Asterias Vulgaris Research Articles

Effect of different predators on the escape response of Placopecten magellanicus

To assess whether giant scallops, Placopecten magellanicus use distinct escape strategies to respond to their seastar and crustacean predators, escape responses to two major seastar predators, Asterias vulgaris and Leptasterias polaris, two seastars with little predatory impact, Crossaster papposus and Solaster endeca, and two crustacean predators, Cancer irroratus and Hyas araneus were compared. A glass rod served as a mechanical control. The responses of juvenile [2+ year (y), ~36-mm shell height (SH)] and adult (6+ y, ~100-mm SH) scallops from the Magdalen Islands, Quebec, Canada, were assessed in early summer 2005. The predatory seastars evoked the strongest response, in terms of both response latency and minimum interval between phasic contractions and numbers of phasic contractions, particularly early in the escape response. Both the minor seastar predators and crabs stimulated stronger responses than the mechanical control. Juvenile scallops were livelier than adult scallops. As P. magellanicus consistently responded to predators with an initial flurry of phasic contractions that tapered off to spaced phasic contractions separated by increasingly long tonic contractions, only the intensity of the escape response seems to have been modified by selection.

Behavioural mechanisms of sea stars ( Asterias vulgaris Verrill and Leptasterias polaris Müller) and crabs ( Cancer irroratus Say and Hyas araneus Linnaeus) preying on juvenile sea scallops ( Placopecten magellanicus (Gmelin)), and procedural effects of scallop tethering

We compared predation rates and behaviours of sea stars ( Asterias vulgaris and Leptasterias polaris) and crabs ( Cancer irroratus and Hyas araneus) preying on juvenile sea scallops ( Placopecten magellanicus, 25-35 mm shell height) in laboratory. These predatory species co-occur with sea scallops on the sea bed of the Gulf of St. Lawrence, Canada, and limit scallop survival in seeding operations. We also examined, under controlled conditions, the effect of tethering scallops on predator–prey interactions. Predation rates, time budgets and encounter behaviours observed for A. vulgaris and C. irroratus preying on free (untethered) scallops were comparable to previous studies. C. irroratus were more effective predators as they consumed 3.1 scallops predator − 1 day − 1 , although they spent only 0.9% of their time searching for prey. A. vulgaris consumed 0.9 scallops predator − 1 day − 1 and spent 7.6% of their time searching. Sea stars L. polaris had a lower predation rate (0.02 scallop predator − 1 day − 1 ) than A. vulgaris. The frequent avoidance behaviour of L. polaris and its low ability to capture scallops support the notion that scallops are not a main component of this sea star's diet. Crabs H. araneus had similar predation rates (1.3 scallops predator − 1 day − 1 ) and behaviours to C. irroratus, although the probability of consumption upon capture was affected by relatively high numbers of rejections and post-capture escapes of scallops. As expected, the tethering procedure increased predation rate of L. polaris (about 19 times higher), but surprisingly did not significantly affect that of A. vulgaris. Examination of behaviours indicated that A. vulgaris offered tethered scallops tended to have a higher probability of capture, but spent less time searching for prey (possibly because satiation was reached) than A. vulgaris offered free scallops. Predation rates and behaviours of both crab species were not affected by tethering, since encounter rate was the primary determinant of crab-scallop interactions. Identification and quantification of behaviours underlying the predation process allowed us to mathematically model predator-related mortality for the four predator species.

Poor phenotypic integration of blue mussel inducible defenses in environments with multiple predators

Aquatic prey encounter an array of threat cues from multiple predators and killed conspecifics, yet the vast majority of induced defenses are investigated using cues from single predator species. In most cases, it is unclear if odors from multiple predators will disrupt defenses observed in single‐predator induction experiments. We experimentally compared the inducible defenses of the common marine mussel Mytilus edulis to waterborne odor from pairwise combinations of three predators representing two attack strategies. Predators included the sea star, Asterias vulgaris (=Asteriasrubens), and the crabs Carcinus maenas and Cancer irroratus. The mussels increased adductor muscle mass in response to cues from unfed Asterias (a predatory seastar that pulls mussel shells open) and increased shell thickness in response to unfed Carcinus, a predatory crab that crushes or peels shells. However, the mussels did not express either predator specific response when exposed to the combined cues of Asterias and Carcinus, and mussels did not increase shell thickness when exposed to cues from Cancer alone or any pairwise combination of the three predators. Shell closure or ‘clamming up’ did not occur in response to any predator combination. These results suggest that predator‐specific responses to the Asterias and Carcinus are poorly integrated and cannot be expressed simultaneously. Simultaneous cues from multiple predators affect the integration of predator specific defenses and predator odors from functionally similar predators do not necessarily initiate similar defenses. Ultimately, the degree that prey can integrate potentially disparate defenses in a multiple predator environment may have ecological ramifications and represent a seldom explored facet of the evolution of inducible defenses.

