Agonistic interactions between invasive green crabs, Carcinus maenas (Linnaeus), and sub-adult American lobsters, Homarus americanus (Milne Edwards)

The invasive green crab, Carcinus maenas, has recently expanded its range into the Southern Gulf of St. Lawrence, where there is potential for substantial niche overlap with juvenile American lobsters, Homarus americanus. We used two experiments to elicit, record and analyze the agonistic interactions of adult green crabs (carapace width of 63-75 mm) and sub-adult (carapace length of 55-70 mm) lobsters. The first experiment gave each animal equal access to a limited food resource. The green crabs were first to the food in significantly more trials, spent a significantly greater proportion of time with the food, and were able to successfully defend the food from attacks by the heavier lobsters. In the second experiment, we allowed the lobsters to gain possession and initiate feeding on the food before releasing the green crabs. In these trials, the lobsters spent significantly more time with the food, and were able to defend the food from the green crabs. The results of both experiments are discussed in the context of game theory. The different behaviour of the crustaceans in the two experiments is consistent with the “bourgeois” strategy in a hawk and dove game simulation. With this strategy, an animal acts like a hawk if in possession of a resource, but acts like a dove if the other animal is in possession of the resource. The fact that the green crabs were able to physically compete with, and in many cases dominate the larger, heavier lobsters supports the potential for competitive impacts of green crabs on sub-adult lobsters.     

Predatory impact of the green crab (Carcinus maenas Linnaeus) on post-settlement winter flounder (Pseudopleuronectes americanus Walbaum) as revealed by immunological dietary analysis

Predation on flatfish during the early juvenile stage is an important factor regulating year-class strength and recruitment. In this study, immunological dietary analysis was performed on green crabs (Carcinus maenas) collected from the Niantic River, Connecticut, in an effort to evaluate the predatory impact of this species on post-settlement winter flounder (Pseudopleuronectes americanus). Through the use of species-specific antiserum, winter flounder proteins were identified in 4.8% of the green crab stomachs analyzed (n = 313, size range = 14-74 mm carapace width, CW), revealing that crabs ≥ 29 mm CW are predators of post-settlement winter flounder in natural populations. The most significant factor underlying the predator-prey interaction was the relative size relationship between species, such that the incidence of winter flounder remains in the stomach contents of green crabs was positively correlated with predator-to-prey size ratio. Results from dietary analysis were incorporated into a deterministic model to estimate the average daily instantaneous mortality and cumulative mortality of winter flounder owing to green crab predation. Accordingly, green crabs may account for 0.4% to 7.7% (mean = 2.2%) of the daily mortality of winter flounder and consume 1.1% to 32.3% (mean = 10.2%) of the flounder year-class. Model simulations further indicate that variations in green crab abundance and size-structure account for the greatest variability in winter flounder mortality. Relative to other macro-crustacean predators, however, predation by green crabs has a minimal effect on winter flounder survival, due in large part to the low densities of these crabs in temperate estuaries.     

Spatial overlap and interaction between sub-adult American lobsters, Homarus americanus, and the invasive European green crab Carcinus maenas

Invasive species are considered one of the major threats to biological diversity, and they can negatively impact species of particular ecological and/or economic importance. In this study, we investigate the potential threat that the European green crab, Carcinus maenas, represents to sub-adult lobsters, Homarus americanus, in Passamaquoddy Bay, Canada, northwest Atlantic. We conducted transect surveys using SCUBA diving to evaluate spatial overlap between these species inside, and immediately adjacent to, a lobster nursery area, and ran two laboratory experiments to investigate interactions between them in conditions of limited food and shelter availability. We found marked spatial overlap between these species in nature, particularly between adult green crabs and sub-adult lobsters in the shallower parts of the nursery area. It was not uncommon to see green crabs and sub-adult lobsters less than 1-2 m from one another, and we estimated mean nearest-neighbor values as low as 0.5-1 m on certain transects and months, with a grand mean of 5.17 ± 1.62 m (SE). Monte Carlo randomization of positional data revealed that green crabs and lobsters were randomly distributed relative to one another; individuals of one species did not seem to aggregate or segregate relative to individuals of the other species. In the lab, green crabs did not negatively affect survival (one exception), growth, activity, feeding, or shelter use of sub-adult lobsters. In fact, sub-adult lobsters regularly preyed upon green crabs in the first lab experiment, where heterospecific individuals were matched for body mass. Our results suggest that green crabs do not pose a significant threat to sub-adult lobsters in Passamaquoddy Bay, and may in fact represent a food subsidy for them.     

