Range and dispersal of a tropical marine invader, the Asian green mussel, Perna viridis, in subtropical waters of the southeastern United States.

ABSTRACT The tropical Asian green mussel, Perna viridis (Bivalvia: Mytilidae) is a recent invader in the Caribbean Basin, including the subtropical southeastern United States. In this study we examined the (1) range of P. viridis in the southeastern United States, (2) relative abundance of P. viridis across habitats and (3) density of P. viridis in Tampa Bay, FL. The invasion and spread of P. viridis in the southeastern United States was estimated by a combination of first-hand qualitative sampling and second-hand observations. There were apparently at least two discrete introductions, each followed by natural dispersal. The initial invasion was discovered in Tampa Bay in 1999, and was followed by rapid spread of P. viridis south as far as Marco Island, FL, but limited spread northward. In 2002, a second invasion occurred in northeast Florida, separated from the west Florida population by 650 km of coastline. On the east coast, P. viridis appeared to be distributed discontinuously between South Carolina and the Indian River Lagoon, FL by 2007. The literature concerning native distribution, habitat use and invasion history, and vectors of P. viridis is also reviewed.

KEY WORDS: Asian green mussel, Perna viridis, invasion, Florida

INTRODUCTION

Taxonomy and Related Species

The Asian green mussel, Perna viridis (Linnaeus 1758) is an economically important marine bivalve, native to the tropical Indo-Pacific The species was first observed in Florida, USA, in 1999 and has since spread to other areas in the state (Benson et al. 2001, Ingrao et al. 2001).

The use of common names in the literature has caused P. viridis to be confused with other members of the same genus or unrelated mussels that have a similar common name. The green-lipped mussel, P. canaliculus (Gmelin 1791), which very closely resembles P. viridis, is restricted to New Zealand where it is cultured and widely marketed as the Greenshell[R] mussel (New Zealand Mussel Industry Council 2001). The common name green-lipped mussel is occasionally applied in the literature to P. viridis and green mussel to P. canaliculus, but the vast majority of the literature on P. viridis uses green mussel. The American Fisheries Society (AFS) suggests using the common name green mussel instead of Senhouse mussel for the nonindigenous Musculista senhousia (Turgeon et al. 1998) but this usage should be reviewed following the North American invasion by P. viridis. Older reports place P. viridis in the genus Mytilus, as reviewed by Siddall (1980). Zvyagintsev (2003) placed P. viridis in the genus Chloromytilus, but this appears to be a misspelling of Chloromya, in which P. viridis was once placed (Siddall 1980). Some reports use the synonym Mytilus smaragdinus (Chemnitz 1785) (e.g., Luangpan 1982, Baluyut 1989, Merino 2002).

The brown mussel, P. perna, has invaded the western Gulf of Mexico, including Texas and northern Mexico (Hicks & Tunnell 1993, Hicks & Tunnell 1995, Hicks & McMahon 2002). The range of P. perna does not currently overlap with P. viridis except in the southern Caribbean (Agard et al. 1992, Rylander et al. 1996). The AFS common name for P. perna is Mexilhao mussel; mexilhao simply means mussel in Portuguese.

Florida lacks native members of the genus Perna; specimens from the closely related genus Mytilus are rare, although juvenile M. edulis have been reported in Georgia (Power et al. 2004) and north Florida (Camp et al. 1998). The hooked mussel, Ischadium recurvum (Rafinesque 1820) and members of the genus Brachidontes both occur in Florida (Bergquist et al. 2006, Lee & O Foighil 2004); these are the most closely related to Perna based on accepted subfamilies (Coan et al. 2000). According to Lee and O Foighil (2004) Brachidontes cf. exustus (Linnaeus, 1758) from the east and west coasts of the Florida peninsula are separate species, but the taxonomic treatment of these species in Florida has not yet been completed. Brachidontes are usually less than 20 mm in shell length and I. recurvum may rarely reach 60 mm (Abbott 1974) but usually occurs in waters with salinities below 15 ppt (Turgeon 1968, Bergquist et al. 2006). Perna viridis is easily distinguishable from the above mytlids by its green color and lack of ribs.

Distribution and Prior Invasions of the Asian Green Mussel

Native Distribution

The reported native distribution of P. viridis extends from the Persian Gulf to all of the Philippines plus Sumatra, Borneo, Bali, and Sulawesi but excludes the remainder of Indonesia, northeast Vietnam, and China (Fig. 1) (Siddall 1980, Vakily 1989). Siddall (1980) noted the presence of P. viridis in Hong Kong but regarded this stock as introduced. Secondary sources include the island of New Guinea in the native range of P. viridis (Academy of Natural Sciences of Philadelphia 2003, Gulf States Marine Fisheries Commission 2003, FAO Fisheries 2006). Perna viridis is known from the Musandam Peninsula of Oman (Gindy et al. 2001) but its distribution and abundance at the western edge of its range is poorly documented. Additional localities not reported by Siddall (1980) or Vakily (1989) include the Andaman Islands (Appukuttan 1977, Dorairaj & Soundararajan 1998), Vietnam (Academy of Natural Sciences of Philadelphia 2003, Holmyard 2003), Java (Setyobudiandi 2001a, 2001b), and southern Sulawesi (Sharifuddin Bin Andy & Gassing Sitepu 1997). Asian green mussels are fouling organisms and ships have been sailing these waters for millennia; it is possible (though not easily testable) that some of the current Indo-Pacific range includes ancient human introductions. Nonnative occurrences of P. viridis are summarized in Table 1 and reviewed later.

