Recruitment Of Marine Invertebrates Research Articles

Eco-Plastics in the Sea: Succession of Micro- and Macro-Fouling on a Biodegradable Polymer Augmented With Oyster Shell

Impacts of Marine Plastic Debris (MPD) on marine ecosystems are among the most critical environmental concerns of the past three decades. Virgin plastic is often cheaper to manufacture than recycled plastics, increasing rates of plastic released into the environment and thereby impacting ecosystem health and functioning. Along with other environmental effects, MPD can serve as a vector for marine hitchhikers, facilitating unwanted organisms’ transport and subsequent spread. Consequently, there is a growing demand for more eco-friendly replacements of conventional plastic polymers, ideally with fit-for-purpose properties and a well-understood life cycle. We enriched polybutylene succinate (PBS) with three different concentrations of oyster shell to investigate the dynamics of biofouling formation over 18 weeks at the Nelson Marina, Aotearoa/New Zealand. Our study focused on oyster shell concentration as a determinant of fouling assemblages over time. While generally considered as a waste in the aquaculture sector, we used oyster shells as a variable of interest to investigate their potential for both, environmental and economic benefits. Using bacterial 16S and eukaryotic 18S rRNA gene metabarcoding, our results revealed that following immersion in seawater, time played a more critical role than substrate type in driving biofouling community structures over the study period. In total, 33 putative non-indigenous species (NIS) and 41 bacterial families with putative plastic-degrading capability were detected on the different substrates. Our analysis of NIS recruitment revealed a lower contribution of NIS on shell-enriched substrates than unadulterated polymers samples. In contrast, the different concentrations of oyster shells did not affect the specific recruitment of bacterial degraders. Taken together, our results suggest that bio-based polymers and composites with increased potential for biodegradability, recyclability, and aptitude for the selective recruitment of marine invertebrates might offer a sustainable alternative to conventional polymers, assisting to mitigate the numerous impacts associated with MPD.

Interactive effects of light and substrate colour on the recruitment of marine invertebrates on artificial materials

Marine invertebrate larvae are highly phototactic during early stages of larval period and greatly influenced by light, colour and other physio-chemical properties of the substrates. In the present study, the effects of light and substrate colour on recruitment of marine invertebrates were investigated. Four different colour panels (red, green, blue and yellow acrylic sheets) and colourless panel (clear transparent acrylic sheet) were suspended at 2 m depth in the central Red Sea for 4 weeks during two seasons (spring and summer). The effects of substrate colour on invertebrate recruitment were tested under light and shade conditions. The results revealed significant variations in the recruitment of invertebrates between different colour panels and the panels exposed to light and shade. Higher densities of invertebrates were observed on the panels exposed to shade but the bryozoans abundance was high on the panels exposed to light. Tube worms and bryozoans preferred blue colour panels and the abundance of barnacles and bivalves were high on red colour panels. Significant interaction between substrate colour and light was observed in the abundance of barnacles, tube worms, bivalves and bryozoans. In conclusion, this study indicated the ability of some marine invertebrate larvae to discriminate coloured substrates depending on the light intensity.

Abalone Recruitment in Low-Density and Aggregated Populations Facing Climatic Stress

Understanding spatial and temporal patterns in the recruitment of marine invertebrates with complex life histories remains a critical knowledge gap in marine ecology and fisheries. As marine invertebrates are facing multiple stressors from overfishing and climatic stress, it is important to evaluate the conditions that facilitate recruitment in low-density populations. The red abalone Haliotis rufescens historically supported an economically important fishery in California, but the fishery was sequentially closed as stocks declined, and the last fished area was closed in 2018 following the collapse of the kelp forests in Northern California. Here, red abalone recruitment was evaluated annually from 2012 to 2016 and monthly from 2016 to 2017 in Central California where red abalone naturally occur in highly aggregated but low-density populations because of sea otter predation. Trends in wind-driven upwelling, temperature, wave forces, and food resources (kelp) were evaluated over the same time period as factors that could affect recruitment patterns. Recruitment was annually consistent except in 2015, when recruitment declined by 76%, likely because of reproductive failure during the second year of the North Pacific marine heat wave. The monthly recruitment assessment was the first field assessment of red abalone recruitment over a full year, and it showed that red abalone can recruit year-round. There were no clear recruitment patterns associated with seasonal wind-driven upwelling or relaxation patterns, and kelp density was constant over the study period; however, conditions at the study sites included three key features that may provide optimal conditions for consistent recruitment: (1) spatial abalone aggregation, (2) a sheltered embayment that may retain larvae, and (3) persistent algal food resources. These results can inform statewide and global abalone recovery and management programs.

