Intertidal Populations Research Articles

Temporal and Spatial Dynamics of a Lower-Intertidal Lancelet Population in the Seto Inland Sea, Japan

The population dynamics of Branchiostoma japonicum, formerly known as B. belcheri, were Investigated from September 2003 to August 2005 in the intertidal zone at Takehara (Hiroshima Prefecture), Seto Inland Sea, Japan. The intertidal population appeared from spring to autumn and disappeared during winter. A laboratory experiment showed that exposure to temperatures below 1 degree C for 2 hours resulted in severe mortality. This low temperature corresponds to the minimum temperature in sediments in the study area. This result suggested either that the intertidal population collapses in winter because of low temperature, or that the lancelets escape from the intertidal to the subtidal zone. Throughout the research period, no lancelets smaller than 10 mm in body length were found, indicating that no larvae settled in the intertidal zone. The intertidal population is probably maintained by the influx of individuals from the neighboring subtidal population. The mean annual density of the lancelets was greatest (10.6 individuals/m(2)) at station 1 nearest the low water mark, and lowest (0.3 Individuals/m(2)) at station 3 furthest from the low water mark. In summer, the water content of the sediments was remarkably lower at station 3 (20.2%) than at station 1 (25.8%). Another laboratory experiment showed that higher mortality occurred from exposure to sediments with a water content less than 25% for 2 hours, comparable to the water content at station 3, suggesting that the spatial distribution of the lancelets upward in the intertidal is restricted by sediment dryness.

Survey of mussel (Mytilus)species at Scottish shellfish farms

Mytilus trossulus and its hybrids with the native species Mytilus edulis and also Mytilus galloprovincialis have been recently detected in both farmed and natural populations of mussels in Scotland, adding complexity to the known M. edulis×M. galloprovincialis hybrid zone in Europe. Mytilus trossulus was first found in Scotland at aquaculture sites, where it was associated with significant losses in production and reductions in profitability. Mytilus trossulus and its hybrids have been identified at higher frequencies in farm stock than in natural intertidal populations, raising concerns about the potential wider impact of M. trossulus genotypes at mussel aquaculture sites in Scotland. The present study clarifies the composition of Mytilus species in cultivation at Scottish mussel aquaculture farms, by screening samples from 41 mussel aquaculture sites using a high-throughput real-time PCR methodology. Mytilus edulis alleles were detected at all sites. Mytilus galloprovincialis alleles were detected from samples collected at 39 sites, and M. trossulus alleles were found at five sites. Overall, M. trossulus genotypes were restricted in their geographical distribution. It is suggested that, in order to support the sustainability of the Scottish shellfish cultivation industry, the economic impact arising from the presence of M. trossulus could be limited through good management practice.

Larval dispersion of the estuarine crab Neohelice granulata in coastal marine waters of the Southwest Atlantic

The estuarine brachyuran crab Neohelice granulata export their larvae from the parental intertidal population of the Mar Chiquita coastal lagoon, and probably other populations, to marine waters. The degree of larval dispersion or self-recruitment of populations is unknown. We evaluated the presence of all larval stages of N. granulata in coastal waters of Argentina between 37.9° and 35.8° S, at two different spatial scales: a broad scale of tens to hundreds of kilometers from the Río de la Plata estuary in the north, to Mar Chiquita lagoon in the south, and a small scale of hundreds of meters to some kilometers around the mouth of Mar Chiquita, during spring and summer. Additionally, we registered the larval composition and density at San Clemente creek population, in Samborombon Bay (Río de la Plata estuary), every 3 h along a 30-hour period. Evidence indicates that larval release of N. granulata is temporally synchronized with nocturnal ebb tides and all development from Zoea I to Zoea IV occur in areas close to the parental population, even with very different oceanographic characteristics. A possible mechanism based on salinity selection and wind-driven transport is proposed for such behavior, and some considerations related to the connectivity of present populations are made.