[FIGURE 1 OMITTED]

China

Perna viridis stocks in Hong Kong are among the best studied (Huang et al. 1983, Seed & Richardson 1999, Wong & Cheung 2001). Some secondary sources include China in the native range of P. viridis (NIMPIS 2002, Tang et al. 2002) but Siddall (1980) considered Chinese stocks to be introduced. This view seems to be borne out by Ye (1997) who described a series of purposeful introductions to the extreme southwest China coastline for fishery enhancement, starting in 1964. We could not determine from this information whether the earlier mentioned introductions were also the source of Hong Kong stocks via subsequent natural dispersal, or whether Hong Kong stocks represent a separate introduction. Currently, P. viridis is abundant in Chinese coastal waters at least as far north as Xiamen (Fujian) on the mainland coast, at about 25 [degrees] 30'N (Du et al. 2000). Perna viridis also occurs in western Taiwan, where it is commercially important and probably nonindigenous, although we could find no reports of introduction (Kubota et al. 1999, Chen et al. 2001).

Japan

The invasion of Japanese waters by P. viridis is well documented. Perna viridis was first reported in Japan in 1967 (Hanyu & Sekiguchi 2000). International shipping has been proposed as the vector of introduction and initial invasions were associated with major Honshu ports such as Osaka, Nagoya, and Tokyo (Hanyu & Sekiguchi 2000). In these northern latitudes, P. viridis populations persist in thermal effluents from industry, spread in spring, but die out in surrounding waters each winter (Umemori & Horikoshi 1991, Kazuhiro & Sekiguchi 2000, Zvyagintsev 2003). According to Zvyagintsev (2003), P. viridis "disappeared" after initial reports but was again observed in Tokyo Bay in 1986. In the 1990s, during a period of elevated sea temperature, P. viridis began to spread through the Seto Inland Sea (Matsuyama 1999) and recent work has confirmed that the species is reproducing in the Inland Sea of Japan (Yoshiyasu et al. 2004). Perna viridis has been cultured in Okinawa since 1983, when it was purposefully introduced from the Philippines (Hanyu et al. 2001, Kotani et al. 2001).

South Pacific

Introductions of P. viridis for aquaculture were made in 1972 from Manila Bay in the Philippines to a site in New Caledonia, where it established and became locally abundant (Eldredge 1994). Further introductions from Manila Bay were made to Fiji in 1975 (Vereivalu 1990) and Tonga in 1976 (Eldredge 1994). Perna viridis from New Caledonia was used to establish populations in the Society Islands (Tahiti) in French Polynesia in 1978 (de Gaillande 1979, Coeroli et al. 1984, Eldredge 1994). Tahiti P. viridis populations were subsequently used to restock Fiji in 1981 (Vereivalu 1990), Western Samoa in 1982 (Bell et al. 1983) and the Cook Islands in 1984 (Eldredge 1994). Exact locations on the above islands are usually not reported. All introductions, except the Cook Islands, appear to have resulted in established populations, although some involved multiple inoculations (Eldredge 1994). It is possible that P. viridis was stocked in Fiji as early as 1969 (the reference is vague on that detail) but, in any case, the Fiji stocks did not become a commercial success (Vereivalu 1990). Most of these island habitats lie well within the thermal range of the native habitat of P. viridis. There are no reports of subsequent accidental invasions of P. viridis from these introductions.

In 2001, P. viridis was found on international merchant vessel hulls in Cairns, a seaport of Queensland, Australia. An aggressive eradication and monitoring program was established. Individual P. viridis were found on buoys and test substrata as late as 2004 (NIMPIS 2002, CRC Reef Research Center 2002, Peebles 2004) but the species is since believed to have died out or been eradicated at that location (Hayes et al. 2005).

Atlantic Basin

The first known Atlantic basin invasion of P. viridis occurred in western Trinidad (southern Caribbean), shortly prior to 1990 (Agard et al. 1992). The vector of invasion is undetermined but we could find no reports of deliberate introductions or aquaculture attempts for this species anywhere in the Atlantic Basin prior to 1999. By 1992 P. viridis had spread along the entire western coast of Trinidad on a variety of substrata and, in 1993, appeared in Venezuela, across the Gulf of Paria. By 1996 P. viridis had spread across the northern coastline of the Araya Peninsula as far west as Punta Araya, and to Isla de Margarita (Rylander et al. 1996); it has subsequently become a fishery species in Venezuela (Malave & Prieto Arcas 2005) where it is displacing the nonnative P. perna (Penchaszadeh & Velez 1996, Segnini de Bravo et al. 1999). This spread is consistent with larval dispersal via local oceanic currents (Agard et al. 1992), but does not rule out human mediated dispersal or additional introductions.

In February 1998, P. viridis was reported in Kingston Harbor, southern Jamaica (Ingrao et al. 2001, Buddo et al. 2003). The Jamaican introduction, which was presumed unintentional, was limited to Kingston Harbor as of 2003 (Buddo et al. 2003).

Perna viridis was deliberately introduced to the Cape Verde Islands (eastern Atlantic) from China in 1999 for aquaculture (Merino 2002). As of January 2004, P. viridis had not been recovered from the wild in that location (S. Merino pers. comm.)

The first USA report was in July 1999 from the seawater cooling system of an electrical power facility in TamPa Bay, FL. Limited molecular genetic data suggests that the Trinidad and Florida invasions share a single source population (Benson et al. 2001). The invasion vector is not known for certain, though ballast water has been suggested (Benson et al. 2001). By August 2000, this species had spread south as far as Charlotte Harbor (100 km) but north from Tampa Bay only to Boca Ciega Bay (5 km) (Benson et al. 2001, Ingrao et al. 2001). This coastal distribution is consistent with larval dispersal in local oceanic currents (Pickard & Emery 1982). The present study in Florida began in 2001.

