Spat Settlement Research Articles

Effect of diet on larval settlement, growth, and spat survival of the oyster Crassostrea gigas (Thunberg, 1793)

This study aimed to assess the impact of microalgal concentrations in diets on the settlement and metamorphosis stages of pediveliger larvae and spat of Crassostrea gigas oysters. Diets containing microalgal concentrations of 8, 12, and 16×104 cells mL-1 were administered for seven days during the larval settlement phase of pediveliger larvae. In the metamorphosis and postlarval cultivation phases, diets with microalgal concentrations of 8, 16, 24, and 32×104 cells mL-1 and a control group without food were tested for 14 and 21 days, respectively. Growth, yield, and survival were assessed every seven days. The diet comprised 30% Isochrysis galbana and 70% Chaetoceros muelleri microalgae. In the metamorphosis phase, the results revealed no significant differences in larval metamorphosis rate and survival across the tested diets. The concentrations exhibited similar survival in the spat with an initial average shell height of 0.657 ± 0.05 mm, with a significant difference only in growth. In the spat with an initial average shell height of 0.830 ± 0.12 mm, no differences in survival were observed among the tested concentrations. We concluded that diets of 12 and 16×104 cells mL-1 provide high rates of larval metamorphosis and spat yield during the larval settlement and metamorphosis phases. During the spat cultivation phases, diets of 32 and 24×104 cells mL-1 could optimize the cultivation time and yield of C. gigas spat in the laboratory. However, when considering survival alone, no advantage was found in providing a diet with a microalgal concentration above 8×104 cells mL-1 across all C. gigas oyster cultivation phases.

Efficiency of different collectors for spat settlement of Mediterranean mussel (Mytilus galloprovincialis) in the southeastern Marmara Sea, Türkiye

ABSTRACT Mediterranean mussel have spread almost throughout Turkish seas. The objective of this study was to set up longline systems to determine spat efficiency by two different systems (buoy and pipe) consisting of three different materials, new rope, used rope and used net, in Erdek Bay. The second objective was to identify whether waste materials could be used in mussel culture. Sea water temperature, salinity, dissolved oxygen, chlorophyll-a, total suspended, inorganic and organic matter were measured at monthly intervals. Results revealed that the highest settlement (4196 spat) was observed in the collectors made of used nets in the pipe system (October 2016). In contrast, the lowest value (2 spat) was recorded from the collectors made of new ropes in the buoy system (August 2016). Waste materials presented relatively better results on settlement than the new ropes. The optimum production was observed in the mussel farm with 1239 spat in the buoy and 1607 spat in the pipe systems, which were commissioned in April 2016. It is concluded that the spat performance was affected by collectors and materials. Therefore, we propose the use of used ropes and used nets, which will allow more effective attachment surfaces to achieve higher spacing settlement.

Spatial and Temporal Variability in Oyster Settlement on Intertidal Reefs Support Site-Specific Assessments for Restoration Practices

As some of the most threatened ecosystems in the world, the declining condition and coverage of coastal habitats results in the loss of the myriad ecosystem services they provide. Due to the variability in physical and biological characteristics across sites, it is imperative to increase location-based information to inform local management projects, which will potentially help to reestablish functions of coastal habitats. Since oysters are often used in restoration projects, this study quantified spatial and temporal patterns in eastern oyster spat settlement in a bar-built estuary in northeast Florida, USA that is host to a robust population of intertidal oyster reefs. Spat settlement was found to occur from April to October with small peaks in the spring and large ones around September. Inter-annual differences in spat settlement were likely influenced by existing environmental conditions and heavily affected by large-scale events such as tropical cyclones. Variations in regional spat settlements are possibly driven by the residence times of the watersheds, the density of adult populations, and the location of the spat collectors. The results of this study illustrate place-based variability in oyster settlement patterns and underscore the importance of local monitoring for oyster resource management, restoration, and research.

