotypes. This pattern of mitochondrial variation suggests that these population differences have arisen recently and may reflect the historical interplay between the restricted dispersal capabilities of H. erythrogramma and the climatic and geological changes associated with Pleistocene Ice Ages.

Are eels (Anguilla anguilla L.) planktonic feeders? Evidence from parasite communities

The metazoan parasite communities of eels Anguilla anguilla were investigated in three riverine localities and one lake in Devon. Altogether 21 species of parasite, the majority of the British eel parasite fauna, were recorded. Parasite communities in the three riverine localities were more similar to each other than to that of the lake. The four parasite component communities were typical and representative of those from eels in other parts of the U.K. They were characterized by low species diversity and domination by a single species: Pseudodacytlogyrus anguillae in the rivers and Diplostomum spathaceum in the lake. Both the dominant species and the majority of parasite species, and hence individuals, in any locality infected eels directly by free swimming planktonic larval stages or indirectly by the ingestion of infected planktonic copepod intermediate hosts. Examination of other species of fish in each locality showed that only Raphidascaris acus utilized a fish species as an intermediate or paratenic host to infect eels. Increases in prevalence and intensity of infection with size of eel by helminths such as Camallanus lacustris and Paraquimperia tenerrima were observed in localities in which transmission of these parasites could not have involved another fish species. Over 63% of the specialist, specific parasites of eels throughout Europe employ planktonic transmission to their hosts, as do several species of the commoner generalists. A study of eel parasite communities thus leads to the conclusion that eels are widely and closely associated in their habits with plankton and that eels of all sizes feed regularly upon planktonic copepods: the widely accepted view of eels as being more or less exclusively benthic in habit and feeding behaviour requires re‐appraisal.

Regional differences in duration of the planktonic larval stage of reef fishes in the eastern Pacific Ocean

Regional variation in the duration of the planktonic larval phase of three species of reef fishes, Thalassoma lucasanum (Labridae), Stegastes flavilatus, and Microspathodon dorsalis (Pomacentridae) was investigated between 1982 and 1991 at several sites in the tropical eastern Pacific over a distance of 3500 km, encompassing virtually their entire range of distribution. Durations of the larval phase, determined from counts of daily otolith increments, were significantly different (1.3 to 1.6 x) between sites. Populations of all three species had a consistently shorter larval life at the most northern site, Cabo San Lucas (Mexico) compared to Panama and the offshore islands of Galapagos and Cocos. Analyses of otolith increment width over the precompetent period revealed that this disparity in larval duration primarily reflected differences in larval growth rates: faster growing fish spent less time in the plankton. In T. lucasanum, some of the variation in larval duration between Panama and offshore sites (Galapagos Islands and Cocos Island) may be accounted for by a higher frequency of individuals delaying metamorphosis at the offshore sites. These data indicate that conditions in the planktonic environment are not homogeneous throughout the tropical eastern Pacific and may have a profound effect on aspects of the larval ecology of reef fishes in this region.

Decapod Larvae: Dispersal, Mortality, and Ecology. A Working Hypothesis

TWO topics on decapod larval biology are discussed: retention and recruitment of decapod larvae to the parental population and the ecological role of decapod larvae in the water column. Most decapods have retained a planktonic larval phase which is generally interpreted as a mechanism for increased dispersal. Evidence of restricted gene flow and biological/physical interaction research have suggested that larvae can be retained and recruited to the parental population via mesoscale processes. To fully understand recruitment processes improved estimates of mortality rates for planktonic larval stages will be required. Recent evidence suggests that mortality rates are not constant over the complete larval developmental period but decrease with time. During some seasons meroplankton including decapod l arvae can constitute more than 50% of the plankton biomass. The quantity of energy transferred into the water column can be significant. Their role in planktonic ecology may be significant and additional research is required.

Mitochondrial DNA variation and phylogenetic relationships of Jasus spp. (Decapoda: Palinuridae)

The seven species of rock lobster in the genus Jasus have a fragmented circumpolar distribution, inhabiting continental or island waters of the Southern Ocean. Gene flow between nominal species is possible as the planktonic larval stages of Jasus are widely dispersed in major oceanic gyres. Restriction endonuclease analysis of mitochondrial DNA (mtDNA) from five species (J. verreauxi, J. novaehollandiae, J. edwardsii, J. lalandii and J. tristani) was used to assess taxa previously defined only by morphological criteria. Intraspecific mtDNA nucleotide sequence diversity was generally high (0˙33–0˙99%). An absence of episodic population bottlenecks and extinctions, attributable to a teleplanic (far wandering) and prolonged pelagic stage, may be a significant factor contributing to this variation. New Zealand (J. edwardsii) and Australian (J. novaehollandiae) populations appear to be conspecific and should be referred to as J. edwardsii; however, a significant difference in the magnitude of mean sequence diversities between these populations may indicate restrictions to gene flow across the Tasman Sea. The genome of J. verreauxi is highly distinct from the genomes of the other species (nucleotide sequence diversity: 14˙92–16˙67%), supporting the existence of ‘verreauxi’ and ‘lalandii’ groups within Jasus. High sequence diversities separating J. edwardsii, J. lalandii and J. tristani (4˙41–7˙36%) indicates longterm reproductive isolation. Hypotheses for the evolution of ‘lalandii’ group Jasus, which suggest a relatively recent divergence of J. lalandii and J. tristani, are not supported by phylogenetic reconstruction. Instead, it gives systematic validity to the grouping of J. lalandii with J. edwardsii as proposed by the existing taxonomy.

