Larval Dispersal Distances Research Articles

Successful Determination of Larval Dispersal Distances and Subsequent Settlement for Long-Lived Pelagic Larvae

Despite its importance, we still have a poor understanding of the level of connectivity between marine populations in most geographical locations. Taking advantage of the natural features of the southeast coast of New Zealand's North Island, we deployed a series of settlement stations and conducted plankton tows to capture recent settlers and planktonic larvae of the common intertidal gastropod Austrolittorina cincta (6–8 week larval period). Satellite image analysis and ground truthing surveys revealed the absence of suitable intertidal rocky shore habitat for A. cincta over a 100 km stretch of coastline between Kapiti Island to the south and Wanganui to the north. Fifteen settlement stations (3 replicates×5 sites), which were used to mimic intertidal habitat suitable for A. cincta, were deployed for two months around and north of Kapiti Island (at 0.5, 1, 5, 15, 50 km). In addition, we also conducted plankton tows at each settlement station when the stations were first deployed to collect A. cincta larvae in the water column. On collection, all newly settled gastropods and larvae in the plankton samples were individually isolated, and a species-specific microsatellite marker was used to positively identify A. cincta individuals. Most of the positively identified A. cincta settlers and larvae were collected at the first three sampling stations (<5 km). However, low numbers of A. cincta settlers and larvae were also recorded at the two more distant locations (15 and 50 km). Dispersal curves modeled from our data suggested that <1% of gastropod larvae would travel more than 100 km. While our data show that most larvae are retained close to their natal populations (<5 km), a small proportion of larvae are able to travel much larger geographic distances. Our estimates of larval dispersal and subsequent settlement are one of only a few for marine species with a long-lived larva.

Larval connectivity in an effective network of marine protected areas.

Acceptance of marine protected areas (MPAs) as fishery and conservation tools has been hampered by lack of direct evidence that MPAs successfully seed unprotected areas with larvae of targeted species. For the first time, we present direct evidence of large-scale population connectivity within an existing and effective network of MPAs. A new parentage analysis identified four parent-offspring pairs from a large, exploited population of the coral-reef fish Zebrasoma flavescens in Hawai'i, revealing larval dispersal distances ranging from 15 to 184 km. In two cases, successful dispersal was from an MPA to unprotected sites. Given high adult abundances, the documentation of any parent-offspring pairs demonstrates that ecologically-relevant larval connectivity between reefs is substantial. All offspring settled at sites to the north of where they were spawned. Satellite altimetry and oceanographic models from relevant time periods indicated a cyclonic eddy that created prevailing northward currents between sites where parents and offspring were found. These findings empirically demonstrate the effectiveness of MPAs as useful conservation and management tools and further highlight the importance of coupling oceanographic, genetic, and ecological data to predict, validate and quantify larval connectivity among marine populations.

The utility and limitations of size and spacing guidelines for designing marine protected area (MPA) networks

Marine protected areas (MPAs) implemented to conserve biodiversity must protect many species with a broad range of movement characteristics. To meet that goal, size and spacing guidelines have been used in MPA network design as a proxy for explicitly representing connectivity and species movement. However, there has been no assessment of the biological effects of these simple rules. We evaluated these guidelines by estimating population persistence with a spatially explicit population model over: (1) an idealized coastline and (2) an example from California, USA. Persistence of a species within an MPA network depends strongly on its movement characteristics; therefore we used the number of combinations of larval dispersal distances and adult home range diameters as an index representing the number of species that could be protected by a MPA network. The index of species protected usually increased steadily with increasing MPA size. By contrast, decreasing MPA spacing only produced large increases in the index when spacing became close enough to allow species persistence via network connectivity rather than self replenishment. Species persistence also depended on the exploitation rate outside MPAs. Size and spacing guidelines are a simple and useful way to begin the MPA network design process, but meeting these guidelines does not guarantee persistence for all populations one may wish to protect. The use of spatially explicit population dynamics models that evaluate population persistence directly provides a more comprehensive basis for comparing proposed MPA network designs.

Detecting larval export from marine reserves.