Invertebrate recruitment on artificial substrates in the Red Sea: role of substrate type and orientation

AbstractSurface physical properties, hydrodynamics, biochemical cues, orientation and temporal scales play an important role in invertebrate larval recruitment on artificial substrates. In the present study, invertebrate recruitment on four different substrates (acrylic, stainless steel, ceramic and concrete panels) was investigated in two different orientations (vertical and horizontal) in the central Red Sea. Results showed significant variations in the abundance of benthic invertebrates between the different substrates. While barnacles and bivalves preferred panels placed in vertical positions, the abundance of bryozoans was high on horizontal panels. Artificial panel submersion season plays a significant role in the recruitment of benthic invertebrates on surfaces in the Red Sea. In conclusion, this study supports the overall notion that marine invertebrate recruitment on hard substrates is regulated by a combination of factors which include substrate type, orientation and submersion season.

First large-scale ecological impact study of desalination outfall reveals trade-offs in effects of hypersalinity and hydrodynamics

Desalination is an increasingly common method of meeting potable water demands, but the associated ecological risks are not well understood. Seawater desalination plants discharge large volumes of hypersaline brine directly into the ocean, raising concerns about potential impacts to marine life. In order to reduce impacts of brine, newer desalination outfalls are often fitted with high-pressure diffusers that discharge brine at high velocity into the water column, increasing the mixing and dilution of brine with ocean water. However, there are few published studies of marine impacts of desalination brine, and no well replicated before-after designs. Here we report a six-year study testing for impacts and subsequent recovery of sessile marine invertebrate recruitment near a desalination outfall with high-pressure diffusers. We used a Multiple Before-After-Control-Impact (MBACI) design to test for impacts and recovery at two distances (30 m and 100 m) from a 250 ML/day plant outfall, as well as a gradient design to test the strength of impacts relative to distance from the outfall. The diffusers achieved the target of less than 1 psμ salinity difference to surrounding ambient waters within 100 m of the discharge outfall, but sessile invertebrates were nonetheless impacted. Polychaetes, bryozoans and sponges reduced in cover as far as 100 m from the outfall, while barnacles showed the opposite pattern and were more abundant near the discharging outfall. Ecological impacts were disproportionate to the relatively minor change in salinity (∼1 psμ), suggesting a mechanism other than salinity. We propose that impacts were primarily driven by changes in hydrodynamics caused by the diffusers, such as higher near-bed flow away from the outfall. This is consistent with flow preferences of various taxonomic groups, which differ due to differences in settlement and feeding abilities. High-pressure diffusers designed to reduce impacts of hypersalinity may inadvertently cause impacts through hydrodynamics, leading to a trade-off in minimizing combined salinity and hydrodynamic stress. This study provides the first before-after test of ecological impacts of desalination brine on sessile marine communities, and rare insight into mechanisms behind impacts of a growing form of human disturbance.

Disentangling settlement responses to nutrient-rich contaminants: Elevated nutrients impact marine invertebrate recruitment via water-borne and substrate-bound cues

