Sea Star Pisaster Ochraceus Research Articles

A Dynamic Energy Budget (DEB) model for the keystone predator Pisaster ochraceus.

We present a Dynamic Energy Budget (DEB) model for the quintessential keystone predator, the rocky-intertidal sea star Pisaster ochraceus. Based on first principles, DEB theory is used to illuminate underlying physiological processes (maintenance, growth, development, and reproduction), thus providing a framework to predict individual-level responses to environmental change. We parameterized the model for P. ochraceus using both data from the literature and experiments conducted specifically for the DEB framework. We devoted special attention to the model’s capacity to (1) describe growth trajectories at different life-stages, including pelagic larval and post-metamorphic phases, (2) simulate shrinkage when prey availability is insufficient to meet maintenance requirements, and (3) deal with the combined effects of changing body temperature and food supply. We further validated the model using an independent growth data set. Using standard statistics to compare model outputs with real data (e.g. Mean Absolute Percent Error, MAPE) we demonstrated that the model is capable of tracking P. ochraceus’ growth in length at different life-stages (larvae: MAPE = 12.27%; post-metamorphic, MAPE = 9.22%), as well as quantifying reproductive output index. However, the model’s skill dropped when trying to predict changes in body mass (MAPE = 24.59%), potentially because of the challenge of precisely anticipating spawning events. Interestingly, the model revealed that P. ochraceus reserves contribute little to total biomass, suggesting that animals draw energy from structure when food is limited. The latter appears to drive indeterminate growth dynamics in P. ochraceus. Individual-based mechanistic models, which can illuminate underlying physiological responses, offer a viable framework for forecasting population dynamics in the keystone predator Pisaster ochraceus. The DEB model herein represents a critical step in that direction, especially in a period of increased anthropogenic pressure on natural systems and an observed recent decline in populations of this keystone species.

Sea Otters Homogenize Mussel Beds and Reduce Habitat Provisioning in a Rocky Intertidal Ecosystem

Sea otters (Enhydra lutris) are keystone predators that consume a variety of benthic invertebrates, including the intertidal mussel, Mytilus californianus. By virtue of their competitive dominance, large size, and longevity, M. californianus are ecosystem engineers that form structurally complex beds that provide habitat for diverse invertebrate communities. We investigated whether otters affect mussel bed characteristics (i.e. mussel length distributions, mussel bed depth, and biomass) and associated community structure (i.e. biomass, alpha and beta diversity) by comparing four regions that varied in their histories of sea otter occupancy on the west coast of British Columbia and northern Washington. Mussel bed depth and average mussel lengths were 1.5 times lower in regions occupied by otters for >20 years than those occupied for <5 yrs. Diversity of mussel bed associated communities did not differ between regions; however, the total biomass of species associated with mussel beds was more than three-times higher where sea otters were absent. We examined alternative explanations for differences in mussel bed community structure, including among-region variation in oceanographic conditions and abundance of the predatory sea star Pisaster ochraceus. We cannot discount multiple drivers shaping mussel beds, but our findings indicate the sea otters are an important one. We conclude that, similar to their effects on subtidal benthic invertebrates, sea otters reduce the size distributions of intertidal mussels and, thereby, habitat available to support associated communities. Our study indicates that by reducing populations of habitat-providing intertidal mussels, sea otters may have substantial indirect effects on associated communities.

Differences in relative abundance and size structure of the sea stars Pisaster ochraceus and Evasterias troschelii among habitat types in Puget Sound, Washington, USA

We surveyed patterns in the relative abundance and size structure of the sea stars Pisaster ochraceus and Evasterias troschelii in five habitat types of varying structural complexity and prey availability (sand/cobble, boulder, and rocky intertidal; pilings; and floating docks) in Puget Sound and the San Juan Islands, Washington. For both species, small sea stars were most abundant in the most structurally complex habitat type (boulder), where they occurred almost exclusively under boulders during low tide. Larger individuals became more abundant as structural complexity decreased, occurring more frequently in open habitat types (rocky shores, pilings, and docks) known to have greater abundances of prey resources. Gull foraging observations and experiments demonstrated that exposed small sea stars of both species were highly vulnerable to predation, suggesting that small sea stars require structural complexity (crevice microhabitat) as a predation refuge. Large sea stars, once attaining a size refuge from predation, appear to migrate to more exposed habitat types with more abundant food resources. These results suggest parallel ontogenetic habitat shifts in two co-occurring consumer species related to a shared predation risk at early life stages and demonstrate how the relative importance of top-down and bottom-up processes may differ with ontogeny.