Mapping the Potential Spread of the Zebra Mussel (Dreissena polymorpha) in Ontario

The European zebra mussel (Dreissena polymorpha) is now firmly established in the lower Great Lakes and in isolated locations in the upper Great Lakes. Dispersal is aided by high fecundity, a planktonic larval stage, and ability of the mussel to be transported via shipping and recreational boating. Almost all waterbodies in Ontario which are downstream of established colonies or which can be reached through boating are vulnerable to invasion. However, there are ecological requirements (derived from European literature) which limit successful zebra mussel reproduction. Using data from over 6000 inland lakes in the province, we have prepared maps which show the distribution of pH, calcium, and major access routes and which indicate our estimated potential for success or failure of zebra mussel colonization. The utility of the maps is dependent on the validity of the ecological requirements for successful development of zebra mussel larvae, which were developed in Europe. There is a need to test the chemical requirements of the zebra mussel in North American waters.

Drifting and dispersal of small bivalves and gastropods with direct development

Quantitative evidence that several marine benthic invertebrates lacking a planktonic larval stage disperse as juveniles and small adults by drifting in the water column has been obtained using off-bottom intertidal collectors. These collectors were mimics of a habitat commonly used by small molluscs in intertidal seaweed communities: red turf algae. Taxa caught in collectors include three brooding bivalves [ Musculus sp., Lasaea sp., Transennella tantilla (Gould, 1853)] and small gastropods that produce benthic egg capsules and crawl-away juveniles ( Barleeia spp.). Dispersal is accomplished by juveniles—and even sexually mature adults in the case of small-sized species — drifting with water currents using mucous threads in at least some of the taxa found. Some of these molluscs have also been caught in off-bottom collectors placed away from the shore, in the middle of a bay. Findings indicate that, contrary to what is suggested by their mode of development, these invertebrates have alternative and likely effective dispersal mechanisms (at least at a local scale). It is probable that frequent drifting excursions in these taxa enhance rafting opportunities, which in turn may favor long distance dispersal. Large numbers of juveniles of several other species of molluscs whose life cycles include a planktonic larval stage were also caught in off-bottom collectors. These results, coupled with those from previous studies, show that postmetamorphic drifting is a common feature of the life history and ecology of many marine bivalves and gastropods, regardless of their mode of development.

Physiological ecology of mussel larvae

Mussels produce large numbers of planktotrophic larvae that spend several weeks in the surface waters. This planktonic stage in the species' life-history is a critical phase concerned with feeding, growth and development, dispersal and the selection of a suitable site for the sedentary adult life. The process of development, from the small 70 μm egg through to the pediveliger larva (ca. 300 μm shell length) which settles and undergoes metamorphosis, is outlined. A maximum growth rate of ca. 12 μm day −1 in shell length results in a minimum larval life of 3 weeks at 18°C, but this can be extended to 3 months depending upon endogenous and exogenous (environmental) factors. However, the planktonic larval stage and the immediate post-settlement stage are accompanied by high mortality rates (ca. 0.1 to 0.2 day −1). Therefore any factor that reduces growth rate, and thus extends the period of larval development in the water column will have a major effect on mortality and the chances of survival to the settlement stage and beyond. The effects of environmental factors, such as food availability, temperature, oxygen concentration, salinity, and pollutants, on the processes of feeding, metabolism and growth of mussel larvae are reviewed and discussed in this context. Once the larvae have reached the pediveliger stage and are ‘competent’ to settle, then the process of settlement and metamorphosis can be delayed for up to 7 weeks until a suitable substratum is found. However, any delay will result in a further decline in the number of offspring surviving.

Life history and population structure of the spionid polychaete Paraprionospio sp. (form A)

The life history and population structure of Paraprionospio sp. (form A), an indicator of organic pollution in Japanese coastal waters, were studied in Kumihama Bay over a 19-month period. Breeding of Paraprionospio sp. (form A) occurred at a low level throughout the year, but there were two peaks, namely, in spring and in late summer through autumn. However, intense recruitment was restricted to a short period between October and November, when recovery of levels of dissolved oxygen in the bottom water began. After 3 to 4 months worms began to develop their germ cells, and then they became indistinguishable from the adult population. The life span of individuals is estimated to be 1 yr or longer. This species has no brood protection, a large brood size with a maximum of 43 000 oocytes · individual which is positively correlated to body size, and a long planktonic-larval stage that lasts for nearly 2 months. These features suggest that this species is not opportunistic but belongs to a group of species that is able to respond to large-scale disturbances and can take advantage of the timing of oxygen recovery.