A review of methods for labeling and tracking marine invertebrate larvae

Determining the significance of pelagic versus juvenile dispersal of larvae in a coastal mussel

Patterns of pelagic dispersal of the early stages of wild mussels are important ecologically for maintaining population connectivity, and economically for supplying wild seed for mussel aquaculture. However, it is difficult to trace the pelagic pathways of mussels due to their minuscule size, high abundance and interactions with the ocean environment. Microchemical methods can be used to infer locations of mussels during their pelagic journey by matching the trace metals sequentially deposited during the formation of the shells of the early stages of mussels to the chemical composition of the seawater in which the shell developed. This method was used to reconstruct the pelagic pathways for recently-settled green-lipped mussels, Perna canaliculus, sampled for two sequential 5-week periods, back to their natal locations for 22 sites covering a ∼110 km long coastal embayment in northern New Zealand. The majority (∼61 %) of the mussel settlers arrived at a specific part (∼25 km) of the coastline, and most of the settlers (∼84 %) were >0.8 mm in shell length, indicating they were mostly juveniles undergoing secondary pelagic dispersal. Sequential microchemical analyses of the shells indicated that these juvenile mussels mainly (∼82 %) originated as larvae from a small length of the coastline (i.e., ∼12 km) with their larvae settling in the vicinity, prior to their subsequent pelagic dispersal as juveniles throughout the embayment. The mean net distance travelled by all sampled mussels was about 3-8 km higher than the mean dispersal distance to reach their primary settlement site (∼20 km) in these two sampling periods, suggesting that secondary pelagic migration by plantigrades extends the overall dispersal range in this species by about 15-41 %. These novel results allowed us to separate primary and secondary pelagic dispersal phases, and highlight the significance of the pelagic dispersal of juveniles subsequent to the initial dispersal of mussel larvae.

Comparative Population Genomics Unveils Congruent Secondary Suture Zone in Southwest Pacific Hydrothermal Vents

How the interplay of biotic and abiotic factors shapes current genetic diversity at the community level remains an open question, particularly in the deep sea. Comparative phylogeography of multiple species can reveal the influence of past climatic events, geographic barriers, and species life history traits on spatial patterns of genetic structure across lineages. To shed light on the factors that shape community-level genetic variation and to improve our understanding of deep-sea biogeographic patterns, we conducted a comparative population genomics study on seven hydrothermal vent species co-distributed in the Back-Arc Basins of the Southwest Pacific region. Using ddRAD-seq, we compared the range-wide distribution of genomic diversity across species and discovered a shared phylogeographic break. Demogenetic inference revealed shared histories of lineage divergence and a secondary contact. Low levels of asymmetric gene flow probably occurred in most species between the Woodlark and North Fiji basins, but the exact location of contact zones varied from species to species. For two species, we found individuals from the two lineages co-occurring in sympatry in Woodlark Basin. Although species exhibit congruent patterns of spatial structure (Eastern vs. Western sites), they also show variation in the degree of divergence among lineages across the suture zone. Our results also show heterogeneous gene flow across the genome, indicating possible partial reproductive isolation between lineages and early speciation. Our comparative study highlights the pivotal role of historical and contemporary factors, underscoring the need for a comprehensive approach-especially in addressing knowledge gaps on the life history traits of deep-sea species.

Coloring coral larvae allows tracking of local dispersal and settlement

Quantifying patterns of dispersal and settlement in marine benthic invertebrates is challenging, largely due the complexity of life history traits, small sizes of larvae (<1 mm), and potential for large-scale dispersal (>100 km) in the marine environment. Here, we develop a novel method that allows for immediate differentiation and visual tracking of large numbers of coral larvae (106 to 109) from dispersal to settlement. Neutral red and Nile blue stains were extremely effective in coloring larvae, with minimal impacts on survival and settlement following optimization of incubation times and stain concentrations. Field validation to wild-captured larvae from the Great Barrier Reef demonstrates the efficacy of staining across diverse taxa. The method provides a simple, rapid (<60 minutes), low-cost (approximately USD$1 per 105 larva) tool to color coral larvae that facilitates a wide range of de novo laboratory and field studies of larval behavior and ecology with potential applications for conservation planning and understanding patterns of connectivity.

