Combining heat-transfer and energy budget models to predict thermal stress in Mediterranean intertidal mussels

Recent studies have emphasised that organisms can experience physiological stress well within their geographic range limits. Developing methods for mechanistically predicting the presence, absence and physiological performance of organisms is therefore important because of the ongoing effects of climate change. In this study, we merged a biophysical–ecological (BE) model that estimates the aquatic (high tide) and aerial (low tide) body temperatures of Mytilus galloprovincialis with a Dynamic Energy Budget (DEB) model to predict growth, reproduction and mortality of this Mediterranean mussel in both intertidal and subtidal environments. Using weather and chlorophyll-a data from three Mediterranean sites along the Italian coasts, we show that predictions of sublethal and lethal (acute) stress can potentially explain the observed distribution (both presence and absence) of mussels in the intertidal and subtidal zones, and the maximum size of animals in the subtidal zones. Importantly, our results suggest that different mechanisms limit the intertidal distribution of mussels, and that these mechanisms do not follow a simple latitudinal gradient. At the northernmost site (Palermo), M. galloprovincialis appears to be excluded from the intertidal zone due to persistent exposure to lethal aerial temperatures, whereas at the southernmost sites (Porto Empedocle and Lampedusa) sublethal stress is the most important driver of mussel intertidal distribution. Our predictions provide a set of hypotheses for future work on the role of climate change in limiting intertidal distribution of mussels in the Mediterranean.     

Foraging strategies of the marine iguana,Amblyrhynchus cristatus

Summary Two foraging strategies were found in marine iguanas (Amblyrhynchus cristatus); (1) subtidal feeding: the animals swam out to sea and dived for algae in the subtidal zone; (2) intertidal feeding: the animals foraged around low tide in the intertidal zone on more or less exposed algae. Most marine iguanas were very consistent in their foraging strategy and so could be classified as subtidal feeders (SFs) or intertidal feeders (IFs). Feeding strategy was weight-related (Fig. 1), not sexspecific. Animals 1,200 g were IFs, animals >1,800 g SFs. Some iguanas in between followed a mixed foraging strategy. SFs foraged independently of the tides, IFs always around low tide (Figs. 2, 3). Feeding time patterns of IFs and SFs are described (Table 1). Sea motion seemed to have little effect on the foraging pattern of SFs, but strongly influenced that of IFs (Fig. 2). The smaller a marine iguana, the faster it cooled when immersed in water (Fig. 4). The difference between water temperature and core temperature of animals returning from foraging was significantly greater in IFs than SFs (Fig. 5). SFs swimming in very cold water regulated their body temperature to prevent excessive cooling. Possible costs and benefits of the two foraging strategies are discussed. Only part of a marine iguana population lives really amphibiously and only ca. 5% of a 24 h day is spent close to or in the water. All social activities, including mating, take place on land. These life history characteristics preclude those adaptations to an amphibious way of life that would at the same time reduce the iguanas' ability to be maximally active at their typical terrestrial body temperature of 35° C.     

Temperature relationships of some tropical Pacific intertidal gastropods

Tissue and environmental-temperature measurements were made using thermistors in Hawaii, Guam, Palau and Singapore in the summer of 1970. Limpets have a very limited ability to regulate body temperature, and several species possessed tissue temperatures greater than those of the substratum. Some neritids and the trochid Monodonta labio possessed tissue temperatures as much as 2.2° to 2.8°C below substratum temperature. Littorinids attached by a mucus film to the substratum were found to have relatively lower body temperatures than those attached by the foot. Among neritids there is a tendency for high intertidal species to be better temperature regulators than low intertidal species. A very important factor in the temperature regulation of intertidal snails appears to be the extent of the area of contact between the animal and the substratum, which is greatest in limpets and least (negligible) in littorinids. Evaporative cooling and light coloration of the shell play a role in certain species and groups.     

Nitrogen regeneration by the surf zone penaeid prawn Macropetasma africanus

Nitrogen excretion of individual Macropetasma africanus (Balss) from an exposed beach/surf zone in Algoa Bay, South Africa was monitored under laboratory and field conditions in relation to body mass, temperature and feeding during 1985. Excretion rate experiments were performed on starved prawns at 15°, 18°, 20° and 25°C, as well as on individuals fed on four different diets (mussel, fish, shrimp and natural diet) at 15° and 20°C. The ratios of the excreted compounds to total nitrogen excreted were similar for the four diets despite differences in their nitrogen content and in the amount of food consumed. At 15° and 20°C, ammonia excretion rates of fed individuals were four to seven times higher than in starved prawns. the excretion rates were not correlated with nitrogen content of diets. M. africanus recycles 1 557 g NH₄–N per metre strip per year or 1 832 g total nitrogen m^-1 yr^-1, which constitute 12 and 14%, respectively, of total phytoplankton requirements of the surf zone. This study indicates that large motile crustaceans, when abundant, can play an important role in nutrient recycling in turbulent marine environments.     

