Learning in an invasive and a native predatory crab

The invasion of the green crab Carcinus maenas in the northeastern U.S. and its competition with the native blue crab Callinectes sapidus and other native crustaceans has been well-documented and researched. Various reasons for the invader’s success against native crabs have been examined (juvenile predation, food source flexibility, etc.), but another possibility is a difference in the learning ability of invasive versus native crab species. In this study, the learning ability of C. maenas and C. sapidus was tested by their increased speed in locating hidden food over successive days. The data suggest that C. maenas possesses a learning ability significantly greater than that of C. sapidus, which may partially contribute to its success.     

Marine invasive species: validation of citizen science and implications for national monitoring networks

Approximately 1,000 volunteers assessed the presence of invasive (Carcinus maenas and Hemigrapsus sanguineus) and native crabs within the intertidal zone of seven coastal states of the US, from New Jersey to Maine. Identification of crab species and determination of the gender of the observed crabs was documented at all 52 sites across a 725-km coastal transect. Using quantitative measures of accuracy of data collected by citizen scientists, a significant predictor of a volunteer’s ability was determined and eligibility criteria were set. Students in grade three and seven had the ability to differentiate between species of crabs with over 80% and 95% accuracy, respectively. Determination of gender of the crabs was more challenging and accuracy exceeded 80% for seventh grade students, while 95% accuracy was found for students with at least 2 years of university education. We used the data collected by citizen scientists to create a large-scale standardized database of the distribution and abundance of the native and invasive crabs. Hemigrapsus sanguineus dominated the rocky intertidal zone from Sandy Hook, New Jersey to Boston Harbor, Massachusetts while C. maenas dominated the northern extent of the sampled coastline. A citizen scientist of this monitoring network detected a range expansion of H. sanguineus. We identified obstacles to creating a national monitoring network and proposed recommendations that addressed these issues.     

Impact scenario for the invasive red king crab <Emphasis Type="Italic">Paralithodes camtschaticus</Emphasis> (Tilesius, 1815) (Reptantia,Lithodidae) on Norwegian,native, epibenthic prey

Large invasive predators like the king crab, Paralithodes camtschaticus, deserve particular attention due to their potential for catastrophic ecological impact on recipient communities. Conspicuous, epibenthic prey species, such as the slow growing commercial scallop Chlamys islandica, are particularly exposed to the risk of local extinction. A research program integrating experiments and field monitoring is attempting to predict and track the impact of invasive king crab on scallop beds and associated fauna along the north Norwegian coast. The claw gape of the crab shows no limitations in handling the flat-bodied scallop. However, the potential impact of the crab on scallop may depend on the availability of other calcified prey associated with scallop beds, such as the sea star, sea urchin, and blue mussel, all species recorded in the diet of P. camtschaticus. To address this issue, a laboratory experiment on foraging behaviour of P. camtschaticus was conducted. The experimental results show that all size classes of red king crab prefer scallops, but small juveniles and medium sized crabs demonstrate active selection for starfish (Asterias rubens) that equals or surpasses the electivity of the large crab. The selection of sea urchin (Strongylocentrotus droebachiensis) and blue mussel (Mytilus edulis) is slightly positive or neutral for the three crab size classes. These results suggest that scallop beds with a rich associated fauna are less vulnerable to red king crabs predation and possibly more resilient than beds with few associated species. Also, crab size distribution is likely relevant for invasion impact, with increasing abundance of small and medium sized crabs being detrimental for alternative calcified prey associated with scallop beds. Successive stages of crab invasion will see an acceleration of scallop mortality rates associated with (i) decreasing availability of alternative prey, due to protracted predation pressure intensified by recruitment of juvenile crabs, and (ii) increased number of large crabs. Estimates of crab density and intake rates suggest that the accelerated loss rates will eventually endanger scallop beds persistence.     