Anthropogenic contaminants, including nutrient enrichment, frequently alter environmental conditions in marine systems and affect the development of communities on hard-substrata. Biofilms can influence the settlement of marine invertebrates and hence impact on the structure of fouling communities. Few studies have examined bacteria, invertebrates and nutrient-rich contaminants in concert, with none yet to examine the effects of nutrient-rich contaminants on both biofilms and the recruitment of sessile invertebrate communities in-situ to ascertain the mechanistic basis behind observed impacts. Biofilm treatments were allowed to develop under manipulated environmental conditions of either ambient or enriched nutrient levels. Enrichment conditions were elevated via slow-release fertiliser and invertebrate recruitment was prevented during initial biofilm development. Biofilm treatments (including a no film control) were then subject to either ambient or enriched water-borne nutrients (in a fully-factorial design) during a period of invertebrate colonisation in the field. Effects of nutrient-rich contaminants on invertebrate recruitment were observed as changes to community composition and the abundances of taxonomic groups. Communities on no biofilm control treatments differed from those with pre-developed biofilms. Naturally developed biofilms promoted recruitment by all organisms, except barnacles, which preferred nutrient-enriched biofilms. Water-borne nutrients increased the recruitment of ascidians and barnacles, but suppressed bryozoan, serpulid polychaete and sponge recruitment. The direct and indirect impacts observed on biofilm and invertebrate communities suggest that increasing nutrient levels via nutrient-rich contaminants will result in structural community shifts that may ultimately impact ecosystem functioning within estuaries.

Contaminant cocktails: Interactive effects of fertiliser and copper paint on marine invertebrate recruitment and mortality

Understanding interactive effects of contaminants is critical to predict how human activities change ecosystem structure and function. We examined independent and interactive effects of two contaminants (fertiliser and copper paint) on the recruitment, mortality, and total abundance of developing invertebrate communities in the field, 2, 4, 6, and 8weeks after substrate submersion. Contaminants affected community structure differently, and produced an intermediate community in combination. Fertiliser increased recruitment and decreased mortality of active filter feeders (ascidians and barnacles), while copper paint decreased recruitment and increased mortality of some taxa. Contaminants applied together affected some taxa (e.g. Didemnid ascidians) antagonistically, as fertiliser mitigated adverse effects of copper paint. Recruitment of active filter feeders appears to be indicative of nutrient enrichment, and their increased abundance may reduce elevated nutrients in modified waterways. This study demonstrates the need to consider both independent and interactive effects of contaminants on marine communities in the field.

Interactive effect of elevated pCO2 and temperature on the larval development of an inter-tidal organism, Balanus amphitrite Darwin (Cirripedia: Thoracica)

The projected increase in the pCO2 and temperature level by the end of the 21st century, is expected to influence the life cycle of marine invertebrates. Experiments were carried out with an acorn barnacle, Balanus amphitrite a dominant inter-tidal organism, to investigate the effect of these variables on its larval development and metamorphosis. Four alternate combinations of temperature and pCO2: (1) control (~30°C and ~400μatm); (2) elevated pCO2 (~30°C and ~750μatm); (3) elevated temperature (~34°C and ~400μatm); and (4) synergistic (~34°C and ~750μatm) were tested. Apart from their direct effect, the diet-mediated influence was also evaluated by providing a diatom feed (Chaetoceros calcitrans) that was grown under similar conditions as that of nauplii. In both the direct and diet-mediated effects, irrespective of pCO2 levels, elevated temperature favoured faster naupliar development. However, in the direct effect, the synergistic condition yielded poor quality cyprids (non-feeding, pre settlement larval stage) as evidenced by lower RNA:DNA ratio, that resulted in lower metamorphosis. On the contrary, in the diet-mediated evaluation, the feed grown in the synergistic condition had higher protein content that facilitated faster naupliar development, better quality of cyprids (higher RNA:DNA ratio) and yielded a higher percentage of metamorphosis. The results suggest that future climatic conditions will affect the quality of food consumed by invertebrate larvae, thus influencing their development and metamorphosis. Therefore, future studies evaluating the climate change scenarios to depict the changes in settlement and recruitment of marine invertebrates should be advocated to integrate the changes in the food chain.