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.     

The geography of crushing: Variation in claw performance of the invasive crab Carcinus maenas

The major claws of predatory, durophagous decapods are specialized structures that are routinely used to crush the armor of their prey. This task requires the generation of extremely strong forces, among the strongest forces measured for any animal in any activity. Laboratory studies have shown that claw strength in crabs can respond plastically to, and thereby potentially match, the strength of their prey's defensive armor. These results suggest that claw strength may be variable among natural populations of crabs. However, very few studies have investigated spatial variation in claw strength and related morphometric traits in crabs. Using three geographically separate populations of the invasive green crab in the Gulf of Maine, we demonstrate, for the first time, geographic variation in directly measured claw crushing forces in a brachyuran. Despite variation in mean claw strength however, the scaling of claw crushing force with claw size was consistent among populations. We found that measurements of crushing force were obtained with low error and were highly repeatable for individual crabs. We also show that claw mass, independent of a linear measure of claw size, and carapace color, which is an indicator of time spent in the intermoult, were important predictors of claw crushing force.     

Patterns of morphological and genetic variability in UK populations of the shore crab, Carcinus maenas Linnaeus, 1758 (Crustacea: Decapoda: Brachyura)

Previous research has identified extensive inter-population variability in the morphology of the shore crab (Carcinus maenas L.). To determine the source of this variation (genetic or environmental), morphological and genetic data were analysed from crabs collected from eight sites around the coast of the UK. Ten morphometric traits were measured from over 800 crabs and the degree of morphological similarity among sites was calculated using multivariate techniques. Allozyme electrophoresis was used to investigate patterns of genetic similarity. Extensive morphological variability was detected: eight out of the ten morphometric traits analysed were useful when discriminating between crabs from each site. Discriminant function analysis revealed that over 35% of individuals could be classified to their site of origin on the basis of their morphology. In contrast, the allozyme analysis revealed low levels of genetic variability, both within the meta-population and among the crab population at each site. Pairwise comparisons revealed a moderate correlation between the degree of morphological and genetic similarity of crabs at each site, which suggests that the observed phenotypic variability has a genetic component. However, only around 20% of the phenotypic variability detected was associated with the patterns of genetic similarity. This means that patterns of morphological variability in this species are largely determined by the local environmental conditions: local factors could have a within-generation selective influence on mean trait values or C. maenas may exhibit phenotypic plasticity.     

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.     

Exposure to the steroid hormone 20-hydroxyecdysone modulates agonistic interactions in male Homarus americanus

In this study we present evidence that 20-hydroxyecdysone (20E) affects agonistic behavior in male American lobsters and that male and female animals differ in their response to the hormone. Thirty-minute staged fights were conducted between large males exposed either to artificial seawater (ASW) or 20E and small, anosmic opponents. The nephropores of both combatants were blocked. Fights were videotaped and quantitatively analyzed for aggressive, defensive and avoidance behaviors using an ethogram in which behaviors are ranked according to aggressiveness. Unlike female lobsters, exposing male lobsters to 20E did not increase their aggressive behavior; however, there was a marginally significant trend toward an increase in defensive behaviors with a lower aggressive content than in their ASW-exposed counterparts. The opponents of 20E-exposed animals performed significantly more aggressive behaviors than their counterparts. In fights with 20E-exposed animals, the overall aggressive intensity of the fight was increased and the animals performed a greater number of avoidance behaviors. Unlike the effects of 20E on females, where exposure to 20E caused an increase in overall agonistic arousal, males only exhibited a change in frequency of their behaviors. These findings suggest that while 20E affects both males and females in agonistic encounters, the nature of the effect is different for the two sexes.     