Collapsed oyster populations in large Florida estuaries appear resistant to restoration using traditional cultching methods—Insights from ongoing efforts in multiple systems

AbstractObjectiveDepressed eastern oyster Crassostrea virginica populations in the northern Gulf of Mexico have been the target of numerous post‐Deepwater Horizon restoration projects. These projects primarily have focused on replacing oyster cultch (substrate) to promote spat settlement, increase recruitment, and bolster adult oyster populations. This study assessed the outcomes of six such efforts, which used different cultch types and densities between 2015 and 2022 in three estuaries on the Florida panhandle (Pensacola, St. Andrew, and Apalachicola bays). Total restoration costs for these projects were more than US$14 million.MethodsUsing generalized linear models, we analyzed oyster count data collected from diver surveys in three size‐classes (spat, seed, and adult). We tested whether oyster population responses in the six restoration efforts varied over time, location, or study design.ResultOyster counts did not persistently increase after restoration, regardless of cultch type or density. Positive responses to restoration efforts were irregular and short‐lived and seemed only to occur for spat‐size oysters immediately after restoration. None of the restoration efforts significantly improved the abundance of oysters of any size‐class in any of the study estuaries. Factors contributing to these results likely include design and implementation elements, such as the materials used and the height of the restored reefs. However, monitoring programs have not been able to deliver a clear picture of what is hindering restoration success.ConclusionFor oyster restoration efforts to succeed, changes are needed—both in their implementation and in the way they are monitored—in order to promote continuous learning and improvement in restoration outcomes.

Evaluation of the green mussel floating cage related to the enlightenment aspect through the analysis of the Perna viridis spat settlement & population structure

A floating cage is one of the methods of cultivating green mussels that can potentially be used as a medium for maritime education because it is easily accessible to people of various age groups, especially in green mussel cultivation related to mangrove ecotourism. This study aims to analyze the design of floating cages from light and shaded zones based on the spat density and the population structure. The research was conducted in floating cages operated in the Java Sea, Banyuurip Village, Ujungpangkah District at coordinates 06°52’18.84” South Latitude and 112°29’41.19” East Longitude. The spat density was calculated per collector rope per depth (1-4m). When the green mussels were harvested in five-month cultivation from each zone, the population structure was analyzed with Excel 2010 software and measured for the population frequency distribution to investigate the percentage of mussels that reached consumer size. The differences between open and shaded population variables were evaluated by t-test (α=5%). The results of this study show that the density of spat attachment to the collector in the open space was 17.044±724 individuals/m2, and in the shaded area was 16.085±4634 individuals/m2. The density of the spat at a depth of 1m (22,538±4,536 individuals/m2) was significantly different (p<0.05, with three observations) from the density at a depth of 4m (10,830±1,016 individuals/m2) in a light area. However, the spat settlement between each depth was not significantly different in the shaded area. The population structure of green mussels in both areas had seven length classes (class I-VII) with a 7.8 mm interval. The percentage of the class VI population (47.4-55.20 mm) in the light area (31%) was significantly different (p<0.05) from the shaded area (25.79%). It means that the portion of green mussels suitable for harvest (length 40-80 mm) in the light zone is more than in dark zone. We conclude that lighting is an essential aspect of the rate of spat attachment, survival, and growth. The proportion and position of the shaded area in the floating cages must be adjusted so that the growth of the spat is not blocked.

Demography of Oysters Pre‐ and Postcollapse in Apalachicola Bay, Florida, Using Stage‐Based Counts

AbstractThe collapse of oyster populations and the fisheries they support has been a worldwide phenomenon, but studies of oyster demography in situ prior to and after the collapse have been rare. We used time series of stage‐based counts of eastern oysters Crassostrea virginica in Apalachicola Bay, Florida, to help understand how abundance and demographic rates may have changed in the decade after the 2012 collapse relative to the period before the collapse. We relied on a Bayesian hierarchical model in which the latent stage structure of the oyster population (i.e., densities of spat, sublegal oysters, and legal oysters) was governed by a system process and where the count data represented summaries of that latent structure. Count data were sufficient to conduct this on two large oyster bars that had some of the highest precollapse oyster densities. We also examined nine other bars with less data for any temporal trends in postcollapse abundance that might be associated with recent restoration efforts. Among the 11 bars examined, oyster densities were often increasing prior to the collapse and were very low, without detectable trends, afterward. Based on our demographic analyses, mortality rates of Apalachicola Bay oysters in the decade after the collapse generally exceeded (often greatly so) those during the precollapse period for all oyster stages. On the other hand, spat settlement rates apparently were increasing prior to the collapse and remained high during the postcollapse period. Simulations of postcollapse demography suggest that without improved survival rates, further declines of the oyster population can be expected. We discuss these findings in light of ongoing restoration and management efforts and suggest ways in which rapid transitions to undesirable socio‐ecological regimes might be avoided in the future.