Short timescale rate maximization by gulls and implications for predation on size-structured prey

The timescale over which a predator estimates changes in prey encounter rates will play an important role in maximization of energetic returns from foraging in habitats where prey availability is highly variable through time. However, studies that explicitly test the temporal scale over which foragers track changes in prey availability are surprisingly rare. The increasingly well-recognized impact of terrestrial predators (e.g., birds) on intertidal food webs is likely to depend on their ability to track prey fluctuations in these highly variable environments. Here, we compare the predictions of 2 optimal diet models: a “classic” model in which prey encounter rate estimates are based on long-term, site-level abundance averages, and a model in which encounter rate estimates change at regular intervals throughout the foraging period. We parameterized these models using data from a field study on glaucous-winged gulls (Larus glaucescens) foraging on various sizes of the sea star Pisaster ochraceus. Predictions from the classic model, which assumes constant diet breadth throughout the tide cycle, did not match field obser vations of diet breadth. The “tide-sensitive” model, which assumes that gulls track tide-related changes in prey abundance, provided a better fit to observational data, explaining the full range of Pisaster sizes consumed by gulls. We conclude that gulls track short-term changes in prey encounter rates within a single low tide period to maximize foraging returns. We also present data for high rates of Pisaster removal by gulls, challenging the view of this sea star as a top predator in its intertidal communities. Key wor ds: diet breadth, glaucous-winged gull, intertidal ecology , Pisaster ochraceus, prey encounter rate. [Behav Ecol]

Keystone intimidators in the intertidal: non-consumptive effects of a keystone sea star regulate feeding and growth in whelks

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 450:107-114 (2012) - DOI: https://doi.org/10.3354/meps09567 Keystone intimidators in the intertidal: non-consumptive effects of a keystone sea star regulate feeding and growth in whelks J. Stephen Gosnell1,*, Steven D. Gaines1,2 1Department of Ecology, Evolution, and Marine Biology, and 2Bren School of Environmental Science and Management, University of California, Santa Barbara, California 93106, USA *Email: gosnell@lifesci.ucsb.edu ABSTRACT: Predators can exert strong controls on community structure through both consumptive and non-consumptive effects. When one of these impacts on diversity is large, such predators have been labeled keystone predators and keystone intimidators respectively. Here we demonstrate that some predators play both roles simultaneously. We studied the species for which the term keystone predator was originally coined, the ochre sea star Pisaster ochraceus. We observed non-consumptive effects that were not mediated through its well-known invertebrate prey, the competitive dominant mussel Mytilus californianus, but instead through whelks Nucella emarginata, another important intertidal predator. Whelks exposed to effluent from stars in the laboratory consumed significantly less, grew significantly less, and developed less reproductive and digestive tissue. Star presence also altered the feeding preferences of whelks. Results suggest these responses are non-linear with respect to the number of predators and thus predator presence may be a larger determinant of non-consumptive effects than density. A second experiment found, in agreement with previous field surveys, that these large non-consumptive effects occurred even though whelk mortality due to consumption by stars is extremely low. These results offer a new mechanism to explain previous field experiments that found whelks have little impact on mussel mortality in the presence of stars yet large effects when stars are absent. They demonstrate the potential for non-consumptive effects to play a major role in community regulation by a keystone predator. They also demonstrate that non-consumptive interactions may have large effects on species even when rates of consumptive mortality are exceedingly low. KEY WORDS: Keystone · Non-consumptive effects · Pisaster · Nucella · Mytilus · Rocky intertidal Full text in pdf format PreviousNextCite this article as: Gosnell JS, Gaines SD (2012) Keystone intimidators in the intertidal: non-consumptive effects of a keystone sea star regulate feeding and growth in whelks. Mar Ecol Prog Ser 450:107-114. https://doi.org/10.3354/meps09567 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 450. Online publication date: March 29, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Energy intake, kleptoparasitism risk, and prey choice by Glaucous-winged Gulls (Larus glaucescens) foraging on sea stars