Spawning synchrony and aggregative behaviour of cold-water echinoderms during multi-species mass spawnings

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 361:161-168 (2008) - DOI: https://doi.org/10.3354/meps07415 Spawning synchrony and aggregative behaviour of cold-water echinoderms during multi-species mass spawnings John H. Himmelman1,*, Clément P. Dumont1,2, Carlos F. Gaymer1,2, Catherine Vallières1, David Drolet1 1Département de Biologie, Université Laval, Québec City, Québec G1K 7P4, Canada 2Center for Advanced Studies in Arid Zones (CEAZA), Departamento de Biologia Marina, Universidad Catolica del Norte, Larrondo 1281, Casilla 117, Coquimbo, Chile *Email: john.himmelman@bio.ulaval.ca ABSTRACT: Although many marine invertebrates reproduce by releasing gametes into the water, major spawning events have rarely been observed for cold-water invertebrates. We documented mass spawnings by the sea star Asterias vulgaris and the ophiuroid Ophiopholis aculeata on 6 July 1996 and 2 July 1998, and by these 2 species plus a second ophiuroid Ophiura robusta and the sea urchin Strongylocentrotus droebachiensis on 3 July 2003. In each year, the abrupt spawning events coincided with the first major intrusion of warm surface water into a region normally dominated by cold upwelling. The behaviours of the ophiuroids and sea stars favoured fertilization success. First, they spawned from elevated surfaces, with the central disc raised, to increase suspension and mixing of gametes. Second, many individuals pseudocopulated, which involved crawling over one another in the case of the sea star, and individuals holding their central discs together in the case of the 2 ophiuroids. Models of fertilization success need to incorporate such behaviours to better understand fertilization in the sea. KEY WORDS: Ophiopholis aculeata · Ophiura robusta · Asterias vulgaris · Strongylocentrotus droebachiensis · Mass spawning · Behaviour · Pseudocopulation · External fertilization · Northern Gulf of St. Lawrence Full text in pdf format Supplementary appendix PreviousNextCite this article as: Himmelman JH, Dumont CP, Gaymer CF, Vallières C, Drolet D (2008) Spawning synchrony and aggregative behaviour of cold-water echinoderms during multi-species mass spawnings. Mar Ecol Prog Ser 361:161-168. https://doi.org/10.3354/meps07415Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 361. Online publication date: June 09, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Specificity of induced defenses in Mytilus edulis and asymmetrical predator deterrence

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 334:145-153 (2007) - doi:10.3354/meps334145 Specificity of induced defenses in Mytilus edulis and asymmetrical predator deterrence Aaren S. Freeman* Zoology Department, University of New Hampshire, 46 College Road, Durham, New Hampshire 03824, USA *Email: afreeman@unh.edu ABSTRACT: Induced defenses of prey have become widely recognized in several marine taxa, yet their specificity to particular predators and impacts on subsequent predation are seldom investigated. In this study, Mytilus edulis showed highly specific induced defenses in response to predators with different attack strategies. The mussels developed significantly heavier shells only in the presence of waterborne cues from Carcinus maenas, a crushing crab predator, and significantly heavier adductor muscles only in the presence of waterborne cues from Asterias vulgaris (= Asterias rubens), a predatory sea star that pries bivalves open. However, mussels effectively thickened their shells in response to cues from predators by either increasing allocation to shell (in response to C. maenas), or reducing linear shell growth (in response to A. vulgaris and, to a lesser extent, the predatory whelk Nucella lapillus). These different mechanisms of shell thickening in response to all 3 predators defended the mussels from subsequent crab predation, increasing handling times of mussels by predatory C. maenas by more than 48%. In contrast, only mussels with increased adductor muscle weight (an induced response to A. vulgaris) were subsequently protected from the sea star. These results suggest that although induced defenses in M. edulis are specific to predators with different attack strategies, diffuse selection by C. maenas may allow predator specific responses to be adaptive even when predator composition changes. KEY WORDS: Blue mussel · Asterias vulgaris · Carcinus maenas · Nucella lapillus · Asterias rubens · Inducible defense · Phenotypic plasticity Full text in pdf format PreviousNextExport citation Mail this link - Contents Mailing Lists - RSS Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 334. Online publication date: March 26, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

