Evasterias Troschelii Research Articles

Lethal and sublethal implications of low temperature exposure for three intertidal predators

Benthic invertebrate predators play a key role in top-down trophic regulation in intertidal ecosystems. While the physiological and ecological consequences of predator exposure to high temperatures during summer low tides are increasingly well-studied, the effects of cold exposure during winter low tides remain poorly understood. To address this knowledge gap, we measured the supercooling points, survival, and feeding rates of three intertidal predator species in British Columbia, Canada — the sea stars Pisaster ochraceus and Evasterias troschelii and the dogwhelk Nucella lamellosa — in response to exposure to sub-zero air temperatures. Overall, we found that all three predators exhibited evidence of internal freezing at relatively mild sub-zero temperatures, with sea stars exhibiting an average supercooling point of −2.50 °C, and the dogwhelk averaging approximately −3.99 °C. None of the tested species are strongly freeze tolerant, as evidenced by moderate-to-low survival rates after exposure to −8 °C air. All three predators exhibited significantly reduced feeding rates over a two-week period following a single 3-h sublethal (−0.5 °C) exposure event. We also quantified variation in predator body temperature among thermal microhabitats during winter low tides. Predators that were found at the base of large boulders, on the sediment, and within crevices had higher body temperatures during winter low tides, as compared to those situated in other microhabitats. However, we did not find evidence of behavioural thermoregulation via selective microhabitat use during cold weather. Since these intertidal predators are less freeze tolerant than their preferred prey, winter low temperature exposures can have important implications for organism survival and predator-prey dynamics across thermal gradients at both local (habitat-driven) and geographic (climate-driven) scales.

Effects of ocean acidification and ocean warming on the behavior and physiology of a subarctic, intertidal grazer

The global ocean is expected to both acidify and warm concurrently; thus, multiple-stressor manipulative experimentation is an emergent area of study that ultimately aims to examine the individual and interactive effects of these factors on marine organisms. We characterized the physiological responses to acidification and warming of the intertidal grazer Lottia scutum, and examined how these ocean change variables influenced predator-prey dynamics with Evasterias troschelii, a key sea star predator. Specifically, we conducted a laboratory experiment where we exposed limpets to factorial combinations of temperature (11 and 15°C) and pH (7.6 and 8.0), and measured effects on thermal tolerance, metabolic rate, cortisol concentrations, and behavioral responses to the predator. We found that ocean warming (OW) decreased the critical thermal maxima (CTmax) and increased cortisol levels in L. scutum, whereas ocean acidification (OA) increased the mass-specific metabolic rate in this species. Additionally, we found that there was no significant effect of OA or OW on the anti-predator behavior of L. scutum when exposed to E. troschelii. These results highlight the need for future studies to integrate multidisciplinary experimental designs (i.e. behavior and physiology) that span multiple levels of biological organization to make ecologically relevant predictions for how marine organisms will respond to ocean change.

Evasterias troschelii

This datasheet on Evasterias troschelii covers Identity.

Emergent effects of two rocky intertidal predators on prey: Interaction of crabs and sea-stars may reduce mussel mortality

Two predator – one prey systems are widespread in nature, and have been important to both ecological theory and practice. A primary question for these systems is whether the two predators produce “emergent effects”: greater or less prey mortality than expected based on the two predators acting alone. We conducted an in-lab experiment on a two predator (the crab Cancer productus and the sea-star Evasterias troschelii), one prey (the bivalve Mytilus trossulus) system common to the marine rocky intertidal of the northeastern Pacific. In 156 mesocosm experimental trials, mortality rates of M. trossulus were compared between Crabs only, Sea-stars only, and a Mixed treatment with both predators present. The experiment used a substitutive design, i.e., predator density was held constant. Based on a Linear Mixed Effects Model, the interactive effects of the number of predator species and water temperature had the greatest effect on mussel mortality. For the observed temperature range, the two predators in the Mixed treatment killed fewer mussels than expected based on the average mussel mortality of the one-predator treatments. As the two predators were never observed to interfere with each other directly, the reduced prey mortality is possibly a trait-mediated indirect interaction, involving behavioral modification (mutual avoidance) on the part of the predators. Temperature may mediate these results; crabs consumed more mussels at higher temperatures while sea-stars did so at lower temperatures. As these experiments were conducted over a large part of the known temperature range for this region, these results suggest that mussels may experience seasonal shifts in both predation rates and the identity of the predator, which should be investigated in future field studies. Increasingly warmer temperatures anticipated in the coming century could increase mussel mortality due to predation by cancrid crabs, which appear to have temperature responsive feeding rates. Understanding this two-predator one-prey system better is particularly important given that mussels are important foundational species, and recent climate change has affected abundances of the taxa involved.

