Planktonic Larvae Research Articles

Searching for sea lice: surveillance to assess environmental infection pressures to model and inform sea lice infestation management

The expansion and development of sustainable salmon aquaculture requires research, innovation and, where necessary, regulation to manage environmental impacts. Sea lice are a key concern for marine salmonid fish aquaculture as these parasites adversely affect farmed and wild salmonids. Reducing the risk of harmful infestation levels requires appropriate data to support modelling and inform adaptive management processes. Throughout their life cycle, sea lice occur as either planktonic larvae or as ectoparasites on farmed, wild or sentinel fish host populations. Limited resources require surveillance methodologies and strategies for these stages to be optimized to obtain data that meet the specific needs of control. In this review we assess the different surveillance strategies available to inform the appropriate management of sea lice impacts on wild salmonids. We advocate modelling as the most effective way to use surveillance data, with subsequent model improvements informed by the continued input of collected data. A feedback loop is proposed of identifying/collecting empirical data to improve models, which in turn will direct more focused surveillance for future data collection and so drive adaptive management processes. In the future, surveillance monitoring, as part of an adaptive management regime in Scotland, should build on existing links between stakeholders and policy makers, and use both models and data to help the sustainable development of the aquaculture industry.

Changes in the Characteristics of Zooplankton Communities in Response to Shifts in the Aquatic Environment in the Shallow Waters of Northern Liaodong Bay, China

The characteristics of zooplankton communities and the relationships with the aquatic environment in the shallow waters of northern Liaodong Bay were investigated. Spot sampling surveys were carried out in April, June, September, and November 2018 to assess zooplankton species composition and diversity, abundance, biomass, and dominant species, and the associated relationships with environmental factors. A total of 45 species of zooplankton were recorded in the survey, comprising 18 Copepoda, 2 Amphipoda, 1 Mysidacea, 1 Decapoda, 1 Chaetognatha, 7 Hydrozoa, 1 Tunicate, and 14 planktonic larvae. Overall, the most dominant species was Aidanosagitta crassa (Tokioka, 1938), with copepods and planktonic larvae also dominating the zooplankton community. However, there was a seasonal alternation of species dominance. A cluster analysis showed that the zooplankton community in spring differed from other seasons and was mostly influenced by suspended particulate matter. Bioenv analysis indicated the main environmental factor affecting the zooplankton community in spring was suspended particulate matter. In summer, the determining variables were temperature, dissolved inorganic nitrogen (DIN), nitrate, and sediment pH. In autumn, temperature, DIN, and nitrate were determining variables, and dissolved oxygen (DO) and DIN in winter. Zooplankton abundance and biomass were influenced by salinity, suspended particulate matter, chemical oxygen demand (COD), chlorophyll, and water and sediment pH. In general, the shallow sea area north of Liaodong Bay is rich in zooplankton species and exhibits significant seasonal variations. Human activities have disturbed the biological community to a certain extent, and the environmental factors in this area are closely related to the diversity of zooplankton species.

Unexpected mismatches in population structure among marine mussel life‐history stages reveal the true scales of planktonic larval dispersal

AbstractStudies investigating gene flow in sessile or sedentary marine species typically draw conclusions about larval dispersal by investigating genetic structure of adults. Here, we generated microsatellite data from adults, recruits, settlers and planktonic larvae of the brown mussel, Perna perna, from the southeast coast of South Africa, and identified a consistent mismatch in genetic structure between the adults and all earlier life stages. While adults could be assigned to two major geographical groups (western and eastern), most of the early‐stage mussels were strongly affiliated with the eastern group. This suggests that few of the early‐stage individuals present in the western portion of the sampling range will eventually establish themselves in the adult population, highlighting the importance of post‐recruitment processes as drivers of population structure. Our findings caution against the exclusive use of genetic data generated from adults to assess population connectivity facilitated by the dispersal of planktonic propagules.