Temperature and salinity: two climate change stressors affecting early development of the New Zealand sea urchin Evechinus chloroticus

Temperature and salinity are important environmental factors affecting the normal functioning of marine animals, particularly animals such as sea urchins living in shallow waters and tide pools. Here, we evaluated the effect of different combinations of temperature and salinity on early embryos of the endemic New Zealand sea urchin Evechinus chloroticus. Animals were collected at Matheson’s Bay (36o18′17′′S; 174o47′51′′E) in north-eastern New Zealand in February 2013. Embryos were exposed to five salinities (29, 31, 34, 35 and 37 ppt) and two temperatures (18 and 21 °C) during the first 24 h of development. Low salinity (29 ppt) affected all parameters (fertilization, development rate, gastrulation and normal development), with ca. 50 % of embryos surviving at 29 ppt, whereas seawater temperature only affected development rate and gastrulation. An increase in temperature from 18 to 21 °C minimized the negative effect of low salinity (≤31 ppt) on development rate and gastrulation of E. chloroticus. Overall, the results of this study suggest that early embryos of E. chloroticus have developmental plasticity to withstand reductions in salinity up to 29 ppt; however, it is still unknown whether the surviving embryos will be able to complete larval development at low salinities, particularly whether the embryos and larvae are carried into extreme environments such as estuaries where salinity is even lower. Multistressor studies are very important for climate change research as multiple environmental factors will act together in the wild, having major consequences for development and recruitment of marine invertebrates.

Field assessment of the predation risk-food availability trade-off in crab megalopae settlement.

Settlement is a key process for meroplanktonic organisms as it determines distribution of adult populations. Starvation and predation are two of the main mortality causes during this period; therefore, settlement tends to be optimized in microhabitats with high food availability and low predator density. Furthermore, brachyuran megalopae actively select favorable habitats for settlement, via chemical, visual and/or tactile cues. The main objective in this study was to assess the settlement of Metacarcinus edwardsii and Cancer plebejus under different combinations of food availability levels and predator presence. We determined, in the field, which factor is of greater relative importance when choosing a suitable microhabitat for settling. Passive larval collectors were deployed, crossing different scenarios of food availability and predator presence. We also explore if megalopae actively choose predator-free substrates in response to visual and/or chemical cues. We tested the response to combined visual and chemical cues and to each individually. Data was tested using a two-way factorial design ANOVA. In both species, food did not cause significant effect on settlement success, but predator presence did, therefore there was not trade-off in this case and megalopae respond strongly to predation risk by active aversion. Larvae of M. edwardsii responded to chemical and visual cues simultaneously, but there was no response to either cue by itself. Statistically, C. plebejus did not exhibit a differential response to cues, but reacted with a strong similar tendency as M. edwardsii. We concluded that crab megalopae actively select predator-free microhabitat, independently of food availability, using chemical and visual cues combined. The findings in this study highlight the great relevance of predation on the settlement process and recruitment of marine invertebrates with complex life cycles.

Manipulating thermal stress on rocky shores to predict patterns of recruitment of marine invertebrates under a changing climate

For rocky intertidal organisms, temperature is often considered the most influential factor governing early survival and growth. Nevertheless, our review of the literature revealed that few studies have manipulated temperatures in the field to test for effects on these critical early life history processes. Here, we present the results from a novel manipulation of substratum tempera- ture using settlement plates of different colour (black, grey and white) and infrared measurements of temperature to test hypotheses that temperature influences the early survival and growth of recent settlers of the intertidal barnacle Tesseropora rosea. Mean surface temperatures of black and grey plates were as great as 5.8°C (on average 2.2°C) and 4.8°C (on average 1.6°C) hotter than white plates across the sampling period, respectively. Cooler, white plates had significantly greater settlement and early growth than hotter, black plates, but differences in plate temperature did not significantly influence early survival or recruitment, though patterns were consistent with thermal variability. Comparisons between grey coloured natural rock and plates indicate that grey plates thermally mimic natural rock. Nevertheless, on average, more than twice as many larvae settled on plates than on natural rock, but early post-settlement survival on natural rock was double that on plates, suggesting that this artificial surface may not adequately capture the natural variability in early life history processes. Regardless, our simple and repeatable thermal manipulation repre- sents a useful tool for experimentally investigating the effects of temperature on recruitment pro- cesses and simulating future temperature variability associated with climate change.