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.

The ‘Expansion-Contraction’ model of Pleistocene biogeography: rocky shores suffer a sea change?

Approximately 20,000 years ago the last glacial maximum (LGM) radically altered the distributions of many Northern Hemisphere terrestrial organisms. Fewer studies describing the biogeographic responses of marine species to the LGM have been conducted, but existing genetic data from coastal marine species indicate that fewer taxa show clear signatures of post-LGM recolonization. We have assembled a mitochondrial DNA (mtDNA) data set for 14 co-distributed northeastern Pacific rocky-shore species from four phyla by combining new sequences from ten species with previously published sequences from eight species. Nuclear sequences from four species were retrieved from GenBank, plus we gathered new elongation factor 1-alpha sequences from the barnacle Balanus glandula. Results from demographic analyses of mtDNA for five (36%) species (Evasterias troschelii, Pisaster ochraceus, Littorina sitkana, L. scutulata, Xiphister mucosus) were consistent with large population expansions occurring near the LGM, a pattern expected if these species recently recolonized the region. However, seven (50%) species (Mytilus trossulus, M. californianus, B. glandula, S. cariosus, Patiria miniata, Katharina tunicata, X. atropurpureus) exhibited histories consistent with long-term stability in effective population size, a pattern indicative of regional persistence during the LGM. Two species of Nucella with significant mtDNA genetic structure showed spatially variable demographic histories. Multilocus analyses for five species were largely consistent with mtDNA: the majority of multilocus interpopulation divergence times significantly exceeded the LGM. Our results indicate that the LGM did not extirpate the majority of species in the northeastern Pacific; instead, regional persistence during the LGM appears a common biogeographic history for rocky-shore organisms in this region.

Association and distribution of the ciliate Orchitophrya stellarum with asteriid sea stars on the west coast of North America

The association of the scuticociliate Orchitophrya stellarum with 3 species of asteriid sea stars from the west coast of North America was studied by flushing the gonopore region with seawater and spawning the sea stars, along a latitudinal gradient of 2549 km between Pigeon Point, California, and Kodiak, Alaska. Asterias forbesii and A. rubens from the Isles of Shoals, New Hampshire (east coast), were also sampled. The ciliate was found on the aboral surface of both sexes of reproductively ripe Evasterias troschelii, Leptasterias spp., and Pisaster ochraceus with a maximum intensity of association occurring at Cape Arago, Oregon, and Clallum Bay and Manchester Dock, Washington. A survey of gonad smears and hematoxylin and eosin-stained sections indicated that the ciliate was only present in males. Spring-spawning E. troschelii and P. ochraceus are more negatively impacted by the ciliate than are winter-spawning Leptasterias spp. as judged by a skewed sex ratio and sex size differences, which may be associated with seasonal differences in water temperature affecting the growth rate of O. stellarum. The external morphology of O. stellarum appears to be similar throughout the geographical range surveyed.

Genetic structure and mode of reproduction in five species of sea stars (Echinodermata: Asteroidea) from the Alaskan coast

Genetic structure was studied in five sea star species with diverse patterns of reproduction. Eleven sea star samples from six locations in Alaska, representing five species and three genera in the family Asteriidae, were analyzed for allozyme variation at 16–25 loci. Levels of intra- and inter-population variation were determined for three brooding species (Leptasterias hexactis, Leptasterias epichlora, and Leptasterias polaris) and two free-spawning species with long planktonic larval periods (Evasterias troschelii and Pisaster ochraceus). Population divergence of L. epichlora (FST = 0.156) was much higher than that off. ochraceus (FST = 0.006) or E. troschelii (FST = 0.023). Earlier work in our laboratory found that L. hexactis also showed significant interpopulation differences in allele frequencies. Expected heterozygosity was 0.050 in E. troschelii, 0.083 in L. polaris, 0.092 in P. ochraceus, 0.135 in L. epichlora, and 0.151 in L. hexactis, and was unrelated to mode of reproduction.