Mechanism of barotaxis in marine zooplankton

Hydrostatic pressure is a dominant environmental cue for vertically migrating marine organisms but the physiological mechanisms of responding to pressure changes remain unclear. Here, we uncovered the cellular and circuit bases of a barokinetic response in the planktonic larva of the marine annelid Platynereis dumerilii. Increased pressure induced a rapid, graded, and adapting upward swimming response due to the faster beating of cilia in the head multiciliary band. By calcium imaging, we found that brain ciliary photoreceptors showed a graded response to pressure changes. The photoreceptors in animals mutant for ciliary opsin-1 had a smaller sensory compartment and mutant larvae showed diminished pressure responses. The ciliary photoreceptors synaptically connect to the head multiciliary band via serotonergic motoneurons. Genetic inhibition of the serotonergic cells blocked pressure-dependent increases in ciliary beating. We conclude that ciliary photoreceptors function as pressure sensors and activate ciliary beating through serotonergic signalling during barokinesis.

Mechanism of barotaxis in marine zooplankton.

Hydrostatic pressure is a dominant environmental cue for vertically migrating marine organisms but the physiological mechanisms of responding to pressure changes remain unclear. Here, we uncovered the cellular and circuit bases of a barokinetic response in the planktonic larva of the marine annelid Platynereis dumerilii. Increased pressure induced a rapid, graded, and adapting upward swimming response due to the faster beating of cilia in the head multiciliary band. By calcium imaging, we found that brain ciliary photoreceptors showed a graded response to pressure changes. The photoreceptors in animals mutant for ciliary opsin-1 had a smaller sensory compartment and mutant larvae showed diminished pressure responses. The ciliary photoreceptors synaptically connect to the head multiciliary band via serotonergic motoneurons. Genetic inhibition of the serotonergic cells blocked pressure-dependent increases in ciliary beating. We conclude that ciliary photoreceptors function as pressure sensors and activate ciliary beating through serotonergic signalling during barokinesis.

Mitochondrial markers revealed genetic panmixia in the data-deficient yellowfin snapper, Lutjanus xanthopinnis (Actinopterygii: Eupercaria: Lutjanidae), from a hotspot of the southern region of the South China Sea

Understanding the genetic structure and diversity of marine fish is crucial for a sustainable management program. We examined the genetic diversity and historical demographics of the yellowfin snapper, Lutjanus xanthopinnis Iwatsuki, Tanaka et Allen, 2015, in the coastal waters of east Peninsular Malaysia which is bordered by the southern part of the South China Sea using the mitochondrial genes (mtDNA) D-loop and Cytochrome b (Cyt-b). A total of 99 (D-loop) and 78 (Cyt-b) specimens of L. xanthopinnis were successfully sequenced from six locations within the range of species distribution along the Malaysian South China Sea. In the presently reported study, the lack of genetic differentiation among populations can be attributed to historical demographic events, eggs and planktonic larvae’ ability to disperse, spawning patterns, and the absence of physical barriers in the geographical landscape. Maximum likelihood gene trees demonstrated that the populations under study had limited structuring and formed a panmictic population that lacks support for internal clades. The AMOVA (Analysis of Molecular Variance) and population pairwise ФST values indicated high genetic exchange between the study areas. A high level of haplotype diversity (D-loop: 0.948–1.000; Cyt-b: 0.542–0.928), low nucleotide diversity (D-loop: 0.0095–0.0159; Cyt-b: 0.0022–0.0049) and starlike haplotype network indicates a recent expansion of L. xanthopinnis populations in Malaysian South China Sea. However, neutrality and goodness of fit tests revealed non-significant values. Furthermore, the BSP (Bayesian skyline plot) analysis estimated population expansion events during the late Pleistocene. During this epoch, the fluctuation in sea level may have led to an increase in the abundance of resources and favorable habitats for the yellowfin snapper. The presently reported findings could initiate efficient management strategies for L. xanthopinnis along the coastal areas of the Malaysian South China Sea and other nearby nations that share the same waterways.

A juvenile journey: Using a highly resolved 3D mooring array to investigate the roles of wind and internal tide forcing in across‐shore larval transport