Geographic Distribution and Intertidal Population Status for the Olympia Oyster,Ostrea luridaCarpenter 1864, from Alaska to Baja

Despite the recently renewed interest by ecologists and government agencies to reestablish historical populations of the Olympia oyster, focus has been limited to projects located at the north of this species' range, with little or no attention to southern California and Baja populations. In addition, historical information on the status of natural populations across the range has been mainly qualitative in nature, with no comprehensive information on the current status of natural populations. The focus of this study was to conduct the first large-scale quantitative biogeographic survey of remnant populations of the Olympia oyster and to identify suitable sites in southern California for future restoration projects. We surveyed intertidal populations at 24 historical sites during spring and summer 2005 and summer 2006, established presence/absence and collected data on densities and percent cover. Average maximum densities ranged from 0.0–36.7 ± 12.1 oysters per 0.25 m2. In southern California, intertidal populations were present in all bays and estuaries south of Morro Bay and most showed evidence of regular recruitment. Thus, all southern California sites could present favorable opportunities for restoration projects. At the north end of the range, intertidal populations were more often absent from sites, though there was evidence of subtidal populations. Populations were absent from intertidal sites at the northern endpoint of its distribution in Sitka, Alaska. We speculate that the current northern range limit of this species is located in northern British Columbia. Intertidal populations were also absent at two CA sites, Morro Bay, and Big Lagoon; anecdotal evidence further suggests that subtidal populations were also absent. This study represents the first comprehensive biogeographic survey of intertidal populations of the Olympia oyster, Ostrea lurida†, and identifies sites in southern California as suitable locations for future restoration projects.

Immune- and Pollution-mediated DNA Damage in Two Wild Mya arenaria Clam Populations

In aquatic environments, genotoxicity results from the effects of pollution combined with the inflammatory response triggered by the immune system. Indeed, the production of nitrosylated DNA and proteins are though to arise from the production of peroxinitrite during phagocytosis and inflammation. The purpose of this study was to examine new DNA biomarkers that differentiate between immune- and pollution-mediated genotoxicity in wild clam populations. Intertidal clam populations were sampled and analyzed for gonadal DNA strand breaks, DNA nitrosylation and xanthine oxidoreductase (XOR) activity (purine salvage pathway). The clam weight-to-shell-length ratio, the gonado-somatic index (GSI), age status, lipid peroxidation, xenobiotic conjugation activity (glutathione S-transferase (GST) and phagocytic activity were examined to shed light on their relationships with the observed genotoxic endpoints. XOR activity and DNA strand breaks were generally elevated at polluted sites and correlated significantly with clam weight-to-shell-length ratios and DNA nitrosylation. DNA nitrosylation was also higher at some sites and correlated significantly with phagocytic activity and with DNA strand breaks. This study showed that DNA strand breaks were associated with both immune-and pollution-mediated effects. This suggests that there is a loss of DNA repair capacity due to the combined effects of aging, pollution and immune response in wild clam populations that are impacted by anthropogenic activity.

Feeding behavior of the infaunal bivalve Tagelus dombeii (Lamarck, 1818). Suspension vs. deposit feeding

The feeding behavior of two populations of the bivalve Tagelus dombeii (Lamarck, 1818) was studied through laboratory experiments, with the purpose of establishing a possible dual feeding strategy (suspensivore/detritivore). Individuals from both the intertidal population of Coihuin (near Puerto Montt, Chile) as well as those from the subtidal of Corral Bay (near Valdivia) showed continuous suspension feeding with total absence of deposit feeding when immersed, and always showed their open siphons flush with the sediment surface. These observations were confirmed by the high clearance rates shown by both populations of T. dombeii, similar to the ones described for obligate suspension feeding bivalves. Different results were obtained during emersion, since T. dombeii individuals from both populations displayed their siphons on the sediment (3-5 cm), thus behaving as deposit feeders. Deposit feeding was observed in up to 80% of individuals. The present study allows to conclude that T. dombeii is primarily a suspension-feeder, which might be related to the dominance of sand (>92%) in the habitat of both populations studied. Growth coincides with periods of suspended particulate food supply increase in the natural environment. This suggests that T. dombeii would be competing for the particulate food supply with the community of obligate suspension feeding bivalves in these habitats.