No evidence for homeoviscous adaptation in intertidal snails: analysis of membrane fluidity during thermal acclimation, thermal acclimatization, and across thermal microhabitats

Many eurythermal organisms alter composition of their membranes to counter perturbing effects of environmental temperature variation on membrane fluidity, a process known as homeoviscous adaptation. Marine intertidal gastropods experience uniquely large thermal excursions that challenge the functional integrity of their membranes on tidal and seasonal timescales. This study measured and compared membrane fluidity in marine intertidal snail species under three scenarios: (1) laboratory thermal acclimation, (2) thermal acclimatization during a hot midday low tide, and (3) thermal acclimatization across the vertical intertidal zone gradient in temperature. For each scenario, we used fluorescence polarization of the membrane probe DPH to measure membrane fluidity in individual samples of gill and mantle tissue. A four-week thermal acclimation of Tegula funebralis to 5, 15, and 25°C did not induce differences in membrane fluidity. Littorina keenae sampled from two thermal microhabitats at the beginning and end of a hot midday low tide exhibited no significant differences in membrane fluidity, either as a function of time of day or as a function of thermal microhabitat, despite changes in body temperature up to 24°C within 8 h. Membrane fluidities of a diverse group of snails collected from high, middle, and low vertical regions of the intertidal zone varied among species but did not correlate with thermal microhabitat. Our data suggest intertidal gastropod snails do not exhibit homeoviscous adaptation of gill and mantle membranes. We discuss possible alternatives for how these organisms counter thermal excursions characteristic of the marine intertidal zone.     

Gridded meteorological data as a resource for mechanistic macroecology in coastal environments

Gridded weather data were evaluated as sources of forcing variables for biophysical models of intertidal animal body temperature with model results obtained using local weather station data serving as the baseline of comparison. The objective of the study was to determine which gridded data are sufficient to capture observed patterns of thermal stress. Three coastal sites in western North America were included in this analysis: Boiler Bay, Oregon; Bodega Bay, California; and Pacific Grove, California. The gridded data with the highest spatial resolution, the 32-km North American Regional Reanalysis (NARR) and the 38-km Climate Forecasting System Reanalysis (CFSR), predicted daily maximum intertidal animal temperature most similarly to the local weather Station data. Time step size was important for variables that change rapidly throughout the day, such as solar radiation. There were site-based differences in the ability of the model to predict daily maximum intertidal animal temperature, with the gridded data predictions being the closest to local weather station predictions in Boiler Bay, Oregon. In a review of gridded data used as part of ecological studies, there was broad use of the data across subject areas and ecosystems so the recent improvements in the spatial (from 2 degrees to 32 km) and temporal scales (from 6 hours to 1 hour) of gridded data will further add to the applicability within the ecological community particularly for mechanistic studies.     

The ecological energies of growth,respiration and assimilation in the intertidal American oyster Crassostrea virginica

Seasonal variations in the growth, respiration and assimilation of the intertidal oyster Crassostrea virginica (Gmelin) of different sizes were determined. The instantaneous growth rates for intertidal oysters decreased with increasing size and with lower temperatures. Q₁₀ values computed from instantaneous growth rates were approximately 2 during the warm growing season, but were higher in the colder months. Oxygen consumption increased with temperature and body size. A model was developed to predict oxygen consumption at any environmental temperature from 10° to 30°C for oysters ranging in weight from 0.1 to 100.0 g. Q₁₀ values computed from oxygen-consumption rates decreased with increasing temperature and increasing body size. Intertidal oysters utilize a large proportion of their assimilated energy in growth.Supported by a Belle W. Baruch Fellowship in Marine Ecology.     

