Intertidal tussles: a shifting balance

As an omnivore with a research-oriented palate, I delight in consuming many different food types.  High on my list are crustaceans – in particular the American lobster, Homarus americanus.

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A juvenile American lobster, Homarus americanus. Credit: C. Baillie.

However, another crustacean, the invasive Asian shore crab, Hemigrapsus sanguineus, threatens to disrupt my epicurean delight, by interfering with the growth and development of juvenile lobsters in the low intertidal zone in the north Atlantic. Christopher Baillie and Jonathan Grabowski have explored interactions between these lobsters and crabs to unravel how they might be influencing each other.

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The invasive Asian shore crab, Hemigrapsus sanguineus. Credit: Rhode Island Marine and Estuarine Invasive Species Site.

The Asian shore crab was first detected off the New Jersey coast in 1988 and quickly spread from North Carolina to Maine. Their increase has coincided with a sharp decrease in the abundance of their rival green crabs over the same range. Baillie and Grabowski were concerned that the Asian shore crab could also be adversely affecting lobster populations. They did monthly surveys (May-October) of both lobster and crab densities in Dorothy Cove in Masachusetts, USA, between 2013 and 2017, and discovered that crab populations were increasing sharply at the same time that lobster populations were decreasing steadily.

BaillieFig1

Annual average densities of Asian shore crabs (dark gray) and American lobsters (light gray) from surveys at Dorothy Cove, Nahant, Massachusetts, USA, between 2013 and 2017. Error bars are 1 standard error.

The researchers wanted to know whether the increased number of Asian shore crabs was responsible for the lobster decline. Perhaps the two species competed with each other for shelter. Baillie and Grabowski set up experimental tanks, each containing a wire mesh bottom with a rectangular opening cut in the center, so that a burrow could be excavated.  They then introduced a single lobster or crab to the tank, and allowed it to dig a burrow in the cutout center (we’ll call this individual the resident).

Slide1

In one shelter experiment, the researchers compared the behavior of larger (mean carapace length = 24.7 cm) and smaller (mean carapace length = 9.3 cm) juvenile lobsters in the presence and absence of a variable number of crabs. They discovered that both larger and smaller lobsters spent most of the time in their burrow when no crabs were in the tank. However, introducing crabs was a major disruptor to their mellow existence, with both lobster size classes being more likely to abandon their residences when crabs were present.

BaillieFig3final

Mean (+ standard error) percentage of time spent in shelter by large juvenile lobsters (top graph) and small juvenile lobsters (bottom graph) in relation to absence (Control) or presence of different numbers of crabs.  Different letters above the bars indicate that the means are statistically different from each other.

The reasons for the decline in residence time were very different for large vs. small lobsters.  In an experiment with one large lobster pitted against one crab, resident lobsters initiated an average of 18.00 attacks against crabs, while resident crabs initiated an average of only 0.20 attacks against lobsters. Even if crabs were allowed to establish residency, when a lobster was introduced, it usually picked a fight with the resident crab. So large resident lobsters left their burrows to challenge intruding crabs. Lobsters managed to kill and eat two intruding crabs.

In contrast, smaller lobsters had a much different experience. Crabs attacked resident small lobsters and were able to displace them from their burrow. This was particularly the case when a greater number of crabs were added to the tank.  When eight crabs were added, the poor lobster was kicked out of its burrow, on average, almost 20 times within a six-hour trial.  Under these conditions, crabs attacked the resident lobster almost 40 times per trial.

BaillieFig4

Crab behavior towards a resident lobster in relation to the number of crabs (heterospecific competitors) introduced into the tank. (A) Mean number of times the lobster is displaced. (B) Mean number of fights initiated by an intruder crab. Error bars are 1 standard error. Different letters above the bars indicate that the means are statistically different from each other.

Baillie and Grabowski also conducted feeding trials – but only with a larger lobster pitted against an individual crab (a blue mussel – a preferred food item for both species – was the prey).  Lobsters were much more successful feeders than crabs, and actually increased their feeding rates in the presence of crabs, presumably having no interest in sharing the mussel with its competitor. Taken together, the shelter and feeding experiments suggest a reversal in dominance structure occurs over the course of lobster development.  The abundant Asian shore crab outcompetes small juvenile lobsters for shelter, but once lobsters attain a certain size, they can outcompete crabs for both shelter and food. We still don’t know, for sure, whether the decline in lobsters in the low intertidal zone at the study site was caused by the increase in crabs; the Asian shore crab may still be expanding its range, so it may be possible to more directly study changes in distribution at other sites both north and south of its current range. Fortunately for lobsters (and for lobster consumers), juveniles can also grow and flourish in deeper ocean waters, where Asian shore crabs are much less of a threat.

note: the paper that describes this research is from the journal Ecology. The reference is Baillie, C. J. and Grabowski, J. H. (2018), Competitive and agonistic interactions between the invasive Asian shore crab and juvenile American lobster. Ecology, 99: 2067-2079. doi:10.1002/ecy.2432. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

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