Spatiotemporal spatfall dynamics and prevailing estuarine conditions for optimal oyster (Crassostrea tulipa) spat availability in selected Gulf of Guinea brackish systems

Uncertainties associated with wild harvests of seed and adult oysters due to unknown oceanographic oscillations are a major challenge in oyster fisheries and aquaculture development. In contribution to addressing this challenge, we proffer clarity on the spatiotemporal variations in spatfall (number of spat/m2) of the mangrove oyster Crassostrea tulipa (Lamarck, 1819) in four estuaries along the Gulf of Guinea coast. By monthly deployment of artificial substrates affixed to bamboo racks over 12 months, we find significant differences in spatfall among and within the brackish systems, and across months and seasons. Spatfall regimes were unique in each ecosystem albeit with an overall preponderance of dry season availability of spat. Locations with reef oysters had superior spatfall to mangrove root-adapted-oyster areas. Narkwa, a relatively small lagoon with reef oysters had the highest annual mean spatfall, which was 1.3, 2.5, and 9.8 folds the spatfall in Densu Delta, Benya Lagoon and Whin Estuary, respectively. Spatfall varied significantly by depth as the more frequently exposed top collectors harvested much less spat than submerged collectors. There was a year-round availability of spat, confirming continuous spawning in C. tulipa. Spatfall variability was significantly driven by fluctuations in prevailing dissolved oxygen and salinity. Prevailing dissolved oxygen and salinity levels in the estuaries for optimal spat settlement were 1.68 – 3.40 mg L-1 and 11.00 – 29.33 ppt (parts per thousand), respectively. The findings of this study are recommended as empirical reference points for sustainable seed procurement for aquaculture production and management of C. tulipa fishery in the region of the study.

Settlement experiment of spat using three different collectors in two contrasting coastal sites from Chile (SE Pacific Ocean)

Most of the marine bivalve production comes from aquaculture and only 11% comes from the wild fishery. For example, mussel aquaculture still strongly relies on spat - the recently settled juveniles. Gaps of information on the recruitment and settlement of spat are important drawbacks for the development of mussel aquaculture in several geographic regions. We here study these processes using three collectors (Anchovy mesh, Rope and Raschel mesh) throughout one year in two coastal sites of Central Chile (Coliumo and Punta Lobería) at three depths (2, 4 and 6 m). Five spat species were settled in the collectors, i.e. Aulacomya atra, Choromytilus chorus, Mytilus chilensis, Mytilus galloprovincialis and Semimytilus algosus. Recruitment was consistently higher during summer and autumn at both sites. Different seasonal recruitment patterns were observed regarding the spat species. Spat collection of the former mytilid species occurs from spring to autumn with the exception of M. galloprovincialis in Coliumo and M. chilensis in Punta Lobería. M. galloprovincialis is settled mostly in autumn and summer, and M. chilensis during winter. There were also differences when considering the collector type, mostly due to A. atra and S. algosus which peaked in nylon rope. Spat collection of the commercial species M. chilensis should be preferentially carried out in nylon rope during autumn in Coliumo and summer and autumn in Punta Lobería. During summer and autumn, also S. algosus will be settled. These results will enable the implementation of management measures to promote mytilid culture on the coastal zone of the Biobío region.

Taura kuku: prioritising Māori knowledge and resources to create biodegradable mussel spat settlement lines for shellfish restoration in Ōhiwa harbour

ABSTRACT Mātauranga Māori (Māori knowledge) is grounded in place-based, multi-generational knowledge and the connection of that knowledge with the environments from which it is derived. It takes for granted that all elements of the natural world are related, and it is upon those relationships that survival depends. A Māori worldview advocates for the responsibility of each generation to pass onto their descendants at least as good a supply of resources as they, themselves, had inherited. For Māori, the wellness of the environment is a direct reflection on the wellness of the people. This article provides an overview of a practical mātauranga Māori-led marine restoration project in Ōhiwa harbour. The transdisciplinary project worked with a traditional Māori master weaver and kaumātua (tribal elders) to develop biodegradable taura kuku (green-lipped mussel spat settlement lines, hereafter taura kuku) made from traditional Māori plant biowaste and other natural materials. The taura kuku proved a successful tool for the recruitment and settlement of wild mussel spat assisting shellfish restoration and increasing marine biodiversity in the culturally and ecologically important mahinga kai (traditional food basket) of Ōhiwa harbour.