ABSTRACT. Energy gain is thought to play a central role in prey selection by most foragers, but it may conflict with food theft avoidance and be constrained by undeveloped foraging skills. We investigated predation by the Glaucous-winged Gull (Larus glaucescens) on the ecologically important Ochre Sea Star (Pisaster ochraceus). We tested the hypothesis that Pisaster size selection by gulls was based on energetic profitability, defined as energy provided per unit handling time. We then examined the degree to which profitability interacts with intraspecific kleptoparasitism risk and age-related foraging efficiency (i.e., energy intake rate, prey capture success) to produce the patterns of prey choice observed in a natural setting. Behavioral observations of free-living gulls revealed moderate to high (32.5–91.6%) occurrences of Pisaster in gull diets. We used handling time data and bomb calorimetry to determine the relationship between Pisaster size and energetic profitability, which informed prey offer exp...

Effects of brackish water incursions and diel phasing of tides on vertical excursions of the keystone predator Pisaster ochraceus

The physical factors that constrain the vertical foraging excursions of the keystone predator, the sea star Pisaster ochraceus, hold considerable interest because they indirectly shape the vivid patterns of zonation of rocky shore communities by impeding or enhancing the ability of P. ochraceus to traverse the intertidal zone. In this paper, we describe a study conducted in the Pacific Northwest of North America in which we examined, in the field and laboratory, the abiotic factors that can affect vertical excursions by P. ochraceus. Our field observations revealed that the extreme upward reach and average shore level height reached by P. ochraceus were significantly lower for daylight high tides than nocturnal high tides. Based on diver observations following a severe storm, it would also appear that these diurnal movements can be impeded by freshwater incursions into the intertidal zone; a regularly occurring event in the Pacific Northwest. As part of an experimental investigation into this phenomenon, we observed that sea stars maintained in tall cylindrical aquaria, without tidal flux, remained near the bottom during daylight and moved to the top of the column at night, suggesting that photoperiod alone can influence the cycle of vertical movement. Adding a freshwater layer to the aquaria restricted these vertical excursions. Our results suggest that on rocky coastlines susceptible to fresh water incursions, the suppression of foraging may be an important factor in the spatial and temporal variation in the intensity of predation. Furthermore, given the relative increase in frequency and intensity of freshwater incursions in the Pacific Northwest and the intolerance of P. ochraceus to lowered salinity, there is the long-term potential to significantly alter patterns of species zonation in this essential marine habitat.

An Intertidal Sea Star Adjusts Thermal Inertia to Avoid Extreme Body Temperatures

The body temperature of ectotherms is influenced by the interaction of abiotic conditions, morphology, and behavior. Although organisms living in different thermal habitats may exhibit morphological plasticity or move from unfavorable locations, there are few examples of animals adjusting their thermal properties in response to short-term changes in local conditions. Here, we show that the intertidal sea star Pisaster ochraceus modulates its thermal inertia in response to prior thermal exposure. After exposure to high body temperature at low tide, sea stars increase the amount of colder-than-air fluid in their coelomic cavity when submerged during high tide, resulting in a lower body temperature during the subsequent low tide. Moreover, this buffering capacity is more effective when seawater is cold during the previous high tide. This ability to modify the volume of coelomic fluid provides sea stars with a novel thermoregulatory "backup" when faced with prolonged exposure to elevated aerial temperatures.

