DURHAM, N.C. – The green porcelain crab (Petrolisthes armatus) is an invasive crab hailing from South America. This filter feeder tends to reside in available oyster reefs, creating population densities of over 1,000 crabs per square meter. Such densities are unheard of anywhere in the world, even in the crab’s native environment.
First sighted in Florida in 1994, this crab has spread throughout the South Atlantic Bight, an area of coastline ranging from Cape Hatteras, N.C. to Cape Canaveral, F.L. Even though the blue crab (Callinectes similis), common mud crab (Panopeus herbstii), and mummichog (Fundulus heteroclitus), a small fish, readily consume this invader, nothing has truly slowed its movement. Researchers worriedly speculate about the environmental and commercial effects of such densities on American oyster reefs, a major fishery in the Southeast United States.
The green porcelain crab represents a double edged sword: increasing the population of oysters at current reefs, while decreasing their growth rate. Yet, the decreased physical growth offsets any oyster population growth. Thus, P. armatus represents a danger to all reef restoration efforts, as they remove nutrients and space required for reproducing oyster habitats.
P. armatus has demonstrated an affinity for hard substrates currently used in reef restoration projects. However, there is no research on whether this crab is likely to inhabit other substrates used in reef restoration efforts as well. Such information would prove useful in avoiding new crab colonies in future artificial reefs.
According to Wilber, “this study should tell us whether altering the method of oyster reef restoration will have an impact on this invasive species. This can hopefully be compiled with data on oyster recruitment and material cost to generate the optimum strategy for restoring oyster reefs in the South Atlantic region.”
The research focuses on determining the effects of different substrates on populations of P. armatus. His proposal outlines the construction of four artificial reefs at five to eight intertidal sites chosen by the South Carolina Oyster Restoration and Enhancement program (SCORE), the targeted funding body.
The artificial reefs will consist of two naturally occurring substances (oyster and whelk shells), and two artificial substances (cinderblocks and cement treated crab traps). The oyster and whelk reefs will have a higher density due to the bagged shells in chicken wire, while the cinderblock and concrete reefs will have a lower density. After creating each artificial reef, the total area will be mapped and physically sectioned off. Every seven days during a two to three month summer period (as the crabs die in the winter, according to Wilber), a section will be chosen at random and removed at low tide. The inhabitants of the section will be identified and recorded. After acquiring data from the section, the inhabitants and section of the reef will be returned to their original location.
The SCORE reefs will be used as a control for the experiment, in order to determine the efficiency of the artificial reefs as opposed to the current ones.
Reef restoration efforts are often costly and manpower intensive. Wilber hopes to determine the most effective and most cost efficient material(s) for use in artificial reef construction. He notes that such information will allow resource managers to maximize ecological benefit and best utilize manpower in reef construction, by noting which materials prove the least desirable for P. armatus.
Further lines of suggested research include a determination for preference and avoidance of certain materials as indicated by avoidance of predators and an analysis of symbiotic relationships between the oysters and P. armatus over a longer period of time.
-By Evan Schwartz