Invasion Biology and Vectors for the Asian Green Mussel

The invasion vector for P. viridis into Florida is undetermined, but there are several reasonable possibilities, discussed later. The initial invasion by P. viridis was not directly observed. Without observation of further inoculations, we cannot assign relative probabilities to any of the vectors discussed later.

Natural Dispersal

Perna viridis has a biology similar to other mytilids. It uses a byssus to attach to a variety of hard substrata, is dioecious with planktonic fertilization (broadcast spawning) and larvae, and has successively declining motility in juvenile and adult stages (Vakily 1989, Tuaycharoen 1991, Chen et al. 2001). The observed larval period in culture varies widely from as brief as 8-12 days (Tan 1975) to 15-18 days (Sreenivasan et al. 1988, Manoj Nair & Appukuttan 2003), to as long as 24-29 days (Tan 1997). The larvae and postlarvae of P. viridis are superficially similar to those of Mytilus species but no one has determined whether P. viridis has a planktonic postlarval phase like some other members of the family (Baker & Mann 1997). Natural pelagic dispersal of adults on driftwood or other natural objects is possible but less likely to occur on a large scale in modern times because of anthropogenic factors (Baker et al. 1999). Plastic and other human debris, on the other hand, may be an increasingly important vector for fouling organisms such as P. viridis (Winston 1982, Aliani & Molcard 2003, Barnes & Fraser 2003).

Reported growth estimates for P. vh'idis (for specimens under 70 mm shell length) include: 8.8 mm. mo 1 in the Philippines (Walter 1982); 7.7-8.8 mm x [mo.sup.-1] in eastern India (calculated from Rajagopal et al. 1998); and 6.6-9.2 mm. mo i in Hong Kong (Lee 1986). Gametogenesis has been reported in specimens as small as 21.3 mm in Thailand (Tuaycharoen 1991) and 28.6 mm in Florida, USA (Barber et al. 2005) but 50 mm is a more typical minimum size for production of mature gametes (i.e., spawning) in Taiwan (Chen et al. 2001). If we accept a minimum shell size of 50 mm for reproduction, combined with the growth rates reported previously, it suggests that a newly recruited P. viridis cohort will need 5.5 7.5 mo to reach reproductive size. The ability of P. viridis to expand its invasion by more than one generational pulse annually depends, therefore, on the length of the growing season in the host habitat. Once it becomes reproductive, P. viridis in Florida and other regions appear to spawn twice in a year (Barber et al. 2005).

Artificial Invasion Vectors

Deliberate introduction (Eldredge 1994) and hull fouling (Coutts & Taylor 2001, Peebles 2004) are known artificial vectors for the dispersal of P. viridis, including elsewhere in the Atlantic (Merino 2002). There are no reports of P. viridis being imported alive to Florida, either as food or for culture. This species is well known as a fouling organism on a variety of substrata, however, (Nair et al. 1988, Low et al. 1991) and some studies have implicated hull fouling as an invasion vector for bivalves for at least the past five centuries (Carlton & Hodder 1995, O Foighil et al. 1998). This raises the possibility that the range of P. viridis was altered by ancient mariners long before naturalists began to document the species. A variety of vessels cross the Caribbean, from private yachts to large barges, and hull fouling is a reasonable possibility for the introduction of P. viridis to Florida from the southern Caribbean.

Ballast water has also gained recognition as an important vector of marine and freshwater introductions (Carlton & Geller 1993, Subba Rao et al. 1994, Collinetti et al. 2001) and has been indicted for the introduction of other bivalve species (Carlton 1993, Carlton 1999). The United States Coast Guard requires high seas ballast water exchange to reduce the risk of introduction of alien species from foreign coastal waters, but compliance was less than 100% in 2000, around the time of the reported P. viridis invasion (Everett 200l, Murphy & Ruiz 2001). The United States is the leading exporter of phosphate fertilizers and the leading importer of phosphate rock. The Port of Tampa handles the largest volume of fertilizer materials in the world, and phosphate materials account for about 90% of exports by weight (Huettel 1994, Tampa Bay Port Authority 2006). Fertilizer products are exported to many countries including India, China, and Brazil (Jasinski 2006). Bulk carriers, and other ballasted vessels, return to Tampa Bay stabilized with ballast water from distant locales. Ballast water, therefore, is a reasonable vector for P. viridis introduction.

Habitat Use

Habitat data for P. viridis are sparse in either native or introduced ranges. Fisheries publications sometimes refer to the species as estuarine without mentioning the specific substratum (Appukuttan 1977, Nagaraj & Neelakantan 1982, Rajagopal et al. 1998). Qasim et al. (1977) report P. viridis as abundant on laterite boulders; it is unclear whether said boulders were naturally occurring or artificially placed. Substratum is sometimes mentioned in the context of fouling or aquaculture. Examples of such substrata include artificially placed granite boulders (Narasimham 1980), bamboo or palm poles (Vakily 1989), polypropylene, polyethylene, or nylon rope or cord (Vakily 1989), and steel and concrete (Nair et al. 1988). Nair et al. (1988) also reported sparse settlement of P. viridis onto teak fouling panels after a full year, even though P. viridis dominated surrounding coastal habitats, but other wood substrata have been used by wild populations of P. viridis including red mangrove (Rhizophora mangle Linnaeus 1753) prop roots in Trinidad (Agard et al. 1992), and the palm logs mentioned previously.

Environmental tolerances of P. viridis have been reviewed by Vakily (1989). Previous studies suggest that P. viridis successfully adapts to salinities between 19 and 44 ppt. The lower thermal limit for 50% survival is 10 [degrees] C and the reported upper limit is 37 [degrees] C to 42 [degrees] C. The maximum reported depth inhabited by P. viridis is 42 m in a power plant intake canal in India (Nair & Murugan 1991).