Effects of temperature, season and locality on wasting disease in the keystone predatory sea star Pisaster ochraceus

This study investigates wasting disease in the northeast Pacific keystone predatory sea star Pisaster ochraceus on the outer west coast of Vancouver Island (British Columbia, Canada). To quantify the effects of temperature, season and locality on the vulnerability of P. ochraceus to wasting disease, we conducted surveys and experiments in early and late summer. To test the prediction that a small increase in temperature would result in heightened infection intensities, we housed sea stars at different temperatures in the laboratory and caged sea stars subtidally at 2 depths. Prevalence and infection intensity were always higher in warm temperature treatments and did not differ between the sexes or with increasing size. Disease effects also varied with season and locality. Specimens held in aquaria displayed significantly higher disease prevalence and infection intensity in June versus August. Furthermore, sea stars from a sheltered inlet showed markedly higher prevalence of the disease in late summer, while wave-exposed sites had consistently low disease prevalence. Seasonal changes in reproductive potential, host condition and/or physiological acclimation, as well as differences in environmental regime among localities, may impact the dynamics of wasting disease. These results demonstrate that small increases in temperature could drive mass mortalities of Pisaster due to wasting disease, with vulnerability possibly reaching a peak in spring and in populations from sheltered localities. This is the most northern report of wasting disease in the class Asteroidea on the west coast of North America.

A non‐lethal method for estimation of gonad and pyloric caecum indices in sea stars

Abstract. Gonad and pyloric caecum indices are widely used indicators of reproductive effort and nutritional condition in asteroids. Current methods of quantification generally require sacrificing multiple animals and the resulting reduction in local sea star density could have an unintended impact on benthic communities. Using the intertidal sea star Pisaster ochraceus, we developed and tested a method for estimating organ indices through the non‐lethal sampling of single arms. Indices estimated via dissections of single arms accurately predicted the values obtained by sacrificing whole animals. In laboratory and field trials, we compared two methods of sampling single arms: (1) arm removal, and (2) organ extraction through an incision (without arm removal). Two years after these treatments, organs had regenerated in the affected arms of most sea stars, but were small relative to those in unmanipulated arms. The extent of organ recovery did not differ between the two treatments, but sea stars in the arm removal group were relocated in the field more frequently than those in the arm incision group. Our results suggest that sampling via the removal of single arms can be an effective, non‐lethal method for estimating gonad and pyloric caecum indices in asteroids.

Elevated water temperature and carbon dioxide concentration increase the growth of a keystone echinoderm

Anthropogenic climate change poses a serious threat to biodiversity. In marine environments, multiple climate variables, including temperature and CO(2) concentration ([CO(2)]), are changing simultaneously. Although temperature has well-documented ecological effects, and many heavily calcified marine organisms experience reduced growth with increased [CO(2)], little is known about the combined effects of temperature and [CO(2)], particularly on species that are less dependent on calcified shells or skeletons. We manipulated water temperature and [CO(2)] to determine the effects on the sea star Pisaster ochraceus, a keystone predator. We found that sea star growth and feeding rates increased with water temperature from 5 degrees C to 21 degrees C. A doubling of current [CO(2)] also increased growth rates both with and without a concurrent temperature increase from 12 degrees C to 15 degrees C. Increased [CO(2)] also had a positive but nonsignificant effect on sea star feeding rates, suggesting [CO(2)] may be acting directly at the physiological level to increase growth rates. As in past studies of other marine invertebrates, increased [CO(2)] reduced the relative calcified mass in sea stars, although this effect was observed only at the lower experimental temperature. The positive relationship between growth and [CO(2)] found here contrasts with previous studies, most of which have shown negative effects of [CO(2)] on marine species, particularly those that are more heavily calcified than P. ochraceus. Our findings demonstrate that increased [CO(2)] will not have direct negative effects on all marine invertebrates, suggesting that predictions of biotic responses to climate change should consider how different types of organisms will respond to changing climatic variables.

Shell Traits of a Marine Mussel Mediate Predation Selectivity by Crabs and Sea Stars

ABSTRACT Crabs and sea stars are known to preferentially select mussels with morphological traits that diminish the predators' searching or handling times. I compared two distinct morphotype of mussels (Mytilus trossulus; Gould, 1850) from Howe Sound, British Columbia, Canada, by dissection and measurement. Then, I experimentally offered mussels of the two morphotypes on the same patch to crabs (Cancer productus; Randall, 1839) and sea stars (Pisaster ochraceus; Brandt, 1835) to quantify the extent to which these predators select prey based on morphological features. Sea stars preferentially consumed mussels with gaps in shell closures, although these mussels also had larger adductor muscles compared with mussels rejected. Gaps at the shell margin presumably allowed sea stars easier access between shell valves to insert their stomachs and begin digestion. Small crabs preferentially consumed mussels with thin shells, which are easier to crush, whereas, large crabs consumed more thick-shelled mussels, possi...