AbstractThe across‐shore transport of meroplanktonic larvae is predominantly driven by coastal physical processes, resulting in episodic recruitment of benthic species. Historically, due to the sampling challenges associated with resolving these advective mechanisms across the continental shelf, relevant components of larval transport have been difficult to isolate and understand. We use three‐dimensional temperature and velocity data from an array of 29 moorings to identify fundamental physical processes that could have generated successful across‐shore transport and settlement of meroplankton. The dense spatial and temporal sampling from this array allows us to use Lagrangian particle tracking to estimate the influences of wind conditions and the internal tide on the across‐shore transport of planktonic larvae. Settlement was found to be episodic at all depths studied. Above mid‐water, modeled larvae were successfully transported onshore by the internal tide during wind relaxations. Surprisingly, abundant pulses of shallow‐water larvae were supplied to the coast on occasions when strong, upwelling‐favorable winds (> 4 m s−1) drove offshore‐flowing surface waters, revealing a complex, potentially topographically influenced flow. These intense upwelling‐favorable winds also contributed to subsurface onshore flows that created large pulses of larval settlement in deeper waters (> 20 m). Our analyses from this highly resolved data set provide novel insights into the interactions of physical drivers in creating episodic pulses of coastal larval recruitment.

Vertical distribution of Pacific oyster Crassostrea gigas larvae and modeling larval transport in Hiroshima Bay, Japan

Understanding vertical distribution of planktonic larvae is essential for elucidating larval dispersal and recruitment processes. We investigated the vertical distribution and horizontal transport of Pacific oyster Crassostrea gigas larvae by field observations and numerical simulations during their main spawning season in Hiroshima Bay, Japan. In field observations, despite horizontal differences and slight diurnal/semi-diurnal changes depending on larval sizes, most larvae were distributed in the upper 3 m layer. The relationship between C. gigas larvae and environmental conditions revealed that larval density increased with increasing temperature and chlorophyll a concentration, and the density peaked at salinity of approximately 20 for all larval sizes. The observed results suggest that the distribution characteristics of C. gigas larvae are suitable for survival in an estuarine area, where environmental conditions are potentially favorable but hydrodynamic conditions can drastically change over the short term due to variations in river discharge. To examine the effect of high river discharge on larval transport, numerical simulations were conducted using a particle-tracking model incorporating the vertical motion of C. gigas larvae. The simulation results reproduced the spatio-temporal dynamics of planktonic and settled larvae after the high river discharge. Although most particles simulating larvae outflowed from the main spawning area, an area of high particle density at the end of simulation corresponded with the offshore area for seedling collection. The present study suggests the role of vertical distribution of C. gigas larvae for recruitment, and the prospect of sustainability in oyster aquaculture with respect to seedling collection despite the frequent heavy rainfall associated with climate change.

Seasonal and synoptic spatio-temporal variability on larval delivery mechanisms inferred from mussel settlement patterns in the Portuguese coast

Demographic connectivity is the process by which population persistence and growth are affected by the dispersal of individuals. In sessile marine species that disperse by planktonic larvae and form metapopulations, the interaction between oceanographic forcing and larval biology drives the spatial extent and magnitude of larval delivery to suitable habitat patches. The present study investigates the supply of Mytilus galloprovincialis larvae to rocky intertidal habitats located within and around two marine protected areas (MPA) in central Portugal, one located on the exposed coast and the other one in a sheltered bay, using time series of settlement intensity and oceanographic variables. The results indicate that larval delivery at short time scales (days) is strongly dependent on the interaction of upwelling circulation with coastline topography. In the exposed MPA, located on a coastal island, the magnitude of mussel larval delivery is limited by the dynamics of an upwelling front and filament that intercepts the island as upwelling increases and decreases. In the sheltered bay, under an upwelling shadow, the upwelling front promotes recirculation of the water mass, intensifying larval retention and settlement. The results are also consistent with larval transport distances of tens of kilometres and ongoing connectivity between the two MPA. This study highlights the relevance of coastal topography and its interaction with mesoscale hydrodynamics in the modulation of larval delivery, crucial for the persistence of populations and communities and in systematic conservation planning.

A First Look at Internal Waves in the Great Barrier Reef Lagoon

The Great Barrier Reef lagoon is a large, relatively shallow area of the Australian continental shelf, isolated from the open ocean by a dense matrix of coral reefs. As the lagoon is generally vertically well mixed by strong tidal currents and wind, it is perhaps not surprising there is no mention in the open literature of the occurrence of internal waves there. Nevertheless, high-resolution satellite imagery is shown in this article to reveal the characteristic surface expressions of nonlinear internal waves in the lagoon. The waves are confined to periods of low winds in austral spring and summer, making them a potentially important mechanism for the dispersal of algae and planktonic larvae. The imagery suggests a link between the waves and tidally forced submesoscale jets and vortices, but the actual mechanism generating the internal waves is unclear and requires investigation.