MATERIALS AND METHODS

The spread of P. viridis in Florida was estimated by a combination of first-hand qualitative sampling and secondhand observations. Surveys were conducted over coastline stretches of 100 500 m; for this reason, positions are reported here only to the nearest minute. Sampling for P. viridis range was usually limited to public access points (bridges, public parks and boat ramps, and breakwaters), and this also limited the coastal distance available to sample. Much of Florida's coastline is inaccessible because of private control and, especially after September 11, 2001, security restrictions around ports, government lands, and major industrial sites.

Qualitative sampling was conducted by us in 2002 and 2003 at most coastal public access points between Shired Island (29 [degrees] 24'N, 83 [degrees] 12'W) and Marco Island (25 [degrees] 55'N, 81 [degrees] 44'W) on the west coast of Florida, and Virginia Key (25 [degrees] 44'N, 80 [degrees] 10'W) and the Florida-Georgia border (30 [degrees] 42'N, 81 [degrees] 26'W) on the east coast of Florida. Additional sampling was conducted in 2004 in the Florida Keys at Summerland Key (24 [degrees] 39'N, 81 [degrees] 27'W), Key West (24 [degrees] 34'N, 81 [degrees] 49'W), Woman Key (24 [degrees] 32'N, 81 [degrees] 59'W), and the Dry Tortugas (24 [degrees] 38'N, 82 [degrees] 52'W). Many of the sites were visited again in 2005 and 2006. Perna viridis shells are vividly and uniquely colored, at least at the growing margin, so visual surveys are effective in determining presence or absence. Survey methods were as follows. When access permitted, we surveyed shorelines for approximately 500 m at or near low tide. Visual searches concentrated on subtidal rocks, shells, pilings, and other firm substrata. Beaches were surveyed for dead shells and living specimens, particularly attached to firm substrata in subtidal areas. If we could access a pier, we examined the submerged portions of floats and lines.

Sites between Marco Island and the Florida Keys in southwest Florida were not sampled by us because of logistic constraints but observations were provided by marina operator contacts in Everglades City (25 [degrees] 51'N, 81 [degrees] 23'W) and by Florida Sea Grant Extension Service for all of Monroe County, which included the Everglades and Florida Keys. Additional reports, including from areas outside Florida, came from individual observations reported to us. The Florida Fish and Wildlife Research Institute (W. Arnold & D. Marelli) and Mote Marine Laboratory (D. Ingrao) separately compiled sighting reports from second-hand sources, including photographs when available. In most cases, Asian green mussels are sufficiently different from native Florida bivalves that a color photograph is sufficient for identity confirmation if the specimen was collected alive. Dead specimens over 7 cm long could have been discarded shells of the related temperate P. canaliculus (sold in Florida as a frozen seafood product) and were treated as "possible" sightings if they could not be examined first-hand by us. Second-hand reports without specimens or photographs, even when collected by marine researchers, were also treated as "possible" sightings.

Observations from areas outside, but adjacent to, the known range of P. viridis were collected in 2005 and 2006 to determine if any additional spread of the species had occurred. If sites could not be visited first-hand by us, information was solicited from other coastal researchers, Sea Grant extension agents, wildlife refuge managers, clam farmers, and marina operators.

Two sites in Tampa Bay, the Interstate Freeway 275 Sunshine Skyway Bridge (27 [degrees] 36' 16"N, 82 [degrees] 38'59"W) and the US Highway 92 Gandy Bridge (27 [degrees] 52'55"N, 82 [degrees] 34'17"W), were surveyed in 2004 by teams in scuba gear to observe P. viridis depth distribution. Bridge pilings were visually examined from the surface to the bottom (about 10 m) and any apparent change in density with depth was recorded.

RESULTS

Perna viridis Range in the Southeast United States

In west Florida (Gulf of Mexico coastline), P. viridis was common to abundant in a wide range of sizes at most mesohaline or polyhaline sites examined between Clearwater (27 [degrees] 58'N, 82 [degrees] 48'W) and Marco Island (25 [degrees] 55'N, 81 [degrees] 43'W). No specimens were observed north of Anclote Key (near Tarpon Springs) in the Gulf of Mexico. No specimens were reported south of Marco Island, including at sites examined in Florida Bay, the Florida Keys, and the Dry Tortugas; a follow-up survey in 2005-2006 still found no specimens in these areas. The one exception consisted of P. viridis specimens on a gill net entangled on the bill of a sawfish, Pristis pectinata Latham 1794 (Table 2), but the sawfish could have traveled a considerable distance in a short time. In east Florida, P. viridis occurred from Ponce de Leon Inlet to near the Georgia-Florida border within a year of its first discovery, but did not spread south of Cape Canaveral (28 [degrees] 27'N, 80 [degrees] 31 'W) until 2006 (Table 2).

The first living P. viridis on the Atlantic coast was found by our group in late 2002 (Table 2). On the Atlantic coast, P. viridis specimens were locally common in several size classes between the Sebastian Inlet (27 [degrees] 51' N, 80 [degrees] 26'W) and the mouth of the St. Johns River (30 [degrees] 24'N, 81 [degrees] 23'W), but this distribution appears to be discontinuous; P. viridis has never been observed in some portions of the Intracoastal Waterway connecting these sites--especially near the mouth of the St. Johns River. A single living P. viridis specimen was found in 2004 in the southern Mosquito Lagoon (28 [degrees] 46'N, 80 [degrees] 46'W), but none in subsequent surveys. April 2006 surveys of the Indian River Lagoon failed to find any P. viridis, and specimens reported in October 2006 were mostly under 5 cm in shell length. Some reports for Georgia have been previously reported by Power et al. (2004).

