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.     

Nature and role of newly described symbiotic associations between a sea anemone and gastropods at bathyal depths in the NW Atlantic

The hormathiid sea anemone Allantactis parasitica was found living as an epibiont on numerous species of gastropods at depths of 725-1100 m along the continental slope of eastern Canada. The proportion of bathyal gastropods hosting 1-6 sea anemones reached 72.5% in a single trawl. Although A. parasitica was occasionally found on other substrata (i.e. empty shells, pebbles), laboratory trials confirmed that they preferably associate with living gastropods. Settlement of planula larvae occurred significantly more often on the shells of live bathyal gastropods than on all other substrata present in the tanks. Juvenile sea anemones (∼ 1 mm diameter) readily moved from the mud or other inert substrata onto shells of burrowed bathyal gastropods. Conversely, larvae, juveniles and adults of A. parasitica never associated with any shallow-water gastropods when given the opportunity. Trials exposing predatory sea stars (Leptasterias polaris) from shallow and bathyal depths to bathyal gastropods (Buccinum undatum) with epibiotic A. parasitica, and to asymbiotic bathyal and shallow-water B. undatum, revealed adaptive behaviours in both prey and predator. Shallow-water gastropods (devoid of epibionts) reacted defensively to L. polaris, whereas bathyal gastropods relied mostly on their epibionts to protect them, thus falling prey to L. polaris when the epibionts were removed. L. polaris from bathyal depths typically ignored symbiotic gastropods, but they consistently preyed on asymbiotic ones, while L. polaris from shallow areas initially attempted to prey on all gastropods, but learned to avoid those harbouring sea anemones. Furthermore, living as epibionts afforded sea anemones a means to escape one of their own predators, the sea star Crossaster papposus. The mutualistic relationship between hormathiid sea anemones and bathyal gastropods from the NW Atlantic may have evolved in response to predation pressure.     

Mussel beds in deeper water provide an unusual situation for competitive interactions between the seastars Leptasterias polaris and Asterias vulgaris

The common seastars Leptasterias polaris and Asterias vulgaris show competitive interactions in shallow subtidal communities in the northern Gulf of St. Lawrence, particularly during summer when aggregations of the two seastars forage on mussel beds at 1-2 m in depth. We examined interactions between the two seastars in a different situation, in a mussel bed at 6 m in depth (a rare situation in this region). In the deeper mussel bed, seastars were three times more abundant than in the shallower beds, and the mussels were larger. The deeper bed disappeared rapidly due to the intense predation. Although decreased prey abundance should have favored interference interactions, we did not detect either partitioning of mussels by size or avoidance of A. vulgaris by L. polaris as previously reported when mussels are in short supply in shallower water. The lack of an avoidance behavior by L. polaris, together with the higher proportion of L. polaris than A. vulgaris that were feeding, suggests that in this situation, the dominance of A. vulgaris (observed in shallower water) is attenuated, or that L. polaris may dominate.     

Effects of substrate on interactions between juvenile sea scallops (Placopecten magellanicus Gmelin) and predatory sea stars (Asterias vulgaris Verrill) and rock crabs (Cancer irroratus Say)

We investigated the effect of substrate (glass bottom, sand, granule, pebble) on predation of juvenile sea scallops (Placopecten magellanicus) by sea stars (Asterias vulgaris) and rock crabs (Cancer irroratus) at two prey sizes (11-15 mm and 24-28 mm shell height), and two prey densities (10 and 30 scallops per aquarium) in laboratory experiments. Specifically, we quantified predation rate and underlying behaviours (proportion of time a predator spent searching for and handling prey, encounter rate between predators and prey, and various outcomes of encounters). We detected a significant gradual effect of particle size of natural substrates on sea star predation: specifically, predation rate on and encounter rate with small scallops tended to decrease with increasing particle size (being highest for sand, intermediate for granule, and lowest for pebble). Substrate type did not significantly affect predation rates or behaviours of sea stars preying on large scallops or of rock crabs preying on either scallop size classes. Other factors, such as prey size and density, were important in the scallop-sea star and scallop-rock crab systems. For example, predation rate by sea stars and crabs and certain sea star behaviours (e.g. probability of consuming scallops upon capture) were significantly higher with small scallops than with large scallops. As well, in interactions between small scallops and sea stars, predation rate and encounter rate increased with prey density, and the proportion of time sea stars spent searching was higher at low prey density than high prey density. Thus, substrate type may be a minor factor determining predation risk of seeded scallops during enhancement operations; prey size and prey density may play a more important role. However, substrate type still needs to be considered when choosing a site for scallop enhancement, as it may affect other scallop behaviours (such as movement).     