Marine reserve theory suggests that where large, productive populations are protected within no-take marine reserves, fished areas outside reserves will benefit through the spillover of larvae produced in the reserves. However, empirical evidence for larval export has been sparse. Here we use a simple idealized coastline model to estimate the expected magnitude and spatial scale of larval export from no-take marine reserves across a range of reserve sizes and larval dispersal scales. Results suggest that, given the magnitude of increased production typically found in marine reserves, benefits from larval export are nearly always large enough to offset increased mortality outside marine reserves due to displaced fishing effort. However, the proportional increase in recruitment at sites outside reserves is typically small, particularly for species with long-distance (on the order of hundreds of kilometers) larval dispersal distances, making it very difficult to detect in field studies. Enhanced recruitment due to export may be detected by sampling several sites at an appropriate range of distances from reserves or at sites downcurrent of reserves in systems with directional dispersal. A review of existing empirical evidence confirms the model's suggestion that detecting export may be difficult without an exceptionally large differential in production, short-distance larval dispersal relative to reserve size, directional dispersal, or a sampling scheme that encompasses a broad range of distances from the reserves.

Marine reserve networks for species that move within a home range

Marine reserves are expected to benefit a wide range of species, but most models used to evaluate their effects assume that adults are sedentary, thereby potentially overestimating population persistence. Many nearshore marine organisms move within a home range as adults, and there is a need to understand the effects of this type of movement on reserve performance. We incorporated movement within a home range into a spatially explicit marine reserve model in order to assess the combined effects of adult and larval movement on persistence and yield in a general, strategic framework. We describe how the capacity of a population to persist decreased with increasing home range size in a manner that depended on whether the sedentary case was maintained by self persistence or network persistence. Self persistence declined gradually with increasing home range and larval dispersal distance, while network persistence decreased sharply to 0 above a threshold home range and was less dependent on larval dispersal distance. The maximum home range size protected by a reserve network increased with the fraction of coastline in reserves and decreasing exploitation rates outside reserves. Spillover due to movement within a home range contributed to yield moderately under certain conditions, although yield contributions were generally not as large as those from spillover due to larval dispersal. Our results indicate that, for species exhibiting home range behavior, persistence in a network of marine reserves may be more predictable than previously anticipated from models based solely on larval dispersal, in part due to better knowledge of home range sizes. Including movement within a home range can change persistence results significantly from those assuming that adults are sedentary; hence it is an important consideration in reserve design.

Population genetic structure in the redefined vermilion rockfish (Sebastes miniatus) indicates limited larval dispersal and reveals natural management units

Recent studies have revealed that the vermilion rockfish Sebastes miniatus is a cryptic species pair. The splitting of this species impacts stock size estimates and draws attention to the unintended consequences of current management policies. Differences in exploitation level between the species necessitated an evaluation of population structure and connectivity among regional management segments of the fishery. Analysis of gene flow and calculations of larval dispersal values were accomplished using 782 bp of DNA sequence data from the mitochondrial cytochrome b gene of 684 vermilion rockfish sampled from 16 sites between Kyuquot Sound, Canada, and San Quintin, Mexico. Significant genetic heterogeneity was found among sample sites (ΦST= 0.0742, p &lt; 0.001 and FST= 0.0899, p &lt; 0.001). Isolation by distance analysis produced a significant correlation, suggesting low average larval dispersal distance. Analysis of molecular variance showed significant partitioning of genetic variance across the biogeographic boundary at Point Conception (ΦCT= 0.0923, p &lt; 0.001 and FCT= 0.0135, p &lt; 0.001) with additional genetic barriers found at Cape Mendocino, Punta Colnett, Santa Monica Bay, and along the coast of Washington. These genetic barriers conform to oceanographic compartments previously proposed for the California Current Ecological Province and suggest natural management units for this species.

Pelagic Larval Duration and Dispersal Distance Revisited

I present dispersal distances for 44 species with data on propagule duration (PD) for 40 of these. Data were combined with those in Shanks et al. (2003; Ecol. Appl. 13: S159-S169), providing information on 67 species. PD and dispersal distance are correlated, but with many exceptions. The distribution of dispersal distances was bimodal. Many species with PDs longer than 1 day dispersed less than 1 km, while others dispersed tens to hundreds of kilometers. Organisms with short dispersal distances were pelagic briefly or remained close to the bottom while pelagic. Null models of passively dispersing propagules adequately predict dispersal distance for organisms with short PDs (<1 day), but overestimate dispersal distances for those with longer PDs. These models predict that propagules are transported tens of kilometers offshore; however, many types remain within the coastal boundary layer where currents are slower and more variable, leading to lower than predicted dispersal. At short PDs, dispersal distances estimated from genetic data are similar to observed. At long PDs, genetic data generally overestimate dispersal distance. This discrepancy is probably due to the effect of rare individuals that disperse long distances, thus smoothing genetic differences between populations. Larval behavior and species' life-history traits can play a critical role in determining dispersal distance.