Perna viridis was reported at a number of locations from Georgia, and from Charleston Harbor in South Carolina (Table 2). Dead specimens were found washed up on the beach at Oregon Inlet, NC (35 [degrees] 46'N, 75 [degrees] 31'W) and Virginia Beach, VA (36 [degrees] 54'N, 76 [degrees] 06'W) (Table 2), but no living or attached specimens have been observed north of South Carolina.

Other Observations

In Tampa Bay, P. viridis occurred on bridge pilings throughout the water column. At the Skyway and Gandy Bridges, P. viridis was observed over the entire length of the pilings, to the bottom at 15 and 10 m, respectively. At the Gandy Bridge site, P. viridis also survived on the sediment surrounding the base of the pilings.

Perna viridis occurred in many of the same habitats as the native eastern oyster, Crassostrea virginica (Gmelin 1791). On many bridge pilings, a layer of C. virginica shells were found beneath P. viridis but living C. virginica were limited to the upper few centimeters of the intertidal (Fig. 2). When intertidal P. viridis died or dropped off bridge piers after a cold-induced mortality event (January 2003), we were unable to find any living C. virginica in the area previously covered by P. viridis. Crassostrea virginica dominated the full intertidal range on nearby seawalls and red mangrove (R. mangle) prop roots, however, with few exceptions. We observed several oyster reefs in the Safety Harbor area of northern Tampa Bay; on one reef, P. viridis covered the reef crest and all C. virginica beneath were dead, but other reefs had fewer P. viridis and more living C. virginica.

Perna viridis occurred with the small native scorched mussels, Brachidontes spp. After reports of P. viridis in the popular media, some of the public misidentified Brachidontes as P. viridis in reports to researchers. Perna viridis at two sites in northeast Florida and Georgia (Mayport, Matanzas River at Marineland, and Sea Camp at Cumberland Island Table 2) occurred with the nonindigenous mussel Mytella charruana (d'Orbigny 1846), first reported in Florida by Lee (1987) and subsequently by Boudreaux and Walters (2006).

DISCUSSION

Perna viridis Invasions in Florida

The invasion of the southeastern United States by the Asian green mussel, P. viridis, apparently occurred as two discrete introductions followed by natural dispersal. The initial 1999 discovery of P. viridis in Tampa Bay has been discussed previously by Benson et al. (2001) and Ingrao et al. (2001). Ballast water, a vector proposed by Benson et al. (2001), is a strong possibility given local shipping patterns (Huettel 1994), but transport on vessel hulls should not be ruled out (Peebles 2004). Tampa Bay harbors numerous barges, for example, which have enormous and infrequently-serviced hulls and long moorage times (pers. obs.). The advantage of a vessel hull as an invasion vector is that it provides a large number of mature P. viridis already in aggregation, thereby bypassing the initial density-dependent spawning limitation that might be encountered by widely dispersed individuals from ballast water (Levitan & Petersen 1995).

[FIGURE 2 OMITTED]

The second invasion in Florida occurred somewhere between the Ponce De Leon Inlet and Jacksonville in northeast Florida on or before 2002, some 650 km by coastline from the nearest known P. viridis in west Florida. The intervening coastline includes considerable habitat apparently suitable to P. viridis, such as the large Indian River Lagoon system, but we consider it unlikely that P. viridis has invaded the intervening region without detection. Perna viridis shells are large, vividly colored, unlike anything else in Florida, and their invasion was well-publicized by the popular press. Not only would we likely have found P. viridis in our surveys of the region if it were present, but it would have been noticed by the numerous amateur collectors in Florida's active shell clubs or by the staff of the various marine science institutions in the region. The most parsimonious explanation is that P. viridis in northeast Florida represent a distinct introduction.

Ballast water and vessel hull fouling are, again, both plausible vectors for the invasion of northeast Florida. Jacksonville, in northeast Florida, is an international deepwater seaport for civilian and U.S. Navy vessels. There is also significant coastal traffic between Tampa Bay and northeast Florida that could carry P. viridis on hulls. For example, beach nourishment (National Oceanic and Atmospheric Administration 2006) is an annual event in northeast Florida and it involves large barges moored for weeks or months just offshore; between contracts, these barges are stored elsewhere in Florida. This scenario presumes, however, that west Florida is the source of P. viridis in northeast Florida, for which we have no evidence besides proximity. Natural dispersal via planktonic larvae seems a remote possibility at best, given the requirement for extremely favorable current regimes, the dilution effect over time, and the 650 km of apparently suitable habitat that would have to be bypassed to reach northeast Florida. Intentional introduction cannot be ruled out, but we have no evidence for it.

Spread of P. viridis in the Southeastern United States

The southward spread of P. viridis in west Florida from the initial invasion would appear to be consistent with coastal currents, based on prior understandings of the Gulf Loop Current (Pickard & Emery 1982). Recent studies of near-shore coastal processes, however (Ruoying He & Weisberg 2002, Ruoying He & Weisberg 2003), suggest that wind patterns drive inshore currents in southwest Florida, whereas the Gulf Loop has little influence. Tidal currents run in both directions along the Intracoastal Waterway between Anclote Key, the northern limit of P. viridis on the west coast, and Sanibel Island (Table 2), but the disproportionate southward spread of P. viridis suggests that south-flowing currents dominate planktonic dispersal. Unfavorable currents, on the other hand, combined with undetermined environmental factors--such as winter temperatures (Vakily 1989)--may be slowing northward expansion. We have no explanation, however, for why P. viridis has not yet spread around the southern tip of Florida. Most of the Everglades-Florida Bay area is difficult to access from the sea and impossible fi'om land, so it is possible that P. viridis has spread somewhat past Marco Island, but it has failed to show up in the one navigable harbor in the area (Everglades City) nor has it been reported anywhere in Monroe County (aside from the sawfish report, Table 2), which encompasses most of the region.