Invasion resistance on rocky shores: direct and indirect effects of three native predators on an exotic and a native prey species

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 378:47-54 (2009) - DOI: https://doi.org/10.3354/meps07870 Invasion resistance on rocky shores: direct and indirect effects of three native predators on an exotic and a native prey species J. S. Shinen1,*, S. G. Morgan2, A. L. Chan2 1Estación Costera de Investigaciones Marinas, Departamento de Ecología, Pontificia Universidad Católica de Chile, Alameda 340, Santiago, Chile 2Bodega Marine Laboratory, Department of Environmental Science and Policy, University of California Davis, PO Box 247, Bodega Bay, California 94923, USA *Email: jlshinen@bio.puc.cl ABSTRACT: Trophic relationships among native and exotic species produce novel direct and indirect interactions that can have wide-ranging community level effects and perhaps confer invasion resistance. We investigated whether native predators have the potential to directly limit the spread of the exotic mussel Mytilus galloprovincialis or mediate interactions among native and invasive mussels at a rocky intertidal invasion front in northern California. Lower survival of M. galloprovincialis in transplanted cultures exposed to predators indicated that the invader was more vulnerable to predators than the numerically dominant native M. californianus. Survival and per capita mortality rate in monocultures and polycultures did not vary for either M. galloprovincialis or M. californianus, suggesting that predator-mediated apparent competition and associational defense did not occur. Complementary laboratory feeding trials determined which among 3 intertidal predators preferred the exotic to 2 native species of mussel. The whelk Nucella ostrina was most selective, consuming the thinner shelled mussels (M. galloprovincialis and the native M. trossulus) rather than the thicker-shelled native species M. californianus. The crab Cancer antennarius and the sea star Pisaster ochraceus showed no preferences among mussel species. N. ostrina were commonly observed among field-transplanted mussels; thus whelk predation may be especially important in limiting the establishment of the invasive mussel. However, 15% of M. galloprovincialis remained intact in the field after 1 yr, suggesting that predation alone may not inhibit establishment of the invader. A tenuous balance between larval settlement and early post-settlement predation likely characterizes the invasion front. KEY WORDS: Invasion · Resistance · Predation · Indirect effects · Rocky intertidal · Mussel · Mytilus Full text in pdf format PreviousNextCite this article as: Shinen JS, Morgan SG, Chan AL (2009) Invasion resistance on rocky shores: direct and indirect effects of three native predators on an exotic and a native prey species. Mar Ecol Prog Ser 378:47-54. https://doi.org/10.3354/meps07870Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 378. Online publication date: March 12, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Predator–prey interactions under climate change: the importance of habitat vs body temperature

Habitat temperature is often assumed to serve as an effective proxy for organism body temperature when making predictions of species distributions under future climate change. However, the determinants of body temperature are complex, and organisms in identical microhabitats can occupy radically different thermal niches. This can have major implications of our understanding of how thermal stress modulates predator–prey relationships under field conditions. Using body temperature data from four different sites on Santa Cruz Island, California, we show that at two sites the body temperatures of a keystone predator (the seastar Pisaster ochraceus) and its prey (the mussel Mytilus californianus) followed very different trajectories, even though both animals occupied identical microhabitats. At the other two sites, body temperatures of predator and prey were closely coupled across a range of scales. The dynamical differences between predator and prey body temperatures depended on the location of pairs of sites, at the extremes of a persistent landscape‐scale weather pattern observed across the island. Thus, the well understood predator–prey interaction between Pisaster and Mytilus cannot be predicted based on habitat‐level information alone, as is now commonly attempted with landscape‐level (‘climate envelope’) models.

