Discussion

            Previous papers have shown that Hemigrapsus sanguineus is indeed proving to be a strong competitor of Carcinus maenas. One experiment was run in sites throughout Connecticut, Rhode Island and Massachusetts between 1997 and 1999 where quadrats were used to count crabs and determine densities. During this time, the observers recorded H. sanguineus becoming the dominant species and increasing in density while C. maenas, which had been the dominant crab species in 1997, decreased in density. The only exceptions to this trend were the two sites which were north of Cape Cod. While C. maenas densities did decrease in the face of growing H. sanguineus populations, C. maenas remained the dominant crab species (O’Connor 2001).

            Another paper, also published in 2001, studied the densities of the two crab species at two sites in Connecticut. This study also found Hemigrapsus sanguineus to be the dominant species, most likely out competing Carcinus maenas for food and habitat. In fact, C. maenas were found at “extremely low numbers at both sampling sites where historical data indicates they were once abundant” (Cassanova 2001, p. 17).

            Clearly, Hemigrapsus sanguineus is asserting its dominance over Carcinus maenas at most sites across Southern New England. Furthermore, both authors found that H. sanguineus reached maximum densities in the mid to lower intertidal zone. One could argue that this appears to indicate a classic example of competitive displacement. In Joseph Connel’s groundbreaking paper in 1972, he documented the results of competition between two species of barnacle in Scotland’s intertidal. Through removal experiments, he proved that the dominant competitor (Semibalanus balanoides) was forcing the subordinate competitor (Chthamalus fragilis) to reside in a suboptimal refuge habitat (Connell 1972). S. balanoides was out competing C. fragilis for the optimal area in the intertidal. The weaker organism was forced to eek out its existence in the stressful upper intertidal. Since the upper intertidal is left exposed to air longer, organisms which dwell there are subjected to more extreme temperatures, increased desiccation time and longer exposure to bird predators. Marine organisms typically live lower in the intertidal unless they just can’t compete with the organisms that are already there. This yields a typical pattern of zonation in the intertidal where an organism’s upper limit is restrained by the physical stress it can endure while its lower limit is determined by biological factors, such as competition and predation (Bertness 1999). This might be demonstrated by the two crab species under consideration: H. sanguineus may be colonizing the lower intertidal and forcing C. maenas to move higher.

            This explanation may be valid, but a relationship between the crabs, the seaweed and the bio-geographical differences between areas north and south of Cape Cod does yield a seemingly more credible answer. Hemigrapsus sanguineus has been expanding its range very rapidly and wreaking havoc upon Carcinus maenas populations in Southern New England. H. sanguineus had already become established north of Cape Cod by 1999, and in the five years since O’Connor’s study, one would have expected to see a dominance of H. sanguineus over the other crab species, but this didn’t happen.

            Perhaps of considerable importance is that while Cassanova was finding Hemigrapsus sanguineus to be dominating over Carcinus maenas in Connecticut, “In areas of dense Fucus sp. cover, no medium-to-large… H. sanguineus were found. However, C. maenas were present in the Fucus sp. occupied areas” (Cassanova 2001, p. 17). Maybe, for Carcinus maenas, seaweed cover is the optimal habitat, regardless of its position in the intertidal. After all, Ascophyllum nodosum can form very thick canopies providing cover from desiccation, waves and temperature stresses to any organisms which dwell within and underneath it (Bertness 1999). Despite the wide ranges of color patterns which both crab species are capable of, only dark colored greenish-brown morphs were found in this study. Perhaps this is because of the great protection that the seaweed canopy provides for them in this habitat, and their color represents natural selection’s influence on their morphology. They survive better living in the seaweed canopy than anywhere else, even if it requires living higher in the intertidal. Their dark color in Nahant is their adaptation to avoid predation by birds, given the longer exposure to predation from living higher in the intertidal. Other crab species in the area, such as Cancer sp., have had to retreat below the low tide mark in order to avoid bird predation (Bertness 1999).

            Rocky outcroppings are indeed more prevalent north of Cape Cod (www.mass.gov). Also, the cold Labrador currents from the north and the warmer Gulf Stream currents from the south create contrasting coastal conditions between Northern and Southern New England (www.msp.umb.edu).

Here is the cold Labrador current,
 making its way towards Massachusetts.

This is the Gulf of Maine current,
bringing cold Northern waters
 towards Massachusetts.

Here is the Gulf Stream current which brings warm, Southern water up the Eastern coast.

(Current pictures from http://www.aquatic.uoguelph.ca/oceans/AtlanticOceanWeb/NACurrents/currentmain.htm)

            These contrasting conditions create a change which gives Carcinus maenas an advantage in the North. Perhaps with the change in coastal geography, there is a change in the zonation pattern of the intertidal. If H. sanguineus’s preferred habitat is rendered unsuitable, these crabs are forced to try to move into the algal canopy and share it with C. maenas. As in Cassanova’s observation in the Fucus sp. canopies in Southern New England, this does not work out to H. sanguineus’s favor. Figure 2 demonstrates that H. sanguineus does not have the upper hand in this competition.

Carcinus maenas is better suited to living in and under seaweed canopies. This creates a situation where the Ascophyllum nodosum canopy is the optimal habitat. The upper intertidal at Nahant, bare of most seaweed, is still too stressful for the crabs, but the lower intertidal, which is the least stressful physically but still fairly lacking in seaweed cover, has become the refuge habitat. H. sanguineus may be the dominant competitor south of Cape Cod, but north of the Cape, it is the subordinate competitor. Like C. maenas in Southern New England, H. sanguineus is being out competed from the optimal habitat and is forced to survive in a refuge habitat.

            The coastal ecosystem in Nahant, MA appears to now be stable: H. sanguineus populations have grown as large as they can. Partly becuase Carcinus maenas has a further northern limit than H. sanguineus, it appears to be better suited to the conditions here. H. sanguineus has finally moved to the edge of its range; any further north would be too altered from the conditions of its natural home range to be suitable.

            Of course, further studies are required to determine which theory may be correct. Removal experiments (like Connel’s) of Hemigrapsus sanguineus could determine whether or not H. sanguineus was the dominant competitor: if Carcinus maenas moved into the new territory, it was likely being displaced by H. sanguineus. Additionally, more studies of the relationship between the crabs and the seaweeds as well as the seaweed distributions across the New England shoreline are required to better understand this system.

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