Tidal height influences the levels of enzymatic antioxidant defences in Mytilus edulis

We investigated the potential variability of enzymatic antioxidant activities in blue mussels Mytilus edulis from a single intertidal population but living at different tidal heights. Activity levels of antioxidant enzymes (Cu/Zn superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase, glutathione transferase) were measured in the gills and digestive gland of mussels sampled at high shore (HS, air-exposure > 6 h/12 h) and low shore (LS, air-exposure < 2 h/12 h) of an intertidal zone (Yport, Normandie, France) for two consecutive autumns. In both tissues, levels of each enzymatic activity (except GST) were clearly higher in HS mussels than in LS for the two years. These results suggest an ability to acclimate the enzymatic antioxidant defences to the degree of undergone stress, confirming the importance of environmental conditions in the antioxidant responses. Therefore, the location of organisms on the shore should be taken into account in sampling for ecotoxicological studies.

Variability of growth, health, and population turnover within mussel beds of Perna canaliculus in northern New Zealand

The growth and health of intertidal mussel (Perna canaliculus) populations were investigated along an exposed coastline at Ninety Mile Beach, northern New Zealand. A marked and recaptured experiment of young mussels (15–35 mm in shell length) over a year at three sites resulted in significant differences in recovery rates, shell growth, condition indices, age, and degree of shell parasitism. A high recovery rate of fast growing mussels coincided with a high condition index and low shell parasitism. Conversely, a low recovery rate of slow growing populations was associated with a low condition index and a high level of shell parasitism. Anomalous shell conditions included bored shells, wear and sloughing of periostracum, shell erosion, and nacre pits and bulges. Aging of adult mussels (70–80 mm) indicates that in areas of fast population turnover rates, mussels reach adulthood in only 1.1 years compared to the 2.5-year-old mussels found in low turnover areas. These differences are attributed to variations in environmental conditions (e.g., food availability, sediment load) throughout the study area, and highlight the importance of site-specific variability for intertidal populations.

Consequences of shoot density and stiffness for ecosystem engineering by benthic macrophytes in flow dominated areas: a hydrodynamic flume study

Within intertidal areas of European Atlantic coasts the distribution of the small seagrass Zostera noltii and the halophyte Spartina anglica can partially overlap, despite numerous bio- mechanical, demographic and ecophysiological differences. Both species are known to be ecosystem engineers that modify their habitat by reducing hydrodynamic energy within their canopies. In this study we investigate the influence of biomechanical (i.e. shoot flexibility) and demographic (i.e. shoot density) characteristics of these intertidal plants on their interaction with unidirectional currents to (1) understand their differences in ecosystem engineering capacity and (2) identify which physical traits explains these differences. In a flume tank, hydrodynamic variables were measured within trans- planted S. anglica and Z. noltii meadows, and their corresponding simplified mimics. The results revealed that stiff canopies had a larger potential capacity (relative to flexible ones) to trap sediment, as in these vegetations velocity reduction within the canopy combined with a sufficient volumetric flow rate to provide sediment for settling. Flexible canopies were most efficient at reducing erosion by reconfiguration of their leaves. Shoot density increased the magnitude of these effects when val- ues were moderate. However, the capacity to increase sediment accretion disappeared when the maximum velocity attenuation was reached and the flow of water was relocated on top of the canopy. These habitat modifications may provide ecological benefits for saltmarsh and seagrass species. For saltmarsh plants, the rigid shoots allow lateral expansion of their populations via increased sedimen- tation. For seagrasses, the dense and flexible shoots typical of temperate intertidal populations pro- vide efficient protection from erosive forces, while at the same time helping to avoid stresses, such as drag forces and high sedimentation rates.

Does cannibalism of larvae by adults affect settlement and connectivity of mussel populations?