Genetic differentiation in the intertidal zone in populations of the alga Enteromorpha linza (Ulvales: Chlorophyta)

Genetic differentiation within the intertidal zone was examined in six populations of the asexually reproducing alga Enteromorpha linza growing in the Long Island Sound, USA. Four of the five populations sampled in 1981 showed significant differentiation between high and low intertidal positions with respect to the GOT-2 locus. The pattern of differentiation was consistent for samples collected at several times during the year with some seasonal modifications. Four additional polymorphic loci, resolved in 1982, identified a total of 13 five-locus genotypes or clones. Four of the six populations sampled in 1982 showed significant differences in clone frequency between high and low intertidal positions. Laboratory experiments revealed differences in response to temperature among the clones. At 24°C a high intertidal associated clone showed an increase in growth, while low intertidal associated clones showed decreased growth compared to growth at 15°C. These results suggest that the microgeographic differentiation observed for E. linza in the intertidal zone may in part be due to the differential adaptation of clones to different intertidal environments. Additional demographic information is needed for individual clones in order to determine the role longevity, reproductive output, recruitment and interclonal competition play in maintaining the observed differentiation.     

Effect of high temperatures on the ultrastructure of Leydig cells in Mytilus galloprovincialis

Individuals of the mussel Mytilus galloprovincialis (L), collected from the Black Sea, were subjected to relatively high temperatures (25°, 28° and 30°C) for 60 h Ultrastructural studies on Leydig cells of the mantle indicated progressive modifications in their structure. Mainly, we established that a continuous decrease of glycogen quantity in the cells took place as the temperature raised, while the lipid content increased, up to 28°C. Between 28° and 30°C the lipid content decreased as a result of a pronounced degradation process. An increase of membrane permeability and the disturbance in the synthesis of mucoproteic secretory granules were observed. It is concluded that for Mytilus galloprovincialis the thermal adaptation zone ranges between 25° and 28°C. 28°C is the upper tolerable limit temperature, while 30°C acts as a lethal temperature.     

A finite element method to solve a population dynamics stage-structured model of intertidal barnacles

Many marine organisms are fixed or highly sedentary as adults but the adult population may be strongly dependent on the oceanic transport of planktonic larvae. In order to assess interactions between oceanographic and biological processes that determine the population dynamics of marine organisms with a sessile adult phase restricted to the coastline and a planktonic larval phase, we present a stage-structured finite element model for the barnacle Balanus glandula that inhabits the rocky intertidal zone of central California, USA.     

Improved survival under heat stress in intertidal embryos (<Emphasis Type="Italic">Fucus</Emphasis> spp.) simultaneously exposed to hypersalinity and the effect of parental thermal history

Intertidal organisms exposed to thermal stress normally experience other stresses simultaneously, but how these combined stresses modify tolerance to heat, especially for embryos, is poorly understood. Tolerance of fucoid algal embryos to heat, with and without acclimation to a sublethal temperature and with simultaneous exposure to hypersaline media, was examined. Embryos of Fucus vesiculosus L. (mid-intertidal zone) were less tolerant than embryos of Fucus spiralis L. (upper intertidal zone); without acclimation and with a growth temperature of 14°C, about half of the embryos survived 3 h exposure to 33°C in F. vesiculosus and of 35°C in F. spiralis. Conditions experienced by parental thalli (4°C versus 14°C storage) significantly affected the heat tolerance of embryos grown for 24 h post-fertilization at 14°C in F. vesiculosus, a result that is important for biologists using fucoid algae as model systems. Acclimation to a sublethal temperature (29°C) or exposure to the LT₅₀ (33°C, F. vesiculosus; 35°C, F. spiralis) in 100 psu seawater (2850 mmol kg^–1 osmolality) resulted in 30–50% higher levels of embryonic survival. Higher levels of HSP60s were found in embryos exposed to 29–33°C than to 14°C; lower levels of HSP60s were present in embryos exposed to the LT₅₀ under hypersaline conditions than in normal seawater. Contemporaneous studies in 1995–1996 of substratum temperature and desiccation levels were made at Schoodic Point, Maine (USA) underneath F. spiralis and F. vesiculosus canopies and in Semibalanus balanoides patches. This study extends the bioindicator utility of heat-shock proteins in studies of intertidal organisms and demonstrates the importance of integrated stress responses in survival of a single stress factor (e.g. temperature).Electronic Supplementary Material Supplementary material is available in the online version of this article at     

Patterns of mussel recruitment in southern Africa: a caution about using artificial substrata to approximate natural recruitment

Quantifying sessile marine invertebrate recruitment often requires destructive sampling or extrapolation from artificial substrata, the latter introducing the danger of artifacts. We measured intertidal mussel recruitment into mussel beds and into brushes at three-month intervals for five years across 3,200 km of southern Africa and determined substrata effects on recruitment rate. Recruitment into mussel beds showed a strong, coast-wide gradient, with high recruitment on the West coast, diminishing on the South coast, and increasing slightly on the East coast. At scales of 10 s of km, brushes reflected natural temporal recruitment variability, with a strong significant linear correlation between recruitment into brushes and into mussel beds. However, the relationship became semi-logarithmic when comparing among locations at a scale of 100 s of km. Artificial substrata thus reflect local natural settlement well but may be a poor indicator of it when spatial scales are large, particularly when mussel bed topography is complex, or localities have very different recruitment densities.     