SEDIMENT SELECTION BY JUVENILE SEA SCALLOPS (CANCER IRRORATUS SAY)

Abstract We examined sediment selection by juvenile sea scallops (Placopecten magellanicus) and their predators, sea stars (Asterias vulgaris) and rock crabs (Cancer irroratus). In laboratory trials, groups of scallops (~30 mm shell height) were simultaneously offered four sediment types in the presence and absence of a sea star or a rock crab: (1) glass representing a homogeneous, hard bottom; (2) sand; (3) granule and (4) pebble. As well, individual predators were offered the four sediment types without scallops. The number of scallops and the proportion of time predators spent on each sediment type were monitored over time. When compared with the expected distribution, scallops avoided glass and tended to select granule and pebble sediments when alone. In the presence of a rock crab, scallops also avoided glass. However, scallop distribution was similar to the expected distribution when a sea star was present, because sea star encounters are an important trigger of scallop swimming, leading to frequent...

Successional development of fouling communities on open ocean aquaculture fish cages in the western Gulf of Maine, USA

Growth of fouling organisms on suspended fish cages is an impediment to aquaculture projects in coastal waters around the world. The present study characterized ecological succession of fouling communities on the netting of fish cages at an open ocean aquaculture site 10 km east of New Hampshire, USA in the western Gulf of Maine. Ecological succession can be defined as the process by which a community moves from a simple level of organization to a more complex community. Routine cleaning of the cages causes loss of organisms and initiation of ecological succession. Experimental panels of nylon net material were deployed at different times of the year and for different durations from September 2002 to September 2003 (eleven sets of 1-month panels, four sets of 3-month panels, two sets of 6-month panels, and one set of 1-year panels), with four replicates of each deployment. Panels were randomly arranged on a grid that was attached to a fish cage at a water depth of ∼ 15 m. There were substantial and significant differences in density and biomass of the total communities of most successional sequences when comparing panels deployed during May–September to those deployed during the cooler months, October–April. However, the blue mussel, Mytilus edulis, dominated in density and biomass in almost every sequence, regardless of time of initiation or duration. Other species that occurred in high numbers and/or biomass were the amphipods Caprella sp. and Jassa marmorata, the molluscs Hiatella arctica and Anomia sp., the seastar Asterias vulgaris, and the anemone Metridium senile. Juveniles and adults of some species were also present in some early (1-month) successional sequences, indicating that migration may be an important process in community development. Some of the dominant species listed above were present in all successional stages (early, intermediate and late), differing only in relative abundances in the community. The consistent dominance of M. edulis, and other differences in successional patterns compared to what has been typically observed for epifaunal communities in the region, were hypothesized to be the result of a combination of factors: a lack of predators such as seastars and fish that typically consume mussels in natural communities, excessive predation by nudibranchs on those species (e.g., Tubularia sp.) normally abundant in early successional stages, year-round availability of mussel larvae, and cage cleaning protocols that do not remove all the organisms present. The introduction of predatory fishes or seastars into or onto the cages might provide some amount of control on the growth of fouling organisms.

Biochemical content, energy composition and reproductive effort in the broadcasting sea star Asterias vulgaris over the spawning period

To assess sex differences in reproductive effort, we examined the biochemical composition and energetic content of the principal body components of the broadcast spawning sea star Asterias vulgaris in the Mingan Islands in the northern Gulf of St. Lawrence, eastern Canada. The body wall was the most stable body component, showing no variations in mass or in lipid and protein content (and total energetic content) between sexes or during spawning. Patterns in the gonads differed between sexes and with spawning. The lipid, protein and carbohydrate content of the ovary dropped during spawning, while only the protein content of the testis decreased significantly. Reproductive effort, expressed as loss of energy in the gonads during spawning for an individual weighing 10 g in underwater mass (8.2 cm in radius), was six times greater in females (49.5 kJ) than males (7.9 kJ). The energetic content of the pyloric caeca also decreased during spawning, by 17.7 kJ in females and 21.5 kJ in males, mainly due to a decrease in lipids. If this decrease is included as reproductive effort, it lessens the gender difference. The caecum decrease possibly represented expenditures due to formation of aggregations or the expulsion of gametes during spawning. Effectively, we observed aggregations during a massive spawning in this population. The sex ratio did not differ from 1:1 in all size classes sampled. This suggests that, unless males suffer higher mortality, females manage to allocate as much energy to somatic growth as males, possibly by feeding at higher rates to compensate for their higher reproductive effort. Stomach protein content tended to be higher in females than males and may indicate greater muscular development to facilitate digestion.