Intertidal population dynamics are driven by a complex series of processes, including larval supply and the possibility of larval predation by benthic animals such as filter-feeders. We hypothesised that cannibalism by adults could play a major role in the population connectivity of mussel populations by removing larvae as they attempt to settle in the adult habitat. Specifically, we tested hypotheses that consumption of mussel larvae by adults removes a significant proportion of potential settlers and is influenced by both settlement intensity and tidal state (flooding or ebbing). Predation of mussel larvae by adult mussels was investigated on incoming and ebbing tides during four spring tides by analysing the gut contents of adult Perna perna and Mytilus galloprovincialis collected from the low intertidal mussel zone between October 2005 and January 2006. Consumption rates were then compared with estimates of successful settler densities on natural beds. The results showed that mortality of competent mussel larvae through adult ingestion removes up to 77% a of potential settlers. Rates of larval consumption were highest during months of intense settlement, suggesting that mussels feed opportunistically, filtering a relatively fixed volume of water and removing particles, including larvae, in proportion to their densities in the water. Rates of larviphagy were also higher during receding than incoming tides. We suggest that this is due to changes in larval density or, more probably, in adult filtration efficiency that are related to the state of the tide. Despite significant effects of both tidal state and settlement intensity on rates of larval ingestion, neither had a significant effect on the proportion of potential settlers removed. During settlement more than half of all potential settlers are lost through cannibalism, with potentially serious consequences for population maintenance. The results highlight the paradoxical nature of the evolution of settlement mechanisms in mussels, which must balance the advantages of settlement in habitats favourable to adults against the consumption of larvae by adults.

Morphometric Relationships Between the Hermit Crab Isocheles sawayai (Forest &amp; Saint Laurent) (Crustacea Anomura Diogenidae) and its Shell from Southern Brazil

A study on morphometric relationships between the hermit crab Isocheles sawayai and its shell in a population from Parana-Santa Catarina States coast was carried out. Hermit crabs from the by-catch of shrimp fishing (4-15m depth) were collected. Cephalothoracic shield length (CL) and width (CW) from hermit crabs and total length (SL), aperture length (AL) and weight (SW) from shells were taken. A total of 266 hermit crabs occupying eight species of shell - Buc- cinanops lamarckii, Dorsanum moniliferum, Oliva reticularis, Olivancillaria steeriae, O. urceus, Polinices hepaticus, Semicassis granulatum and Stramonita haemastoma - were obtained. Males (N = 189) that dominated strongly (1M : 0.4 F) occupied eight shell species, while females (N = 76) occupied only six. There was only one intersex individual that oc- cupied a Stramonita shell. Contrarily to intertidal populations, O. urceus was the most frequently occupied shell by males (57.14%) and females (62.33%) in this studied area. Due to the smaller size of females (mean CL 5.19±0.84mm), these ones occupied shells of a smaller size than males (mean CL 6.49±1.14mm). The highest O. urceus shell frequency of oc- cupation is connected to the hermit crabs life style: it is a filter feeding species and its shell remains totally buried in the sandy substratum having only its cephalothoracic appendages free. The heavy shell of this gastropod (highest SW/SL) fa- vors the stability in strong current water conditions, as it functions as an anchorage for this semi-buried hermit crab.

Scales of Mytilus spp. population dynamics: importance of adult displacement and aggregation

The control of mussel population dynamics by larval supply and by more local processes affecting post-recruitment survival and growth has received considerable attention. However, immigration and emigration of adults and their contribution to colonization dynamics remain largely unknown. We manipulated the location and density of marked mussels in experimental transplants in the St. Lawrence Estuary (Quebec). Our results reveal that colonization by Mytilus spp. is influenced by (1) passive adult movement through wave disturbance and immigration, (2) spatial aggregation and (3) topographic heterogeneity. Results show that ~90% of colonization in experimental plots was by individuals of 4 to 32 mm shell length and was associated with both withintransplant and local ( 100 m. The passive displacement distribution of marked and wave-dislodged individuals further revealed movement distance ranging from 1 to 150 m and disturbance mortality from <20 to 40%. Together our results reveal that colonization in subarctic mussel populations can be largely controlled by cycles of adult displacement and further aggregation at the landscape level (~100 m). Such positive feedbacks involved in disturbance and colonization could explain strong fluctuations in abundance and challenge the interpretation of intertidal populations as systems mostly limited by larval supply and post-recruitment survival.