Introduced versus native subspecies of Codium fragile: how distinctive is the invasive subspecies tomentosoides?

After its introduction, the green alga Codium fragile (Sur.) Hariot ssp. tomentosoides (van Goor) Silva has spread widely on several temperate-zone, rocky shores where non-weedy conspecific subspecies occur (N.E. Atlantic, N.E. Pacific, S. Pacific). To determine how phenologically and morphologically distinctive the invasive alga was relative to native subspecies, I compared marine intertidal populations of C. fragile ssp. tomentosoides and the native C. fragile ssp. novaezelandiae (J. Ag.) Silva (hereafter referred to as ssp. tomentosoides and ssp. novae-zelandiae respectively on New Zealand shores in 1992, 1993 and 1995. On the North Island, the invasive ssp. tomentosoides is sparsely distributed on low intertidal benches on wave-protected shores in the Hauraki Gulf (east coast) in spring and summer, and thalli die back to the perennial holdfast in autumn. In contrast, the native ssp. novaezelandiae forms dense beds within the low intertidal mussel zone on wave-swept shores of Maori Bay (west coast), and fronds are perennial. Whereas ssp. tomentosoides has only a few fronds arising from the spongy basal hold-fast, ssp. novae-zelandiae thalli are composed of many fronds. The ssp. tomentosoides from the Hauraki Gulf is significantly more branched than comparably sized native conspecifics from Maori Bay. These phenological and morphological differences were used to predict the subspecific identity of C. fragile from three other locations on the North Island, two locations on the South Island, and four locations on S.E. Australian shores; microscopic examination of utricles was used to check the predictions. Seasonality and number of fronds per thallus are the most reliable characters for field identification of native vs invasive subspecies: perennial intertidal thalli with large numbers of fronds are indicative of native subspecies for different geographic regions.     

Geographic range limitation of the intertidal gastropods Littorina sitkana and L. planaxis

Geographic ranges of marine intertidal organisms are not entirely governed by discontinuities in temperature gradients. This is especially true in the eastern north Pacific Ocean, where temperature gradation is less steep than in the western north Atlantic Ocean. The southern limit of distribution of Littorina sitkana (Philippi, 1845) and the northern limit of L. planaxis (Philippi, 1847) occur at Charleston, Oregon (Latitude 43.4°N). No discontinuity in air or water temperature can be correlated with these distributional boundaries. A third species, L. scutulata (Gould, 1849), has the widest range, overlapping with L. planaxis in the south and L. sitkana in the north (Latitude 27.5° to 58°N). L. planaxis transplanted to northern Washington survived for 4 years, grew and produced planktonic larvae. Either larval mortality or south-flowing currents during planktonic life prevent this species from dispersing northward. L. sitkana has direct development and occurs only in habitats where adults are sheltered from waves and egg masses from desiccation. Lack of suitable habitat, coupled with the presence of the predatory crab Pachygrapsus crassipes (Randall, 1839) prevent the southward establishment of L. sitkana.     

Metabolic rates of epipelagic marine zooplankton as a function of body mass and temperature

