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biological control, but this approach has not yet been perfected.

iv. atlantic rangia (Rangia cuneata) The Atlantic rangia is a characteristically estu-arine clam native to the Gulf Coast. Adults are 2-6 cm long (Fig. 21.4, color plate 8). In the summer, adults release eggs and sperm into the water, where fertilization occurs. In the James River (Virginia), juveniles settled chiefly in the fall and winter (Cain, 1975). Adults are burrowing suspension-feeders, and may live for ten to fifteen years. Atlantic rangia live in estuaries where the salinity is usually less than 20 ppt. Adults can survive in fresh water, but larvae cannot develop, so the landward border of rangia populations is in areas with some sea salt.

Populations of Atlantic rangia often reach densities >100 adults/m2 and may dominate benthic biomass in low-salinity estuaries. Further information on the Atlantic rangia was summarized by LaSalle and de la Cruz (1985), on which much of this account was based.

Atlantic rangia are widespread and abundant along the Gulf Coast. Fossil shells are commonfrom NewJersey to Florida, but living animals weren't reported along the East Coast until 1955. Since 1955, the species has become widespread and common in estuaries from Florida to the Hudson River. It is unclear whether this spread represents an entirely new introduction of populations from the Gulf Coast or a resurgence of the population from a post-glacial refuge along the East Coast. In either case, the spread of rangia from estuary to estuary almost certainly was aided by human movement of animals in ballast water, for bait or food, and with oyster shells used in oyster re-establishment programs (Carlton, 1992). Atlantic rangia were first seen in the Hudson River in 1988. Although the population has not been systematically studied, Atlantic rangia is abundant in Haverstraw Bay and theTappanZee (Llanso etal., 2001), andisfoundas far north as Newburgh (Strayer and Smith, 2001). It is not known how rangia reached the Hudson, but humans probably carried the species into the river in some way.

Atlantic rangia is important both ecologically and economically. Like the zebra mussel, Atlantic rangia may have large, far-reaching effects on aquatic ecosystems. The ecological impacts of rangia in the Hudson have not been studied, but the shallow, well-mixed waters of Haverstraw Bay would be susceptible to impacts from a benthic suspension feeder. Atlantic rangia is an important food for waterfowl and some fish and crabs. Atlantic rangia also is edible and is sometimes harvested commercially (taking and eating rangia from the Hudson is illegal and possibly hazardous). Finally, Atlantic rangia is so abundant along the Gulf Coast that it is harvested for use in road building, where its shells are used as a substitute for gravel (another common name for the species is the "Louisiana road clam"). In 1967, 21.2 million tons of rangia (living and dead) were taken along the Gulf Coast (LaSalle and de la Cruz, 1985).

v. asian shore crab (Hemigrapsus sanguineus)

This small crab (Fig. 21.4) is native to east Asia, where it lives on rocky shores from Hong Kong to Sakhalin Island (McDermott, 1998a). It lives in the upper intertidal zone to the upper subtidal zone, and often hides under rocks. It feeds on a mixed diet of algae and small crustaceans and mollusks (Lohrer and Whitlatch, 1997; McDermott, 1998a; Ledesma and O'Connor, 2001). H. sanguineuspro-duces two to five broods during the summer (McDermott, 1998b). It takes about two months for an egg to develop into a small crab, during which time the planktonic larvae may disperse widely.

H. sanguineus was first seen in the United States in New Jersey in 1988. It is spreading rapidly, and by 1996 was common on rocky shores from North Carolina to Cape Cod Bay (McDermott, 1998a). Based on its Asian range, the species may spread along the East Coast from Florida to the Gulf of St. Lawrence, andprobablywilloccupymesohaline estuaries as well as the open coast (Ledesma and O'Connor, 2001). H. sanguineus was first seen in the Hudson in 1995, and is now common along the piers on the Lower West Side of Manhattan (Cathy Drew, personal communication).

H. sanguineus may have strong impacts on populations of competitors and prey (Jensen, McDonald, and Armstrong, 2002; Lohrer and Whitlatch, 2002a,b). It lives in many of the same habitats as the European green crab (Carcinus maenus - another abundant alien species that was introduced to the East Coast in the early nineteenth century, and which is itself presumed to have strong ecological impacts; Grosholz and Ruiz, 1996) and mud crabs, and feeds on broadly similar foods. H. sanguineus is dominant over co-occurring C. maenus and displaced this species from intertidal habitats in the Northeast (Jensen et al., 2002; Lohrer and Whitlatch, 2002a). It further appears that H. sanguineus may cause populations of the blue mussel (Mytilus edulis) to decline (Lohrer and Whitlatch, 2002a). In the Hudson, Cathy Drew has noticed that green crabs and mud crabs have become scarce on the upper parts of piers, perhaps as a result of the shore crab invasion (personal communication).

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