The Impacts of Human Visitation on Mussel Bed Communities Along the California Coast: Are Regulatory Marine Reserves Effective in Protecting These Communities?

Rocky intertidal habitats frequently are used by humans for recreational, educational, and subsistence-harvesting purposes, with intertidal populations damaged by visitation activities such as extraction, trampling, and handling. California Marine Managed Areas, particularly regulatory marine reserves (MRs), were established to provide legal protection and enhancement of coastal resources and include prohibitions on harvesting intertidal populations. However, the effectiveness of MRs is unclear as enforcement of no-take laws is weak and no regulations protect intertidal species from other detrimental visitor impacts such as trampling. The goal of this study was two-fold: (1) to determine impacts from human visitation on California mussel populations (Mytilus californianus) and mussel bed community diversity; and (2) to investigate the effectiveness of regulatory MRs in reducing visitor impacts on these populations. Surveys of mussel populations and bed-associated diversity were compared: (1) at sites subjected to either high or low levels of human use, and (2) at sites either unprotected or with regulatory protection banning collecting. At sites subjected to higher levels of human visitation, mussel populations were significantly lower than low-use sites. Comparisons of mussel populations inside and outside of regulatory MRs revealed no consistent pattern suggesting that California no-take regulatory reserves may have limited effectiveness in protecting mussel communities. In areas where many people visit intertidal habitats for purposes other than collecting, many organisms will be affected by trampling, turning of rocks, and handling. In these cases, effective protection of rocky intertidal communities requires an approach that goes beyond the singular focus on collecting to reduce the full suite of impacts.

Colonization and substratum preference of an introduced burrowing crustacean in a temperate estuary

Habitat use in marine invertebrates is often influenced by multiple abiotic and biotic factors. Substratum composition is one factor known to have a dramatic effect on habitat selection. The Australasian burrowing isopod ( Sphaeroma quoianum, H. Milne Edwards 1840) is a common introduced species in many estuaries on the Pacific coast of North America. S. quoianum burrows into a variety of firm substrata including marsh banks (composed of peat, clay, and/or mud), wood, friable rock, and Styrofoam floats. In some areas, isopods achieve high densities and may accelerate the rate of shoreline erosion and damage marine structures; thus, understanding the substratum preference of this species may be important for conservation and management efforts. Field experiments were conducted in Coos Bay, Oregon to examine substratum preference, burrowing rates, and the life stage of colonizers. In three experimental trials (Fall 2005, Spring 2006, Fall 2006), replicates of four intertidal substrata (marsh banks, decayed wood, sandstone, Styrofoam) were deployed near intertidal populations of S. quoianum. The numbers of burrows created in each substratum were enumerated weekly or daily (depending on trial). After the trials were completed, the total numbers of isopods inhabiting each substratum were counted. In weeks, S. quoianum extensively burrowed the substrata but exhibited a distinct preference for decayed wood. Significantly more isopods were present in wood than the other substrata at the end of the experiments and rates of burrowing were greatest in wood, although significance varied across time in one trial. Nearly 90% of colonizing isopods were under 5 mm in length suggesting that juvenile isopods primarily colonize intertidal substrata. Differences between burrow densities measured in the field and the results from these preference trials may indicate other factors, such as relative availability of substrata, recruitment and dispersal limitations, and possible gregarious behavior also influence local isopod densities.

Rates of fission, somatic growth and gonadal development of a fissiparous sea star, Allostichaster insignis, in New Zealand