Behavioural mechanisms underlying functional response of sea stars Asterias vulgaris preying on juvenile sea scallops Placopecten magellanicus

The functional response characterises the relationship between prey density and the consumption rate of individual predators. Typically, it is studied by fitting a model to observations of predation rate at different prey densities. The behavioural mechanisms underlying a functional response are not well understood, and estimates of model parameters seldom conform to observations of behaviour. We have developed a mechanistic approach that directly incorporates behavioural observations into characterisation of the functional response. Laboratory experiments were used to record predation rates and observe foraging behaviour of sea stars Asterias vulgaris preying on juve- nile sea scallops Placopecten magellanicus at different densities. Experiments were conducted in tanks with no sediment and tanks with sediment. Behavioural data from the experiments were used to calculate parameters of functional response models Th (handling time per prey) and a (rate of suc- cessful search). On both substrates, Th remained constant across prey density, while a was density- dependent. An inverse quadratic was used to describe a and was incorporated into a functional response model. Estimates of a were also obtained by fitting the functional response model to the pre- dation rate data using regression analysis. These estimates of a were highly consistent with the esti- mates calculated from behavioural data. On both substrates, sea stars preying on scallops had a Type III sigmoid-shaped functional response; on sediment, predation rate decreased at high prey densities. Sea star ability to capture attacked prey was probably the mechanism underlying the observed responses. In general, behavioural information can lead to better understanding of observed func- tional responses.

SUBSTRATE EFFECTS ON SURVIVAL, GROWTH AND DISPERSAL OF JUVENILE SEA SCALLOP, PLACOPECTEN MAGELLANICUS (GMELIN 1791)

Abstract In the scope of expanding seeding operations of juvenile scallops (Placopecten magellanicus) around Iles-de-la-Madeleine (Quebec, Canada), this study assesses the influence of substrate on growth, survival, dispersal and predation rates in controlled experiments. Three substrates were tested: gravel, sand and sand with empty scallop shells (to investigate the feasibility of modifying the substrate as a management strategy). Dispersal rates were studied with a substrate choice experiment. Predation experiments with two major scallop predators, the rock crab Cancer irroratus and the sea star Asterias vulgaris, were carried out on different substrate types. No significant difference in growth was observed among substrates and no mortality was observed during the experiment. This suggests other factors, such as hydrodynamics, may be more important than substrate. The dispersal rate was significantly greater on sandy substrate, whereas gravel and sand–shells had similar results. Crab predation was low...

Impact of limited short-term sea scallop fishery on epibenthic community of Georges Bank closed areas

On Georges Bank, 2 areas that had been closed to sea scallop fishing since 1994 were opened for a limited harvest from August 2000 to February 2001. The effects of this limited short-term fishery on the epibenthic community were examined using a 'before/after, control/impact' environ- mental design conducted with video surveys. A centric systematic survey with 1379 stations placed on a 1.57 km grid, with 4 video quadrats collected at each station (3.235 m 2 per quadrat equaling 17789 m 2 total sample area), was completed in 2 control and 2 impact areas before and after the fish- ery. The sea scallops Placopecten magellanicus and starfishes (primarily Asterias vulgaris) comprised more than 84% of the fauna. Changes in the number of taxonomic categories and the density of indi- viduals within each category in the areas impacted by the fishery were similar to changes in the con- trol areas that remained closed to fishing. Further, sediment composition shifted between surveys more than epibenthic faunal composition, suggesting that this community is adapted to a dynamic environment. The limited short-term sea scallop fishery on Georges Bank appeared to alter the epibenthic community less than the natural dynamic environmental conditions.