Biotic resistance experienced by an invasive crustacean in a temperate estuary

Communities high in species diversity tend to be more successful in resisting invaders than those low in species diversity. It has been proposed that the biotic resistance offered by native predators, competitors and disease organisms plays a role. In Yaquina Bay, Oregon, we observed very little overlap in the distribution of the invasive European green crab, Carcinus maenas, and the larger red rock crab, Cancer productus. C. productus dominates the more saline, cooler lower estuary and C. maenas, the less saline, warmer upper estuary. Because caged C. maenas survive well in the lower estuary, we decided to test the hypothesis that C. productus prey on C. maenas and thus contribute to their exclusion from the more physically benign lower estuary. A laboratory species interaction experiment was designed to determine whether C. productus preys on smaller C. maenas at a higher rate than on smaller crabs of their own species. Crabs of both species were collected and sorted by weight into three size classes: small, medium and large. Small and medium crabs of both species were paired with C. maenas and C. productus of various sizes. When conspecifics were paired, mortality was less than 14%, even in the presence of larger crabs. Smaller C. productus survived well in the presence of larger C. maenas, but the reverse was not true. When small C. maenas (60–67 mm carapace width) were matched with medium and large C. productus, their mortality increased to 52% and 76%, respectively. A less dramatic pattern was observed for medium C. maenas (73–80 mm) in the presence of medium and large C. productus. Thus on the West Coast of North America, the more aggressive red rock crab, C. productus, has the potential to reduce the abundance of C. maenas in the more saline and cooler lower estuaries.     

Does consumer injury modify invasion impact?

Predicting the impacts of an invasive species solely by its abundance is common, yet it ignores other potentially important moderating factors. One such factor is injury. Severe injury can lead to mortality, which can directly reduce the abundance of the invader. However, more moderate, sublethal injury can also temper the impact of invasive species. Therefore, to predict impacts, it may be useful to examine not only abundance, but also moderating factors (e.g., injury) and predictors of these factors (e.g., density, size). We documented the density, injury (i.e., limb loss), and size of two conspicuous invaders, the European green crab (Carcinus maenas) and the Asian shore crab (Hemigrapsus sanguineus), at thirty sites from Shinnecock County Park, New York to Lubec, Maine. In addition, we used a field experiment to determine how injury influenced the consumption rate of mussels (Mytilus edulis) by each crab species. 31.6% of all C. maenas (1,493/4,721) and 30.7% of H. sanguineus (2,003/6,523) were missing appendage(s). Of the crabs injured, 38.4% (573/1,493) and 30.5% (611/2,003) were missing cheliped(s) for C. maenas and H. sanguineus, respectively. In our experiments, cheliped loss reduced consumption of both species on M. edulis. Injured C. maenas consumed 21% fewer mussels than uninjured crabs. Injury completely eliminated mussel consumption by H. sanguineus. Previous studies have highlighted the detrimental impacts of these two invaders on native bivalve prey. While the loss of a single cheliped can greatly reduce or even eliminate the ability of C. maenas and H. sanguineus to consume M. edulis, our results suggest that injury has a relatively minor role in reducing overall population-level impacts on prey such as mussels. However, injury on an individual-level can play a role in moderating the consumptive impacts of these invaders.     

Home and Away: Comparisons of Resource Utilization by a Marine Species in Native and Invaded Habitats

The exotic Asian shore crab, Hemigrapsus sanguineus, was recently introduced to the northeastern coast of North America and during the 1990's breeding populations were established throughout southern New England. In 1997–1998, ecological studies of several co-occurring brachyuran crabs were conducted and in native (Tanabe Bay, Japan) and invaded (Long Island Sound, USA) habitats of H. sanguineus. Standardized comparisons of H. sanguineus were made between the 2 habitats using data on crab sizes, utilization of space, and food habits. Results revealed that (1) the resource use of H. sanguineus was quite different from that of other resident species in its invaded habitat, and (2) there were no substantial changes in resource utilization by H. sanguineus after it became established in the invaded habitat (relative to native Tanabe Bay). Differing patterns of resource use by H. sanguineus and other crabs in the invaded habitat, the lack of restriction in resource use by H. sanguineus following its introduction, and the climatological and physical similarities between native and invaded regions likely contributed to the successful invasion of H. sanguineus into rocky intertidal habitats in southern New England.     