Climate change in the rocky intertidal zone: predicting and measuring the body temperature of a keystone predator

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 374:43-56 (2009) - DOI: https://doi.org/10.3354/meps07682 Climate change in the rocky intertidal zone: predicting and measuring the body temperature of a keystone predator P. Lauren Szathmary*, Brian Helmuth, David S. Wethey University of South Carolina, Department of Biological Sciences and Marine Science Program, Columbia, South Carolina 29208, USA *Email: szathmary@biol.sc.edu ABSTRACT: Forecasting the responses of populations and ecosystems to climate change requires that we understand both the direct effects of temperature on organism physiology and the indirect effects of temperature change on interactions such as predation and competition. The sea star Pisaster ochraceus is a keystone predator in the rocky intertidal zone with a broad geographic distribution along the west coast of North America. We developed a mechanistic heat budget model that uses environmental data to predict the body temperatures of P. ochraceus. Model accuracy was verified by comparing model output temperatures, generated using measured microclimatic data as input, to the temperatures of live P. ochraceus in the field. The average absolute errors between predicted and measured body temperatures were ~1°C. To continuously monitor sea star body temperatures in the field, we developed data loggers that thermally mimic P. ochraceus. Accuracy of these biomimetic loggers was tested by comparing logger temperatures to P. ochraceus body temperatures, and they were found to mimic body temperatures within ~1°C. Loggers were deployed at different tidal heights at a site in Bamfield, British Columbia, Canada, to investigate within-site body temperature variation. We also explored the relationship between body temperature and vertical distribution of P. ochraceus. A negative correlation was found between maximum body temperatures on Day n and the number of sea stars found in the intertidal on Day n + 1. These results suggest that temperatures reached during aerial exposure at low tide, at least in part, may determine where in the intertidal zone these sea stars are located, which could affect their foraging limits. KEY WORDS: Heat budget · Climate change · Intertidal zone · Pisaster ochraceus · Sea star · Temperature logger Full text in pdf format PreviousNextCite this article as: Szathmary PL, Helmuth B, Wethey DS (2009) Climate change in the rocky intertidal zone: predicting and measuring the body temperature of a keystone predator. Mar Ecol Prog Ser 374:43-56. https://doi.org/10.3354/meps07682 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 374. Online publication date: January 13, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

An Intertidal Sea Star Adjusts Thermal Inertia to Avoid Extreme Body Temperatures

The body temperature of ectotherms is influenced by the interaction of abiotic conditions, morphology, and behavior. Although organisms living in different thermal habitats may exhibit morphological plasticity or move from unfavorable locations, there are few examples of animals adjusting their thermal properties in response to short‐term changes in local conditions. Here, we show that the intertidal sea star Pisaster ochraceus modulates its thermal inertia in response to prior thermal exposure. After exposure to high body temperature at low tide, sea stars increase the amount of colder‐than‐air fluid in their coelomic cavity when submerged during high tide, resulting in a lower body temperature during the subsequent low tide. Moreover, this buffering capacity is more effective when seawater is cold during the previous high tide. This ability to modify the volume of coelomic fluid provides sea stars with a novel thermoregulatory “backup” when faced with prolonged exposure to elevated aerial temperatures.

Overdominant maintenance of diversity in the sea star Pisaster ochraceus

When individuals have higher evolutionary fitness because of being heterozygous at a given gene region, it is known as overdominance. Although overdominant selection could represent an important mechanism for maintaining genetic variation within species, the prevalence of this mode of selection appears to be relatively low. Identification of cases of true single-locus heterozygote advantage are thus useful reference points in our overall understanding of how various forms of balancing selection influence and maintain genetic variation in natural populations. Here we report the apparent long-term maintenance of diversity via overdominant selection with homozygous lethality at an elongation factor locus in the sea star Pisaster ochraceus. Observing this pattern in a gene with such major effects on protein assembly indicates that overdominant selection could be a more prevalent factor in maintaining allelic diversity in the wild than previously recognized.