Successive selection responses of tetraploid Crassostrea angulata: Comparison of growth, survival, sex ratio and ploidy composition between selected and unselected groups

Triploid Crassostrea angulata has the advantages of fast growth, poor fertility and high quality. However, due to poor fertility, direct genetic improvement is difficult. Mating between tetraploid males and diploid females has been shown to be the best method for commercial triploid production, so genetic improvement of tetraploids will directly affect the phenotypic traits of triploid offspring. However, to date, few studies have investigated the genetic improvement of tetraploid oysters. Based on the induced tetraploid C. angulata population, this study carried out three successive generations of growth selective breeding research, and systematically compared the growth, survival, ploidy composition and sex ratio of the selected group and the control group. The results showed that there was no significant difference in the survival rate from planktonic larvae to adult between the two groups. However, the growth of the selected group was significantly higher than that of the control group, that was, the shell height and whole weight of S3 were significantly higher than that of C3, and the selection advantage of shell height increased from 7.80% of S1 to 34.68% of S3. The ploidy composition of the two tetraploid groups was relatively stable before the 90th days, but on the 270th day, some tetraploids underwent chromosome loss and transformed into diploids or triploids, which might be related to gametogenesis of tetraploids. Moreover, with the increase of generations, the number of individuals with chromosome loss in the selected group gradually decreased, and their ploidy became more stable than that in the control group. In addition, both groups were fertile, with a sex ratio of more males than females, and a proportion of hermaphrodite individuals. In conclusion, this study successfully selected a fast growth line of tetraploid C. angulata, which can be used for commercial production of fast-growing triploid C. angulata, and provides a basis and reference for the genetic improvement of tetraploid oysters.

Projecting marine fish distributions during early life stages under future climate scenarios

AbstractChanges to Earth's climate affect organisms globally; in marine systems, these impacts are seen through warming water temperatures, ocean acidification, hypoxia and frequent marine heatwaves. These effects may lead to the movement of species to more favourable conditions. While climate‐driven movement is well studied at the adult stage, how the early life stages of marine fish will respond to future variability is less clear. Many fish species are constrained by specific spawning locations or phenology. Spawning in certain locations allows for local retention of offspring, while precise timing can facilitate transport of offspring to nursery locations through seasonal circulation patterns. Our research investigates how changing oceans impact the location and timing of spawning of Bering Sea groundfishes over the next century. We used ROMS SST and SSS model output and NOAA survey data in species distribution models to hindcast and project distributions and centre of gravity for eggs and larvae of six groundfish species. Our analyses found that most of our study species exhibit flexible geography. However, the speed and direction of egg and larval movement did not track the speed and direction of their respective thermal niches. Hence, the projected distributional patterns of adult stages may be limited by their early life stages. This response is likely to be mirrored globally by other species with planktonic eggs and larvae. These results indicate that life history considerations are critical for the management of commercially important species, as effects on early life stages are strongly connected to the success or failure of adult populations.

Quantifying marine larval dispersal to assess MPA network connectivity and inform future national and transboundary planning efforts

A marine protected area (MPA) network of multiple reserves can protect biodiversity across space, but to be effective, network configuration should support dispersal among MPAs as well as spillover to unprotected habitats. The ability of MPAs to function as an interacting network of populations connected by dispersal, however, is difficult to estimate at broad spatial scales, and therefore, connectivity is often not integrated in MPA design. Here, we simulate passive larval dispersal using a biophysical model to estimate potential transboundary network connectivity of MPAs in western Canada and United States. Drift time was varied to represent generic functional groups of nearshore species with planktonic larvae and sedentary adult stages. We found that MPAs potentially act as an interacting network and meet connectivity design criteria for nearshore invertebrate species, many MPAs (65%–90%) possibly exchange individuals, and a third of larvae from MPAs (25%–32%) source areas of the unprotected coast. This analysis provides a first approximation of multispecies connectivity to inform ongoing transboundary MPA design, and it can be used as a foundation for future model development.