Behavioral Ecology of Podding Red King Crab, Paralithodes camtschatica

Diel activity cycles and foraging dynamics for two pods of 500–800 juvenile red king crab, Paralithodes camtschatica, were monitored for 196 d. (Nov. 1987–June 1988) and 148 d. (Oct. 1988–Feb. 1989) in Womans Bay, Kodiak, Alaska, using SCUBA. Nocturnal foraging and homing behavior of podding red king crab were documented for the first time. Variation in time of pod dispersal into a nightly foraging aggregation was explained (R2 = 0.72) by changes in water temperature, crab weight, and time of sunset. A trend of increased foraging time and movement to deeper, cooler water was apparent after mid-April, as water temperatures reached 4 °C and began a sustained summer increase. Molting occurred only at night, and seasonal variations in molting rate were estimated using molting probability models based on length-frequency distributions. Average pod movement was counter to the direction of strongest water currents, and homing behavior suggested some affinity for a low-light environment in man-made niches. Items most frequently eaten were sea stars (Evasterias troschelii) and macrophytes (Laminaria sp. and Ulva sp.). I discuss the inception of podding as a discontinuity in behavior, perhaps reflecting a break in the fractal continuum of niche availability within red king crab habitat.

Ovothiols, a family of redox-active mercaptohistidine compounds from marine invertebrate eggs.

We have previously reported a novel thiol compound, 1-methyl-N alpha,N alpha-dimethyl-4-mercaptohistidine, or ovothiol, present at high concentration in the eggs of the sea urchin Strongylocentrotus purpuratus [Turner, E., Klevit, R., Hopkins, P. B., & Shapiro, B. M. (1986) J. Biol. Chem. 261, 13056-13063]. Here we report two related compounds, 1-methyl-N alpha-methyl-4-mercaptohistidine, or ovothiol B, from the scallop Chlamys hastata, and 1-methyl-4-mercaptohistidine, or ovothiol A, from the starfish Evasterias troschelii. These two compounds, as well as the S. purpuratus compound now designated ovothiol C, were isolated from eggs or ovarian tissue by S-carboxymethylation with [3H]iodoacetic acid, ion-exchange chromatography and ion-pairing high-pressure liquid chromatography. The structures of S-(carboxymethyl)ovothiols A and B were determined by 1H NMR, and that of ovothiol A was confirmed by comparison with authentic methylhistidine samples after desulfuration with Raney nickel. In the ovary of each species, the predominant methylation form of ovothiol accounts for at least 80% of the total 4-mercaptohistidine. The ovothiol concentration of the ovary far exceeds that of the testis or somatic tissues. The ovothiol C content of unfertilized S. purpuratus eggs is 1.14 mumol/10(6) eggs, equivalent to approximately 4.3 mM average concentration; the glutathione (GSH + GSSG) content is 0.9 mumol/10(6) eggs. In this species, high ovothiol levels persisted for the first 2 weeks of embryonic development. Ovothiol and glutathione account for virtually all of the trichloroacetic acid soluble-SH groups in the egg; these results are compared to several previous studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Starfish oocyte maturation: 1-Methyladenine triggers a drop of cAMP concentration related to the hormone-dependent period

Oocyte maturation (meiosis reinitiation) in starfish is induced by the natural hormone 1-methyladenine (1-MeAde). Oocytes of Evasterias troschelii contain 0.43 pmole cyclic AMP/mg protein and 0.47 pmole cyclic GMP/mg protein. Upon stimulation by 1-MeAde the oocytes undergo a moderate (10–30%) decrease in their cAMP concentration. The concentration of cGMP remains unaltered. Oocytes treated with forskolin, an activator of adenylate cyclase, increase their cAMP concentration over 35-fold, up to 16 pmole cAMP/mg protein. When stimulated by 1-MeAde these forskolin-pretreated oocytes undergo a major (50–70%) decrease in their cAMP concentration. A similar decrease is triggered by mimetics of 1-MeAde, such as dithiothreitol, arachidonic acid (AA), and 8-hydroxyeicosatetraenoic acid (8-HETE), but not by adenine which is inactive. 1-MeAde-stimulated oocytes of Pisaster ochraceus also undergo a decrease in cAMP content, the size of which is increased by forskolin. Although a decrease in cAMP begins at sub-threshold 1-MeAde concentrations, the maximal decrease occurs at the same concentration of 1-MeAde needed for maturation induction and a further 1000-fold increase of the 1-MeAde concentration has no further effect. Upon removal of 1-MeAde, the cAMP concentration immediately increases to its original level. Sequential addition and removal of 1-MeAde triggers a sequential decrease and increase of the cAMP concentration, illustrating the continuous requirement for 1-MeAde for eliciting the decrease. Successive additions of 1-MeAde, however, do not trigger further decreases of the cAMP concentration. The temperature dependences of the cAMP concentration decrease and of the hormone-dependent period (HDP; the time of contact with 1-MeAde required for induction of maturation) are closely related. Forskolin, which increases the cAMP concentration, also increases the duration of the HDP (2.5-fold), delays the time course of protein phosphorylation burst and germinal vesicle breakdown, and inhibits AA- and 8-HETE-induced maturation. We conclude that 1-MeAde triggers a drop in cAMP concentration, which is tightly associated with the hormone-dependent period of oocyte maturation.

