Jul/Aug 2007Marine Animals: Coming in on the Tidesby Marc Rapport

The oceans teem with fearsome wildlife like powerful sharks, jaw-dropping anglerfish and venomous stonefish.

But the sea-faring species that are most likely to disrupt daily life in South Carolina are diminutive crustaceans and mollusks.

Just two months apart in the fall of 2006, the S.C. Department of Natural Resources announced the discovery of two exotic invasive species in Charleston Harbor, despite predictions that neither the mussel nor the barnacle would be able to survive so far from tropical waters.

The first unlikely pilgrim was the Asian green mussel, which left a trail of dead shells surrounding its handiwork: clogged supply lines at a DNR Marine Resources Division facility in Charleston.

"The Asian green mussel has caused big problems along the Florida coast, and it's in South Carolina now," says David Knott, a DNR marine biologist. "It remains to be seen whether it will be able to acclimate to our winter temperatures."

It's usually comfortable in temperatures ranging from 79 to 82 degrees, but tests in Florida have shown that the Asian green mussel can function well in water as cool as 54 degrees. Even after two weeks of exposure to 50-degree water, roughly half of the experimental population survived.

Since DNR temperature records show that Charleston-area water rarely dips below 50 degrees for more than two weeks, the Asian green mussel may prove to be more than just another Lowcountry tourist here for a visit.

Considered a nuisance by power-plant managers even in its native Indo-Pacific waters, the Asian green mussel has a reputation for growing quickly and reaching reproductive maturity at a young age. Because it can spawn at the tender age of two months, moderate mussel populations can produce a progeny of millions.

"These abilities are what make this mussel such a potential threat to coastal structures and enterprises," says Knott.

Planting themselves on almost any hard surface, Asian green mussels can infest buoys, floating docks, piers and pilings. They also settle on intertidal oyster reefs, displacing adult oysters and killing juveniles.

The second new settler, a gigantic barnacle, arrived at the same Folly River dock where the Asian green mussel had taken root. With the onomatopoeic scientific name Megabalanus coccopoma, this stubborn filter feeder dwarfs native coastal barnacles.
Equipped with a strong grip and defensive beak, the species can grow and release larvae in waters with moderate temperatures and high salinity. Giant barnacles thrive in the warm currents surrounding the area of Baja California, Ecuador and Brazil, and recent increases in water temperatures on the South Carolina coast could make it a more attractive destination for these tenacious invaders.

The giant barnacle was first spotted in Texas and Louisiana before moving to the Atlantic Coast, where tennis ball-sized broods create significant drag on boat hulls, propellers and drive shafts. It also prefers to attach to new vessels, leading to potentially devastating economic implications.

"Since it reaches a much larger size than native barnacles in South Carolina, significant settlement and growth of the giant barnacle would require greater maintenance efforts in coastal and high-salinity estuarine areas," Knott says.

Another crustacean nuisance is a tiny isopod, a creature whose strange taxonomy is still the subject of scientific debate. Regardless of whether it hails from the eastern (Synidotea laticauda) or western (S. laevidorsalis) Pacific, the isopod's isolated appearances in both South Carolina and New Jersey in 1999 suggest that it was probably introduced by the maritime industry. With seasonal population surges, it can reach densities of more than 30,000 (fitting, thankfully, in the space of a 2' x 3' tray) in as little as one week.

Two invasive crabs also have scampered onto the beach scene: the green porcelain crab and the swimming spiny hands crab. The tiny green porcelain crab encompasses a broad territory from Peru in the eastern Pacific to tropical western Africa and the Caribbean. It has gained headway in Florida since the 1970s, but its arrival on St. Catherines Island in Georgia in 1994 served as an ecological wake-up call. In less than a year, the green porcelain crab had become the dominant decapod crustacean in its preferred habitat, reigning from the island's rocky substrates to its intertidal oyster bars.

Meanwhile, the larger spiny hands crab was probably introduced to the Caribbean Basin from the Indo-Pacific via shipping routes through the Suez Canal. The crab first reached Florida's Indian River in 1995, but a specimen found at the Southeastern Regional Taxonomic Center (SERTC) dating from 1986 has forced scientists to rethink its pathway into the country. The most documented infestation of spiny hands crabs in South Carolina can be found in Winyah Bay, but their environmental impact remains unknown.

Scientists are also on the trail of an unwelcome relative of the Asian green mussel known as the charrua mussel. Though it has yet to be cataloged in South Carolina waters, the infestation patterns of similar creatures may foreshadow a northward movement in the near future. The United States was spared a massive outbreak when cold winters eliminated the species from Jacksonville, Florida in 1987, but it has since returned to the Indian River Lagoon and Liberty County, Georgia.

Thanks to a voracious appetite and competition with native shellfish, the charrua mussel could leave a wake of economic damage if it migrates to South Carolina.

As for marine fish, the DNR is most concerned about a venomous scorpionfish called the red lionfish. It was first reported in 1992 off the Florida coastline, but a juvenile population ranging from estuaries in New York, New Jersey, Rhode Island and Bermuda indicate that the red lionfish is probably spawning off the southeastern coastline, where larvae can hitch a ride on the Gulf Stream. The red lionfish is the first known non-native marine fish to become a fully established species from an aquarium release.

Invasive marine species of all sorts are continuously cataloged by the SERTC (www.dnr.sc.gov/marine/sertc), which is constructing a searchable database of native and invasive species found from Cape Hatteras to Cape Canaveral. Presently these data are being contributed to a global program that records distributional information on ocean life, but eventually the information will be directly available to scientists worldwide and also to the public. "By becoming familiar with native and non-native species, citizens can help scientists monitor infestations and slow their spread," says Chris Page, the DNR's Aquatic Nuisance Species Program coordinator. "We're asking everyone to learn more about these invaders, and then become part of the solution."

Back to Invasion of the Aquatic Exotics

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