Testing the applicability of the Modelling-Ongrowing Fish farms-Monitoring B (MOM-B) investigation system for assessing benthic habitat quality in the manila clam Ruditapes philippinarum aquaculture areas

The coastal aquaculture areas have been subject to a variety of anthropogenic pressures in recent studies, and reasonable environmental quality assessment is essential for both ecological conservation and production practices. However, there are significant differences between the results produced from various environmental quality assessment regarding the focus of the evaluation and the fundamental methodology. Furthermore, many of these methods are very specific and difficult to adapt to general applications. Here, we utilized the Modelling-Ongrowing fish farms-Monitoring B investigation system (MOM-B), we assessed the benthic habitat quality of benthic bivalve aquaculture areas in the Xiaoqing River estuary located in Laizhou Bay, China. The aim was to validate the accuracy and practicality of this system. The biological, chemical, and sensory parameters of the MOM-B system, temperature, chlorophyll a (Chl-a), food availability, and planktonic larvae were evaluated throughout the investigation area during the summer of 2021. The MOM-B results indicated that the benthic habitat quality in the survey area was good and lightly disturbed, but the quality in the middle tide area began to deteriorate in August, the hottest month of the summer. Environmental factors indicated that the combined effects of high temperatures and fine sedimentation had led to increased environmental stress in the middle tide area. Food availability and population recruitment also suggested that the benthic habitat quality was better in the high tide and low tide areas than in the middle tide area, and more favorable for the survival of Manila clams. The accuracy, sensitivity, and discriminatory ability of the MOM-B system were demonstrated by environmental and biological indicators. This work showed that the MOM-B system is a practical, simple, and sensitive environmental assessment tool that is easy to implement in estuarine and benthic bivalve aquaculture areas. It can be used for long-term continuous monitoring and as an early warning tool for benthic habitat quality.

DNA metabarcoding as a tool for characterising the spatio-temporal distribution of planktonic larvae in the phylum Echinodermata

AbstractMetabarcoding is revolutionising the analysis of biodiversity in marine ecosystems, especially as it provides a means of detecting and identifying cryptic life stages in field samples. The planktonic larval stage of many species underpins the abundance and distribution of adult populations but is challenging to characterise given the small size of larvae and diffuse distributions in pelagic waters. Yet, planktonic larval dynamics are key to understanding phenomena observed in adult populations, such as the boom-and-bust dynamics exhibited by some echinoderms. Rapid changes in echinoderm population density can have significant effects on local benthic ecosystems. For example, outbreaks of the crown-of-thorns sea star (CoTS) on the Great Barrier Reef (GBR) have led to considerable declines in coral cover. Here, we used a DNA metabarcoding approach to investigate the spatio-temporal distribution and diversity of echinoderm larvae on the GBR, including CoTS. Generalised linear mixed models revealed that echinoderm larval richness, was significantly correlated with temporal variables (i.e. season and year) which is consistent with expected fluctuations in larval output based on adult spawning periodicity. However, neither site-specific differences in echinoderm larval richness, nor correlations between larval composition and environmental, temporal, or spatial variables were found. This study validates the utility of metabarcoding approaches for detecting and characterising echinoderm larvae, including CoTS, which could prove useful to future monitoring efforts. Our findings suggest that metabarcoding can be used to better understand the life history of planktonic larvae, and analyses combining environmental (e.g., temperature, nutrients) and oceanographic (e.g., currents) data could deliver valuable information on the factors influencing their spatio-temporal distributions.

Evaluation of light traps for sampling lobster larvae in the German Bight, North Sea

Biological monitoring of planktonic animals is greatly dependent on the deployment of traps. A variety of specialized traps have been designed for surface plankton and vertebrates. However, certain groups, such as planktonic larvae of benthic marine invertebrates remain underrepresented in sampling efforts. Catching them has proven to be more challenging because of their size, swimming ability, location, and abundance. In the present study a successful light trap for sampling American lobster larvae in New Brunswick, Canada, is evaluated on the island of Helgoland (German Bight, North Sea). Our results showed the traps were successful in catching larvae in laboratory experiments but were unable to catch European lobster larvae in the field. Traps deployed in the field were successful in capturing other benthic and pelagic zooplankton predominantly consisting of crustaceans from the orders: Cumacea, Amphipoda, Mysida and Isopoda. The low density of lobster larvae, the island's topography, and their unique photactic response possibly limited the success rate of the light traps. Future research is needed to construct a specialized trap to sample Helgoland's lobster larvae and provide information on the current larval fitness and population numbers.

