Baillie CJ & Grabowski JH (2018) Competitive and agonistic interactions between the invasive Asian shore crab and juvenile American lobster. Ecology.DOI: 10.1002/ecy.2432
Setting the Scene
The year is 1988 and the Asian Shore Crab (Hemigrapsus sanguineus) has just landed in New Jersey. A combination of its aggressive nature and lack of predators to keep the population in check, the crab capitalizes on the situation and expands its range. Flash forward to the present day and the crab can be found as far south as North Carolina and is infringing into Maine. The research conducted by Baillie and colleagues focused in on Asian shore crab populations along the northern front in the waters of New England.
Within the waters of Maine dwells the American lobster, Homarus americanus. The lobster is an ecologically and economically important crustacean to the waters of New England. However, the species faces a multitude of threats; including over-fishing, predation and climate change. As the Asian shore crab increases its range, the lobster may also be vulnerable to the ten-legged assailant. 90% of the crab species found in the intertidal zones of southern New England are comprised of the Asian shore crab and there’s reason to believe the crab will voyage northward into waters historically rich with lobster.
Currently what separates the two species is their preference for habitat. The lobster makes its home in the subtidal waters. Whereas the shore crab is found in shallower waters of the rocky intertidal zone. What separates these two habitats is the water depth and whether or not the tide recedes enough to expose decapods to the harsh conditions of the dry world. However, there is evidence of Asian shore crabs trekking to deeper depths and potentially interacting with more lobster. These initial subtidal zones, where the shore crabs are increasingly encroaching, are noted to be important nurseries for juvenile lobsters. The abundant light and copious number of prey permit juvenile lobsters to grow at faster rates than if they were living at deeper depths.
The Fight is On
The researchers, Baillie and colleagues, were interested in understanding how the populations of the Asian shore crab and the American lobster are interacting in the shallows and what was determining the outcome of these interactions; should we be worried about our important fisheries or can they withstand the invasion?
From 2013-2017 the researchers surveyed areas in northern Massachusetts, where Asian shore crabs are observed in copious numbers already, for abundance of both species. Over the five year period, they found that as the density of Asian shore crabs increased, the density of lobster decreased. Upon collecting this information Baillie and his colleagues ran three experiments, with numerous individuals, to understand how the species were interacting together. Each experiment consisted of both species in a saltwater tank with a food source in the center and either species placed at opposite ends of the tank:
- One individual from each species
- One lobster, with time to adjust to the tank, and varying numbers of shore crabs to mimic natural settings
- Various life stages of lobster, primarily juvenile lobsters
Lobsters that were able to reside in their shelter prior to the introduction of the crabs (Experiment 2) were much more aggressive toward the outsiders than the lobsters who were put in the tank simultaneously with the crabs. This is action is primarily because of “Game Theory“. Game theory predicts that an individual who possesses a resource will discern the value of their habitat and defend the resource more than a passerby.
For all three experiments the lobsters acted aggressively toward the crabs. However, there was a reversal of roles once juvenile lobster are introduced into the experiments (Experiment 3) instead of the adult lobsters. Not only were the crabs more aggressive, the crabs also displaced the lobster. Because the juvenile lobster were smaller than their adult counterparts, the invasive crabs were able to enter the lobster shelters and force out the juvenile lobsters.
Adult lobster may be aggressive toward the new invasive species, and the quality of their magnificent adult lobster life remains intact, but in earlier life stages, the lobster is more vulnerable. The displacement of young lobsters limits their shelter in the shallow intertidal zone exposing them more to predation either directly from the Asian shore crab or other species.
While the shallow intertidal zone is only the brim of lobster habitat, it’s critical to acknowledge the encroachment of the invasive Asian shore crab. The crab species may not displace the adult population of American lobster but the work done by Baillie and colleagues predicts a struggle of important nursery grounds for the juvenile lobster, which may lead to complications in the future. This conflict of the intertidal will only persist as the crab expands its range northward.