The metabolic rates (oxygen uptake, ammonia excretion, phosphate excretion) of epipelagic marine zooplankton have been expressed as a function of body mass (dry, carbon, nitrogen and phosphorus weights) and habitat temperature, using the multiple-regression method. Zooplankton data used for this analysis are from phylogenetically mixed groups (56 to 143 species, representing 7 to 8 phyla, body mass range: 6 orders of magnitude) from various latitudes (habitat temperature range:-1.4° to 30°C). The results revealed that 84 to 96% of variation in metabolic rates is due to body mass and habitat temperature. Among the various body-mass units, the best correlation was provided by carbon and nitrogen units for all three metabolic rates. Oxygen uptake, ammonia excretion and phosphate excretion are all similar in terms of body-mass effect, but differ in terms of temperature effect. With carbon or nitrogen body-mass units, calculated Q₁₀ values are 1.82 to 1.89 for oxygen uptake, 1.91 to 1.93 for ammonia excretion and 1.55 for phosphate excretion. The effects of body mass and habitat temperature on the metabolic quotients (O:N, N:P, O:P) are insignificant. The present results for oxygen-uptake rate vs body mass do not differ significantly from those reported for general poikilotherms by Hemmingsen and for crustaceans by Ivleva at a comparable temperature (20°C). The importance of a body-mass measure for meaningful comparison is suggested by the evaluation of the habitat-temperature effect between mixed taxonomic groups and selected ones. Considering the dominant effects of body mass and temperature on zooplankton metabolic rates, the latitudinal gradient of community metabolic rate for net zooplankton in the ocean is estimated, emphasizing the non-parallelism between community metabolic rates and the standing stock of net zooplankton.     

High-temperature tolerance of photosynthesis in the red alga Chondrus crispus

Chondrus crispus (Stackhouse) is a perennial red seaweed, common in intertidal and shallow sublittoral communities throughout the North Atlantic Ocean. In the intertidal zone, C. crispus may experience rapid temperature changes of 10 to 20C° during a single immerison-emerision cycle, and may be exposed to temperatures that exceed the thermal limits for long-term survival. C. crispus collected year-round at Long Cove Point, Chamberlain, Maine, USA, during 1989 and 1990, underwent phenotypic acclimation to growth temperature in the laboratory. This phenotypic acclimation enhanced its ability to withstand brief exposure to extreme temperature. Plants grown at summer seawater temperature (20°C) were able to maintain constant rates of lightsaturated photosynthesis at 30°C for 9 h. In contrast, light-saturated photosynthetic rates of plants grown at winter seawater temperature (5°C) declined rapidly following exposure to 30°C, reached 20 to 25% of initial values within 10 min, and then remained constant at this level for 9 h. The degree of inhibition of photosynthesis at 30°C was also dependent upon light intensity. Inhibition was greatest in plants exposed to 30°C in darkness or high light (600 mol photons m^-2s^-1) than in plants maintained under moderate light levels (70 to 100 mol photons m^-2s^-1). Photosynthesis of 20°C-acclimated plants was inhibited by exposure to 30°C in darkness or high light, but the degree of inhibition was less than that exhibited by 5°C-grown plants. Not only was light-saturated photosynthesis of 20°C plants less severely inhibited by exposure to 30°C than that of 5°C plants, but the former also recovered faster when they were returned to growth conditions. The mechanistic basis of this acclimation to growth temperature is not clear. Our results indicate that there were no differences between 5 and 20°C-grown plants in the thermal stability of respiration, electron transport associated with Photosystems I or II, Rubisco or energy transfer between the phycobilisomes and Photosystem II. Overall, our results suggest that phenotypic acclimation to seawater temperature allows plants to tolerate higher temperatures, and may play an important role in the success of C. crispus in the intertidal environment.     

Testing the effects of wave exposure,site, and behavior on intertidal mussel body temperatures: applications and limits of temperature logger design

The rocky intertidal mussel Mytilus californianus is exposed to potentially damaging thermal conditions during low tide. However, because the temperatures of ectothermic organisms are driven by multiple climatic factors, we do not fully understand what the body temperatures of intertidal invertebrates are under field conditions, or how thermal stress varies between intertidal sites. We designed a temperature logger that thermally matches (similar size, color, morphology and thermal inertia) living mussels of the species M. californianus, and tested its ability to provide realistic measurements of body temperature in the field. As part of these tests, we examined the propensity of M. californianus to gape, a behavior in which the mussel opens its shell valves, and which may permit evaporative cooling. Because our instruments were unable to mimic this behavior, we tested the degree to which gaping contributes to animal cooling by exposing M. californianus to a range of climatic conditions while recording body temperatures, gaping behavior and water loss. Results indicated no significant influence of gaping on body temperature. Tests comparing temperatures of loggers to those of real mussels under laboratory and field conditions showed that thermally matched loggers recorded temperatures within a few degrees of living animals and that unmodified loggers regularly incurred errors of up to 14°C. We then deployed a series of thermally matched loggers at two sites in central Oregon (Boiler Bay and Strawberry Hill) previously hypothesized to display site-specific differences in aerial temperature, and at various wave-exposure regimes within each site. Significant differences were demonstrated between sites using a subset of temperature metrics in a multivariate analysis. Yearly peaks in maximum temperature, average daily maximum temperature, and degree hours were useful in discriminating between sites. In 2001, wave-exposed sites at Strawberry Hill displayed fewer degree–hours than wave-protected sites, but an equivalent or greater maximum temperature. In 2002 both of these metrics were significantly lower in wave-exposed sites. Boiler Bay and Strawberry Hill differed in thermal regime, but not in a way that would indicate one was hotter than the other.Communicated by P.W. Sammarco, Chauvin     

