Native Mud Crabs Research Articles

The positive effect of coexisting ecosystem engineers: a unique seaweed-mussel association provides refuge for native mud crabs against a non-indigenous predator.

In marine sedimentary bottoms, mussels and macroalgae have long been recognized as important autogenic engineers that create habitat and modify abiotic conditions. The structural complexity added by bivalves and macroalgae may also mediate intraguild predation amongst marine decapod crustaceans. While spatial distributions of these ecosystem engineers frequently overlap, there is limited understanding of compounded effects when more than one engineer is present. Here we demonstrate that the coexistence of two ecosystem engineers may create habitat valuable for the survival of a small native species, the Atlantic mud crab (Panopeus herbstii), in the presence of the invasive green crab (Carcinus maenas). Using laboratory and field habitat mimics, we measured mud crab survival rates as a proxy for refuge quality. We compared the refuge provided by a unique association between shells of blue mussels (Mytilus edulis) and the giant strain of Irish moss (Chondrus crispus) to that provided by bare substrate, and by each engineer alone. These experiments revealed that the association of giant Irish moss with blue mussel shells positively and non-additively increased mud crab survival compared to the other less complex habitat mimics. In contrast, parallel experiments revealed that high habitat complexity was less important for young green crabs to survive predation from large conspecifics. These results suggest that the impact of ecosystem engineers on trophic dynamics should be considered in a broader, whole-community context encompassing multiple habitat-forming species present.

Community disruption in small biogenic habitats: A coastal invader overcomes habitat complexity to alter community structure.

Non-indigenous species are often identified as threats to native species and communities. Yet, the mechanisms that enable many of these invaders to thrive and alter their newly invaded habitats are still not fully understood. This applies to habitats such as widespread sedimentary shorelines characterized by the presence of scattered biogenic clumps of blue mussels (Mytilus edulis) structurally more complex than bare sediments. In Atlantic Canada, some of these shorelines are numerically dominated by native mud crabs (Dyspanopeus sayi) but have been gradually invaded by the European green crab (Carcinus maenas). This study describes between-habitat (mussel clump vs. bare sediment) differences in density and diversity of invertebrates. It also tests the impact of juvenile green crabs in comparison to native mud crabs using two approaches: First, measuring habitat-related differences in these crabs’ feeding rates on a common prey (soft-shell clams, Mya arenaria). Second, measuring their influence on invertebrate communities associated with mussel clumps. The results show that mussel clumps hold higher invertebrate density and diversity than surrounding sedimentary bottoms. In the laboratory, the feeding rates of native mud crabs were dependent on the type of habitat (sand flat > mussel clump), whereas those of green crabs were significantly higher and unrelated to the habitat in which predation occurred. In field experiments, juvenile green crabs were also the only predators that changed community structure in the mussel clump habitat. These results indicate that green crabs can cause a significant impact on native species and communities. Moreover, they suggest that the ability of this species to overcome the refuge provided by complex biogenic habitats for prey may represent an unexplored mechanism to explain this invader’s expansion here and elsewhere.

Sex, size, and prey caloric value affect diet specialization and consumption of an invasive prey by a native predator.

Escaping the control of natural enemies is thought to heavily influence the establishment success and impact of non-native species. Here, we examined how the profitability of alternative prey in combination with the presence of a competitor and predator aggressive behavior explain individual differences in diet specialization and the consumption of the invasive green porcelain crab Petrolisthes armatus by the native mud crab predator Panopeus herbstii. Results from bomb calorimetry estimates show that invasive P. armatus has high caloric value relative to alternative native prey. Laboratory assays indicated that specialization and consumption of invasive P. armatus was mostly exhibited by large, female P. herbstii, but the presence of a competitor and predator aggressiveness did not influence diet and the consumption of P. armatus. Thus, intrinsic factors (e.g., sex and body size) seem to explain consumption of P. armatus and dietary specialization in P. herbstii, more generally. Although there are still many predator individuals that do not consume P. armatus, the proportion of individuals that have begun to specialize on P. armatus suggests that for some, it has become more profitable relative to alternative native prey. Given the high caloric value of P. armatus, we suggest that it is likely that differences in the cost of its consumption, including attack, capture, and handling times relative to alternative prey, determine its net profitability to individual predators.