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.     

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.     

The diet of king eiders wintering in Nuuk,Southwest Greenland,with reference to sympatric wintering common eiders

Coastal and offshore waters of Southwest Greenland are internationally important wintering areas for king eiders (Somateria spectabilis) breeding in eastern Canadian Arctic and in northwestern Greenland. This paper presents the first assessment of their winter diet. Based on esophageal-proventricular samples from 26 females (13 juveniles and 13 older birds) and 15 males (11 juveniles and four older birds) collected in 2000–2002 (November–May) in coastal waters of Nuuk, we identified 28 prey species. The diet consisted of almost equal proportions (aggregate fresh mass) of polychaetes, echinoderms, crustaceans and molluscs. The dominant prey species were Pectinaria spp. (26.8%), Strongylocentrotus droebachiensis (18.4%), Mya eideri (11.2%) and Hyas araneus (9.7%). The polychaetes have previously been identified as important prey for eiders in Greenland, but apparently not outside Greenland. Compared with a diet study of common eiders Somateria mollissima from the same wintering area, the king eiders consumed significantly less bivalves and significantly more echinoderms. This difference corresponded with observations that common eiders were feeding in shallow waters, while king eiders were feeding in deeper waters farther from the shore. Benthic surveys are needed to confirm that diet corresponds with prey availability.     

Effect of dietary history and algal traits on feeding rate and food preference in the green sea urchin Strongylocentrotus droebachiensis

Feeding behaviour is influenced by a variety of factors, including nutritional requirements, the quality of available foods, and environmental conditions. We examined the effect of two factors, food morphology and dietary history, on the feeding rate and preference of the sea urchin Strongylocentrotus droebachiensis. Standardizing food shape and structure did not alter urchins' expected preference for the native kelp Laminaria longicruris over the invasive alga Codium fragile ssp. tomentosoides. However, when foods containing L. longicuris were shaped to mimic the algae, the C. fragile mimic was consumed more rapidly than the kelp mimic. Dietary history had no effect on single diet feeding rate. Urchins feeding on C. fragile consistently consumed twice as much (by mass) as those fed kelp, regardless of their previous diet. Despite higher feeding rates on C. fragile, urchins feeding on this alga were unable to compensate for its low energetic content and ingested less energy. Dietary history had a short-term effect on food preference, with urchins tending to prefer less familiar foods. Our findings suggest that urchins feed on C. fragile at a high rate, due to ease of handling and/or compensatory feeding, and that they do not a have strict preference hierarchy. Rather, food choice appears to reflect active maintenance of a mixed diet.     

Chemoreception by the asteroid Cross aster papposus (L.)

Chemoreception abilities were demonstrated in the solasterid asteroid Crossaster papposus (L.) from a level substratum in the current-swept Crouch Estuary, England. C. papposus showed wide variability of positive and negative responses to crude extracts of prey species and (unattractive) conspecifics. C. papposus has an olfactory (distance chemoreception) ability to whole, live potential prey and conspecifics. This was confirmed by contact chemoreception responses, which are stronger, to the same stimuli. The olfaction tests in the Y-maze yielded variable results in which the initial (rheopositive or rheonegative) responses demonstrated that conspecifics and prey species were avoided yet subsequent arm choices of rheopositive individuals showed significant selection for prey over controls and no avoidance of conspecifics. The arm choices for prey relate to a priority of orientation to food and feeding over intraspecific avoidance in hungry asteroids. Intraspecific avoidance functions more in dispersion than prevention of cannibalism and is less important than feeding to hungry asteroids. Interpretation of chemically-mediated responses of asteroids requires appreciation of their innate behavioural variability which has led to confusion in the literature. This is the first experimental study of contact and distance chemoreception among solasterids. Their chemosensory abilities have previously been contradictory which may be due to intraspecific avoidance in satiated individuals of some species.     