We do not know where P. viridis first invaded on the northeast coast of Florida; it was reported within several months at several widely dispersed locations. Coastal oceanic currents and tidal currents in the Intracoastal Waterway are both reasonable larval dispersal mechanisms for this region. Perna viridis appears to be distributed discontinuously between Georgia and its current reported southern limit of the Indian River Lagoon (Sebastian Inlet, Table 2), but this may be a sampling artifact. Unlike the west coast of Florida, which has large and relatively saline estuaries, northeast Florida has small estuaries or small lagoons, connected by the navigable Intracoastal Waterway. In small estuaries, suitable estuarine bivalve habitat may be hidden in the deeper channels beneath a surface layer of relatively fresh water (Baker et al. 1999). Sampling cryptic habitats for P. viridis was beyond the budget of our investigations, but it is possible that P. viridis is slightly more widespread in northeast Florida than inferred from surface observations. Having reached the relatively large Indian River Lagoon on the central-east Florida coast, however, we predict P. viridis will become very noticeable in that area within several years.

Despite favorable coastal currents, P. viridis has not yet persisted north of Georgia; it will thus be of interest to follow the recently reported P. viridis population from Charleston Harbor in South Carolina. Lower thermal tolerance data for P. viridis in the literature are limited, but observations from other regions (Vakily 1989, Umemori & Horikoshi 1991, Kazuhiro & Sekiguchi 2000, Zvyagintsev 2003) and this study infer cold water does limit P. viridis populations. We cannot predict, without several more years of observations, whether South Carolina represents a true range extension or whether the P. viridis there is extralimital and will die during a normal winter. Complicating the issue of northern range extensions is the possible presence of warm-water effluents from industrial plants. In Japan, warm-water effluents are believed to be responsible for the long-term persistence of P. viridis in regions that are otherwise too cold in the winter (Kazuhiro & Sekiguchi 2000). Range extension by subtropical invertebrates in warm water effluents has also been documented in other locations, including the United Kingdom and the United States (Naylor 1965, Hoagland & Turner 1980).

It is likely P. viridis will continue to expand throughout the greater Caribbean as it has so far: occasional large jumps in range mediated by a human vector, followed by a more paced natural dispersal. It is possible that P. viridis will not expand further in Florida except to infill suitable coastal habitats between the current west and east Florida distributions. Further United States dispersal, such as to Texas where the related P. perna already occurs, may require separate human-mediated introductions.

CONCLUSION

The invasion of the Southern United States by P. viridis bears some resemblances to the invasion of North American freshwater ecosystems by zebra mussels (Dreissena polymorpha) and quagga mussels (D. bugensis) (Nalepa & Schloesser 1992, Dermott & Kerec 1997). Dreissena and Perna species are abundant fouling organisms with the potential to compete with native species and to increase maintenance costs of hulls and water intakes. The differences are also important: we highlight 3 here.

First, P. viridis invaded a subtropical marine ecosystem that is biologically more diverse than North American freshwater ecosystems. Perna viridis faces many more competitors, predators, habitat types, and toxic algae than faced by Dreissena. Consequently, P. viridis is possibly not as dominant, even at its invasion epicenter, as Dreissena species in many of their invaded habitats.

Second, from an economic standpoint, P. viridis merely added to an existing fouling problem, rather than creating one where none existed before, as was the case for Dreissena. We observed P. viridis encrusting vessel hulls and drives, and it was first observed occluding seawater intakes (Benson et al. 2001), so it is clearly fouling, but is by no means as difficult to remove as oysters (Bivalvia: Ostreidae) or balanornorph barnacles (Crustacea: Cirripedia), to give examples of encrusting organisms that vessel owners and industry already deal with.

Third, P. viridis is an edible species of potential commercial value, whereas Dreissena species are valuable only to those who are remunerated to remove them. The edibility of P. viridis is not necessarily desirable from the standpoint of public health officials, however. Most P. viridis in Florida grow in waters that are permanently or provisionally closed to shellfish harvests because of concerns about water pollution (Division of Aquaculture 2006), but their abundance and similarity to the commercial congener P. canaliculata has already lured some Floridians into illegal--and unsafe--harvests from closed waters (McElroy 2003). The interactions of P. viridis with toxic dinoflagellates like Karenia brevis and Pyrodinium bahamense should also be studied further. The State of Florida monitors K. hrevis (Fish and Wildlife Research Institute 2001) but, at this time, there are no Florida standards for monitoring of P. bahamense, a saxitoxin-producing species that can be abundant in areas with P. viridis (Landsberg et al. 2006, Badylak et al. 2007).

ACKNOWLEDGMENTS

Major funding was provided by United States Environmental Protection Agency STAR Grant # R82-8898. Additional funding was provided by a grant from the Tampa Bay Estuary Program. The authors acknowledge the many professionals and volunteers who provided information: a full list is not possible, but they thank Dr. Derk Bergquist of South Carolina Department of Natural Resources, Bill Frank and Dr. Harry Lee of the Jacksonville Shell Club, Dr. Maia McGuire of Florida Sea Grant, and Dr. Alan Power of the University of Georgia Extension Service. E. DeCastro, B. Rimm-Hewitt, and W. Southerland assisted with data collection.

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PATRICK BAKER, (1) JONATHAN S. FAJANS, (2) WILLIAM S. ARNOLD, (3) DEBRA A. INGRAO, (4) DAN C. MARELLI (3) AND SHIRLEY M. BAKER (1) *

(1) Department of Fisheries and Aquatic Sciences, IFAS, University of Florida, 7922 NW 71st St, Gainesville, Florida 32653; (2) Florida Institute of Oceanography, c/o Keys Marine Laboratory, P.O. Box 968, Long Key, Florida 33001; (3) Fish and Wildlife Research Institute, 100 Eighth Avenue SE, St. Petersburg, Florida 33701; (4) Mote Marine Laboratory, 1600 Ken Thompson Parkway, Sarasota, Florida 34236

* Corresponding author. E-mail: [e-mail]

TABLE 1.
Known or suspected artificially mediated introductions
of P. viridis prior to 2001; see text for citations.
Date refers to year P. viridis was introduced or became
established in the locality.