Larvae of native and non-native crabs behave differently in response to chemical cues from potential fish predators and adult crabs

The behavior of planktonic larval invertebrates can play an important role in predator avoidance. Megalopae, the last larval stage in crab development, are strong swimmers yet are vulnerable to predation by fish. The behaviors of megalopae of the native mud crab Dyspanopeus sayi (Smith, 1869) and the Asian shore crab Hemigrapsus sanguineus (De Haan, 1835) in flowing seawater were analyzed in response to chemical cues from potential fish predators and adult crabs. Fish predators consisted of cunner (Tautogolabrus adspersus [Walbaum, 1792]), tautog (Tautoga onitis [Linnaeus, 1758]), and mummichog (Fundulus heteroclitus [Linnaeus, 1766]). Larvae were reared to early and late stages of megalopal development. Individual megalopae were dropped into a horizontal glass tube containing a stream of flowing artificial seawater (control) or seawater containing chemicals from fish or adult crabs. Behaviors of each megalopa were observed and categorized as swimming, walking, tumbling, or inactive. In control seawater, megalopae of both species swam upstream more than in seawater with fish or crab cues. Mud crab megalopae were inactive in the presence of chemicals from all fish species whereas Asian shore crab megalopae were inactive more in the presence of cues from tautog and cunner than from mummichog, a species unrelated to fish predators in its native range. Late-stage D. sayi megalopae walked on the bottom of the tube in response to confamilial adult cues more than H. sanguineus. Both species can therefore detect chemical cues in flowing water, even early in megalopal development, and H. sanguineus can distinguish cues from different potential fish predators.

Species coexistence and the superior ability of an invasive species to exploit a facilitation cascade habitat.

Facilitation cascades generated by co-occurring foundation species can enhance the abundance and diversity of associated organisms. However, it remains poorly understood how differences among native and invasive species in their ability to exploit these positive interactions contribute to emergent patterns of community structure and biotic acceptance. On intertidal shorelines in New England, we examined the patterns of coexistence between the native mud crabs and the invasive Asian shore crab in and out of a facilitation cascade habitat generated by mid intertidal cordgrass and ribbed mussels. These crab species co-occurred in low intertidal cobbles adjacent to the cordgrass–mussel beds, despite experimental findings that the dominant mud crabs can kill and displace Asian shore crabs and thereby limit their successful recruitment to their shared habitat. A difference between the native and invasive species in their utilization of the facilitation cascade likely contributes to this pattern. Only the Asian shore crabs inhabit the cordgrass–mussel beds, despite experimental evidence that both species can similarly benefit from stress amelioration in the beds. Moreover, only Asian shore crabs settle in the beds, which function as a nursery habitat free of lethal mud crabs, and where their recruitment rates are particularly high (nearly an order of magnitude higher than outside beds). Persistence of invasive adult Asian shore crabs among the dominant native mud crabs in the low cobble zone is likely enhanced by a spillover effect of the facilitation cascade in which recruitment-limited Asian shore crabs settle in the mid intertidal cordgrass–mussel beds and subsidize their vulnerable populations in the adjacent low cobble zone. This would explain why the abundances of Asian shore crabs in cobbles are doubled when adjacent to facilitation cascade habitats. The propensity for this exotic species to utilize habitats created by facilitation cascades, despite the lack of a shared evolutionary history, contributes to species coexistence and the acceptance of invasives into a diverse community.

Hide and seek: habitat-mediated interactions between European green crabs and native mud crabs in Atlantic Canada

The non-indigenous European green crab (Carcinus maenas) has well-documented impacts on many native species. In the Atlantic Canada region, the green crab distribution is increasingly overlapping with the distribution of mud crabs (primarily Dyspanopeus sayi), a prominent native species. Despite the potential for antagonistic interactions, the relationship between the two species has not been examined, particularly in the context of the diversity of habitats available in the region. This study used observational beach seine data collected between 2009 and 2013 from the southern Gulf of St. Lawrence to explore the temporal and spatial relationships between mud crabs and green crabs, and detected a negative relationship between these species. Twenty-four-hour laboratory experiments examined their predator–prey interactions and assessed the influence of habitat complexity on the outcomes of these interactions. Mud crabs and similar-sized green crabs collected during July and August of 2010 and 2011 were used as prey for large green crab. These predators consumed almost twice as many mud crab compared with juvenile green crab in the two less structured habitats (no substrate or sandy substrate), but predation rates were statistically similar in oyster bed habitat. In that particular habitat, mud crab mortality dropped by ~65 %, whereas the generally lower mortality affecting juvenile green crabs was unaffected by habitat. These results suggest that complex habitats mediate predator–prey interactions and dampen the effect of green crab prey preference. As green crab continues to invade areas dominated by mud crabs, they may threaten the sustainability of this native species.