Linking long-term, large-scale climatic and environmental variability to patterns of marine invertebrate recruitment: Toward explaining “unexplained” variation

The rate of input of new individuals into communities is a key component of community dynamics. Detection of ecosystem responsiveness to climatic perturbations such as El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO) can provide insight into how ecosystems will respond to climate change. Although these climate patterns are ocean-based, understanding of their influences on key ecosystem processes in temperate coastal marine ecosystems is limited. Recent analyses documented orders-of-magnitude increases in mussel recruitment in the 2000s relative to the previous decade. To evaluate the generality of this pattern across other intertidal species with planktotrophic dispersive larvae, and determine the extent to which variation in recruitment was explained by climatic and environmental (upwelling) variation, we analyzed patterns of barnacle and mussel recruitment across space (at up to 10 sites across 250 km) and through time (up to 20-years). Compared to mussel recruitment, barnacle recruitment varied far less interdecadally, and showed minimal change in seasonality. Using three analytical approaches, multiple regression, quantile regression and wavelet analysis, we found that recruitment of the barnacles Balanus glandula and Chthamalus dalli and mussels Mytilus spp. varied in generally similar ways with climate variation as expressed in ENSO, NPGO, and PDO indices, and with upwelling, as expressed in the Bakun upwelling index. In regression approaches, NPGO and upwelling had the strongest associations with recruitment of all species, with MEI and PDO also having an influence on C. dalli. The strength of the association with the environment varied with recruitment magnitude, however, being generally stronger at intermediate and in some cases high recruitment densities and weak at low recruitment densities. Time series (wavelet) analysis showed that with all environmental indices, the most consistent strong relationships occurred at annual periodicities, but multi-year periodicities were also important with PDO and upwelling indices. Patterns were complex, however, with periods of strong and weak associations at different time periodicities varying differently among the different environmental measures, and in some cases varying from strongly positive to strongly negative associations at the same periodicities through time. These relatively consistent relationships between large-scale climate and upwelling, and recruitment explained a substantial fraction of the variance (37–40%). The ~ 60% of the variance in recruitment not explained in our analyses is likely due to myriad other factors such as shorter-scale variation in upwelling, internal tidal bores, sea breezes, wave forces, larval behavior, and post-settlement processes. Thus, our analysis adds insights into potential influences on recruitment that occur at ocean basin- to regional scales, and complements the body of work that focuses on the more local-scale and shorter-term mechanistic influences that also help determine the pace at which larvae recruit to the adult habitat.

Oyster larval transport in coastal Alabama: Dominance of physical transport over biological behavior in a shallow estuary

Among the various factors affecting recruitment of marine invertebrates and fish, larval transport may produce spatial and temporal patterns of abundance that are important determinants of management strategies. Here we conducted a field and modeling study to investigate the larval transport of eastern oyster, Crassostrea virginica, in Mobile Bay and eastern Mississippi Sound, Alabama. A three‐dimensional larval transport model accounting for physical transport, biological movement of larvae, and site‐ and larval‐specific conditions was developed. A hydrodynamic model was used to simulate physical transport, and biological movement was parameterized as a function of swimming and sinking velocity of oyster larvae. Site‐ and larval‐specific conditions, including spawning location, spawning stock size, spawning time, and larval period, were determined based on the previous studies. The model reasonably reproduced the observed gradient in oyster spat settlement and bivalve larval concentration, although the model results were less dynamic than the data, probably owing to the simplified biological conditions employed in the model. A persistent gradient decreasing from west to east in the model results at time scales of overall average, season, and each survey in 2006 suggests that the larval supply may be responsible for the corresponding gradient in oyster spat settlement observed over the past 40 years. Biological movement increased larval retention near the spawning area, thus providing a favorable condition for local recruitment of oysters. Inclusion of biological movement, however, caused little change in the overall patterns of larval transport and still resulted in a west‐east gradient, presumably because of frequent destratification in the shallow Mobile Bay system.