Modeling no-take marine reserves in regulated fisheries: assessing the role of larval dispersal

We explored the effects of larval dispersal distance on the impact of no-take marine reserves (NTMRs) implemented in fisheries with catch regulations. NTMRs exist in many fisheries with harvest regulated by annual catch limits. In these fisheries, catch is taken from outside NTMRs, potentially resulting in reduced abundance outside NTMRs and an overall reduction in catch. We used a spatial model with two life stages (larvae and adults) to evaluate the effects of larval dispersal distance for fisheries managed by a total allowable catch (TAC) and an NTMR. We examined effects of the timing of density-dependent mortality in relation to larval movement. Abundance reached similar values for populations with long and short larval dispersal distances. Catch declined substantially for stocks with short larval dispersal distances. When larval dispersal distances were long, catch declined to values below maximum sustainable yield (MSY), but stabilized. Catch per unit effort (CPUE) declined to 9% of CPUE at MSY for stocks with short distance larval dispersal after the implementation of an NTMR; with long distance larval dispersal, CPUE declined to approximately 50% or less of the CPUE at MSY. The CPUE did not reflect trends in abundance after the implementation of an NTMR.

Connectivity, sustainability, and yield: bridging the gap between conventional fisheries management and marine protected areas

A substantial shift toward use of marine protected areas (MPAs) for conservation and fisheries management is currently underway. This shift to explicit spatial management presents new challenges and uncertainties for ecologists and resource managers. In particular, the potential for MPAs to change population sustainability, fishery yield, and ecosystem properties depends on the poorly understood consequences of three critical forms of connectivity over space: larval dispersal, juvenile and adult swimming, and movement of fishermen. Conventional fishery management describes the dynamics and current status of fish populations, with increasing recent emphasis on sustainability, often through reference points that reflect individual replacement. These compare lifetime egg production (LEP) to a critical replacement threshold (CRT) whose value is uncertain. Sustainability of spatially distributed populations also depends on individual replacement, but through all possible paths created by larval dispersal and LEP at each location. Model calculations of spatial replacement considering larval connectivity alone indicate sustainability and yield depend on species dispersal distance and the distribution of LEP created by species habitat distribution and fishing mortality. Adding MPAs creates areas with high LEP, increasing sustainability, but not necessarily yield. Generally, short distance dispersers will persist in almost all MPAs, while sustainability of long distance dispersers requires a specific density of MPAs along the coast. The value of that density also depends on the uncertain CRT, as well as fishing rate. MPAs can increase yield in areas with previously low LEP but for short distance dispersers, high yields will require many small MPAs. The paucity of information on larval dispersal distances, especially in cases with strong advection, renders these projections uncertain. Adding juvenile and adult movement to these calculations reduces LEP near the edges in MPAs, if movement is within a home-range, but more broadly over space if movement is diffusive. Adding movement of fishermen shifts effort on the basis of anticipated revenues and fishing costs, leading to lower LEP near ports, for example. Our evolving understanding of connectivity in spatial management could form the basis for a new, spatially oriented replacement reference point for sustainability, with associated new uncertainties.

Influence of habitat discontinuity, geographical distance, and oceanography on fine‐scale population genetic structure of copper rockfish (Sebastes caurinus)