Date      Locality

Unknown   Hong Kong
1964      Southwest China
Unknown   Taiwan
by 1967   Southeast Honshu, Japan
1972      New Caledonia
1975      Fiji
1976      Tonga
1978      Tahiti
1982      Western Samoa
1983      Okinawa, Japan
1984      Cook Islands
by 1990   Trinidad
1993      Northeast Venezuela
1998      Kingston, Jamaica
1999      Tampa Bay, FL
1999      Cape Verde Islands
by 2001   Cairns, Australia

Date      Vector

Unknown   Undetermined
1964      Fishery enhancement (intentional)
Unknown   Undetermined
by 1967   Undetermined
1972      Aquaculture (intentional)
1975      Aquaculture (intentional)
1976      Aquaculture (intentional)
1978      Aquaculture (intentional)
1982      Aquaculture (intentional)
1983      Aquaculture (intentional)
1984      Aquaculture (intentional)
by 1990   Undetermined
1993      Natural dispersal or local shipping
1998      Undetermined - shipping probable
1999      Undetermined - shipping probable
1999      Aquaculture (intentional)
by 2001   Fouling on ship hulls

Date      Result

Unknown   Established, abundant
1964      Established fishery
Unknown   Established fishery
by 1967   Locally common
1972      Established, abundant
1975      Established
1976      Established
1978      Established
1982      Established
1983      Established fishery
1984      Not established
by 1990   Established, abundant
1993      Established fishery
1998      Locally established
1999      Established, abundant
1999      Not established
by 2001   Presumed eradicated or died out

TABLE 2.

Significant P. viridis reports in the United States since 1999,
including sites previously published (see notes for Source).
Sites are clustered by geographic region and are arranged in
order along the coastline following prevailing coastal currents.
Localities for which there has never been a positive report
are italicized. Dates represent either first reported occurrence
of P. viridis (positive finds) or date last examined
(sites with no P. viridis ever reported).

Date        Locality               Lat./
            Name                   Long.

            Gulf of Mexico (Florida) Localities

2002        Pensacola              30[degrees]24'N, 87[degrees]13'W
May 2006    Cedar Key#             29[degrees]08'N, 83[degrees]02'W
June 2006   Green Key#             28[degrees]15'N, 82[degrees]45'W
Nov 2003    Anclote Key            28[degrees]11'N, 82[degrees]49'W
Nov 1999    John's Pass,
              Treasure Is.         27[degrees]47'N, 82[degrees]47'W
Nov 1999    Skyway Br, Tampa Bay   27[degrees]36'N, 82[degrees]38'W
Nov 1999    Gandy Br, Old
              Tampa Bay            27[degrees]53'N, 82[degrees]33'W
Dec 2001    Safety Hbr,
              Old Tampa Bay        28[degrees]00'N, 82[degrees]40'W
Aug 1999    TECO, Hillsborough
              Bay                  27[degrees]48'N, 82[degrees]24'W
Nov 1999    Fort De Soto,
              Tampa Bay            27[degrees]31'N, 82[degrees]38'W
July 1999   Longboat Pass,
              Sarasota Bay         27[degrees]28'N, 82[degrees]42'W
July 1999   Mote Laboratory,
              Sarasota Bay         27[degrees]20'N, 82[degrees]34'W

July 1999   US 41-BR
              Bridge., Venice      27[degrees]06'N, 82[degrees]26'W
July 1999   Boca Grande,
              Charlotte Hbr        27[degrees]43'N, 82[degrees]16'W
Mar 2003    Pine Is. Sound/
              Captiva Is.          26[degrees]28'N, 82[degrees]07'W
Aug 2002    Fort Myers Beach       26[degrees]26'N, 81[degrees]55'W
Dec 2001    Naples                 26[degrees]10'N, 81[degrees]48'W
Jul 2003    Marco Island           25[degrees]55'N, 81[degrees]44'W
May 2006    Chokoloskee#           25[degrees]49'N, 81[degrees]12'W
Nov 2002    Everglades             25[degrees]38'N, 81[degrees]12'W

            South Atlantic Coast Localities

May 2006    Fort Pierce
              Inlet, FL#           27[degrees]28'N, 80[degrees]18'W
Oct 2006    Sebastian Inlet, FL    27[degrees]51'N, 80[degrees]27'W
Sept 2004   Mosquito Lagoon, FL    28[degrees]42'N, 80[degrees]43'W
Oct 2002    Ponce De Leon
              Inlet, FL            29[degrees]04'N, 80[degrees]56'W
Feb 2003    Indian River
              North, FL            29[degrees]02'N, 80[degrees]55'W
Aug 2004    Crescent Beach, FL     29[degrees]46'N, 81[degrees]15'W
Jan 2003    Matanzas R,
              Marineland, FL       29[degrees]40'N, 81[degrees]13'W
Oct 2003    Matanzas R Inlet, FL   29[degrees]42'N, 81[degrees]14'W
Jan 2003    Matanzas R,
              St Augustine, FL     29[degrees]53'N, 81[degrees]18'W
Jan 2003    Vilano Pt,
              Vilano Beach, FL     29[degrees]54'N, 81[degrees]17'W
Mar 2003    Pablo Creek,
              Atlantic Beach, FL   30[degrees]19'N, 81[degrees]26'W
June 2006   Atlantic Blvd,
              Jacksonville, FL#    30[degrees]19'N, 81[degrees]28'W
July 2006   Sherman Point, St.
              Johns R., FL#        30[degrees]22'N, 81[degrees]26'W
June 2006   Mayport Basin,
              Mayport, FL          30[degrees]23'N, 81[degrees]24'W
June 2003   St Johns Jetty,
              Mayport, FL          30[degrees]24'N, 81[degrees]24'W
June 2005   Fort George
              Inlet, FL            30[degrees]25'N, 81[degrees]25'W
July 2003   St Mary's
              Entrance, FL         30[degrees]42'N, 81[degrees]26'W
Mar 2005    Southern Cumberland
                Is, GA             30[degrees]43'N, 81[degrees]26'W
Oct 2006    Sea Camp,              30[degrees]51'N, 81[degrees]28'W
              Cumberland Is., GA
Oct 2003    Brunswick, GA          31[degrees]15'N, 81[degrees]49'W
Jan 2004    Tybee Island, GA       32[degrees]00'N, 80[degrees]51'W
Oct 2006    Ft Johnson,
              Charleston Hbr, SC   32[degrees]45'N, 79[degrees]53'W