Depression of feeding and growth rates of the seastar Evasterias troschelii during long-term exposure to the water-soluble fraction of crude oil

To test the effect of petroleum hydrocarbons on predation by the seastar Evasterias troschelii (Stimpson, 1862) on the mussel Mytilus edulis (L.), we exposed the predator with the prey to six concentrations of the water-soluble fraction (WSF) of Cook Inlet crude oil. Seastars and mussels were collected at Auke Bay, Alaska, in November 1980. During a 28 d exposure in a flow-through system, seastars were more sensitive to the WSF than mussels: the LC50 for the seastars was 0.82 ppm at Day 19 and, although no mussels were exposed to WSF for more than 12 d, none died. Daily feeding rates (whether in terms of number of mussels seastar-1 d-1 or dry weight of mussels seastar-1 d-1) were significantly reduced at all concentrations above 0.12 ppm. At 0.20, 0.28 and 0.72 ppm WSF, daily feeding rates (in terms of dry weight of mussels) were, respectively, 53, 37, and 5% of the control rate; at the two highest concentrations (0.97 and 1.31 ppm WSF), the seastars did not feed. Seastars at concentrations greater than 0.12 ppm WSF grew slower than individuals from the control group and the 0.12 ppm-treatment group combined. These laboratory results show that E. troschelii is more sensitive to chronic low levels of the WSF of crude oil. The possibility that such oil pollution could reduce predation and permit M. edulis to monopolize the low intertidal zone of southern Alaska remains to be studied.

Physiological response of symbiotic polychaetes to host saponins

Two species of the genus Arctonoe occur as host-isolated populations in the tidal and subtidal areas of Puget Sound. A saponin has been isolated from Evasterias troschelii (Stimpson) which has deleterious physiological effects on Arctonoe pulchra (Johnson), a species not associated with the asteroid. This same substance has no adverse effect on the viability and only a slight positive effect on the respiration of Arctonoe fragilis (Baird), the normal companion of Evasterias. These results suggest a possible mechanism for the strict isolation of these polychaetes and provide additional evidence for the biological significance of asterosaponins.

Effects of in situ tidal salinity fluctuations on osmotic and lonic composition of body fluid in Southeastern Alaska Rocky intertidal fauna

Tidal fluctuations in salinity and temperature were monitored at a location on Lynn Canal, north of Juneau, Alaska. Organisms were collected from 4 tidal levels during each slack water over a period of 24 h. Body-fluid milliosmolality as well as sodium, potassium, calcium, magnesium and chloride levels were determined. Mytilus edulis were collected at the +1.0 m tidal level, where ambient salinity ranged from 7.9 to 25.1% and body fluids from 356±62 to 730±17 mOsm/kg water. Cucumaria vegea and Katherina tunicata were collected at the +0.6 m level, where ambient salinity ranged from 13.5 to 24.9‰ and body fluids from 461±27 to 662±50 and 443±31 to 616±38 mOsm/kg water, respectively. Eupentacta quinquesemita and Evasterias troschelii were collected at the -0.9 m level, where ambient salinity ranged from 11.0 to 28.2‰ and body fluids from 504±32 to 632±51 and 316±31 to 664±37 mOsm/kg water, respectively. Strongylocentrotus droebachiensis were collected subtidally at the -3.7 m level, where ambient salinity ranged from 14.1 to 28.0‰ and perivisceral fluids from 448±35 to 661±51 mOsm/kg water. Ion levels of the perivisceral fluid of c. vegae were not determined. Potassium appeared to be regulated by all animals except Evasterias troschelii. Little regulation occurred for calcium, magnesium, sodium and chloride by the organisms. Eupentacta quinquesemita appeared to be regulating all ions determined, but this may have been due to its microhabitat. Water temperature cycled inversely to salinity at all tidal levels. Ambient-water sodium, chloride and calcium levels deviated from levels that would be expected upon dilution of seawater with deionized water. There was not indication that body-fluid osmoconcentration of the species collected intertidally increased on exposure to air.