Responses of the black sea urchin Tetrapygus niger to its sea-star predators Heliaster helianthus and Meyenaster gelatinosus under field conditions

We ran field experiments to examine the responses of the black sea urchin Tetrapygus niger to predatory sea stars. Trials involving simulated attacks (one or several arms of a sea star being placed on top of half the urchin) showed that the urchin differentiated between the predatory sea stars, Heliaster helianthus and Meyenaster gelatinosus, and a non-predatory sea star, Stichaster striatus, and showed almost no response to a sea star mimic. We further compared the responses of the urchin to different threat levels presented by the two predatory sea stars. The highest threat level was a simulated attack, then mere contact, and subsequently sea stars being placed at different distances from the urchin. All urchins responded to simulated attacks and contact with both sea stars. The proportion responding decreased with distance and more rapidly in trials with H. helianthus (0% at a distance of 30 cm) than with M. gelatinosus (33% at a distance of 50 cm). At each of the threat levels where there was a response to both sea stars, the urchins responded more rapidly to M. gelatinosus than to H. helianthus. In a third experiment where a predatory sea star was added to a circular area (1-m diameter) in which either 4-8 or 11-19 undisturbed urchins were present, the urchins fled the area more rapidly when the added sea star was M. gelatinosus, but the rate of fleeing did not vary with density, as might occur if there was communication among urchins using alarm signals. Our observations suggest that M. gelatinosus presents a stronger predatory threat than H. helianthus. This corresponds to field observations showing that the urchins are more frequently consumed by M. gelatinosus. These are the first field experiments demonstrating distance chemodetection by a marine invertebrate under back-and-forth water flow from wave activity.     

Effect of Lactobacillus plantarum isolated from digestive tract of wild shrimp on growth and survival of white shrimp (Litopenaeus vannamei) challenged with Vibrio harveyi

Two hundred and two strains of lactic acid bacteria (LAB) isolated from digestive tracts of cultivated and wild adult shrimp, including Litopenaeus vannamei, Metapenaeus brevicornis and Penaeus merguiensis were selected based on their antibacterial activity against Vibrio harveyi. LAB strain of MRO3.12 exhibiting highest reduction of V. harveyi was identified as Lactobacillus plantarum MRO3.12 based on the nucleotide sequence of its 16S rDNA, which showed 99% (780/786 bp) homology to L. plantarum strain L5 (GenBank accession number DQ 239698.1). Co-cultivation of V. harveyi and L. plantarum MRO3.12 showed complete reduction of V. harveyi at 24 h under aerobic and anaerobic conditions, whereas L. plantarum increased from 5.29 to 9.47 log CFU ml⁻¹. After 6-week feeding trial with L. plantarum supplemented diet, white shrimp (L. vannamei) exhibited significant differences (p < 0.05) in relative growth rate (% RGR), feed conversion ratio (FCR) and survival compared to the control group fed with non-supplemented diet. LAB-fed group showed 98.89% survival, whereas only 68.89% survival was observed in the control group. LAB from the digestive tract of probiotic-fed shrimp showed higher level of 5.0 ± 0.14 log CFU/g than the non-supplemented ones (3.34 ± 0.21 log CFU/g). However, total bacterial and non-fermenting vibrios counts decreased in shrimps fed on L. plantarum. Ten days after infection with V. harveyi (5.3-5.5 log CFU ml⁻¹), significant survival (p < 0.05) of 77% was observed in LAB supplemented shrimp, while only 67% survival was observed in the control.     

Effect of the degree of autotomy on feeding, growth, and reproductive capacity in the multi-armed sea star Heliaster helianthus

The term autotomy refers to the process by which some species lose limbs or parts of limbs in response to adverse biotic or abiotic conditions, as for example, predation or abnormally high temperatures. The multi-armed sea star Heliaster helianthus is a key predator of the intertidal and the shallow rocky subtidal communities of north-central Chile. Natural populations of this sea star have been found with up to 60% of the individuals showing some degree of autotomy. The present study evaluated the effects of autotomy on feeding rate and growth of juvenile and adult H. helianthus after experimentally induced autotomy of 17% and 33% of their arms, as well as on the energy content of the pyloric caeca and gonads of adults during the reproductive period. Experimental juvenile sea stars were maintained and fed in the laboratory over a period of five months and adult sea stars for one month, Intact individuals were maintained as parallel controls. The results showed that juveniles undergoing 33% autotomy decreased their feeding rates, and as a consequence showed lowered net individual growth. In contrast, adults with 17% and 33% autotomy showed marked reductions in feeding. The results showed that autotomized adults had between five and seven times lower contents of carbohydrates, lipids, and proteins (and thus energetic content) in their pyloric caeca and gonads. The loss of the arms not only decreased the capacity for feeding in sea stars, but also allocated energy away from growth and reproduction into the process of regeneration of arms. This suggests that autotomy reduces the fitness of H. helianthus. Growth was reduced in the juveniles, while adults became limited in their ability to store energy which then limited their reproductive potential. Finally, based on the important effect of autotomy on reducing the feeding capacity of H. helianthus, the role of this sea star as a predator in the environment may be strongly affected.     