Spontaneous alternation in marine crabs: Invasive versus native species

Organisms ranging from paramecia to humans tend to explore places that have been least recently explored, which is referred to as spontaneous alternation. Although organisms rely on different sources of information in alternating between places, the emergent behavioral pattern is likely advantageous during exploration and foraging. Under this rationale, continuous spontaneous alternation performance of the invasive green crab, Carcinus maenas was assessed and compared with the native blue crab, Callinectes sapidus in a plus-maze submerged in seawater. For the first time spontaneous alternation behavior was demonstrated in Crustacea (i.e., C. maenas) and significant interspecific differences in alternation performance were observed between the invasive versus the native species. Carcinus maneas exhibited a pronouncedly higher spontaneous alternation performance than C. sapidus. Carcinus maneas on average alternated at levels higher than chance, which was not the case for C. sapidus. These observations point to an additional behavioral mechanism that might result in the competitive success of green crabs over blue crabs in areas where they co-occur. Most of the subjects exhibited asymptotic alternation performance from the onset; there was no improvement in their performance over the course of the experimental session. This finding implies the innate nature of this behavioral policy.     

Effects of juvenile non-indigenous Carcinus maenas on the growth and condition of juvenile Cancer irroratus

The Atlantic rock crab, Cancer irroratus, is a commercially fished species and a critical prey item for the American lobster, Homarus americanus, in Atlantic Canada. The recent invasion of European green crab, Carcinus maenas, may have significant effects on the growth and condition of native C. irroratus, because both species overlap spatially and temporally and have similar habitat and dietary requirements. To examine such potential effects, we measured the growth of juvenile C. irroratus in the presence of juvenile C. maenas over a period of 4 months (growing season), under the following species combinations: (1) one C. irroratus (10-25 mm CW); (2) two C. irroratus (10-25 mm CW); (3) one C. irroratus (10-25 mm CW) and one C. maenas (10-15 mm CW). Morphological measurements included pre- and post-molt carapace width, chela height, abdomen width (mm), weight (g), and estimates of molt increment (%) and intermolt duration (days). Analysis of the hepatopancreas for % lipid content at the end of the experiment provided an estimate of physiological condition. The effect of the presence of C. maenas on the growth of C. irroratus shifted from negative to positive, when C. irroratus reached CW of 19-22 mm and gained a presumably significant size advantage over C. maenas. The positive effect resulted from increased energy intake through crab consumption. In the absence of crab consumption, the presence of a second crab (conspecific or C. maenas) had no effect on growth. C. irroratus consumed crabs more frequently when the second individual was a green crab than a conspecific. Consumption of C. maenas had a pronounced effect on the growth rate of C. irroratus, resulting in shorter intermolt periods and larger percent molt increments than in the presence of a conspecific. Therefore, the presence of juvenile C. maenas does not appear to have a prolonged negative effect on the growth of C. irroratus; rather, it may provide an additional food item as rock crabs grow, as long as encounters between the two species occur at high enough rates.     

Food sources for the early life history stages of the hydrothermal vent crab <Emphasis Type="Italic">Bythograea thermydron</Emphasis>: a stable isotope approach

We used stable isotope analysis to examine food sources for early life-history stages of the vent crab Bythograea thermydron. During two cruises to a hydrothermal vent site along the East Pacific Rise, we collected a variety of endemic organisms, including approximately 300 specimens of B. thermydron. The crab collection consisted mainly of megalopae and early juveniles, but also included adults and a single zoea. As expected, the carbon-isotope composition of newly released zoeae (−12.2%) was similar to the female (−11.1%) and clearly different from megalopae (21.7%). Because we were unsuccessful in culturing the zoea larvae, we were not able to conduct experiments to determine the effect of diet on isotopic composition. The tissue of megalopae was depleted in ¹³C (−21.7%) when compared to indigenous prey species, but was similar to that expected for carnivorous zooplankton dependent on surface primary production. The nitrogen-isotope composition of megalopae was enriched in ¹⁵N (10%) relative to potential prey species, again suggesting a photosynthetic source of primary production. The mean carbon-isotope composition of newly metamorphosed juvenile crabs (−19.9%) resembled megalopae, while tissue from subsequent juvenile stages was enriched in ¹³C (−10.9%) with values similar to those measured in co-occurring prey species. These results imply that megalopae are dependent on a source of primary production exogenous to the vents, but switch to a vent-based food web soon after metamorphosis to the juvenile stage.     