Feeding Behavior of Asteroids and Escape Responses of their Prey in the Puget Sound Region

Observations were made with scuba on the diet and behavior of 18 species of undisturbed sea stars in their natural habitats along the shores of Washington state through all seasons. Some sea stars are specialists. Hippasteria spinosa feeds almost exclusively on a sea pen; Solaster stimpsoni eats holothurians; full—sized Orthasterias koehleri consume the venerid clam Humilaria; Solaster dawsoni eats its congener S. stimpsoni. Others show remarkably variable diets. In different habitats, Dermasterias imbricata specializes on either anemones, holothurians or sea pens, but within these habitats its diet is consistent throughout the year. The diet of Mediaster aequalis varies with both habitat and season. Pycnopodia helianthoides feeds on sea urchins on rocky substrata but digs clams from sand and cobble. Many other sea stars, including Luidia foliolata, Pteraster tesselatus, Pisater ochraceus, Evasterias troschelii and Leptasterias hexactis, are quite generalized in their diet and, though often demonstrating preferences in laboratory studies, will feed on a variety of prey determined largely by relative abundance of prey species in the particular habitat. The areal and seasonal variation in the diet of sea stars coupled with the reluctance of some to eat any prey in the laboratory makes extension of laboratory observations to the field diet suspect. Laboratory observations can be used to provide a more detailed understanding of field observations. Several previously undescribed behavioral mechanisms of food capture in asterioids and escape or defense responses or prey are described: Orthasterias pulls chips of shell off Humilaria until an opening is made, thus allowing exploitation of a prey species unavailable to other clam—eating asteroids. A number of sea stars, such as Luidia, Hippasteria, Mediaster, Pisaster brevispinus, Orthasterias and Pycnopodia dig into mud, sand or cobble substrata to obtain infaunal prey. The locomotory patterns of S. dawsoni and Crossaster papposus tend to allow encounter with their prey in such a way as to reduce the effectiveness of the escape response. A pushing response by Solaster stimpsoni and autotomy in Pycnopodia and Evasterias are thought to be effective responses to S. dawsoni predation. The similar swimming responses to two anemones, a nudibranch, and a holothurian are discussed in relation to asteroid predation. The avoidance responses that many invertebrates show to asteroids are correlated with predator—prey relationships. Two mechanisms may obscure this correlation. The very success of the response may effectively remove the responding organism from the predator's diet. Thus scallops, although abundant, form a very inconspicuous portion of starfish diet. Also biochemical similarities between the predator and other organisms may make a prey species unable to distinguish only its predator. Thus Pycnopodia will move rapidly away from Crossaster and Solaster stimpsoni as well as from S. dawsoni, although the latter is its only asteroid predator. The similarities may be based either on a close taxonomic relationship with the predator, or on the diet of the other organism including species closely related to the responding prey species.

The mantle response of Diodora aspera

Diodora aspera, the common keyhole limpet of the North American Pacific coast, displays a characteristic response to seven species of forcipulate seastars, Pisaster ochraceus, Pisaster brevispinus, Pisaster giganteus, Leptasterias aequalis, Evasterias troschelii, Orthasterias Koehleri, and Pycnopodia helianthoides, to one seastar of the order Phanerozonia, Hippasteria spinosa, to one seastar of the order Spinulosa, Patiria miniata, to one ophiuroid, Gorgonocephala eucnemis, and to one echinoid, Strongylocentrotus purpuratus. The typical reaction consists of a protrusion of the siphon from the “keyhole”, and an extension and reflexing of the middle mantle fold over the shell margin, with a movement of the mantle fold dorsomedially to cover most of the shell. Observations made on ten individuals of D. aspera kept in an aquarium with two P. ochraceus seastars for six days indicated that the reaction was effective in protecting the limpet from being eaten by this seastar, which is possibly one of its predators. The reaction was evoked by actual contact with the seastar, and also as a result of proximity without contact. The limpet reacted in a similar manner to seawater in which a seastar of a stimulating species had previously been kept; this water proved no longer effective after it was boiled for three minutes, or after standing for ten hours at 12 °C. It is assumed that the seastar produces a stimulating substance which is soluble in seawater, heat-labile, and which tests indicated did not act through a dialyzing membrane. Repeated stimulation of the same limpet, at regularly timed intervals alternating with timed rest periods in running seawater, resulted in a drop in starting reaction time for the second and third stimulation, and complete cessation of resonse after 320 to 390 minutes. The behaviour of D. aspera kept in the aquarium with P. ochraceus for six days suggested some kind and degree of adaptation.