Biological activity of l-2-azetidinecarboxylic acid,isolated from Polygonatum odoratum var. pluriflorum, against several algae

The biological activities of an aqueous fraction extracted from Polygonatum odoratum var. pluriflorum Owhi and of l-2-azetidinecarboxylic acid (AZC), purified from the extract, on the growth of several types of algae were tested. The aqueous fraction was prepared by methanol extraction of P. odoratum var. pluriflorum rhizomes followed by reverse partitioning with butanol. The aqueous extraction inhibited growth of the green alga Chlorella vulgaris by less than 10% at a concentration of 1000 mg L⁻¹. However, growth of the blue-green alga Microcystis aeruginosa was inhibited by 22.0%, 67.9%, and 87.1%, respectively, at 3.1, 6.2, and 12.5 mg extract L⁻¹. AZC was isolated from the aqueous extract and was shown to be the major active substance inhibiting algal growth. AZC concentrations higher than 25 μM inhibited growth, while at 400 μM, growth of the green algae C. vulgaris and Scenedesmus spp. was inhibited by 71.2% and 70.4%, respectively. In contrast, growth of the blue-green algae Anabaena affinis and M. aeruginosa was inhibited at concentrations greater than 1.6 and 0.2 μM, respectively, whereas 92% control required concentrations of 6.3 and 1.6 μM, respectively. AZC also suppressed the growth of the red-tide microalga Cochlodinium polykrikoides by 86.9% and 100% at concentrations of 6.3 and 12.5 μM, respectively. Azetidine and 2-azetidinone showed little activity on the tested algae. The results demonstrate that AZC selectively inhibits algal growth at low concentrations. The green algae C. vulgaris and Scenedesmus spp. were tolerant, whereas M. aeruginosa, A. affinis, and C. polykrikoides were relatively sensitive. Thus, extract and AZC, prepared from P. odoratum rhizomes, showed a potential as natural selective algicide for the control of harmful algae in laboratory assay.     

Assessing the potential benthic-pelagic coupling in episodic blue mussel (Mytilus edulis) settlement events within eelgrass (Zostera marina) communities

Coastal marine seagrass ecosystems are important nursery grounds for commercially and recreationally important species, and they serve as key settlement and recruitment sites for other species. We investigated several years (2001-2003) where episodic settlement events of blue mussels (Mytilus edulis) occurred in Barnegat Bay, NJ, USA. Population assessment indicated that blue mussels settled in eelgrass beds (Zostera marina) in late spring with peak densities exceeding 170,000 m⁻². Based on calculated filtration rates of M. edulis, we determined that for at least 53 days in 2001, the density and size distribution of M. edulis were sufficient to filter the water column volume in excess of twice a day, with maximum calculated filtration rates exceeding 8 m³ water m⁻² day⁻¹. While the settlement event in 2001 was very localized, in 2003, the settlement event was considerably more widespread throughout the bay, with maximum settling densities exceeding 175,000 individuals m⁻². Associated with these high densities, maximum calculated filtration rates exceeded 15 m³ water m⁻² day⁻¹. This filtration potential may have impeded the localized development of a brown-tide (Aureococcus anophagefferens) bloom in 2001, which occurred in other regions of the bay, but the widespread settlement event seen in 2003 may have impeded the development of any brown-tide blooms in Barnegat Bay during that summer. The decline in mussel densities throughout the summer may be a result of elevated water temperatures in this back bay, but at one site, the high settlement of M. edulis was followed by a substantial migration (>40 individuals m⁻²) of small sea stars (Asterias forbesii). In 2001, A. forbesii was a significant factor in reducing M. edulis density by the end of the summer at the Barnegat Inlet site and a community level assessment showed significant positive correlations between mussel aggregations and sea star densities (r=0.68-0.73, P<0.001). At this same site in 2003, the sea stars were again present in high densities (26 m⁻²) and were a potential mechanism for mussel decline. In other regions of the bay, sea star densities are very low, but numerous other predatory species exist, including blue crabs (Callinectes sapidus), green crabs (Carcinus maenus), spider crabs (Libinia spp.), and several Xanthid crabs. Given the high mussel densities seen in this study and the considerable predation by sea stars and other benthic predators, the benthic-pelagic coupling which these mussels provide in this system contributes to the high secondary production in these grass beds.     