Seasonal changes in population structure and incidence of fission were measured in intertidal and subtidal populations of Allostichaster insignis, a fissiparous sea star. Population size structure was stable over the course of the 1-year study. Sea stars in the subtidal zone attained greater maximum size (mean arm length, R = 35 mm) than those in the intertidal population (20 mm). Fission rates were greatest among small individuals (R < 20 mm). The frequency of fission ranged from 5 to 32% with peaks in early austral summer in the intertidal zone, and in autumn and winter in the subtidal zone. Sexual reproduction occurred in early spring in sea stars larger than 12 mm. The populations were heavily biased toward males. In the laboratory, A. insignis of three size classes (small, R = 9–13 mm; medium, 19–21 mm; and large, 29–31 mm) were fed mussels ad libitum or starved (not fed macroscopic food) for ∼1 year in a 3 × 2 factorial experiment. Small and medium-sized sea stars divided throughout the experiment and the ramets of most individuals regenerated sufficiently to divide again after 6–9 months. Unfed sea stars did not undergo fission (with one exception), had a higher mortality rate, and did not grow. Small, fed sea stars grew significantly faster than medium-sized or large individuals. At the end of the experiment, the pyloric caeca index (a measure of nutritional condition) was greater in fed than in unfed animals. Gonads (only testes were observed) developed in medium-sized and large, fed sea stars. Our field and laboratory results indicate that asexual reproduction in A. insignis predominantly occurs in small, well-nourished individuals. Ramets grow gradually through repeated fission and regeneration to a size (mean length of regenerating arms, Rr ∼ 20 mm) at which they begin to switch to sexual reproduction as the dominant reproductive mode.

Abundance, growth and recruitment of Mytilus galloprovincialis on the west coast of South Africa in relation to upwelling

Upwelling has the potential to influence the structure of intertidal populations and communities by enhancing productivity and thereby food supplies, and by influencing the transport and abundance of propagules. We investigated alongshore variability in Mytilus galloprovincialis populations in relation to upwelling intensity at 11 sites spread across ~1000 km of the west coast of South Africa. Mussels grew faster at upwelling centres than at downstream sites characterized by weaker upwelling, presumably due to greater availability of food supply provided by organic material from nearshore subtidal kelp beds and/or phytoplankton. Maximum sizes reached by mussels were correlated with growth rate. Large among-site variability existed in mussel recruit- ment, density, percentage cover, biomass, mean size, condition and shell thickness. However, contrary to predictions of bottom-up regulation, none of these differences were consistent with differences in upwelling intensity. High growth rates did not, therefore, influence local population structure. In general, where densities were low, mean size was larger and shells were thinner. This implies that although upwelling influences growth rate and maximum size of M. galloprovincialis, these effects are overridden by other factors such as recruitment and intraspecific competition so that fast growth does not translate into greater abundance, biomass, cover or improved condition at upwelling sites.

Gametophyte-sporophyte coalescence in populations of the intertidal carrageenophyte Mazzaella laminarioides (Rhodophyta)

Mazzaella laminarioides has consistently been reported as a typical coalescent/clump species with a triphasic life history of the Polysiphonia-type in which the haploid gametophyte is the predominant phase with respect to the diploid sporophyte. Preliminary observations of intertidal populations revealed that, in some instances, cystocarpic and tetrasporic fronds emerged from the same clump (G-T clumps), implying a coalescent process of haploid and diploid thalli by fusion of their corresponding adjacent basal holdfasts. Population surveys at three sites in Coliumo Bay, central Chile, were carried out to characterize frond demography as well as to asses the frequency of gametophyte-tetrasporophyte (G-T) coalescence. Visual and resorcinol methods were employed to determine the phases of the fronds collected over central transects of 15 randomly sampled clumps. Coalescence of G-T clumps was infrequent, with gametophytes dominating over tetrasporophyte thalli.

Is reproductive failure responsible for reduced recruitment of intertidalMytilus edulis L. in the western Dutch Wadden Sea?