The Feeding Behavior of the Seastar Evasterias troschelii Stimpson

Kymograph recordings of shell movements of 4.0–5.5 cm long clams (Protothaca staminea) during attacks by the seastar (Evasterias troschelii) showed: (1) in 21 experiments the valves were pulled apart at a mean rate of about 1.0 mm per minute, as measured at the mid‐ventral edge of the clams; (2) clams constantly tried to close during the initial opening of them by the seastars and then at longer intervals thereafter for a period usually not exceeding 4 hours.A specially constructed, false lucite “clam” containing Mytilus meat as bait was opened by Evasterias against known resistances applied to the approximate center of the upper, free‐moving valve of the “clam.” Kymograph recordings of shell movements of this “clam” during seastar attacks showed: (1) the curves showing the openings agree closely with those showing openings of live clams; (2) an average sized Evasterias may exert forces in excess of 5000 grams; (3) having lifted such weights, the seastar may hold them lifted for several hours; (4) in 14 experiments, weights varying from 700 to 5500 grams were lifted at an average speed of about 0.8 mm per minute, equal to an increase in gape of about 1.2 mm per minute during the lift. Maximum lifts varied from 2.8 to 13.5 mm with a mean of 6.6 mm.Experiments and observations showed that only the longitudinal musculature of the tube feet is responsible for the forces exerted by the seastar in opening bivalves. However, there are strong indications that the seastar is capable of “locking” itself in a rigid state before commencing the pull.If prevented from applying force, Evasterias will still succeed in feeding on bivalves, but the digestion period is considerably prolonged due to difficulties in inserting all lobes of the stomach into the prey.

Studies in the physiology of commensalism; the polynoid genera Arctonoë and Halosydna.

1. Water from an aquarium from which a host Evasterias troschelii has been removed 24 hours before no longer possesses an attraction for commensal Arctonoe fragilis. Similar results were obtained five hours after removal.2. Water from an aquarium from which a host Evasterias has been removed with difficulty, so that tube feet were torn from it, has no attraction for commensals almost immediately after removal of the host. Such water appears to repel them.3. Water from an aquarium from which a host Evasterias has been removed with comparative ease (involving less disturbance to the starfish) retains its attraction for commensals immediately after removal, but shortly thereafter the attraction appears to weaken.4. If a host Evasterias is suspended in a test aquarium in a dialyzing bag for two hours, at the end of this time not enough attractant has passed through to attract commensals.5. Experiments to determine whether attractant will diffuse through a dialyzing bag in effective concentrations overnight ma...

Studies in the physiology of commensalism. I. The polynoid genus arctonoe.

1. Apparatus was developed which made it possible to determine whether commensals of the polynoid genus Arctonoe are attracted by specific substances to their hosts and whether they can distinguish between water coming from an aquarium containing their host and sea-water alone or water from non-host.2. Arctonoe fragilis (Baird) commensal with the star Evasterias troschelii Stimpson are clearly attracted to their host and can distinguish between water coming from their host and sea-water alone. Arctonoe (pulchra-vittata?) commensal with the cucumber Stichopus are likewise attracted to their host.3. Arctonoe fragilis commensal with Evasterias are not attracted to the non-host star, Pisaster, occurring in the same immediate environment as their host. Arctonoe (pulchra-vittata?) commensal with the cucumber Stichopus are likewise not attracted to the non-host Cucumaria.4. Arctonoe (pulchra-fragilis?) commensal with the mud-star Luidia are not attracted, under these experimental conditions at least, to their ow...