Conceptualisation of multiple impacts interacting in the marine environment using marine infrastructure as an example

The human population is increasingly reliant on the marine environment for food, trade, tourism, transport, communication and other vital ecosystem services. These services require extensive marine infrastructure, all of which have direct or indirect ecological impacts on marine environments. The rise in global marine infrastructure has led to light, noise and chemical pollution, as well as facilitation of biological invasions. As a result, marine systems and associated species are under increased pressure from habitat loss and degradation, formation of ecological traps and increased mortality, all of which can lead to reduced resilience and consequently increased invasive species establishment. Whereas the cumulative bearings of collective human impacts on marine populations have previously been demonstrated, the multiple impacts associated with marine infrastructure have not been well explored. Here, building on ecological literature, we explore the impacts that are associated with marine infrastructure, conceptualising the notion of correlative, interactive and cumulative effects of anthropogenic activities on the marine environment. By reviewing the range of mitigation approaches that are currently available, we consider the role that eco-engineering, marine spatial planning and agent-based modelling plays in complementing the design and placement of marine structures to incorporate the existing connectivity pathways, ecological principles and complexity of the environment. Because the effect of human-induced, rapid environmental change is predicted to increase in response to the growth of the human population, this study demonstrates that the development and implementation of legislative framework, innovative technologies and nature-informed solutions are vital, preventative measures to mitigate the multiple impacts associated with marine infrastructure.

Molecular identification and larval morphology of spionid polychaetes (Annelida, Spionidae) from northeastern Japan

Planktonic larvae of spionid polychaetes are among the most common and abundant group in coastal meroplankton worldwide. The present study reports the morphology of spionid larvae collected mainly from coastal waters of northeastern Japan that were identified by the comparison of adult and larval 18S and 16S rRNA gene sequences. The molecular analysis effectively discriminated the species. Adult sequences of 48 species from 14 genera (Aonides Claparède, 1864; Boccardia Carazzi, 1893; Boccardiella Blake & Kudenov, 1978; Dipolydora Verrill, 1881; Laonice Malmgren, 1867; Malacoceros Quatrefages, 1843; Paraprionospio Caullery, 1914; Polydora Bosc, 1802; Prionospio Malmgren, 1867; Pseudopolydora Czerniavsky, 1881; Rhynchospio Hartman, 1936; Scolelepis Blainville, 1828; Spio Fabricius, 1785; Spiophanes Grube, 1860) and larval sequences of 41 species from 14 genera (Aonides; Boccardia; Boccardiella; Dipolydora; Laonice; Paraprionospio; Poecilochaetus Claparède in Ehlers, 1875; Polydora; Prionospio; Pseudopolydora; Rhynchospio; Scolelepis; Spio; Spiophanes) of spionid polychaetes were obtained; sequences of 27 of these species matched between adults and larvae. Morphology of the larvae was generally species-specific, and larvae from the same genus mostly shared morphological features, with some exceptions. Color and number of eyes, overall body shape, and type and arrangement of pigmentation are the most obvious differences between genera or species. The morphological information on spionid larvae provided in this study contributes to species or genus level larval identification of this taxon in the studied area. Identification keys to genera and species of planktonic spionid larvae in northeastern Japan are provided. The preliminary results of the molecular phylogeny of the family Spionidae using 18S and 16S rRNA gene regions are also provided.

Islands in a Sea of Mud: Insights From Terrestrial Island Theory for Community Assembly on Insular Marine Substrata

Most marine hard-bottom habitats are isolated, separated from other similar habitats by sand or mud flats, and can be considered analogous to terrestrial islands. The extensive scientific literature on terrestrial islands provides a theoretical framework for the analysis of isolated marine habitats. More individuals and higher species richness occur on larger marine substrata, a pattern that resembles terrestrial islands. However, while larger terrestrial islands have greater habitat diversity and productivity, the higher species richness on larger marine hard substrata can be explained by simple surface area and hydrodynamic phenomena: larger substrata extend further into the benthic boundary, exposing fauna to faster current and higher food supply. Marine island-like communities are also influenced by their distance to similar habitats, but investigations into the reproductive biology and dispersal ability of individual species are required for a more complete understanding of population connectivity. On terrestrial islands, nonrandom co-occurrence patterns have been attributed to interspecific competition, but while nonrandom co-occurrence patterns have been found for marine fauna, different mechanisms are responsible, including epibiontism. Major knowledge gaps for community assembly in isolated marine habitats include the degree of connectivity between isolated habitats, mechanisms of succession, and the extent of competition on hard substrata, particularly in the deep sea. Anthropogenic hard substrata of known age can be used opportunistically as "natural" laboratories to begin answering these questions.