            Mid Atlantic Coast Localities

Mar 2003    Oregon Inlet, NC       35[degrees]46'N, 75[degrees]31'W
Dec 2001    Virginia Beach, VA     36[degrees]54'N, 76[degrees]06'W

Date        Locality               Source   Most Recent
            Name                            Known Status

            Gulf of Mexico (Florida) Localities

2002        Pensacola                6      1 Juvenile specimen,
                                              unconfirmed
May 2006    Cedar Key#               2      no P. viridis
June 2006   Green Key#               6      no P. viridis
Nov 2003    Anclote Key             2,6     Common-abundant
Nov 1999    John's Pass,
              Treasure Is.          1,2     Common
Nov 1999    Skyway Br, Tampa Bay    1,2     Common-abundant
Nov 1999    Gandy Br, Old
              Tampa Bay             1,2     Sbundant
Dec 2001    Safety Hbr,
              Old Tampa Bay          2      Abundant
Aug 1999    TECO, Hillsborough
              Bay                   1,6     Abundant
Nov 1999    Fort De Soto,
              Tampa Bay             1,2     Common
July 1999   Longboat Pass,
              Sarasota Bay           1      Several specimens on buoy
July 1999   Mote Laboratory,
              Sarasota Bay          l, 4    Common-abundant

July 1999   US 41-BR
              Bridge., Venice        1      Several specimens on buoy
July 1999   Boca Grande,
              Charlotte Hbr          1      Rare-absent
Mar 2003    Pine Is. Sound/                 Common on
              Captiva Is.           3, 6      aquaculture buoys
Aug 2002    Fort Myers Beach         2      Common-abundant
Dec 2001    Naples                   3      Occasional-common
Jul 2003    Marco Island             2      Occasional-common
May 2006    Chokoloskee#             6      No P. viridis
Nov 2002    Everglades               4      Many specimens attached
                                              to gill net entangled
                                              on Pristis pectinata
                                              Latham 1794 rostrum.
            South Atlantic Coast Localities

May 2006    Fort Pierce
              Inlet, FL#             2      no P. viridis
Oct 2006    Sebastian Inlet, FL      2      Juveniles common
Sept 2004   Mosquito Lagoon, FL      6      Single specimen
Oct 2002    Ponce De Leon
              Inlet, FL             2, 6    Common
Feb 2003    Indian River
              North, FL              7      Common
Aug 2004    Crescent Beach, FL       2      Large clusters
                                              on beach in 2004
Jan 2003    Matanzas R,
              Marineland, FL         2      Abundant on floating pier
Oct 2003    Matanzas R Inlet, FL    2, 7    Common
Jan 2003    Matanzas R,
              St Augustine, FL     2, 5, 6  abundant
Jan 2003    Vilano Pt,
              Vilano Beach, FL       5      common
Mar 2003    Pablo Creek,
              Atlantic Beach, FL     5      common-abundant
June 2006   Atlantic Blvd,
              Jacksonville, FL#      2      no P. riridis
July 2006   Sherman Point, St.
              Johns R., FL#          2      no P. riridis
June 2006   Mayport Basin,
              Mayport, FL            6      abundant
June 2003   St Johns Jetty,
              Mayport, FL            5      common
June 2005   Fort George
              Inlet, FL              5      2 specimens
July 2003   St Mary's
              Entrance, FL           5      common
Mar 2005    Southern Cumberland             common in subtidal,
                Is, GA               5        died in winter
Oct 2006    Sea Camp,                5      common on navigation
              Cumberland Is., GA               buoys
Oct 2003    Brunswick, GA           1, 5    occasional-common
Jan 2004    Tybee Island, GA         1      common in summer
Oct 2006    Ft Johnson,
              Charleston Hbr, SC    6, 8    common in summer

            Mid Atlantic Coast Localities

Mar 2003    Oregon Inlet, NC         5      Single record, "several"
                                              dead specimens
Dec 2001    Virginia Beach, VA       6      Single record, many
                                              dead specimens
Data Sources:

(1) = previously published reports (Benson et al.
2001; Ingrao et al. 2001; Power et al. 2004)
(2) = primary investigators (S. Baker & staff)
(3) = Florida Fish and Wildlife Research Institute report
      (W. Arnold, D. Marelli & staff)
(4) = Mote Marine Laboratory report (D. Ingrao & staff)
(5) = Jacksonville Shell Club report (W. Frank & H. Lee)
(6) = other second hand report to us
(7) = USGS NAS Database (A. Benson)
(8) = South Carolina Department of Natural Resources. 2006.