Evaluation of the sample size used for the rapid bioassessment of rivers using macroinvertebrates

Experiments were designed to investigate selective predation by medium (40–55 mm carapace width: CW) and large (55–70 mm CW) Carcinus maenas when feeding on four bivalves of contrasting shell morphology. Size-selection was examined by presenting individual crabs with a wide size range of Mytilus edulis, Ostrea edulis, Crassostrea gigas and Cerastoderma edule. Medium-sized crabs preferred mussels 5–15 mm shell length (maximum shell dimension: SL) and cockles 5–10 mm SL, whereas large crabs preferred mussels 15–25 mm and cockles 10–20 mm SL. Crabs generally showed no preference for any particular size of either oyster species. Species-selection was examined by presenting individual crabs with paired combinations of the four bivalves in various proportions. When offered mussels and oysters simultaneously, both size categories of crabs consistently selected mussels, and food choice was independent of prey relative abundance. By contrast, C. maenas selected mussels and cockles as expected by the frequency in which each size category of crab encountered the preferred size ranges of prey. Crab preference clearly paralleled the rank order of prey profitability, which in turn was mainly determined by prey biomass, suggesting that active selection takes place at some point of the predation cycle. Experiments with epoxy resin models showed that initial reluctance of crabs to attack oysters was not associated with the ultimate energy reward. Moreover, they suggest that foraging decisions are partly based on evaluations of overall prey shape and volume, and that the minimum dimension of the shell constitutes an important feature which crabs recognise and associate with prey value.     

A food web analysis of the juvenile blue crab, Callinectes sapidus, using stable isotopes in whole animals and individual amino acids

The stable isotope compositions (C and N) of plants and animals of a marsh dominated by Spartina alterniflora in the Delaware Estuary were determined. The study focused on the juvenile stage of the Atlantic blue crab, Callinectes sapidus, and the importance of marsh-derived diets in supporting growth during this stage. Laboratory growth experiments and field data indicated that early juvenile blue crabs living in the Delaware Bay habitat fed primarily on zooplankton, while marsh-dwelling crabs, which were enriched in ¹³C relative to bay juveniles, utilized marsh-derived carbon for growth. In laboratory experiments, the degree to which juvenile blue crabs isotopically fractionated dietary nitrogen, as well as the growth rate, depended on the protein quality of the diet. The range of δ¹³C of amino acids in laboratory-reared crabs and their diets was almost 20‰, similar to the isotopic range of amino acids of other organisms. In laboratory studies, the δ¹³C of nonessential and essential amino acids in the diet were compared to those in juvenile crabs. Isotopic fractionation at the molecular level depended on diet quality and the crabs' physiological requirements. Comparison of whole-animal isotope data with individual amino acid C isotope measurements of wild juvenile blue crabs from the bay and marsh suggested a different source of total dietary carbon, yet a shared protein component, such as zooplankton. Received: 1 July 1998 / Accepted: 15 March 1999     

Exotic Spartina alterniflora provides compatible habitats for native estuarine crab Sesarma dehaani in the Yangtze River estuary