Solar radiation plays a role in habitat selection by the sea star Pisaster ochraceus

In order to avoid intense abiotic stresses at low tide, motile animals in the intertidal zone must identify and move into 'safe' microhabitats before the tide drops. We experimentally investi- gated the role of solar radiation in the microhabitat selection of the sea star Pisaster ochraceus, an ecologically important predator in the Pacific Northwest. Specifically, we tested the response of P. ochraceus to UVB radiation (290 to 315 nm), UVA radiation (315 to 400 nm), and photosynthetically active radiation (PAR; 400 to 700 nm) to determine whether specific wavelength ranges prompted specific responses. Sea stars were exposed to ambient summer light levels in shallow water in out- door aquaria in which one-half of the tank was covered with a light-filtering material. We established 4 light filtration treatments: filter control (+UVB +UVA +PAR), no UVB (-UVB +UVA +PAR), no UVR (-UVB -UVA +PAR), and shade (-UVB -UVA -PAR), plus a no-filter control treatment (+UVB +UVA +PAR). P. ochraceus did not avoid UVB or UVA, but they did avoid PAR: individuals in tanks with shaded areas spent significantly more time under the filters than did individuals in the tanks with fil- ters that blocked UV radiation. In field surveys, 85% of emersed P. ochraceus encountered during low tide were located in areas completely shaded from both PAR and UV radiation (by rocks or by the intertidal kelp Hedophyllum sessile). This study demonstrates that the presence of PAR in shallow water conditions prompts P. ochraceus to seek out shaded microhabitats that have more benign low- tide abiotic conditions than exposed rock benches.

Body temperature during low tide alters the feeding performance of a top intertidal predator

Although the direct effects of climate change on species distribution and abundance have become increasingly apparent, considerably less is known about the potential for thermal variations to influence community structure indirectly through altered species interactions. We examined how the low tide body temperature of the rocky intertidal sea star Pisaster ochraceus affected the rate at which this keystone species fed on the mussel Mytilus californianus. Patterns of intertidal distribution and sea star body temperatures were recorded in the field at Bodega Bay, California, in 2006 and 2007. Under controlled laboratory conditions, we separately examined the roles of short‐term (acute) and long‐term (chronic) exposures to high aerial body temperatures in driving patterns of feeding and growth. Upper limits to thermal tolerance were also quantified in the laboratory. Integrating field data on the vertical distribution, microhabitat, and body temperature of sea stars indicated that individuals typically experienced relatively high body temperatures over a series of short periods of time (acute exposures of a few days) and that they escaped from situations where they would have experienced such temperatures for longer periods (chronic exposure). Although sea stars never approached their upper thermal limit (35°C), laboratory trials demonstrated that chronic exposure to realistic peak aerial body temperatures (≥23°C) negatively affected both feeding and growth rates, whereas acute exposure had positive consequences on feeding rates but no effects on growth. Variations in aerial body temperature, and more specifically the frequency of exposure to high body temperatures, can influence interaction strength in this important predator‐prey system.

Reproductive output and consistency of source populations in the sea star Pisaster ochraceus

Although a common goal of marine reserves is to protect areas of high reproductive out- put, few studies have tested whether these source populations are spatially and temporally consistent. For 6 years between 1995 and 2005, we examined nutrient storage and gonad production in the sea star Pisaster ochraceus at 2 sites on the central coast of Oregon, USA. Sea stars accumulated energy in the pyloric caeca during a short season of intensive feed- ing (May to September). Within a site, the size of the pyloric caeca in the fall accurately predicted repro- ductive output during the following spring. In accord with hypotheses regarding bottom-up effects, the pyloric caeca were generally larger at the site where mussels were more abundant in the diet, indicating a greater accumulation of surplus energy. However, gonad indices were consistently ~50% lower at this site, suggesting striking differences in allocation to reproduction. This difference between populations was maintained despite marked variation among years in oceanographic conditions and reproductive output. These results suggest that (1) source popula- tions may be consistent spatially, but (2) identifying those sources may be more complex than simply tar- geting areas with high quality resources and strong bottom-up influences. Rather, predicting patterns of per capita reproductive output may require a more detailed understanding of the causes of spatial varia- tion in allocation to maintenance, growth, and repro- duction.