Planktonic larvae of the invasive bivalves Dreissena spp. and Limnoperna fortunei: review of their effects on freshwater communities

Planktonic larvae of the invasive bivalves Dreissena spp. and Limnoperna fortunei: review of their effects on freshwater communities

Coastal topography and rates of mussel recruitment: a comparison of bay versus open-coast sites

The recruitment of intertidal benthic organisms that have planktonic larvae is influenced by hydrodynamics at multiple nested spatial scales, ranging from tens of centimetres to kilometres. Intertidal mussel populations on the south coast of South Africa are known to exhibit higher cover within bays than on open-coast shorelines. This study tested the hypothesis that topographically driven differences in hydrodynamics will result in comparatively higher recruitment within bays. Specifically, we estimated recruitment rates of the native brown mussel Perna perna and the invasive Mediterranean mussel Mytilus galloprovincialis (family Mytilidae) over five spring tidal cycles at bay and open-coast sites at both Algoa Bay and St Francis Bay, and the data were then correlated with upwelling indices derived from wind data. We recorded 3 313 P. perna and 2 485 M. galloprovincialis recruits individually and found that mussel recruitment within bays was as much as two orders of magnitude higher than on open coast. The relationship between recruit numbers and upwelling indices was positive, although non-significant, and markedly stronger for the open-coast sites (where r 2 values were 0.51–0.59). These results indicate that better mussel cover within bays can be at least partially explained by higher recruitment rates.

Fjord circulation permits a persistent subsurface water mass in a long, deep mid-latitude inlet

Abstract. Fjords are deep nearshore zones that connect watersheds and oceans, typically behaving as an estuary. In some fjords, strong katabatic winds in winter (also known as Arctic outflow wind events) can lead to cooling and reoxygenation of subsurface waters, with effects lasting until the following autumn, as observed in 2019 in Bute Inlet, British Columbia, Canada. We used high-resolution, three-dimensional ocean model summer simulations to investigate the mechanisms allowing for the persistence of these cool, oxygen-rich subsurface conditions in Bute Inlet. The slow residual circulation underneath the brackish outflow (and consequent slow advection) in this long, deep fjord is a main reason why the cold subsurface water mass stays in place until conditions change in autumn (i.e., start of stronger wind mixing and reduced freshwater forcing). Another mechanism is a positive feedback provided by the presence of this subsurface water mass, since it further reduces the already weak residual circulation. These findings are applicable to any similar long, deep fjord that experiences katabatic winds in winter, and they could have implications not only for the preservation of water masses but other possible subsurface features (e.g., pollutant spills, planktonic larvae). Furthermore, the identification of mechanisms that permit persistent cold and oxygenated conditions is key to understanding potential areas of ecological refugia in a warming and deoxygenating ocean.

Quantifying the effects of fragmentation of connectivity networks of deep‐sea vulnerable marine ecosystems

AbstractAimProtection of vulnerable marine ecosystems (VMEs) in the high seas has focussed on identifying concentrations of indicator species and prohibiting the operation of bottom‐contact fishing gears where those occur in significant concentrations. Most such species have planktonic larvae and depend on dispersal networks for inter‐generational persistence. Yet, connectivity amongst patches of VME has seldom been considered when spatial management measures are introduced. Here, the relative importance of individual patches for the maintenance of their connectivity networks is evaluated, and a prioritization scheme for management action is proposed. Effective conservation measures should maintain approximately natural network configurations whenever possible.LocationGrand Bank and Flemish Cap, Northwest Atlantic Ocean.Methods3‐D Lagrangian particle tracking was used to model larval dispersal connections between known patches of each of seven groups of benthic invertebrate taxa, previously recognized as indicators of VME. Connectivity networks were constructed and the effects of habitat loss simulated by systematic removal of whole patches, to determine the importance of each patch to connectivity within its respective network.ResultsThe various patches differed widely in their contributions to network connectivity. Each taxon group had both some patches that, if removed from the network, would result in a major decline in connectedness but also several which could be lost with negligible consequences for the remainder.Main ConclusionsWhile protecting each patch of VME has conservation value, the wide variation in connectedness shows that some patches are much more critical than others to the long‐term persistence of the taxa, providing a foundation for prioritization of conservation actions.