Although the impact of plant invasions on benthic communities, especially burrowing crabs, has received increasing attention, the results from past studies are mixed. The exotic plant Spartina alterniflora has become the most abundant species in the salt marshes of the Yangtze River estuary since it was first found just over a decade ago, but its effects on crabs in the salt marshes is largely unknown. To examine whether the invasions of this exotic plant affected native crabs, we compared the biomass and abundance of the dominant burrowing crab Sesarma dehaani in an exotic Spartina marsh, native Phragmites australis marsh and mudflats of the Yangtze River estuary, China. To explain the differences of S. dehaani populations between different habitats, feeding preference of S. dehaani for Spartina and Phragmites was investigated. Results showed crab abundance and biomass in the Spartina marsh were significantly greater than those in the Phragmites marsh and mudflats. Soil water content and plant community characteristics in the Spartina marsh also significantly differed in the Phragmites marsh and mudflats. Moreover, the feeding preference experiment showed that crabs consumed Spartina more than twice as much as Phragmites. In summary, this study showed that Spartina provided compatible habitats for native crab S. dehaani through offering suitable food source and moderate environmental conditions.     

An invasive crab alters interaction webs in a marine community

Over the last decade, the non-native, filter-feeding crab Petrolisthes armatus invaded oyster reefs of the South Atlantic Bight at densities of thousands m⁻². Mesocosm and field experiments demonstrated that P. armatus at ∼10–75% of mean summer densities: (1) suppressed growth of small oysters, biomass of benthic microalgae, and recruitment of native mud crabs, (2) enhanced oyster, mussel, and total bivalve recruitment, macroalgal cover, and survivorship of predatory oyster drills, but (3) did not affect native taxonomic richness. Laboratory feeding assays, field tethering experiments, and population changes in field and mesocosm experiments suggest that P. armatus is a preferred prey for native mud crabs and other consumers, thus relieving predation on native species and enhancing recruitment or survival of bivalves and oyster drills. In contrast, the invasive crab can consume crustacean larvae and via this feeding may suppress recruitment of native mud crabs. Our findings should be conservative given the low densities of P. armatus seeded into experimental plots and our inability to run longer-term experiments due to controls rapidly being colonized by non-native crabs recruiting from the plankton. Invasive crabs commonly impact native communities via predation, but community impacts of this invasive crab may be as much due to its role as a preferred prey of native consumers as to its predation on native prey. Given that oysters are foundation species for shallow reefs in the South Atlantic Bight, the long-term effects of this invasion could be considerable.     

Evidence for a free-living life stage of the blue crab parasitic dinoflagelate, Hematodinium sp.

Hematodinium sp. is a parasitic dinoflagellate reported to cause disease and death in a variety of crustacean species including the blue crab (Callinectes sapidus). However, because of difficulties in the culture of Hematodinium sp. associated with blue crabs, little is known about its life cycle or mode of transmission. Here, we report the first detection of this organism outside of a metazoan host and provide evidence that this life stage can act as an infective agent. Observations of dinospores in crab hemolymph samples suggest that dinospores may be responsible for waterborne disease transmission. Additionally, we developed and validated a quantitative Real Time PCR assay for the detection of Hematodinium sp. inside and outside of a host organism that will be useful for future investigations of Hematodinium biology and Hematodinium sp.-infection etiology. Based on the observations of a free-living form of Hematodinium sp. and the association of this parasite with a widespread epizootic in blue crab populations, we propose that Hematodinium sp. be considered a Harmful Algal Bloom species.     

Host specificity of <Emphasis Type="Italic">Sacculina carcini</Emphasis>, a potential biological control agent of the introduced European green crab Carcinus maenas in California

The European green crab, Carcinus maenas, is an introduced marine predator established on the west coast of North America. We conducted laboratory experiments on the host specificity of a natural enemy of the green crab, the parasitic barnacle Sacculina carcini, to provide information on the safety of its use as a possible biological control agent. Four species of non-target, native California crabs (Hemigrapsus oregonensis, H. nudus, Pachygrapsus crassipes and Cancer magister) were exposed to infective larvae of S. carcini. Settlement by S. carcini on the four native species ranged from 33 to 53%, compared to 79% for green crabs. Overall, cyprid larvae tended to settle in higher numbers on individual green crabs than on either C. magister or H. oregonensis. However, for C. magister this difference was significant for soft-shelled, but not hard-shelled individuals. Up to 29% of the native crabs arrested early infections by melanizing the rootlets of the parasite. Most native and green crabs settled on by S. carcini became infected, especially when settled on by >3 cyprids. Infected green crabs died at more than twice the rate of uninfected green crabs. In contrast to green crabs, all infected native crabs died without producing an externa (reproductive sac). At high settlement intensities, infected native crabs frequently exhibited neurological symptoms (twitching, loss of movement) before death. These results indicate that use of S. carcini as a biological control agent could result in the death of native crabs. The magnitude of this effect would be proportional to the density of infected green crabs in the environment and the probability that cyprids would contact native crabs in the wild. Potential benefits of biological control should be assessed in relation to these potential non-target effects.     