Settlement of oyster Magallana bilineata (Röding, 1798) spat in the natural environment: seasonality and substrate texture preference

ABSTRACT This study determined the seasonality of Magallana bilineata spat settlement in a natural oyster bed and established surface texture preference of competent larvae by comparing the numbers of spat settling on the smooth and rough surfaces of ceramic tiles over a period of 31 months. This will guide farmers when to deploy cultches to maximise spat harvest. Regardless of tile orientation, spat numbers were significantly higher on the rough side (129.07 ± 9.3 m−2) than on the smooth side (66.95 ± 7.0 m−2). Spat collection was highest in March, June and July in each of 2015, 2016 and 2017. There was no significant correlation between the number of spat settling in each month with temperature, salinity, pH, dissolved oxygen (DO), sulphide, phosphate, nitrate, nitrite and ammonia concentrations, plankton density measured at deployment and retrieval of tiles, except for DO during deployment. The only significant correlations with mean shell length (SL) were with DO and ammonia during tile retrieval. Mean SL did not significantly differ between tile orientations and surfaces but differed between months, being highest in August 2015 and August 2016. From these results, it is best to deploy roughened cultches in this particular area in March, June and July.

Characterization of the effects of triclosan on sperm and embryos of Mytilus and Perna mussel species.

The Greenshell™ mussel (GSM), Perna canaliculus, is a culturally and commercially important species in New Zealand. Declines in spat settlement of GSM have been observed in important growing areas and the cause(s) have not been identified. One hypothesis is that chemical contaminants could be a contributing factor. The aim to this study was to investigate the effects of acute exposure on early life stages using the anti-microbial triclosan (TCS) as a benchmark toxicant and the blue mussel (BM), Mytilus galloprovincialis, as a reference species. Sperm and embryos of BM and GSM were exposed to TCS for 1h and 48h, respectively. Following exposures, a range of parameters were investigated including spermatozoa cellular characteristics via flow cytometry, fertilization success, larval mortality and size. Exposure to TCS negatively impacted functional parameters of sperm, reduced the fertilization success and larval size, and increased larval mortality in both BM and GSM with LC5048h of 94.3 and 213µg L-1, respectively. Triclosan increased sperm ROS production in both species, which could cause destabilisation of mitochondrial and other cellular membranes, resulting in reduced mitochondrial membrane potential (BM) and increased sperm size (GSM), leading to apoptosis in both species. Fertilization success of GSM was only affected at the highest TCS concentration tested (391µg L-1), but development of larvae derived from exposed sperm was affected from the lowest concentrations tested (0.5 and 5.2µg L-1) in both species. This highlights the importance of assessing the sensitivity of contaminants across developmental stages. Results of this study confirm that TCS causes oxidative stress and has membranotropic effects, and that early life stages of the endemic GSM are suitable to assess ecotoxicity of contaminants such as TCS.

Settlement of Bivalve Spat on Artificial Collectors (Net Bags) in Two Commercial Mussel Parks in the North-Western Adriatic Sea

Among aquaculture activities, shellfish culture is considered more sustainable and beneficial in terms of food security. Currently, only a few bivalve species are reared and there is a need to explore the possibility to introduce new candidates for shellfish farming. Due to the lack of information on bivalve recruitment in the North-Western Adriatic Sea, in this study, the possibility to collect natural spat of commercial species was investigated. Artificial collectors (net bags) were deployed in two sites, Pellestrina and Caleri (North-Western Adriatic Sea), within two commercial mussel parks, during the spring–summer and summer–autumn periods. At both sites, collectors were placed at a distance of 1 m from each other, from 5 to 14 m depth. The influence of season, site and depth on bivalve recruitment was inspected and the presence of invasive species was also evaluated. In all, 28 bivalve taxa were found, and a higher settlement rate was observed in summer–autumn compared to the spring–summer period. Mytilus galloprovincialis, Flexopecten glaber, Mimachlays varia and Aequipecten opercularis were the most abundant species in spring–summer. In the summer–autumn period, in both sites analysed, a very high quantity of Anadara transversa and F. glaber were found. Indeed, these species were dominant at Pellestrina and Caleri, respectively. Another non-indigenous species, Arcuatula senhousia, was also detected. Relevant amounts of Pectinidae spat, F. glaber in particular, were collected and the optimal depth range for the scallop spat collection was found to be between 8 and 14 m. Our results highlight the relevant potential of Pectinidae spat collection along the North-Western Adriatic coasts, even though the presence of invasive species needs to be monitored.