The copper rockfish is a benthic, nonmigratory, temperate rocky reef marine species with pelagic larvae and juveniles. A previous range-wide study of the population-genetic structure of copper rockfish revealed a pattern consistent with isolation-by-distance. This could arise from an intrinsically limited dispersal capability in the species or from regularly-spaced extrinsic barriers that restrict gene flow (offshore jets that advect larvae offshore and/or habitat patchiness). Tissue samples were collected along the West Coast of the contiguous USA between Neah Bay, WA and San Diego, CA, with dense sampling along Oregon. At the whole-coast scale (approximately 2200 km), significant population subdivision (F(ST) = 0.0042), and a significant correlation between genetic and geographical distance were observed based on 11 microsatellite DNA loci. Population divergence was also significant among Oregon collections (approximately 450 km, F(ST) = 0.001). Hierarchical amova identified a weak but significant 130-km habitat break as a possible barrier to gene flow within Oregon, across which we estimated that dispersal (N(e)m) is half that of the coast-wide average. However, individual-based Bayesian analyses failed to identify more than a single population along the Oregon coast. In addition, no correlation between pairwise population genetic and geographical distances was detected at this scale. The offshore jet at Cape Blanco was not a significant barrier to gene flow in this species. These findings are consistent with low larval dispersal distances calculated in previous studies on this species, support a mesoscale dispersal model, and highlight the importance of continuity of habitat and adult population size in maintaining gene flow.

Genetic Isolation by Distance among Populations of the Netted Dog Whelk Nassarius reticulatus (L.) along the European Atlantic Coastline

Estimates of the average distances by which marine larvae disperse are generally poorly described, despite the central role that larval dispersal plays in the demographic connectivity of populations across geographic space. Here, we describe the population genetic structure and average dispersal distance of the netted dog whelk Nassarius reticulatus (L.) (Mollusca, Gastropoda, Prosobranchia), a widespread member of European intertidal communities, using DNA sequence variation in a fragment of the mitochondrial gene cytochrome c oxidase subunit I (COI). An analysis of 156 individuals from 6 locations spread across approximately 1700 km of the European Atlantic coastline revealed weak and nonsignificant population structure (overall Phi(ST) = 0.00013). However, pairwise Phi(ST) values revealed a slight but significant increase in genetic isolation with geographic distance (IBD), suggesting that populations are not panmictic across the sampled geographic range. If we assume that the isolation by distance is maintained by a stable, stepping stone model of gene flow, then the slope of the IBD is consistent with an average larval dispersal distance of approximately 70 km per generation. The spatial scale of larval dispersal in N. reticulatus is consistent with the life cycle of the species (planktotrophic veliger lasting 30-60 days before competent to settle). A mismatch analysis of the COI sequences revealed a signature of an ancient demographic expansion that began 61 500-160,000 years ago, well before the most recent Pleistocene glaciation event. The greatest levels of genetic diversity occur within the middle latitudes of the whelk's geographic range, consistent with the notion that historic populations of N. reticulatus might have expanded northward and southward from the centrally located Bay of Biscay.

Genetic population structure of the endemic fourline wrasse (Larabicus quadrilineatus) suggests limited larval dispersal distances in the Red Sea

The connectivity among marine populations is determined by the dispersal capabilities of adults as well as their eggs and larvae. Dispersal distances and directions have a profound effect on gene flow and genetic differentiation within species. Genetic homogeneity over large areas is a common feature of coral reef fishes and can reflect high dispersal capability resulting in high levels of gene flow. If fish larvae return to their parental reef, gene flow would be restricted and genetic differentiation could occur. Larabicus quadrilineatus (Labridae) is considered as an endemic fish species of the Red Sea and Gulf of Aden. The juveniles of this species are cleaner fish that feed on ectoparasites of other fishes. Here, we investigated the genetic population structure and gene flow in L. quadrilineatus among five locations in the Red Sea to infer connectivity among them. To estimate genetic diversity, we analysed 369 bp of 237 mitochondrial DNA control region sequences. Haplotype and nucleotide diversities were higher in the southern than in the northern Red Sea. Analysis of molecular variance (amova) detected the highest significant genetic variation between northern and central/southern populations (Phi(CT) = 0.01; P < 0.001). Migration analysis revealed a several fold higher northward than southward migration, which could be explained by oceanographic conditions and spawning season. Even though the Phi(ST) value of 0.01 is rather low and implies a long larval dispersal distance, estimates based on the isolation-by-distance model show a very low mean larval dispersal distance (0.44-5.1 km) compared to other studies. In order to enable a sustainable ornamental fishery on the fourline wrasse, the results of this study suggest that populations in the northern and southern Red Sea should be managed separately as two different stocks. The rather low larval dispersal distance of about 5 km needs to be considered in the design of marine protected areas to enable connectivity and self-seeding.

Temperature control of larval dispersal and the implications for marine ecology, evolution, and conservation.