Behavioural mechanisms underlying functional response of sea stars Asterias vulgaris preying on juvenile sea scallops Placopecten magellanicus

Behavioural mechanisms underlying functional response of sea stars Asterias vulgaris preying on juvenile sea scallops Placopecten magellanicus

PREDATION POTENTIAL OF THE INVASIVE GREEN CRAB (CARCINUS MAENAS) AND OTHER COMMON PREDATORS ON COMMERCIAL BIVALVE SPECIES FOUND ON PRINCE EDWARD ISLAND

The prey and size-class preferences of four marine epibenthic predators was examined in a laboratory study by determining the mortality rates of four commercial bivalve species in 4-d trials where predators were present or absent. Bivalve species used were quahogs (Mercenaria mercenaria), eastern oysters (Crassostrea virginica), blue mussels (Mytilus edulis) and soft-shell clams (Mya arenaria). Prey size-classes investigated were 0–15, 15–25 and 25–40 mm. A first set of experiments compared the mortality rates of prey in the presence of the invasive green crab (Carcinus maenas) and the native rock crab (Cancer irroratus). The second set of experiments compared the predation behavior of the green crab, the common starfish (Asterias vulgaris) and the moon snail (Euspira heros). Single- and multiple-choice experiments were carried out in relation to the prey species being challenged by the predator. Results from the first set of experiments showed that green crabs preyed on all prey species from all size-classes in single and multiple-choice trials. Mussels and clams were the preferred prey species. Rock crabs preyed on mussels in single- and multiple-choice trials. Rock crabs preyed heavily on soft-shell clams in single-choice trials, whereas no clams were eaten in multiple-choice trials. Rock crabs did not prey on quahogs in single- and multiple-choice trials. In both trials, small individuals were preyed on more often by both crab species. The second set of experiments confirmed results observed for the green crab in the first set of experiments. The common starfish was a very active predator as well. Blue mussels were the preferred prey species of the starfish. Moon snails displayed a much lower predation activity than the other predators. The multiple-choice experiment showed different results in that predation rates were lower than in cases where predators were facing one prey species at a time. Overall, our results showed that the two crab species and the common starfish displayed a generalist feeding behavior. Results also showed that the invasive green crab might represent a new predation threat to commercial bivalves as well as a competing threat to the native rock crab.

Prey selection and the functional response of sea stars ( Asterias vulgaris Verrill) and rock crabs ( Cancer irroratus Say) preying on juvenile sea scallops ( Placopecten magellanicus (Gmelin)) and blue mussels ( Mytilus edulis Linnaeus)

Predators in nature include an array of prey types in their diet, and often select certain types over others. We examined (i) prey selection by sea stars ( Asterias vulgaris) and rock crabs ( Cancer irroratus) when offered two prey types, juvenile sea scallops ( Placopecten magellanicus) and blue mussels ( Mytilus edulis), and (ii) the effect of prey density on predation, prey selection, and component behaviours. We quantified predation rates, behavioural components (proportion of time spent searching for prey, encounter probabilities) and various prey characteristics (shell strength, energy content per prey, handling time per prey) to identify mechanisms underlying predation patterns and to assess the contribution of active and passive prey selection to observed selection of prey. Sea stars strongly selected mussels over scallops, resulting from both active and passive selection. Active selection was associated with the probability of attack upon encounter; it was higher on mussels than on scallops. The probability of capture upon attack, associated with passive selection, was higher for mussels than for scallops, since mussels can not swim to escape predators. Sea stars consumed few scallops when mussels were present, and so did not have a functional response on scallops (the target prey). Rock crabs exhibited prey switching: they selected mussels when scallop density was very low, did not select a certain prey type when scallop density was intermediate, and selected scallops when scallop density was high relative to mussel density. The interplay between encounter rate (associated with passive selection) and probability of consumption upon capture (associated with both active and passive selection) explained observed selection by crabs. Scallops were encountered by crabs relatively more often and/or mussels less often than expected from random movements of animals at all scallop densities. However, the probability of consumption varied with scallop density: it was lower for scallops than mussels at low and intermediate scallop densities, but tended to be higher for scallops than mussels at high scallop densities. When mussels were absent, crabs did not have a functional response on scallops, but rather were at the plateau of the response. When mussels were present with scallops at relatively low density, crabs exhibited a type II functional response on scallops. Our results have implications for the provision of protective refuges for species of interest (i.e., scallops) released onto the sea bed, such as in population enhancement operations and bottom aquaculture.