Estuarine fouling communities are dominated by nonindigenous species in the presence of an invasive crab

Interactions between anthropogenic disturbances and introduced and native species can shift ecological communities, potentially leading to the successful establishment of additional invaders. Since its discovery in New Jersey in 1988, the Asian shore crab (Hemigrapsus sanguineus) has continued to expand its range, invading estuarine and coastal habitats in eastern North America. In estuarine environments, H. sanguineus occupies similar habitats to native, panopeid mud crabs. These crabs, and a variety of fouling organisms (both NIS and native), often inhabit man-made substrates (like piers and riprap) and anthropogenic debris. In a series of in situ experiments at a closed dock in southwestern Long Island (New York, USA), we documented the impacts of these native and introduced crabs on hard-substrate fouling communities. We found that while the presence of native mud crabs did not significantly influence the succession of fouling communities compared to caged and uncaged controls, the presence of introduced H. sanguineus reduced the biomass of native tunicates (particularly Molgula manhattensis), relative to caged controls. Moreover, the presence of H. sanguineus favored fouling communities dominated by introduced tunicates (especially Botrylloides violaceous and Diplosoma listerianum). Altogether, our results suggest that H. sanguineus could help facilitate introduced fouling tunicates in the region, particularly in locations where additional solid substrates have created novel habitats.

A comparison of reproductive patterns and adult dispersal in sympatric introduced and native marine crabs: implications for species characteristics of invaders

The introduction and translocation of nonindigenous marine species is widespread and can pose severe threats to biodiversity and ecosystem functioning. Predicting which species are potential invaders is of particular interest to ecologists. One approach is to identify characteristics that predispose a species to becoming a successful invader. Since its introduction in the 1980’s, the invasive Asian shore crab, Hemigrapsus sanguineus, has shown a remarkable ability to colonize rocky intertidal habitats along the east coast of the United States. In Long Island Sound H. sanguineus occurs sympatrically with the functionally equivalent, but non-invasive, native Atlantic mud crab, Panopeus herbstii. The presence of both species at the same site allowed us to make a detailed, simultaneous assessment of life history traits and adult dispersibility of co-occurring invading and native crab species. We investigated fecundity and maturation rates, length of breeding season and brood production for both species, and conducted field experiments using mark–recapture techniques to determine mobility patterns. Our results show that the nonindigenous Asian crab has a greater reproductive potential than the native mud crab as evidenced by a longer breeding season, multiple brood production and higher fecundity rates. Field experiments confirmed previous studies indicating H. sanguineus is a highly mobile crab, and further demonstrated that adult Asian crabs are more likely than mud crabs to disperse from their shelter/refuge sites. Recovery rates for native mud crabs were significantly higher than those for Asian crabs in three experimental trials, across sites and years. This work provides new information about life history characteristics of both species and supports the hypothesis that high reproductive potential combined with high adult dispersal ability may be important factors associated with the invasion/establishment success of the Asian shore crab. More study is needed, however, to determine the applicability of these findings to other highly successful marine invaders.

Predation risk predicts use of a novel habitat

Predator–prey interactions are often highly co‐evolved, with selection over time for prey with morphological and behavioral traits that minimize predation risk. Consequently, in many environments prey choose among potential habitats according to their refuge value. It is unclear, however, when presented with new habitats, if prey are able to evaluate the predation risk of these relative to familiar habitats and utilize these in accordance with their value. We tested whether, along the east coast of the USA, native mud crabs Panopeus herbstii utilize the non‐native alga Gracilaria vermiculophylla according to its relative refuge value. Experiments examining predation by blue crabs Callinectes sapidus on mud crabs revealed that the non‐native alga had an intermediate refuge value relative to native oysters, which were the most protective, and unvegetated sediment, which was the least. In subsequent choice experiments, mud crabs selected oysters over alga over unvegetated sediment, in accordance with habitat refuge values. Further, in field experiments, the use of Gracilaria by mud crabs was inversely related to the proximity of the alga to the preferred habitat type, oysters, and was reduced by the presence of a blue crab predator. Consequently, mud crabs are utilizing the non‐native alga Gracilaria in accordance with its intermediate refuge value. The relative refuge value of non‐native vs native habitat‐forming species may provide a baseline expectation against which to measure the speed of learning and opportunism in the response of native prey to novel protective habitats.