Temperature controls the rate of fundamental biochemical processes and thereby regulates organismal attributes including development rate and survival. The increase in metabolic rate with temperature explains substantial among-species variation in life-history traits, population dynamics, and ecosystem processes. Temperature can also cause variability in metabolic rate within species. Here, we compare the effect of temperature on a key component of marine life cycles among a geographically and taxonomically diverse group of marine fish and invertebrates. Although innumerable lab studies document the negative effect of temperature on larval development time, little is known about the generality versus taxon-dependence of this relationship. We present a unified, parameterized model for the temperature dependence of larval development in marine animals. Because the duration of the larval period is known to influence larval dispersal distance and survival, changes in ocean temperature could have a direct and predictable influence on population connectivity, community structure, and regional-to-global scale patterns of biodiversity.

Scaling of Connectivity in Marine Populations

Defining the scale of connectivity, or exchange, among marine populations and determining the factors driving this exchange are pivotal to our understanding of the population dynamics, genetic structure, and biogeography of many coastal species. Using a high-resolution biophysical model for the Caribbean region, we report that typical larval dispersal distances of ecologically relevant magnitudes are on the scale of only 10 to 100 kilometers for a variety of reef fish species. We also show the importance of the early onset of active larval movement mediating the dispersal potential. In addition to self-recruitment, larval import from outside the local area is required to sustain most populations, although these population subsidies are very limited in particular systems. The results reveal distinct regions of population isolation based on larval dispersal that also correspond to genetic and morphological clines observed across a range of marine organisms.

Accounting for uncertainty in marine reserve design

Ecosystems and the species and communities within them are highly complex systems that defy predictions with any degree of certainty. Managing and conserving these systems in the face of uncertainty remains a daunting challenge, particularly with respect to developing networks of marine reserves. Here we review several modelling frameworks that explicitly acknowledge and incorporate uncertainty, and then use these methods to evaluate reserve spacing rules given increasing levels of uncertainty about larval dispersal distances. Our approach finds similar spacing rules as have been proposed elsewhere - roughly 20-200 km - but highlights several advantages provided by uncertainty modelling over more traditional approaches to developing these estimates. In particular, we argue that uncertainty modelling can allow for (1) an evaluation of the risk associated with any decision based on the assumed uncertainty; (2) a method for quantifying the costs and benefits of reducing uncertainty; and (3) a useful tool for communicating to stakeholders the challenges in managing highly uncertain systems. We also argue that incorporating rather than avoiding uncertainty will increase the chances of successfully achieving conservation and management goals.

Distribution, Adult Morphology, and Larval Development of Sacculina Sinensis (Cirripedia: Rhizocephala: Kentrogonida) in Hong Kong Coastal Waters

Abstract Sacculina sinensis (Cirripedia: Rhizocephala) is a parasitic barnacle first reported in Hong Kong by Boschma (1933) to infect the intertidal crab, Leptodius exaratus. Since then, no studies have been conducted to investigate its distribution and morphology, which are important in providing background information for further taxonomic and ecological studies. In the present study, S. sinensis has shown to have a limited distribution in Hong Kong, being confined to only four out of the nine sheltered boulder shores sampled and with a mean infestation rate of 11.3 percent. The localized distribution of S. sinensis in Hong Kong is probably due to its short larval development times (four naupliar stages and one cypris stage, completed in about six days) and hence limited larval dispersal distance. The externa of S. sinensis is ovoid in shape with two distinct shoulders, and the outer cuticle surface is spinous. From histological studies, the male receptacles are located posteriorly in the visceral mass...

Evidence for limited larval dispersal in black rockfish (Sebastes melanops): implications for population structure and marine-reserve design

Although dispersal distances of marine larvae influence gene flow and the establishment of population structure, few data on realized dispersal distances exist for marine species. We combined otolith microstructure and micro chemistry of black rockfish (Sebastes melanops) to assess their potential to provide relative estimates of larval dispersal distance. In 2001 and 2002 we measured trace elements at discrete otolith regions, representing the (i) egg/early-larval, (ii) pelagic larval, and (iii) late-larval/early-juvenile periods of fish collected at three locations 120–460 km apart. Discriminant-function analyses based on geochemical signatures at each otolith region accurately grouped an average of 85% (jackknife = 67%) and 87% (jackknife = 81%) of the fish to collection location in 2001 and 2002, respectively. Age at collection ranged from 83 to 174 days and parturition dates within each site were spread over a 22- to 66-day period. Therefore, individuals within sites were not released at similar times. A probable explanation of these data is that larvae from different geographic locations did not mix during ontogeny and possibly did not disperse long distances alongshore. Larval dispersal distances may be appreciably shorter, &lt;120 km, than previously assumed based on models of passive dispersal.