Culturing larvae of marine invertebrates

Larvae of marine invertebrates cultured in the laboratory experience conditions that they do not encounter in nature, but development and survival to metamorphic competence can be obtained in such cultures. This protocol emphasizes simple methods suitable for a wide variety of larvae. Culturing larvae requires seawater of adequate quality and temperature within the tolerated range. Beyond that, feeding larvae require appropriate food, but a few kinds of algae and animals are sufficient as food for diverse larvae. Nontoxic materials include glass, many plastics, hot-melt glue, and some solvents, once evaporated. Cleaners that do not leave toxic residues after rinsing include dilute hydrochloric or acetic acid, sodium hypochlorite (commercial bleach), and ethanol. Materials that can leave toxic residues, such as formaldehyde, glutaraldehyde, detergents, and hand lotions, should be avoided, especially with batch cultures that lack continuously renewed water. Reverse filtration can be used to change water gently at varying frequencies, depending on temperature and the kinds of food that are provided. Bacterial growth can be limited by antibiotics, but antibiotics are often unnecessary. Survival and growth are increased by low concentrations of larvae and stirring of large or dense cultures. One method of stirring large numbers of containers is a rack of motor-driven paddles. Most of the methods and materials are inexpensive and portable. If necessary, a room within a few hours of the sea could be temporarily equipped for larval culture.

Connectivity and resilience of coral reef metapopulations in marine protected areas: matching empirical efforts to predictive needs

Design and decision-making for marine protected areas (MPAs) on coral reefs require prediction of MPA effects with population models. Modeling of MPAs has shown how the persistence of metapopulations in systems of MPAs depends on the size and spacing of MPAs, and levels of fishing outside the MPAs. However, the pattern of demographic connectivity produced by larval dispersal is a key uncertainty in those modeling studies. The information required to assess population persistence is a dispersal matrix containing the fraction of larvae traveling to each location from each location, not just the current number of larvae exchanged among locations. Recent metapopulation modeling research with hypothetical dispersal matrices has shown how the spatial scale of dispersal, degree of advection versus diffusion, total larval output, and temporal and spatial variability in dispersal influence population persistence. Recent empirical studies using population genetics, parentage analysis, and geochemical and artificial marks in calcified structures have improved the understanding of dispersal. However, many such studies report current self-recruitment (locally produced settlement/settlement from elsewhere), which is not as directly useful as local retention (locally produced settlement/total locally released), which is a component of the dispersal matrix. Modeling of biophysical circulation with larval particle tracking can provide the required elements of dispersal matrices and assess their sensitivity to flows and larval behavior, but it requires more assumptions than direct empirical methods. To make rapid progress in understanding the scales and patterns of connectivity, greater communication between empiricists and population modelers will be needed. Empiricists need to focus more on identifying the characteristics of the dispersal matrix, while population modelers need to track and assimilate evolving empirical results.

Quantifying the "bio-" components in biophysical models of larval transport in marine benthic invertebrates: advances and pitfalls

Biophysical models are being used increasingly, both as predictive tools of larval dispersal for a particular system and for general evaluation of the role of different factors in larval transport. In the results of such models, larval duration, mortality, and behavior in the water column have exhibited pronounced effects on larval dispersal of marine benthic invertebrates. The parameterization of these processes has broadly reflected values from laboratory experiments, but the accuracy of these values is unknown. The pelagic larval duration used in models should be determined by laboratory, or preferably field, studies and should incorporate environmentally dependent variability. For mortality, in situ estimates are now feasible and, likely, more accurate than the currently used values. Larval behavior can be measured in the field, by high-frequency sampling of distributional changes relative to features in the water column or by controlled larval releases in tractable systems. To successfully validate the outcomes of these models, we must either improve our techniques for measuring larval abundance at the end of larval transport immediately before settlement, or incorporate components for settlement into the models. We must also address the mismatch in sampling resolution between biological and physical processes. If used with caution, this powerful approach can significantly advance our understanding of larval transport.

Complex larval connectivity patterns among marine invertebrate populations

Based on the belief that marine larvae, which can spend days to months in the planktonic stage, could be transported considerable distances by ocean currents, it has long been assumed that populations of coastal species with a planktonic larval stage are demographically open and highly "connected." Such assumptions about the connectivity of coastal populations govern approaches to managing marine resources and shape our fundamental understanding of population dynamics and evolution, yet are rarely tested directly due to the small size and high mortality of marine larvae in a physically complex environment. Here, we document a successful application of elemental fingerprinting as a tracking tool to determine sources of settled invertebrates and show that coastal mussel larvae, previously thought to be highly dispersed, can be retained within 20-30 km of their natal origin. We compare two closely related and co-occurring species, Mytilus californianus and Mytilus galloprovincialis, and determine that, despite expected similarities, they exhibit substantially different connectivity patterns. Our use of an in situ larval culturing technique overcomes the previous challenge of applying microchemical tracking methods to species with completely planktonic development. The exchange of larvae and resulting connectivities among marine populations have fundamental consequences for the evolution and ecology of species and for the management of coastal resources.