Temporal Dynamics of Eastern Oyster Larval Abundance in Great Bay Estuary, New Hampshire

Eastern oyster Crassostrea virginica populations have been declining steadily over the past several decades across the North American East coast. The Great Bay Estuary (GBE), located in New Hampshire, is experiencing this loss and restoration efforts have been put into effect. This paper characterizes larval abundances of settled spat and two early stages of C. virginica, D-hinge and veliger, in GBE from 2018 to 2020. Abundances are compared based on date of sampling, year, collection site, and the physicochemical data recorded on each sampling date. It was found that overall, D-hinge larval abundances have declined significantly from 2018 to 2020, whereas veliger abundances have remained steady or increased. Although the physicochemical factors are known to play a role in larval abundance, very little significance was found, suggesting future study may need to be modified to include a broader range of factors (e.g., more temporal sampling). This study indicates that both D-hinge, veliger, and spat settlement occur in GBE before sampling traditionally has started (June), suggesting an earlier than previously thought first spawn of C. virginica in GBE. This finding can be used to enhance restoration efforts as it suggests that spat brought in to augment current sites of active restoration should be released earlier in the season and that recruitment devices should be deployed before the previously thought first spawn of each season.

Population, Aquaculture and Transplantation Applications of Critically Endangered Species Pinna nobilis (Linnaeus 1758) in the Mediterranean Sea

The population of fan mussel, Pinna nobilis across the Mediterranean Sea has been affected by factors such as overfishing, fisheries processes, environmental pollution, destruction of habitat, tourism, etc. Therefore, the species P. nobilis was taken under protection by the Decisions of the Council of Europe and Barcelona Convention. However, its mortality rates of 100% have been reported to be due to Haplosporidium pinnae, a parasite in different Mediterranean regions. The status of P. nobilis has thus been revised to be reduced from “Vulnerable” to “Critically Endangered” and the importance of all the studies on the species further increased. The aim of the study is to present the current status of P. nobilis, the native to the Mediterranean, by combining the relevant studies on ecology, aquacultural process (larvae, spat settlement and rearing), culture methods and transplantation. The present study has provided comprehensive knowledge on the current status P. nobilis population, aquaculture and transplantation activities. Except for studies to determine stocks, in particular, those on collecting young individuals from nature and planting and growing them in predetermined sites as well as their production through various cultures from their larval phase onwards are of great importance in terms of rehabilitation and sustenance of the damaged P. nobilis population. Therefore, alternative and potential habitats should be created thanks to transplantation and aquaculture. Marine protected areas should be determined to enable a healthy P. nobilis population to be sustained.

Predominant factors limiting the recovery of the eastern oyster (Crassostrea virginica) in western Mississippi Sound, USA

In addition to suitable substrate, successful oyster recruitment requires an adequate supply of planktonic larvae, followed by the subsequent survival and growth of early post-settlement stages. Substrate limitation, recruitment limitation and post-settlement limitation each played key roles in limiting the recovery of the eastern oyster, Crassostrea virginica (Gmelin 1791) within western Mississippi Sound between 2016 and 2019. Effects of a mortality event which eliminated the oyster spawning stock biomass from two major oyster reefs of western Mississippi Sound in 2016 carried over to the 2018 recruitment period. But the supply of larvae from outside the affected reefs was sufficient to provision spat to restored and reference sites within the 2018 study area. Thus, substrate limitation was inferred to be the primary limitation to early oyster recruitment in 2018. Moreover, variability in spat metrics from settlement samples implied potential post-settlement limitation in 2018. Spat density, mortality, size, and inferred growth varied on temporal, subregional, and microhabitat scales. In 2019, oyster spawning stocks were completely decimated throughout the western Mississippi Sound region by unprecedented freshwater discharge from the Bonnet Carré spillway. Consequently, early oyster recruitment was effectively eliminated, as evidenced by the lack of spat settlement across the entire oyster metapopulation of western Mississippi Sound. Within the course of a single year, the predominant deterrent to oyster recovery had shifted from substrate limitation to recruitment limitation.