Subtidal-intertidal trophic links: American lobsters [Homarus americanus (Milne-Edwards)] forage in the intertidal zone on nocturnal high tides

Homarus americanus (Milne-Edwards), the American lobster, is a predator in New England subtidal communities, feeding on ecologically important grazers (sea urchins), mesopredators (crabs), and basal species (mussels). In this study, we provide the first report of adult American lobsters foraging in rocky intertidal habitats during nocturnal high tides. Censuses by SCUBA divers in the low intertidal (Chondrus crispus Stackhouse) zone showed mean densities of 2.2 lobsters/20 m² on nocturnal high tides, with contrasting low densities of 0.18/20 m² during diurnal high tides. Nocturnal high-tide intertidal densities were 62% of those reported in a previous study of lobsters in nearby subtidal rocky areas (Novak, 2004). The average carapace length of lobsters in the intertidal at night was > 50 mm. These lobsters were actively foraging in the intertidal with collected individuals having a mean stomach fullness of 67%. Prey found in the stomach contents primarily consisted of crabs, mussels and snails. Field experiments showed that lobsters rarely fed on medium to large size individuals of the common intertidal snail, Littorina littorea (L.). In contrast, experiments with local crab species demonstrated that lobsters actively and readily prey on Cancer irroratus (Say) and Carcinus maenas (L.), but were significantly less likely to consume Cancer borealis (Stimpson). The abundance of Carcinus maenas and blue mussels (Mytilus edulis L.) in the intertidal zone may explain the upshore movement of lobsters. Since nocturnal migration of Homarus americanus into the intertidal zone has not been documented before, our understanding of the dynamics of New England intertidal communities needs to be expanded to include this predator.     

An historical overview of the Republic of Ireland's lobster (Homarus gammarus Linnaeus) fishery,with reference to European and North American (Homarus americanus Milne Edwards) lobster landings

Historical developments of the Republic of Ireland's Homarus gammarus fishery are reviewed and compared in the context of landings in Europe and the North American H. americanus fishery. H. gammarus fishing has provided essential income for over a century to many Irish coastal communities. There have been significant fluctuations (greater than 460%) in recorded landings over this time and the relative importance of H. gammarus fishing has changed with the establishment of ancillary crustacean fisheries. In 1994, H. gammarus landings reached an all-time high of 714 tonnes, with a landed value of IR £6.3 million; in 1999 landings were 597 tonnes valued at IR £6.15 million. Current Republic of Ireland lobster fishery regulations are: (a) minimum carapace length of 85 mm, (b) ban on landing lobsters that have been `V-notched' or have a mutilated tail fan, and (c) prohibition of capturing lobsters by SCUBA diving. Chronological details on recent legislation, regulations and landings relating to Ireland's lobster fishery sector are reviewed. In the last 40 years landings of H. gammarus in the Republic of Ireland exhibit signs of an overexploited stock characterised by a reduction in catch per unit effort and there has been a general decline of total landings for H. gammarus throughout Europe. This contrasts with North America, where many regulations govern the H. americanus fishery and landings appear to be in a healthier state.     

Immunological detection of winter flounder (Pseudopleuronectes americanus) eggs and juveniles in the stomach contents of crustacean predators

Predation on the early life history of fish is an important factor regulating year-class strength. Verifying predation events, however, is difficult when analyses rely on visually identifying the remnants of partially digested fish in the stomachs of suspected predators. The objective of this study was to assess the utility of using immunological assays to detect the presence of winter flounder eggs and juveniles (Pseudopleuronectes americanus) in the gut contents of sand shrimp (Crangon septemspinosa) and green crab (Carcinus maenas). After defining assay capabilities, the stomach contents of field-collected shrimp and crabs were examined to determine if these predator-prey relationships occur under natural conditions. Winter flounder-specific antisera developed and used in this study successfully identified homologous antigens (egg or juvenile flounder extracts) without appreciably cross-reacting with antigenic material from predators or nontarget prey. Moreover, antisera detected flounder eggs 10.8-16.4 h after initial feeding by various sized shrimp, and identified juvenile flounder 9.4 and 7.8 h after initial ingestion by shrimp and crabs, respectively. Immuonological dietary analysis of decapod crustaceans collected from Niantic River, Connecticut, revealed that C. septemspinosa and C. maenas are potentially important predators on the early life stages of winter flounder. The temporal trends and magnitude of flounder predator-induced mortality was affected primarily by the spatial and temporal overlap between predator and prey (egg mortality), and the size-dependent relationships underlying crustacean and flatfish predator-prey interactions (juvenile mortality).     