Interactions between behaviour and physical forcing in the control of horizontal transport of decapod crustacean larvae

We summarize what is known of the biophysical interactions that control vertical migration and dispersal of decapod larvae, asking the following main questions: How common is vertical migration in decapod crustacean larvae? What is the vertical extent of the migrations? What are the behavioural mechanisms that control vertical migrations? How does vertical migration interact with the physics of the ocean to control the dispersal of larvae? These questions are analysed by first giving a synopsis of the physical processes that are believed to significantly affect horizontal transport, and then by describing migration patterns according to taxon, to ecological category based on the habitat of adults and larvae, and to stage within the larval series. Some kind of vertical migration has been found in larval stages of virtually all species that have been investigated, irrespective of taxonomic or ecological category. Most vertical migration schedules have a cyclic nature that is related to a major environmental cyclic factor. Tidal (ebb or flood) migration and daily (nocturnal and twilight) migration are the two types of cyclic migration that have been identified. In general, all species show some type of daily migration, with nocturnal migration being the most common, whereas tidal migrations have only been identified in species that use estuaries during part of their life cycle. Moreover, there are several examples indicating that the phasing and extent of migration both change throughout ontogeny. Reported ranges of vertical displacement vary between a few metres in estuaries and several tens of metres (sometimes more than 100 m) in shelf and oceanic waters. Vertical movements are controlled by behavioural responses to the main factors of the marine environment. The most important factors in this respect are light, pressure and gravity, but salinity, temperature, turbulence, current and other factors, also influence behaviour. Many of these factors change cyclically, and the larvae respond with cyclic behaviours. The type of response may be endogenous and regulated by an internal clock, as in the case of some tidally synchronised migrations, but in most cases it is a direct response to a change in an environmental variable, as in diel migration. The reaction of the larvae to exogenous cues depends both on the rate of change of the variable and on the absolute amount of change. A series of dispersal types, involving different spatial and temporal scales, have been identified in decapod larvae: retention of the larval series within estuaries; export from estuarine habitats, dispersal over the shelf, and reinvasion of estuaries by the last stage; hatching in shelf waters and immigration to estuaries by late larvae or postlarvae; complete development on the shelf; and hatching in shelf waters, long-range dispersal in the ocean, and return to the shelf by late stages. In all of these cases, vertical migration behaviour and changes of behaviour during the course of larval development have been related to particular physical processes, resulting in conceptual mechanisms that explain dispersal and recruitment. Most decapod larvae are capable of crossing the vertical temperature differences normally found across thermoclines in natural systems. This ability may have significant consequences for horizontal transport within shelf waters, because amplitude and phase differences of the tidal currents across the thermocline may be reflected in different trajectories of the migrating larvae.

Disjunct distribution of highly diverged mitochondrial lineage clade and population subdivision in a marine bivalve with pelagic larval dispersal

Mitochondrial DNA sequence data for 295 individuals of the marine bivalve Macoma balthica (L.) were collected from 10 sites across the European distribution, and from Alaska. The data were used to infer population subdivision history and estimate current levels of gene flow. Inferred historical biogeography was expected to be congruent with colonization of the Atlantic Ocean from the Pacific Ocean after the opening of the Bering Strait 3.5 Ma. In addition, the last glacial maximum, about 18000 years ago, was expected to have been responsible for most of the present-day distribution of molecular variation within Europe, because the area must have been recolonized after confinement to France and the south of the British Isles during the last glacial maximum. Current gene flow was hypothesized to be high, because the larvae of M. balthica spend 2-5 weeks drifting in the water column. The geographical distribution of one highly diverged haplotype clade was found to be disjunct and was encountered exclusively in samples from the Baltic Sea and Alaska. A molecular clock calibration for marine bivalve cytochrome-c-oxidase I dates this clade as having split off from the other haplotypes 9.8-39 Ma. Multiple colonizations of the Atlantic Ocean from the Pacific by M. balthica may explain the strong differences found between Baltic Sea and other European populations of this species. The sympatric occurrence of the highly diverged mitochondrial lineages in western parts of the Baltic Sea points to secondary admixture. With the use of coalescent analysis, population divergence times for French vs. other non-Baltic European populations ('Atlantic population assemblage') were estimated at a minimum of about 110000 years ago, well before the last glacial maximum 18000 years ago. Signatures of population divergence of M. balthica that appear to have originated during the Pleistocene have thus survived the last glacial maximum. Some of the populations within the Atlantic assemblage are currently isolated, while others appear to be connected by gene flow. Apparently, populations of this species can remain highly subdivided in spite of the potential for high gene flow, implying that their population and evolutionary dynamics can be independent.