Piscivory by the mummichog (Fundulus heteroclitus): Evidence from the laboratory and salt marshes

While piscivory is common in many fishes, there are few accounts for the fundulid, Fundulus heteroclitus (mummichog). We suspected that this species might be involved in several forms of piscivory including predation, cannibalism and scavenging. To evaluate these possibilities we conducted several laboratory experiments and field observations in its primary habitat, salt marshes. We found that digestion of larvae and small juveniles of F. heteroclitus was fast (< 1 h) and this makes detection of small fish prey difficult for any form of piscivory. In addition, laboratory experiments (one-on-one encounters, feeding on dead prey) indicated that both cannibalism and scavenging of conspecific prey were possible. Field observations (n = 2449 stomachs from fish 12-106 mm TL over four years at two salt marshes) also suggested that piscivory occurred frequently (4.3-24.7% of stomachs examined) based on the detection of numerous fish hard parts, especially pharyngeal jaws. These structures allowed us to determine that most prey were F. heteroclitus and to back-calculate the size of the prey and thus the piscivore/prey size relationship. As a result, it appears that both cannibalism (0.2-9.1% of stomachs) and scavenging (0.5-9.9%) are common feeding modes of F. heteroclitus in salt marshes and, subsequently, modes of energy transfer for salt marshes in general. We suggest that future studies of fish feeding consider that cannibalism and scavenging may be frequent possibilities when fish remains are detected in stomach contents.     

Acrosome reaction is subfamily specific in sea star fertilization

In the fertilization process of sea stars, sperm is activated to go through the acrosome reaction before cell fusion. We focused on induction of the acrosome reaction as a key process in fertilization. Six species of sea stars were used in this study: Asterias amurensis, Asterias rubens, Asterias forbesi, Aphelasterias japonica, Distolasterias nipon, and Asterina pectinifera. Acrosome reaction assays indicate that the acrosome reaction can be induced across species within Asteriinae subfamily. However, cross-fertilization assays indicate that sea stars have species specificity in fertilization. Therefore, steps after the acrosome reaction are responsible for the species specificity. To explain acrosome reaction subfamily specificity at the molecular level, the sugar components of egg jelly were examined and analyzed by principal component analysis. A. amurensis and A. forbesi belong to the same induction group of the acrosome reaction. D. nipon and An. pectinifera are in a unique group. Enzyme-linked immunosorbent assays indicate that Asteriinae subfamily share a common glycan structure, the Fragment 1 of Acrosome Reaction-Inducing Substance from A. amurensis. Fragment 1 plays an important role in the subfamily specificity of acrosome reaction induction. In addition, A. amurensis sperm activating peptide was recognized by sperm from the same superorder. These results demonstrate that the specificity of acrosome reaction induction is present at the subfamily level in sea stars.     

Does exogenic food benefit both partners in an ant-plant mutualism? The case of Cecropia obtusa and its guest Azteca plant-ants

In the mutualisms involving the myrmecophyte Cecropia obtusa and Azteca ovaticeps or A. alfari, both predatory, the ants defend their host trees from enemies and provide them with nutrients (myrmecotrophy). A. ovaticeps provisioned with prey and then ¹⁵N-enriched food produced more individuals than did control colonies (not artificially provisioned). This was not true for A. alfari colonies, possibly due to differences in the degree of maturity of the colonies for the chosen range of host tree sizes (less than 3 m in height). Myrmecotrophy was demonstrated for both Azteca species as provisioning the ants with ¹⁵N-enriched food translated into higher δ¹⁵N values in host plant tissues, indicating that nitrogen passed from the food to the plant. Thus, the predatory activity of their guest ants benefits the Cecropia trees not only because the ants protect them from defoliators since most prey are phytophagous insects but also because the plant absorbs nutrients.     