Patterns of mussel recruitment in southern Africa: a caution about using artificial substrata to approximate natural recruitment

Quantifying sessile marine invertebrate recruitment often requires destructive sampling or extrapolation from artificial substrata, the latter introducing the danger of artifacts. We measured intertidal mussel recruitment into mussel beds and into brushes at three-month intervals for five years across 3,200 km of southern Africa and determined substrata effects on recruitment rate. Recruitment into mussel beds showed a strong, coast-wide gradient, with high recruitment on the West coast, diminishing on the South coast, and increasing slightly on the East coast. At scales of 10 s of km, brushes reflected natural temporal recruitment variability, with a strong significant linear correlation between recruitment into brushes and into mussel beds. However, the relationship became semi-logarithmic when comparing among locations at a scale of 100 s of km. Artificial substrata thus reflect local natural settlement well but may be a poor indicator of it when spatial scales are large, particularly when mussel bed topography is complex, or localities have very different recruitment densities.

Shore level differences in barnacle ( Chthamalus dalli) recruitment relative to rock surface topography

Local recruitment of marine invertebrates is often variable and can be an important determinant of population structure and dynamics. Here, we examined recruitment of barnacles relative to rock surface topography (cracks) over 4 years at 5 sites in Hokkaido, Japan. We used recruitment over multiple years to test for differences in barnacle response to rock surface topography among shore levels under natural conditions and varying recruit densities. Our results showed that barnacles exhibit different recruitment patterns in relation to surface topography over small spatial scales (50 cm) with barnacles in upper intertidal associating more with cracks ( p < 0.0001). In general, we found that barnacles tend to recruit more frequently within cracks in the upper shore and this result was not a function of barnacle density or available crack sites.

Molecular species identification of morphologically similar mussel larvae reveals unexpected discrepancy between relative abundance of adults and settlers

Understanding the relative importance of larval supply vs. post-settlement mortality underlies studies of marine invertebrate recruitment, yet is often hampered by researchers' inability to identify species among morphologically similar larvae or early juveniles. In New Zealand, two species of co-occurring intertidal mytilid mussels have morphologically indistinguishable settlers: the blue mussel Mytilus galloprovincialis, which is often numerically dominant in the mid-zone of the rocky intertidal, and the ribbed mussel Aulacomya atra maoriana which is often much less abundant. In this study, we obtained samples of newly settled mussels from 6 sample dates April-May 2005 from the rocky intertidal in Wellington Harbour, New Zealand. We used PCR-RFLP of the cytochrome c oxidase subunit I (COI) mitochondrial gene region to identify settlers to species. Of a total of 224 settlers that could be identified, 64% were identified as Mytilus galloprovincialis and 36% as Aulocomya atra maoriana. The percentage of A. atra maoriana in the samples was unexpectedly high and ranged from 22–50% among the sample dates. This study reinforces the need to quantify larval supply at the species level to understand the relative importance of pre- and post-settlement mortality, and also demonstrates the usefulness of the COI region as a species-specific marker for identifying mussel larvae and juveniles.

Estimation of food limitation of bivalve larvae in coastal waters of north-western Europe

Marine invertebrate recruitment may be affected by food limitation during the pelagic larval life stages. In the present study, field data on abundance of bivalve larvae along with their prey (small phytoplankton) were examined to see whether they were consistent with predictions made by an energetic model of larval requirements. Bivalve larvae were monitored during 2000 at ten different study sites in four different areas (Limfjorden, Sylt-Rømø bight, Western Wadden Sea and Delta area) along the coast of north-western Europe. Calculation of the energetic requirements of the larvae at 15 °C indicated maintenance costs of a 200-μm bivalve larva to be 1.9 × 10 − 5 J larva − 1 d − 1 , while the maximum assimilation rate, resulting in maximum growth, would amount to 6.2 × 10 − 3 J larva − 1 d − 1 . Calculation of potential assimilation rates of larvae in the field resulted in estimates between 10 − 5 and 10 − 3 J larva − 1 d − 1 . Maximum larval concentrations in the field occurred from May to September and ranged between 17 and 392 larvae dm − 3 . Most larvae were able to cover their maintenance costs, but not to attain maximum growth rates. Between April and September, the potential assimilation rate averaged 7–26% of the maximum assimilation rate. Under the assumptions made for the present study, it is suggested that growth of larvae in north-west European waters is often food-limited.