Daily and circadian rhythms of locomotor activity in the American lobster, Homarus americanus

It is widely accepted that American lobsters, Homarus americanus (Milne-Edwards), are nocturnally active. However, the degree to which this rhythm is expressed by different individuals and the underlying causes of lobster activity rhythms, are poorly understood. In order to address these issues we recorded daily patterns of lobster locomotion using two novel techniques. In the first, reed switch assemblies were used to monitor the distance traveled by freely moving lobsters (n=43), each fitted with a small magnet, as they walked around a 1 m diameter racetrack. The advantages of this technique included: (1) lobsters were freely moving; (2) the system could be deployed in laboratory tanks or in the field and; (3) actual distances moved were measured, not just relative activity. The second technique involved placing individual lobsters (n=10) into custom-designed running wheels. This allowed for continuous monitoring of locomotor activity for extended durations (>45 days) under normal light/dark (L/D) cycles, as well as in constant darkness (D/D) and constant light (L/L).Under ambient light conditions lobsters in the racetracks moved an average of 60.1±6.5 m/day in flow-through seawater tanks. Overall, lobsters were significantly more active at night, moving 4.1±0.4 m/h in the dark vs. 1.0±0.2 m/h in the light. However, many of the lobsters moved as much during the day as during the night.Lobsters in the running wheels moved an average of 36.6±11.7 m/day and 80% expressed clear daily rhythms of activity, with a mean periodicity of 24.0±0.1 h under L/D conditions. Under D/D conditions 90% of the animals expressed free-running circadian rhythms with a mean periodicity of 24.2±0.3 h, indicating that this species possesses endogenous rhythmicity. While the running wheel results show that the nocturnal pattern of locomotor activity for this species is strongly influenced by an endogenous circadian clock, the results from the racetracks show that there is remarkable variability in the extent to which they express this pattern under natural conditions.     

Frequency of moulting by shore crabs Carcinus maenas (L.) changes their colour and their success in mating and physiological performance

Male shore crabs, Carcinus maenas [Linnaeus, 1758], compete aggressively for access to receptive females to mate. Size is the single most important factor for the outcome of these conflicts, large males with carapace width (CW) over 60 mm being much more likely to gain access to receptive females than smaller males. To compete aggressively, large male shore crabs decrease moulting frequencies and may potentially terminate moulting to enter a state of anecdysis, in which further growth is suspended to increase reproductive output. This change from a “growth” strategy to a “reproduction” strategy results in the creation of two morphs, which can roughly be separated by their colouration. As the new exoskeleton created during moult is always green in appearance, crabs tend to be green in appearance during periods where they grow rapidly and moult frequently. Green crabs are found in all size classes. However, as the exoskeleton becomes older, the colour gradually changes to a darker red colour, and large crabs that have spent an extended period in intermoult are therefore often red in appearance. Also, the exoskeleton of red crabs exhibits a higher incidence of epibionts and wear and tear. Red crabs can also be found in all size classes, but their relative proportion in the population increases dramatically in size classes above 60-mm CW. Size for size, the red morph has a thicker carapace and larger master chelae than the green morph. Also, the reproductive indices (RI) for red crabs are higher than for green crabs, and they experience higher mating success. However, this mating success appears to be achieved at the expense of a lower physiological tolerance, green crabs being better adapted to deal with changes in the surrounding environment. This increased tolerance is not only observed with regard to natural variations in the habitats where shore crabs live, but green crabs also appear to be more tolerant to variations caused by anthropogenic pollution. Consequently, the shift from growth to reproduction exerts a profound effect on the behaviour, physiology and ecotoxicology of male shore crabs. The present paper reviews the studies conducted so far, proposes a mechanism by which some of these differences between the two morphs are created and discusses their ecological and ecotoxicological significance.     

Laboratory comparison of American lobster,Homarus americanus,foraging responses to invasive green crab,Carcinus maenas,and two traditional finfish baits

The American Lobster (Homarus americanus) is trapped primarily with finfish baits that are becoming more costly and difficult to acquire as stocks dwindle. Thus, alternative bait options should be considered. We made video recordings of lobster foraging responses in the laboratory to compare the attractiveness of the invasive green crab (Carcinus maenas) bait, two traditional finfish baits and an inert control. Solitary lobsters approached and handled all three biological baits with similar latencies and spent similar time in contact with the biological baits. In contrast, lobsters took longer to approach and handle, and spent significantly less time in contact with control bait. As green crabs were statistically indistinguishable in attractiveness from traditional baits in our laboratory setting, we suggest field trials should test this alternate bait in circumstances more directly relevant to the fishery.     