Patterns of larval dispersal and their effect on the maintenance of a blue mussel hybrid zone in southwestern England

The blue mussels Mytilus edulis and M. galloprovincialis hybridize in southwestern England. Within this hybrid zone environmentally based directional selection favors individuals with alleles specific to M. galloprovincialis. What forces are countering this directional selection and allowing for the maintenance of a stable hybrid population are unknown. We used both the genetics of recently settled larvae and a fine-scale model of the physical oceanography of the region to determine the patterns of larval dispersal throughout the hybrid zone and the bordering parental populations. Evidence from both the model and the genetics suggests that the hybrid zone lies between two barriers to dispersal. Start Point separates the M. edulis population from the hybrid zone and allows minimal dispersal from the hybrid zone into the M. edulis population, but none in the other direction. Likewise, the M. galloprovincialis populations along the northern coast of Cornwall regularly receive immigrating larvae from the hybrid zone, but larvae from the M. galloprovincialis population do not enter the hybrid zone. However, larvae settling at hybrid zone sites have high frequencies of alleles specific to M. edulis, suggesting that reproductive barriers, selection in the larval stage, or gene flow from an undetermined source is effectively balancing the directional selection observed in the adults.

Movements of marine fish and decapod crustaceans: process, theory and application

Many marine species have a multi-phase ontogeny, with each phase usually associated with a spatially and temporally discrete set of movements. For many fish and decapod crustaceans that live inshore, a tri-phasic life cycle is widespread, involving: (1) the movement of planktonic eggs and larvae to nursery areas; (2) a range of routine shelter and foraging movements that maintain a home range; and (3) spawning migrations away from the home range to close the life cycle. Additional complexity is found in migrations that are not for the purpose of spawning and movements that result in a relocation of the home range of an individual that cannot be defined as an ontogenetic shift. Tracking and tagging studies confirm that life cycle movements occur across a wide range of spatial and temporal scales. This dynamic multi-scale complexity presents a significant problem in selecting appropriate scales for studying highly mobile marine animals. We address this problem by first comprehensively reviewing the movement patterns of fish and decapod crustaceans that use inshore areas and present a synthesis of life cycle strategies, together with five categories of movement. We then examine the scale-related limitations of traditional approaches to studies of animal-environment relationships. We demonstrate that studies of marine animals have rarely been undertaken at scales appropriate to the way animals use their environment and argue that future studies must incorporate animal movement into the design of sampling strategies. A major limitation of many studies is that they have focused on: (1) a single scale for animals that respond to their environment at multiple scales or (2) a single habitat type for animals that use multiple habitat types. We develop a hierarchical conceptual framework that deals with the problem of scale and environmental heterogeneity and we offer a new definition of 'habitat' from an organism-based perspective. To demonstrate that the conceptual framework can be applied, we explore the range of tools that are currently available for both measuring animal movement patterns and for mapping and quantifying marine environments at multiple scales. The application of a hierarchical approach, together with the coordinated integration of spatial technologies offers an unprecedented opportunity for researchers to tackle a range of animal-environment questions for highly mobile marine animals. Without scale-explicit information on animal movements many marine conservation and resource management strategies are less likely to achieve their primary objectives.

Sharp genetic breaks among populations of Haptosquilla pulchella (Stomatopoda) indicate limits to larval transport: patterns, causes, and consequences

To help stem the precipitous decline of coral reef ecosystems world-wide, conservation efforts are focused on establishing interconnected reserve networks to protect threatened populations. Because many coral reef organisms have a planktonic or pelagic larval dispersal phase, it is critical to understand the patterns of ecological connectivity between reserve populations that result from larval dispersal. We used genetics to infer dispersal patterns among 24 Indo-West Pacific populations of the mantis shrimp, Haptosquilla pulchella. Contrary to predictions of high dispersal facilitated by the strong currents of the Indonesian throughflow, mitochondrial DNA sequences from 393 individuals displayed striking patterns of regional genetic differentiation concordant with ocean basins isolated during periods of lowered sea level. Patterns of genetic structuring indicate that although dispersal within geographical regions with semicontiguous coastlines spanning thousands of kilometres may be common, ecologically meaningful connections can be rare among populations separated by as little as 300 km of open ocean. Strong genetic mosaics in a species with high dispersal potential highlight the utility of genetics for identifying regional patterns of genetic connectivity between marine populations and show that the assumption that ocean currents will provide ecological connectivity among marine populations must be empirically tested in the design of marine reserve networks.