Trophic ecology of the Atlantic blue crab Callinectes sapidus as an invasive non-native species in the Aegean Sea

Invasive non-native crustaceans are a biodiversity and management concern in the Mediterranean Sea. The Atlantic blue crab (Callinectes sapidus) was first recorded in the Mediterranean Sea in 1949, but may have arrived as early as in the 1930’s. Blue crabs in the Mediterranean Sea are of concern due to their presumed potential for negative consumptive and competitive interactions with native fauna. The aim of this study was to provide a first assessment of the trophic ecology of non-native blue crab in the Northern Aegean Sea using stable carbon (δ¹³C) and nitrogen isotope (δ¹⁵N) analysis. We found limited isotopic niche overlap between blue crabs and seven native species examined at Gökçeada Island in April, June, and August of 2017. In addition, the range of calculated trophic positions of blue crabs at Gökçeada Island (2.0 to 4.4), while broad, is in general agreement with prior studies in both native and non-native ranges. We also observe that trophic position declined and the relative importance of pelagic carbon sources to blue crabs increased from April to August. However, we also found that differing assumptions as to the number and type of food web baselines and trophic discrimination factors led to differing estimates of trophic position in blue crabs at Gökçeada Island by as much as one to two trophic levels. These methodical differences make it challenging to directly compare results within and between studies, and thus limit our ability to assess negative consumptive and competitive interactions of invasive blue crab with native coastal species in the Mediterranean Sea.

     

Effect of an invasive crab upon a marine fishery: green crab, Carcinus maenas, predation upon a venerid clam, Katelysia scalarina, in Tasmania (Australia)

Nonindigenous species are increasingly recognized as altering marine and estuarine communities, causing significant changes in abundance and distribution of native species. Such effects are of particular concern to coastal fisheries. We experimentally determined the effect of the nonindigenous European green crab, Carcinus maenas, upon the stepped venerid clam, Katelysia scalarina, the basis for a fledgling clam fishery in Tasmania, Australia. First, we observed a trend of decreased juvenile (<13-mm shell length or SL) abundance of K. scalarina at sites with C. maenas relative to those without this invasive predator. Additionally, relative predation intensity on these juveniles was significantly higher in invaded areas. To better understand the dynamics of predation by this invader, we conducted a number of manipulative experiments. In cage experiments testing per capita predation rates, we found that: (1) of the various sizes of C. maenas, large C. maenas were the most significant predators; (2) the smallest size class of K. scalarina tested (6-12-mm SL) was preferred by C. maenas; (3) C. maenas had much higher predation rates than any native predator tested; and (4) while the native shore crab, Paragrapsus gaimardii, was found to have a constant predation rate over an eightfold range of densities of juvenile K. scalarina (16-128 individuals·m⁻²), C. maenas significantly increased its per capita predation with increasing prey density. Notably, in open field plots at a site where C. maenas was abundant, predation was constant over the range of tested prey densities. We predict, therefore, that the invasion of C. maenas will have significant negative consequences for the Tasmanian K. scalarina fishery.     