Why are intertidal snails rare in the subtidal? Predation, growth and the vertical distribution of Littorina littorea (L.) in the Gulf of Maine

The North Atlantic gastropod Littorina littorea exhibits a characteristic “intertidal” distribution: the snail is abundant in the littoral zone but scarce in the shallow subtidal and the relatively few subtidal individuals are larger (in shell size) on average than those in the intertidal zone. For highly mobile species like L. littorea, this vertical distribution is primarily determined by directional movement. Biotic and abiotic factors vary across tidal heights, and natural selection for movement to shore levels where fitness is maximized provides the ultimate (evolutionary) explanation for vertical distribution patterns. In this study, we asked whether variation in growth rate and/or predation pressure among tidal heights provide an ultimate explanation for vertical gradients in L. littorea size and abundance. We used a cage experiment to compare juvenile growth rate among tidal heights and a series of field and laboratory experiments to examine variation in predation pressure among tidal heights and snail size classes. Juvenile growth rates were highest in the low intertidal zone, declining at both higher and lower levels. Predation risk for tethered L. littorea increased with both decreasing tidal height and decreasing body size (shell height). Almost all tethered prey were consumed by shell- breaking predators and a census revealed that the two most abundant such predators were the crabs Carcinus maenas and Cancer borealis. Laboratory feeding experiments were used to compare size-dependent prey vulnerability and prey-size preferences for these two key predators. We found that L. littorea vulnerability decreased with increasing snail size and increased with increasing size of both predator species. However, whereas C. borealis were capable of consuming even the largest L. littorea, most Carcinus were unable to feed on individuals larger than 10 mm in shell height. Additionally, C. borealis preferred larger sizes of L. littorea than did Carcinus. Thus, Carcinus, which co-occurs with L. littorea in the intertidal, is a much less effective predator than C. borealis, which is found primarily in the subtidal. We conclude that predation on L. littorea by C. borealis and other subtidal consumers has resulted in the scarcity of this ecologically important grazer in the subtidal. This effect has been produced both through direct predation and by imposing strong selection for movement of L. littorea to higher tidal zones.     

Predaceous Behavior and Life History of Odontopharynx longicaudata (Diplogasterida)

The behavior of a California isolate of the predaceous nematode, Odontopharynx longicaudata de Man, was studied in water agar culture. When feeding on an Acrobeloides sp. the predator completed its life cycle in 13 to 14 days at 25 C. Optimum temperature for reproduction of the predator was 25 C, few individuals survived at 10 C, and 30 C was lethal. Males were necessary for reproduction, and at 25 C the sex ratio was about 1:1. All postembryonic stages were voracious feeders. A single female predator consumed 30 individuals of another Acrobeloides sp. in 1.5 days. Juveniles must feed in order to complete their development. Three modes of feeding were observed depending on the prey selected. A high degree of prey selectivity occurred; 6 of 17 nematode prey species were readily consumed by the predator, but there was little or no feeding on the remaining 11 species. Predation percentage varied with prey species. Consumption of Anguina pacificae and the two Acrobeloides spp. was almost 100%, consumption of A. amsinckiae, Pratylenchus vulnus, and Trichodorus sp. was 70-78%. Difference in final predator population densities was obtained after feeding on the two species of Acrobeloides. Final predator population densities increased linearly with increasing inoculum levels of the first Acrobeloides sp.     

Dietary selectivity for the toxic cyanobacterium Lyngbya majuscula and resultant growth rates in two species of opisthobranch mollusc

Trophodynamics of blooms of the toxic marine cyanobacterium Lyngbya majuscula were investigated to determine dietary specificity in two putative grazers: the opisthobranch molluscs, Stylocheilus striatus and Bursatella leachii. S. striatus is associated with L. majuscula blooms and is known to sequester L. majuscula metabolites. The dietary specificity and toxicodynamics of B. leachii in relation to L. majuscula is less well documented. In this study we found diet history had no significant effect upon dietary selectivity of S. striatus when offered a range of plant species. However, L. majuscula chemotype may alter S. striatus' selectivity for this cyanobacterium. Daily biomass increases between small and large size groups of both species were recorded in no-choice consumption trials using L. majuscula. Both S. striatus and B. leachii preferentially consumed L. majuscula containing lyngbyatoxin-a. Increase in mass over a 10-day period in B. leachii (915%) was significantly greater than S. striatus (150%), yet S. striatus consumed greater quantities of L. majuscula (g day^− 1) and thus had a lower conversion efficiency (0.038) than B. leachii (0.081) based on sea hare weight per gram of L. majuscula consumed day^− 1. Our findings suggest that growth rates and conversion efficiencies may be influenced by sea hare maximum growth potential, acquisition of secondary metabolites or diet type.