The relationship between allele frequency and tidal height in a mussel hybrid zone: a test of the differential settlement hypothesis

The blue mussels Mytilus edulis L. and M. galloprovincialis Lmk. hybridize in western Europe. Within hybrid populations nuclear alleles specific to M. galloprovincialis increase in frequency with age and size. This relationship changes with tidal height; alleles from M. galloprovincialis occur more frequently high in the intertidal zone, while M. edulis alleles predominate in the low intertidal zone. We tested the hypotheses that larvae with M. galloprovincialis alleles tend to settle higher in the intertidal zone, or that mussels redistribute themselves with respect to tidal height after initial larval settlement. We sampled recently metamorphosed mussels every 2 weeks in a hybrid mussel population at Whitsand Bay in southwest England throughout the summer of 1996. We observed four cohorts of newly settled mussels. There was no evidence of differential settlement of mussels with different genotypes in connection with tidal height, or into shaded versus unshaded microsites. Therefore, we rejected the preferential settlement hypothesis. There was substantial movement of juvenile mussels in the first 4 weeks following initial settlement, but this “secondary settlement” did not result in genetic differentiation with respect to tidal height. Further, significant differences in allele frequencies were found between primary and secondary spat. This allele frequency change was in the opposite direction of that seen in the adult population, suggesting newly settled larvae may be experiencing different selective pressures than adults. We propose that the genetic structure of hybrid mussel populations with respect to tidal height is the consequence of differences in selection intensity. Received: 30 April 1999 / Accepted: 5 May 2000     

Sex-specific temperature dependence of foraging and growth of intertidal snails

Predicting the biological impacts of climate change requires an understanding of how temperature alters organismal physiology and behavior. Given differences in reproductive physiology between sexes, increases in global temperature may be experienced differently by the males and females of a species. This study tested for sex-specific effects of increased air temperature on foraging, growth, and survival of an intertidal snail, Nucella ostrina (San Juan Island, Washington, 48–30′44″N, 123–08′43″W). Snails exhibited periodic peaks in foraging. Subjecting snails to elevated low tide air temperatures did not alter the timing or magnitude of this pattern. Despite similar temporal patterns in foraging, females foraged more than males, even when the risk of thermal stress was high. While males and females appear to have a similar body temperature threshold for optimal growth, females were more likely to cross that threshold resulting in a loss of body mass when exposed to daily increases in air temperature. These results suggest that the consequences of a warming climate in the short term may be different for males and females of N. ostrina, but also imply longer-term costs of reduced reproductive output, abundance, and distribution of this ubiquitous intertidal predator. Generally, this study points to the possible significance of sex-specific responses in an increasingly warm world.     

Extensive gene flow among mytilid (Bathymodiolus thermophilus) populations from hydrothermal vents of the eastern Pacific

Prior studies of the hydrothermal vent mussel Bathymodiolus thermophilus (Bivalvia: Mytilidae), provided conflicting predictions about the dispersal ability and population structure of this highly specialized species. Analyses of morphological features associated with its larval shells revealed a feeding larval stage that might facilitate dispersal between ephemeral vent habitats. In contrast, an allozyme study revealed substantial genetic differentiation between samples taken from populations 2370 km apart on Galápagos Rift (Latitude 0°N) and the East Pacific Rise (13°N). To resolve the discrepancy between these studies, we examined allozyme and mitochondrial (mt) DNA variation in new samples from the same localities plus more recently discovered sites (9° and 11°N) along the East Pacific Rise. Although analysis of 26 enzyme-determining loci revealed relatively low levels of genetic variation within the five populations, no evidence existed for significant barriers to dispersal among populations. We estimated an average effective rate of gege flow (Nm) of 8 migrants per population per generation. Two common mtDNA variants predominated at relatively even frequencies in each population, and similarly provided no evidence for barriers to gene flow or isolation-by-distance across this species' known range. Larvae of this species appear to be capable of dispersing hundreds of kilometers along a continuous ridge system and across gaps separating non-contiguous spreading centers.