Native species vulnerability to introduced predators: testing an inducible defense and a refuge from predation

To manage the impacts of biological invasions, it is important to determine the mechanisms responsible for the effects invasive species have on native populations. When predation by an invader is the mechanism causing declines in a native population, protecting the native species will involve elucidating the factors that affect native vulnerability. To examine those factors, this study measured how a native species responded to an introduced predator, and whether the native response could result in a refuge from predation. Predation by the green crab, Carcinus maenas, has contributed to the decline in numbers of native soft-shell clams, Mya arenaria, and efforts to eradicate crabs have proven futile. We tested how crab foraging affected clam burrowing, and how depth in the sediment affected clam survival. Clams responded to crab foraging by burrowing deeper in the sediment. Clams at shallow depths were more vulnerable to predation by crabs. Results suggest soft-shell clam burrowing is an inducible defense in response to green crab predation because burrowing deeper results in a potential refuge from predation by crabs. For restoring the native clam populations, tents could exclude crabs and protect clams, but when tents must be removed, exposing the clams to cues from foraging crabs should induce the clams to burrow deeper and decrease vulnerability. In general, by exposing potential native prey to cues from introduced predators, we can test how the natives respond, identify whether the response results in a potential refuge, and evaluate the risks to native species survival in invaded communities.     

Comparative toxicity of Pfiesteria spp., prolonging toxicity of P. piscicida in culture and evaluation of toxin(s) stability

Toxicity of Pfiesteria piscicida (strain CAAE #2200) in the presence of fish (juvenile hybrid tilapia, Oreochromis sp., total length 3–6 cm) has been maintained in the laboratory for 19 months by serial transfer of toxic cells using a modified maintenance protocol. Toxicity was re-induced when toxin-producing P. piscicida cells were separated from fish and cultured on algal prey for 50 days and then re-introduced to new tanks containing fish. We confirmed toxicity in a strain of P. shumwayae (strain CAAE #101272). Toxicity to fish was demonstrated in culture filtrates (0.2 μm) derived from cultures of both Pfiesteria spp., however, it was markedly reduced in comparison to unfiltered water. Filtrates retained toxic activity when stored at −20 °C for up to 6 months. Toxicity to fish was retained when filtrates were held at room temperature for 48 h, at 70 °C for 30 min or at 88–92 °C for 2 h. P. piscicida killed all finfish species tested. Grass shrimp (Paleomonetes pugio; adult 2–3 cm), blue crab (Callinectes sapidus; juvenile 4–7 cm) and brine shrimp (Artemia sp.; 18–24 h post-hatch) were unaffected by concentrations of toxin(s) that killed juvenile tilapia in 4–24 h. Ichthyotoxic activity of filtrates from fish-killing cultures and stability of the toxic activity were similar among P. piscicida and P. shumwayae. These results confirm previously reported observations on toxicity of P. piscicidaand P. shumwayae to finfish. We have maintained toxicity in the laboratory for longer periods than have previously been routinely achieved, and we have demonstrated that the toxic activity is heat stable. In contrast to previous studies with other toxic P. piscicida strains, we did not observe toxic activity to blue crabs or other crustaceans.     

The effects of interspecific competition and prey odor on foraging behavior in the rock crab, Cancer irroratus (Say)

The effects of competitor pressure and prey odor on foraging behavior of the rock crab, Cancer irroratus (Say), were investigated. The Jonah crab, Cancer borealis (Stimpson), was chosen as the interspecific competitor because it shares resources with C. irroratus. Four treatments were tested for their effect on foraging: the presence or absence of a competitor and two types of prey odor; body odor (living mussel) and tissue extract (dead mussel tissue). The presence of Jonah crabs did not influence location time, search time, prey size selected, or handling time of the rock crabs. However, rock crabs responded differently to the presence of body odor and tissue extract cues. The presence of extract odor decreased the time to locate prey while increasing the number of prey manipulated and prey size selected. When prey body odor was present, rock crabs displayed less investigative behaviors than in the presence of extract odor, illustrated by reduced location time. Extract odor provided a stronger and more attractive cue than body odor, but increased prey manipulation and search time. Extract odor induced increases in manipulation and searching for prey but canceled out the benefits of decreased location time, resulting in crabs from both treatments displaying similar search times. These elevated behaviors may be associated with foraging for injured and cracked prey or may indicate an area of conspecific feeding.