Assessment of the risk posed by three antifouling biocides to Pacific oyster embryos and larvae in Hiroshima Bay, Japan.

The Pacific oyster (Crassostrea gigas) is an important species in oyster farming worldwide, including in Japan. Hiroshima Bay is one of the most important oyster farming areas in Japan. We investigated the occurrence of antifouling biocides used worldwide including diuron, Irgarol 1051 (Irgarol), and 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT), which have been detected at sub-ppb levels in seawater in Japan, and estimated their no observed effect concentrations (NOECs). In recent years, the spat settlement of Pacific oysters has become poor, which presents a challenge for oyster aquaculture in Hiroshima Bay; hence, we conducted embryotoxicity and larva settlement tests using Pacific oysters. Compared to diuron and Irgarol, DCOIT exhibited a higher toxicity toward oyster embryos, and the minimum 24-h NOEC toxicity value for the oyster embryos was <3 ng/L. The highest concentrations of diuron, Irgarol, and DCOIT in the environmental seawater in the Seto Inland Sea were 27.6, 3.2, and 24 ng/L, respectively. Considering the NOECs, the environmental concentrations of these biocides suggest that the ecological risks posed by diuron and Irgarol are low, whereas those posed by DCOIT are high. However, the rate of detection of DCOIT was low because it degraded rapidly in the seawater before treatment for chemical analysis, except in the case of the treatment on the research vessel.

Siltation negatively affects settlement and gaping behaviour in eastern oysters

While high levels of siltation are known to be deleterious to eastern oysters (Crassostrea virginica), the collective effect of suspended and bedded sediment is understudied from the perspective of oyster farming and bed restoration. In this study, we used laboratory experiments to explore spat settlement rates on a wild bed proxy substrate (i.e., empty shells on the bottom of experimental tanks) in conditions simulating a siltation event and the presence of suspended spat collectors. Using high-frequency valvometry, we also described the behavioural effects of acute sediment burial on wild adult oysters in situ. The vast majority of larvae settled on bottom substrate as opposed to suspended collectors. Sediment negatively affected overall oyster spat settlement on bottom shell, as spat densities were ≈3 × lower when sediment was present. This negative effect was largely attributed to severely depressed spat densities on the upper side (top) of bottom shells. Settlement on the underside of bottom shell was less affected. Wild adult oyster behaviour was negatively affected by acute burial, which ultimately resulted in death. We suggest that the reduction in settlement in the presence of siltation is likely due to the combined effects of suspended sediment on cue detection and bedded sediment on substrate availability. Given that oysters are ecosystem engineers, the negative effects of siltation on both larval and adult oysters can ultimately result in cascading effects to the surrounding biological community.

Characterization of Spatial and Temporal Variability of Crassostrea spp. Spat Settlement to Ascertain Allocable Sites for Spat Collection at Cox’s Bazar, Bangladesh

AbstractSpat settlement (spat density, recruitment, and mortality) of edible oysters Crassostrea spp. was estimated for three sites (Nunia Chara [NC], Chowfoldandy [CD], and Sonadia Island [SI]) located along the Cox’s Bazar coast of Bangladesh from February 2019 to January 2020 by deploying shellstrings. Field observations were made every 15 d to understand the monthly variation of oyster spat settlement more precisely. Environmental variables varied significantly with the study site × month interaction or across study sites and months. Gonadal maturation and spawning of oysters may depend on certain environmental conditions, which were greatly influenced by seasonality, especially the monsoon. Spat density, recruitment, and mortality rates were significantly different with the study site × month interaction. The NC site offered higher spat density and recruitment than CD and SI during most months; spat density was lower due to increased mortality during the monsoon (late May to mid‐October) at all sites. Spat density was positively related to salinity, pH, and total suspended solids but negatively related to temperature and high‐ and low‐tide water depths. Recruitment was positively related to salinity and negatively related to low‐tide water depth. Spat mortality was positively related to high‐tide water depth but negatively related to low‐tide water depth. Eight different groups of fouling organisms were observed during the study, which could affect spat settlement. Most interestingly, oyster drills at NC and SI and marine macrophytes and bushlike organisms at CD and SI could contribute to increased mortality during the monsoon. This study showed that none of the three sites would be potential locations for spat collection on shellstrings during the monsoon; however, if avoiding the monsoon, NC has a higher potentiality of allocation for spat collection both for commercial farming and oyster restoration efforts.