POPULATION GENETICS, DEMOGRAPHIC CONNECTIVITY, AND THE DESIGN OF MARINE RESERVES

Genetic analyses of marine population structure often find only slight geo- graphic differentiation in species with high dispersal potential. Interpreting the significance of this slight genetic signal has been difficult because even mild genetic structure implies very limited demographic exchange between populations, but slight differentiation could also be due to sampling error. Examination of genetic isolation by distance, in which close populations are more similar than distant ones, has the potential to increase confidence in the significance of slight genetic differentiation. Simulations of one-dimensional stepping stone populations with particular larval dispersal regimes shows that isolation by distance is most obvious when comparing populations separated by 2-5 times the mean larval dispersal distance. Available data on fish and invertebrates can be calibrated with this simulation approach and suggest mean dispersal distances of 25-150 km. Design of marine reserve systems requires an understanding of larval transport in and out of reserves, whether reserves will be self-seeding, whether they will accumulate recruits from surrounding exploited areas, and whether reserve networks can exchange recruits. Direct measurements of mean larval dispersal are needed to understand connectivity in a reserve system, but such measurements are extremely difficult. Genetic patterns of isolation by distance have the potential to add to direct measurement of larval dispersal distance and can help set the appropriate geographic scales on which marine reserve systems will function well.

PRINCIPLES FOR THE DESIGN OF MARINE RESERVES

The theory underlying the design of marine reserves, whether the goal is to preserve biodiversity or to manage fisheries, is still in its infancy. For both of these goals, there is a need for general principles on which to base marine reserve design, and because of the paucity of empirical experience, these principles must be based on models. However, most of the theoretical studies to date have been specific to a single situation, with few attempts to deduce general principles. Here we attempt to distill existing results into general principles useful to designers of marine reserves. To answer the question of how fishery management using reserves compares to conventional management, we provide two principles: (1) the effect of reserves on yield per recruit is similar to increasing the age of first capture, and (2) the effect of reserves on yield is similar to reducing effort. Another two principles answer the question of how to design reserve configurations so that species with movement in various stages will be sustainable: (3) higher juvenile and adult movement lowers sustainability of reserves for biodiversity, but an intermediate level of adult movement is required for reserves for fishery management, and (4) longer larval dispersal distance requires larger reserves for sustainability. These principles provide general guidelines for design, and attention to them will allow more rapid progress in future modeling studies. Whether populations or communities will persist under any specific reserve design is uncertain, and we suggest ways of dealing with that uncertainty.

CONSIDERATIONS IN ESTIMATING LARVAL DISPERSAL DISTANCES FROM OCEANOGRAPHIC DATA

Determination of larval dispersal distances and larval origins is a central challenge in contemporary marine ecology. In this work, the larval dispersal problem is discussed from the perspective of oceanography. Following formulation of the advection- diffusion model, the importance of scale is argued. When considering dispersion parameters at the appropriate population scales, advection is usually weaker than initially anticipated (and often used), and diffusion is stronger than typically used in model studies. Focusing attention on coastal populations, the importance of retention zones is described, and the more general existence of a coastal boundary layer is discussed. The coupling of cross- shore and alongshore dispersion results in a nonlinear relation between alongshore dispersal distance and larval planktonic period for dispersion in a sheared flow. Thus, small changes in cross-shore dispersal, whether due to environmental differences or larval behavior result in significant differences in alongshore dispersal. Finally, the interplay between advection and diffusion is explored, showing the importance of adequately representing the diffusive effects that mitigate alongshore advection. In most cases, diffusion acts to prevent ''wash- out'' of a population and allows for more flexibility in the size and spacing of effective marine reserves. Future challenges must bring oceanographers and ecologists together around specific dispersal problems if there is to be a significant improvement in the notable absence of hard data in this field of enquiry.