The musselMytilus edulis is an abundant bivalve in the Dutch Wadden Sea, both in intertidal areas as well as in subtidal culture plots. From mid 1980’s to late 1990’s, strong declines in mussel intertidal populations have been observed, both in terms of occupied area and in biomass. Despite the efforts since 1999 to preserve intertidal mussel beds, abundance and biomass of mussels in the Dutch Wadden Sea are at present still much lower than in the 1970’1s. In the present paper, we tested whether reproductive failure could have been the cause for the low recruitment ofM. edulis in the western Dutch Wadden Sea. Water temperature is an important factor influencing growth and reproduction in bivalves. The observed increase in mean temperature in the Dutch Wadden Sea during the last decades suggests that an effect of temperature on the population dynamics of mussels might be expected. Nevertheless, no significant relationship was found between body condition and mean winter temperature. Therefore, a decrease in body condition and, consequently, in reproductive output, due to increasing temperature, will not have been a cause for the observed low recruitment in recent years. The seasonal patterns of individual growth supported this view: most adults developed gonads during the spawning season suggesting that reproductive (gametogenic) failure was not a cause for the low recruitment. Nevertheless, reproductive investment by the mussel population studied was not optimal since complete spawning hardly occurred. In addition, the high gonadosomatic ratio at the end of the summer did not result in heavy spawning, suggesting resorption of gonadal tissue due to unfavourable environmental conditions (too high temperature and/or food limitation during summer). Reduced recruitment could be due to poor spawning but other factors that exert their influence after spawning (predation on larvae or postlarvae, lack of settlement substrate) or on the adult population (decrease in number of spawners) are also likely to be involved in the observed low recruitment during the last years.

REPRODUCTION ON THE EDGE: LARGE-SCALE PATTERNS OF INDIVIDUAL PERFORMANCE IN A MARINE INVERTEBRATE

Reproductive output is a central attribute of life history, providing a measure of individual and population performance. The fields of ecology, biogeography, and evolutionary biology take disparate approaches in addressing spatial variation in reproduction, and thus we lack clear predictions for how reproductive output should vary geographically. We empirically investigate these contrasting theoretical approaches by determining geographic patterns in reproductive output for intertidal populations of the purple sea urchin, Strongylocentrotus purpuratus, at 15 sites spanning a large geographic distance (9 degrees span of latitude) from central California, USA, to Baja California, Mexico. Contrary to predictions from biogeography, some of the highest values of reproductive output are at sites near the species' southern range boundary. Additionally, sea urchins do not exhibit a latitudinal gradient in reproduction, but rather show considerable mesoscale variation in reproductive output. Spatial analyses reveal that this variation is correlated with coastal topographic features that are known to influence the pattern of nearshore nutrient flux and circulation. We hypothesize that urchins' reproductive output may be driven by the spatial distribution of their food supply, drift macroalgae, the abundance of which is influenced by both nutrient supply and alongshore transport processes that are coupled to topographic features. Large-scale studies such as this provide valuable insight into the causes of species' range limits, population connectivity, habitat reserve design, and forecasting the effects of climate change on species' distributions.

Vehicles versus conservation of invertebrates on sandy beaches: mortalities inflicted by off‐road vehicles on ghost crabs

AbstractSandy beaches face increasing anthropogenic pressures, with vehicle traffic being ecologically highly harmful. Ghost crabs (Fam. Ocypodidae) are conspicuous on many beaches, and they have been used as a bio‐monitoring tool to measure the ecological responses to human disturbance. However, the mechanisms causing declines in crab numbers are unknown, yet conservation must target the actual impact mechanisms. Therefore, we quantified the magnitude and mechanisms of off‐road vehicle (ORV) impacts on ghost crabs, addressing three key questions: (i) Does abundance of ghost crabs respond to traffic intensity?; (ii) Can burrows protect crabs from vehicles? and (iii) Can mortalities caused by vehicles contribute to population declines? ORV‐impacts were measured on North Stradbroke Island (Australia) for Ocypode cordimanus and Ocypode ceratophthalma. Crab densities were significantly lower in areas subjected to heavy beach traffic, suggesting direct crushing by vehicles. Burrows only partially protect crabs against cars: all individuals buried shallow (5 cm) are killed by 10 vehicle passes. Mortality declines with depth of burrows, but remains considerable (10–30% killed) at 20 cm and only those crabs buried at least 30 cm are not killed by ORVs: these ‘deep‐living’ crabs represent about half of the population. After crabs emerge at dusk they are killed in large numbers on the beach surface. A single vehicle can crush up to 0.75% of the intertidal population. While conservation measures should primarily regulate night traffic, our results also emphasise that the fossorial life habits of sandy beach animals cannot off‐set the impacts caused by ORVs.