Foraging behavior and activity budgets of sea stars on a subtidal sediment bottom community

We dived at regular intervals (8, 12 or 14 h) over periods of 9 to 23 days to quantify the allocation of time to different activities by large adults of the four common sea stars foraging in a sediment bottom community (8 to 11 in depth) in the northern Gulf of St. Lawrence. The relative time spent at different activities was similar for Leptasterias polaris and Asterias vulgaris, the most frequent activity being moving over the bottom (42–46% of the time), presumably in search of prey, secondly being stationary (26–27%), and thirdly digesting prey (17–20%). Capture of prey was less frequent, 13% of the time for L. polaris (mainly digging for bivalves) and 5% for A. vulgaris. Compared to L. polaris and A. vulgaris, Crossaster papposus spent more time being stationary and periods of moving were of shorter duration. Solaster endeca differed from the other sea stars because of the high proportion of time spent digesting prey (40%) and the small proportion of time spent moving (22%). Capture events were not observed for C. papposus and S. endeca, likely because they exclusively fed on epifaunal species, which were rapidly captured. For all four species, the most frequent changes between activities were between moving and being stationary. The number of days per prey eaten (length of a feeding bout) was about 11 for S. endeca, 6.6 for L. polaris, 3.1 for C. papposus and 2.8 for A. vulgaris. All sea stars appear to be energy maximizers that spend most of their time finding, capturing and digesting prey.

Resorption of Gametes in the Testes of the Sea Star Asterina pectinifera (Mueller et Troschel, 1842)

The process of resorption of spermatozoa is described in the testes of the starfish Asterina pectinifera. It is shown that the mode of resorption of the male gametes in this species is different from that in the starfish Asterias vulgaris (see Walker [17, 18]), A. amurensis, and Aphelasterias japonica (see Kas’yanov et al.

Prey selection and predatory impact of four major sea stars on a soft bottom subtidal community

We dived at regular intervals (8-, 12- or 24-h) over periods up to 24 days to quantify the feeding activities of identified sea stars along permanent transects in the upper sediment bottom zone (8–11 m deep). In this habitat, large individuals of four sea stars predominated. The diet of Asterias vulgaris (molluscs and echinoderms) was intermediate between that of Leptasterias polaris (mainly molluscs) and that of Crossaster papposus (mainly echinoderms), whereas the diet of Solaster endeca strongly differed from the other sea stars, as it only fed on holothuroids. Calculation of the Ivlev electivity index indicated little or no selection for many abundant prey species, whereas some rare prey in this zone (e.g., Mytilus edulis) were strongly selected. We quantified prey size selection, and observed evidence of selection on smaller individuals when feeding on infaunal bivalves (possibly the deeper burying of larger individuals limited attacks) and selection of larger individuals when feeding on epifaunal prey (possibly to maximize energetic intake). We observed A. vulgaris feeding on Mya truncata, which had been captured by L. polaris (kleptoparasitism) and this bivalve disappeared from the diet of A. vulgaris in areas where we experimentally removed L. polaris. Estimations of foraging rates over 100 days relative to prey availability indicated strong interactions among the sea stars themselves. C. papposus likely have a strong impact on conspecifics, L. polaris and A. vulgaris, and A. vulgaris on L. polaris. In contrast, <1% of total biomass of the most abundant prey of L. polaris ( M. truncata) and C. papposus ( Strongylocentrotus droebachiensis) was consumed in 100 days.

Role of current and prey odour in the displacement behaviour of the sea starAsterias vulgaris

We performed a factorial experiment to investigate the effects of current and prey odours (mussels) on the displacement behaviour of the sea star Asterias vulgaris (Verrill, 1866). This sea star is a common subtidal predator of sessile and slow-moving animals in the western North Atlantic Ocean. In the presence of current and prey odours in a flume, sea stars oriented themselves upstream and 70% succeeded in finding the prey. Also, the degree of orientation toward the prey increased as the sea star approached the prey. In contrast, only 5% of individuals tested in still water found the prey. Thus, for A. vulgaris the presence of macroscale flow is an essential condition for locating distant prey. Sea stars tested in current alone showed rheotactic behaviour, moving diagonally upstream. This behaviour should enhance the probability of encountering prey odour plumes in the field. Sea stars moved faster and straighter in flowing water than in still water. The slow movement of A. vulgaris in still water probably minimizes costs of foraging when there is a low chance of finding prey and the straight diagonal movement in current should ensure that the sea star continuously samples new areas, rather than resampling the same odour-free area.