Spatial analysis of recruitment of marine invertebrates on arborescent substrata

Juveniles of various species were observed on the perisarc of the dead hydroid Tubularia larynx in the Gulf of St Lawrence, Canada, some at densities up to twenty times more than on other substrata. This suggested active microhabitat selection by settling larvae. Here the results are reviewed of three field experiments using arborescent substrata, which tested whether recruitment of invertebrates could be accounted for by a passive model of encounter, or whether activity of larvae, or settled juveniles, needs to be invoked to explain the distributions. The distribution of inert particles on arborescent structures was examined in a laboratory flume. Both branch diameter and branching heterogeneity of the structures were considered as independent variables. A comparison of laboratory and field results suggested that passive settlement processes (hydrodynamic) were sufficient to explain the recruitment patterns at scales greater than 3 cm. At small scales (ca 1 mm), however, the hypothesis of active larval and/or post‐settlement exploration of the substratum could not be rejected.

Do hydrodynamic factors affect the recruitment of marine invertebrates in a macrotidal area? : The case study of Pectinaria koreni (Polychaeta) in the Bay of Seine (English Channel)

For marine benthic invertebrates exhibiting complex life cycles, changes in populations' distribution and abundance are governed by a large variety of physical, chemical and biological processes. From field observations in the Bay of Seine and laboratory experiments conducted since 1987 on the polychaete Pectinaria koreni, the present study highlights the relative importance of hydrodynamical and biological factors which affect individuals within both the planktonic and benthic phases at different scales of space and time in a macrotidal area. Pectinaria koreni is one of the main macrofaunal component of the Abra alba muddy fine sand community of the eastern Bay of Seine. Despite a highly advective and diffusive environment, a relative larval retention near adult population was reported due to some local hydrodynamics features (e.g. tidal residual circulation, Seine river plume front) and the interaction between the vertical current structure and the larval vertical migration. Although larval retention could be disrupted by wind induced currents, multiple spawning events over the reproductive period increase the likelihood that at least one larval cohort ensures a high recruitment during the life-span. Following a massive settlement whatever the sediment grain size, the newly settled larvae exhibited a high immediate decrease of their densities as a result of postlarval mortality and migration. Postlarval drifting was induced by a combination of physical factors (i.e. tidal currents and swell) and postlarval behaviour in response to sediment texture and adult/settlers interactions. According to the hydrodynamics of the bay, this process may generate a postlarval transport from offshore bottoms to coastal suitable habitats and counteract the demographic effects of larval dispersal. A conceptual model of factors governing the recruitment and population maintenance of Pectinaria koreni is proposed and discussed in comparison with results obtained on another polychaete, Owenia fusiformis, in the same area.

Recruitment of marine invertebrates onto arborescent epibenthic structures:active and passive processes acting at different spatial scales

The hypothesis that passive processes control larval settlement of benthic manne invertebrates on 3-D structures was tested for several groups at both meso (3 cm) and small (mm, pm) scales. Flume experiments were carried out with Inert polyvinylchloride rmcroparticles and 5 different types of silicone-coated 3-D arborescent plastic structures with 5 distal branch diameters. Microparticle attachment a s a function of the distal branch diameter and the proportion of particles attached to the nodal section of each branch were used to predict the initial contact site by living marine benthic invertebrates larvae in field experiments using the same 3-D structures immersed for 2 mo. The flume experiment showed that distal branch diameter had a significant effect on the density of attached particles. The proportion of particles observed in the nodal sections (8%) was much lower than the proportion of the total surface area occupied by the nodes (20%) on each branch. Field experiments examining recruit density (6 sessile species belonging to 4 classes of marine invertebrates) on the same 3-D structures showed the same significant effect of the distal branch diameter observed in the flume. However, individuals were found in the nodal sections much more frequently (35 to 80 %) than would be expected based on the results of the flume experiment (8%). Comparison of results of laboratory and field experiments suggests that passive settlement processes are sufficient to explain early recruitment patterns of the species examined on scales ca 3 cm. However, a t smaller scales (ca mm) the hypothesis of active larval exploration of the substratum could not b e rejected. The likelihood of post-settlement mortality processes potentially explaining the patterns observed is discussed.