Aquatic Invasive Species

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

Biol Invasions  Vol. 10:347–358  (2008)

The green porcelain crab (Petrolisthes armatus) is a species native to South America that has spread up the eastern coast of the United States.  This has alarmed some marine ecologists as the alien crab has invaded the oyster reefs that form the backbone of this region’s marine ecosystems.  Researcher Amanda Hollebone at the Georgia Institute of Technology conducted an experiment to ascertain the possible implications of this invasion on the food webs of these oyster reef habitats.

Hollebone offered P. armatus as a  food option to four common native predators (mummichog, mud crab, blue crab, and spot) and simply looked for consumption.  All but the spot consumed the porcelain crab, indicating that the newly established porcelain crab populations could serve as viable food sources to the native predators.  This could lead to a dramatic increase in the populations of these predators, thus altering the ecological balance of the oyster reef ecosystem and potentially harming the lucrative oyster fisheries of the east coast.

Eli Wilber

Mar Ecol Prog Ser Vol. 336: 211–223, (2007)

The green porcelain crab (Petrolisthes armatus) is an invasive species in the Southeastern United States that is present in staggering densities of several thousand individuals per square meter.  This has generated some concern among ecologists as the crabs are most common in the oyster reef habitats that form the backbone of the area’s marine ecosystem.  Research by Amanda Hollebone at the Georgia Institute of Technology has provided insight into how this non-native species is able to sustain such population densities.  

Hollebone sampled P. armatus populations throughout its new range and found that the females were reproducing at smaller sizes than in the native range.  In addition, she found that the percentage of reproducing females was similar in both ranges despite much higher population densities in the invaded regions.  These results suggest that the expanded P. armatus populations are more than capable of sustaining themselves through reproduction without the support of future immigrations.

The green porcelain crab (Petrolisthes armatus) is a small tropical crustacean that has advanced up the eastern seaboard of the United States since 1990.  Commonly found in the oyster reefs of the South Atlantic Bight, the porcelain crabs appear to be having mixed effects on native species.  Recent research at the Georgia Institute of Technology has demonstrated that the porcelain crabs do not directly prey upon oysters, in fact, they seem to promote the survival of juvenile oysters.  This is due to the fact that the native predators of juvenile oysters appear to prefer porcelain crabs, thus possibly simultaneously increasing the predator population and reducing the predation of juvenile oysters.  However, this positive effect for the oyster population is at least somewhat mitigated by the fact that the researchers also found that the presence of porcelain crabs suppressed the growth of juvenile oysters, supposedly through competition for food as both species are filter feeders.  In light of these conflicting effects, further monitoring and research of the porcelain crab is necessary to ascertain the long term effects its arrival will have for the ecosystems of the southeastern United States.

Due to their roles as nurseries for many species, oyster reefs serve as the foundation of the marine ecosystems of the southeastern United States and so any threat to this valuable habitat warrants serious investigation.  One source of danger is that the green porcelain crab appears to be a preferred prey option to many common native predators of oysters. This could lead to an ecological disaster if the porcelain crab suddenly died out and the increased predator population reverted to its traditional prey.  Also, if the green porcelain crab is truly slowly the development of juvenile oysters, this will have important ramifications for the commercial fishermen who harvest oysters.  As noted in the article, too little is presently known about the long term effects of the green porcelain crab to determine if it poses a significant threat.  However it should be noted that despite the heavy predation, green porcelain crab populations in its new range exist in densities far in excess of what has been observed in its native habitat.  This suggests that ecological management of the green porcelain crab will be costly and difficult should it become necessary to protect the valuable oyster reef ecosystems of the southeast.

Weblink: http://newswise.com/articles/view/533029/

Eli Wilber