Use of refuges by the ophiuroid Ophiopholis aculeata: contrasting effects of substratum complexity on predation risk from two predators

We investigated the effect of substratum complexity on the vulnerability of the ophi- uroid Ophiopholis aculeata to its 2 main predators, the sea star Asterias vulgaris (possibly synony- mous with A. rubens) and the urchin Strongylocentrotus droebachiensis, and further examined behavioral responses of ophiuroids to these predators. Field and laboratory experiments showed that the use of crevices by ophiuroids tended to reduce predation by urchins. The ophiuroids' response to contact with the urchin was to retract deeper into crevices. In contrast, in the laboratory, the ophi- uroids' vulnerability to the sea star was higher on complex substrata. In the laboratory, ophiuroids reacted weakly to contact with the arm tip or ambulacral groove of A. vulgaris, but immediately fled refuges when there was contact with the sea star's stomach. In the field, the density of ophiuroids in 20 cm diameter circular plots placed around sea stars (both feeding and non-feeding) was similar to that in areas without sea stars. However, the estimated density of ophiuroids directly under sea stars was less than in areas without sea stars, and exposed ophiuroids (with the disk outside a crevice) were more abundant near A. vulgaris than in areas without sea stars: both differences were greater when the sea star was feeding. Ophiuroids display risk sensitivity adjusted to the 2 predators. They mainly flee crevices under a sea star when there is contact with the sea star's stomach, but only move a few centimeters. A last-moment departure should increase the probability of capture by sea stars, but reduces time spent on open surfaces where they are vulnerable to urchin attacks.

MASSIVE PREY RECRUITMENT AND THE CONTROL OF ROCKY SUBTIDAL COMMUNITIES ON LARGE SPATIAL SCALES

What drives community change on large spatial scales? An opportunity to address this fundamental question was provided by a massive subtidal recruitment of the mussel, Mytilus edulis, across the southwest Gulf of Maine (GOM), USA, in 1995. Since M. edulis is consumed by many predator species, we hypothesized that the episodic increase in food resources had a bottom-up effect on populations of mussel consumers. The mussel recruitment event was unprecedented in its large spatial extent, high coverage of the sea floor, and depth of penetration into the rocky subtidal zone. During 1995-1997 we tracked mussels and their consumer populations in the area to document what happened. Surveys across a 120-km marine landscape revealed that juvenile mussels covered 33-91% of the bottom (8-12 m depth) at 13 of 17 sites between October 1995 and June 1996. The cover of mussel prey in June 1996 explained significant variation in the density and biomass of mussel predators, the sea stars Asterias spp. and rock crabs Cancer spp., at 16-17 sites 10, 13, and 23 months after mussel recruitment. These results are consistent with the hypothesis that the extensive mussel recruitment had a bottom-up effect on the subtidal food web. Levels of Asterias spp. recruitment, biomass, and density following mussel recruitment were the highest recorded at two monitored sites in 16-18 years. One year after the large pulse of mussel food resources, Asterias spp. recruitment was positively related to the percent cover of mussels, suggesting a feedback between food supply and consumer re- cruitment on a large spatial scale. Crab size-structure data supported the interpretation that recruitment contributed to the high densities of rock crabs recorded in July 1997. We suggest that the bottom-up effects observed in consumer populations were produced by increased growth, elevated reproduction and recruitment, and possibly by immigration. Coupling between bottom-up and top-down control was observed after a lag of 12-14 months post-mussel recruitment when predation by sea stars, rock crabs, and sea urchins eliminated extensive beds of juvenile mussels across the study region. The elimination of Mytilus prey triggered cannibalism in the sea star Asterias vulgaris, which contributed to density-dependent population declines in sea stars between 1996 and 1997. A key as- sumption of a consumer reproduction and recruitment response—that some of the larvae produced by local adult stocks of consumers could be retained in the study region—was tested by investigating the potential larval dispersal of sea stars and crabs in a simulated three-dimensional flow field of the southwest GOM. The simulation supported the as- sumption as 15-75% of particles released below 1-m depth were retained in the region over 2-5 week periods approximating the larval life-span of the consumers. Furthermore, the degree of retention increased dramatically between the surface (1 m) and 15-m depth, implying that the extent of large-scale connectivity by passive larval dispersal is highly dependent on depth. In this study, massive prey recruitment stimulated coupling between bottom-up and top-down forces, resulting in broad-scale changes in local consumer pop- ulations that persisted after the prey-recruitment signal was removed, underscoring the importance of episodic events in marine community dynamics.