Return of the Native: Historical Comparison of Invasive and Indigenous Crab Populations Near the Mouth of Delaware Bay

ABSTRACT An invasive population of the Asian shore crab Hemigrapsus sanguineus was discovered in 1988 near the mouth of Delaware Bay, and populations now occur from North Carolina to Maine. The shore crab H. sanguineus competes with indigenous species and has displaced resident crabs throughout its invasive range. However, there have been few studies that document changes in populations of H. sanguineus after the species has become established.We compare sympatric populations of the Asian shore crab and a native mud crab (Panopeus herbstii) that were monitored initially in 2001 and again in 2011 and 2012. The historical study was conducted in a rocky habitat near Cape Henlopen at the southern terminus of Delaware Bay (38.793° N, 75.158° W). Results showed large differences in the relative abundance of the two species throughout the duration of the study. The Asian shore crab H. sanguineus accounted for 75% of total crab abundance in 2001, but abundance had decreased to less than 25% in both 2011 and 2012....

Predation of the non-native Asian shore crab Hemigrapsus sanguineus by a native fish species, the cunner (Tautogolabrus adspersus)

Predation of non-native species by native predators can impact the success of an invasion. Recently, the Asian shore crab, Hemigrapsus sanguineus, successfully invaded the northeastern coastline of the United States, reaching densities greater than 100crabs·m−2 in rocky intertidal areas. The goal of this study was to determine if a specialist fish predator of invertebrates, the cunner, Tautogolabrus adspersus, consumes the juvenile stage of H. sanguineus. In laboratory prey choice experiments, T. adspersus (10.2–22.1cm total length) consumed greater numbers of native mud crabs (family Panopeidae, 3.0–9.0mm carapace width) than H. sanguineus of similar sizes. T. adspersus also consumed more of the native blue mussel, Mytilus edulis, when paired with similar-sized H. sanguineus. Gut content analysis of 60 T. adspersus captured from intertidal zones with high populations of H. sanguineus in southeastern Massachusetts revealed low consumption of the invasive species; only 5 of 60 guts contained Asian shore crabs. Frequent prey items found in cunner guts included panopeid crabs, small snails, barnacles, and branching and encrusting bryozoans. Our results suggest that, as long as native prey species are present for consumption, T. adspersus will not exert high predation pressure on H. sanguineus.

An invasive crab alters interaction webs in a marine community

Over the last decade, the non-native, filter-feeding crab Petrolisthesarmatus 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.

Population dynamics of the non-native crab Petrolisthes armatus invading the South Atlantic Bight at densities of thousands m–2

The green porcelain crab Petrolisthes armatus recently invaded oyster reefs of the South Atlantic Bight. In 2 estuaries of coastal Georgia, USA, crab densities reached peak densities of 4000 to 11 000 crabs m -2 in some periods and locations. Densities generally were higher in the low versus the high intertidal and in the lower versus upper regions of the estuaries. In warmer months, 20 to 90% of adult females were gravid, recruits were dense, and population levels were high. In colder months, densities dropped by 64 to >99%. Male:female ratios were near 1:1 across times and loca- tions. Maximum mean densities of P. armatus in Georgia were 37 times the highest densities re- corded in the presumptive native range. Crabs in the new range reproduced at a smaller size, and the percentage of gravid females was similar between the old and new range. Thus, population fecundity in Georgia exceeds that of the native range by more than 1 order of magnitude. Densities of native mud crabs in the genera Panopeus and Eurypanopeus were unrelated to, or positively correlated with, densities of the exotic crabs; correlations were never significantly negative. The impact of Petrolisthes armatus on native communities is unclear, but could be considerable if this filter-feeding crab impacts oysters, which are the foundation species of inshore reefs.