Assessment of an alternative Pinctada margaritifera spat collector in French Polynesia

Plastic waste in the oceans is a growing concern due to its size diversity, its ubiquitous nature and its impact on both marine organisms and ecosystems. While threatened by this marine plastic litter, the aquaculture industries also represent one of its major sources. In French Polynesia, black-lip pearl oyster (Pinctada margaritifera) farming is no exception. Wild spat collection has been described as a source of pollution because of the numerous particularly fragile and brittle shade-mesh plastic collectors used locally and often mismanaged when no longer usable. Thus, with the aim of helping to reduce this pollution, the present study is focused on the assessment of reusable plate collector as a potential alternative. We tested, using an in situ approach, the influence of the collecting surface position (i.e. horizontal vs vertical), plate color (i.e. black vs orange) and density (i.e. 25 plates vs 50 plates) on Pinctada margaritifera and Pinctada maculata spat settlement. Our results showed that 50-plates collectors, whatever their color and position, were more efficient to collect both kinds of spat than shade-mesh collectors (P < 0.0001). They also induced a higher number of Pinctada margaritifera spat than on the 25-plates collection device although there was no significant difference in numbers of Pinctada maculata spat (P < 0.0001). However, every type of plate collector presented a decreased mean length of the spat collected (P < 0.0001) compared to the shade-mesh collector. Furthermore, horizontal positioning of the collecting surface greatly improved the spat numbers on plate collectors (P < 0.0001) although there was no effect on shade-mesh collectors (P > 0.05). Finally, our results indicated that, among all the devices tested, the black horizontal 50-plates collector was the most efficient to collect spat and particularly Pinctada margaritifera. These first findings thus tend to suggest that these black re-usable plate collectors could be an efficient alternative to the currently used shade-mesh collectors.

Settlement and culture of post-larvae, spat and juveniles of the West Indian topshell, Cittarium pica (Linne, 1758)

The settlement, survival and growth rates of C. pica post-larvae, spat and juveniles were evaluated under different culture conditions. The factorial effect of six stimuli (low temperature, γ-aminobutyric acid, potassium chloride, hydrogen peroxide, dibromo methane, and coralline algae extract), five types of biofilm substrates (artificial, artificial with coralline algae, artificial with congener mucus, benthic microalgae, and microalgae with congeners mucus) and five stimulation times (6, 12, 24, 36 and 72 h) on the larval settlement of the topshell were assessed. In addition, growth and survival rates of post-larvae fed with artificial biofilm and benthic microalgae were compared. Similarly, growth and survival rates were assessed in spat cultured in different environments (outdoor and indoor), and juveniles grown in concrete tanks with different finished of the inner walls (gray rough finish, and white smooth walls with epoxy paint). A higher larval settlement was observed in substrates with artificial biofilm exposed to cold water (20 °C) for 24 h. Treatments with the stimulus of KCl and dibromo methane had toxic effects on larvae. There were not differences in the shell length of the settled larvae between stimuli, substrata or exposition time. Post-larvae fed with artificial biofilm had survival and growth rates similar to those maintained with benthic microalgae biofilm. The spat that were cultured in outdoor conditions showed a higher growth rate than those kept indoor, but the survival was similar in both cases. Finally, juveniles grown in tanks with rough, gray concrete inner walls had higher growth and survival rates, compared to those held in tanks with smooth white finish. The improvements obtained in growth and survival of post-larvae, spat and juveniles of C. pica suggest that there is a great biological potential for its wild populations restocking and for its aquaculture in sea-based systems.