Sperm ultrastructure, morphometry, and abnormal morphology in American black bears (Ursus americanus)

The objective of this study was to describe sperm ultrastructure, morphometry, and abnormal morphology in American black bears. Electroejaculation was successful in 53.8% (7/13) of the attempts, but urine contamination was common. Epididymal sperm samples were also obtained from five bears. Sperm had a paddle-like head shape and the ultrastructure was similar to that of most other mammals. The most striking particularity of black bear sperm ultrastructure was a tightening of the nucleus in the equatorial region. Although the differences were not significant in all bears, the overall decrease in sperm nucleus dimensions during transport from the caput epididymis to the cauda suggested increasing compaction of the nucleus during maturation. For ejaculated sperm, nucleus length, width, and base width were 4.9, 3.7, and 1.8 μm, respectively, whereas sperm head length, width, and base width were 6.6, 4.8, and 2.3 μm, and midpiece, tail (including midpiece), and total sperm lengths were 9.8, 68.8, and 75.3 μm. Evaluation of sperm cytoplasmic droplets in the epididymis revealed that proximal droplets start migrating toward a distal position in the caput epididymis and that the process was mostly completed by the time sperm reached the cauda epididymis. The proportion of morphologically normal sperm in the ejaculate was 35.6%; the most prevalent sperm defects were distal cytoplasmic droplets and bent/coiled tails. The morphology of abnormal sperm and the underlying ultrastructural defects were similar to that in other large domestic animals thus suggesting similar underlying pathogenesis of specific sperm defects and similar effects on fertility.     

Quantifying factors influencing American lobster (Homarus americanus) predation on the invasive green crab (Carcinus maenas) in Newfoundland,Canada

The European green crab (Carcinus maenas) is a newly invasive species in Newfoundland, where it has likely been present for ≤15 years. The green crab has been found in stomach contents of American lobster (Homarus americanus) in New England and Nova Scotia, Canada, but predation on this species has not yet been quantified in Newfoundland. We conducted feeding experiments to determine whether lobsters from Newfoundland were as likely as those from Nova Scotia (which have coexisted with green crabs for >60 years) to recognize and prey upon this new species. We also performed experiments to determine whether green crabs reach a size refuge from predation and whether factors including starvation, availability of alternate food sources, or habitat complexity would influence the probability of lobster attacking or feeding on green crabs. In our trials, lobster origin had no significant effect on crab predation; lobsters, irrespective of origin, were more likely to consume small (<40 mm carapace width [CW]) and medium (40–65 mm CW) crabs than larger (>65 mm CW) ones. Nevertheless, even small lobsters (73–76 mm carapace length, 300 g) were able to kill and consume the largest green crabs (78 mm CW, 100 g). Green crabs were less likely to be attacked or eaten when an alternative food source was present, suggesting that the lobsters were preying on the crabs, rather than simply killing them in a dispute over territory. The addition of a shelter provided a refuge for the green crabs; however, the crabs were only able to avoid being injured or eaten if this shelter was structurally complex. The green crab is slowly spreading westward around the island of Newfoundland, and so its long‐term effects, interactions with other organisms, and contribution to the diet of Newfoundland lobsters remain to be seen.     

Effects of salinity on competitive interactions between two Juncus species

A glasshouse study investigated the effect of salinity on growth and competitive interactions between two closely related rush species, an Australian native (Juncus kraussii) and an exotic (J. acutus) species. Overall, both species exhibited decreases in height and total biomass with increasing salinity, although tolerance of J. acutus was marginally lower. We observed asymmetric responses at each salinity, due to the presence of the other species. In fresh-water, co-presence of J. kraussii facilitated the growth (increases in height and total biomass) of J. acutus. However, at 10 ppt salinity direct interspecific competition with J. kraussii adversely affected total biomass of J. acutus. When grown with J. acutus, at 5 ppt but not at 10 ppt, salinity reduced total biomass of J. kraussii. We suggest that interspecific interactions vary with salinity, dependant on relative salinity tolerance of each species. It would appear that in areas receiving regular fresh-water inputs, which reduce salinity stress, J. acutus has the potential to displace J. kraussii.