Species Description: The Asian green mussel, Perna viridis, is a large (> 80 mm) bivalve, with a smooth, elongate shell typical of several mytilids (see below). It has visible concentric growth rings and a ventral margin that is distinctly concave on one side. The characteristic green coloration comes from the periostrocum, the proteinaceous outer layer of the shell. It is uniformly bright green in juveniles, but dulls to brown with green margins in mature individuals. The inner surfaces of the valves are smooth and iridescent blue to bluish-green in color. A prominent, kidney-shaped retractor muscle scar is present, but the species lacks anterior adductor muscles. Close examination of the beak (i.e., where the two valves hinge together) reveals a pair of hinge teeth on the left valve that interlock with a single hinge tooth on the right valve (DeVictor and Knott undated, NIMPIS 2002, Rajagopal et al. 2005).
As is typical of most members of the family, P. viridis attaches to hard surfaces by means of proteinaceous byssal threads.
Potentially Misidentified Species: At least 12 different mytilid mussels have been documented within the IRL region. The most commonly encountered of these are readily distinguishable from P. viridis. The scorched mussel (Brachidontes exustus), ribbed mussel (Geukensia demissa), and hooked mussel (Ischadium recurvum) all exhibit prominent radial ribs that are lacking in P. viridis, while the horsemussels (Modiolus spp.) are smaller and possess a shell that is brown on the outside and whitish inside and is partially covered by a mossy periostracum in living specimens (UF/IFAS Green Mussel Homepage). The non-native charru mussel (Mytella charruana) is also readily differentiated from P. viridis; its shell is mostly dark brown to black and exhibits a wavy dark pattern on a lighter background.
Within the genus Perna, karyotyping (counting chromosomes) reveals that P. viridis possesses 30 chromosomes while congeners such as the brown mussel (P. perna) only have 28 (Ahmed 1974).
Regional Occurrence: Green mussels are coastal bivalves, typically occurring at depths of less than 10 m, and shown to be tolerant of a wide range of turbidity and pollution (Power 2004).
The native range of the Asian green mussel broadly encompasses the Asia-Pacific and Indo-Pacific regions. The known introduced range of the species is extensive and includes portions of coastal Australia, Japan, the Caribbean, and North and South America (Benson et al. 2002; NIMPIS 2002).
Atlantic and Caribbean occurrences of P. viridis have been reported from Trinidad and Tobago, Jamaica, and Venezuela (Agard et al. 1992, Rylander et al 1996, Benson et al. 2002, Buddo et al. 2003). In the southeastern U.S., Perna viridis has been reported from coastal Georgia and from both Florida coasts (Power et al 2004).
IRL Distribution: Although the first Florida east coast reports of Perna viridis date back to 2002 from approximately St. Augustine south toward Ponce Inlet, the first positive identification of the organism from the IRL system proper only occurred in 2006. This initial discovery of green mussels was in Mosquito Lagoon within the boundaries of Canaveral National Seashore near the historic Eldora State House. This area is also the epicenter of IRL occurrence for the charru mussel (Mytella charruana). Since the initial discovery, findings of small numbers of new animals have been slow but steady and the number of green mussels thus far found remains lower than the numbers for charru mussels (Dr. Linda Walters, personal communication).
Age, Size, Lifespan: Green mussels are large, with shells typically reaching 80-100 mm in length and occasionally growing larger than 160 mm (Rajagopal et al. 2005). They live for approximately three years (Power et al 2004).
In parts of their native range, rapid growth rates of up to 6-10 mm per month have been reported. In Tampa Bay where the species has been introduced, even more rapid growth rates of 4-5 mm per week have been reported (Power 2004).
Abundance: Baker et al. (2002) report Tampa Bay intertidal densities of Perna viridis attaining peaks ranging between 3,675 and 4,117 individuals per square meter at three study locations. The authors report significantly higher densities of 9,000-12,000 individuals per square meter when they found several layers of mussels on pilings at the mouth of the Little Manatee River.
Reproduction: Reproduction is sexual, sexes are separate, and fertilization is external. Onset of sexual maturity is rapid, occurring at 2-3 months of age in parts of the animal's native range and in as little as 1-2 months in parts of its non-native tange, e.g., Tampa Bay (Power 2004).
Male and female green mussels release gametes directly into the water column. As with many marine bivalves, the presence of gametes in the water can trigger other individuals to release gametes, thereby synchronizing spawning to a degree. Salinity reductions (i.e., such as those often experienced in estuarine environments, particularly in the wet seasons) can also elicit spawning in P. viridis (Stephen and Shetty 1981). Within their native Pacific range, spawning peaks coincide with the monsoon seasons, although the species is known to be capable of reproducing year-round in some locations (Sivalingam 1977, Stephen and Shetty 1981, Walter 1982).
Embryology: Within eight hours of fertilization, Perna viridis larvae enter a ciliated, free-swimming stage known as the trochophore stage. Eight to twelve hours later the larvae have reached a stage known as the veliger stage, which is characterized by the presence of a shell and a ciliated membrane or skirt, called the velum. Metamorphosis and settlement to the benthic habit typically occur within 8-12 days to as many as 20 days. Settlement-stage individuals are capable of secreting byssal threads (Tan 1975, Siddall 1980, Manoj Nair and Appukuttan 2003).
Temperature: Green mussels occur in environments where temperatures range from 10-35 ºC, and exhibit optimal response at temperatures between 26 ºC and 32 ºC (Power 2004). Although the reported native thermal range of the green mussel is broad, reduced temperatures have been demonstrated to significantly negatively impact growth rates (Chatterji et al. 1984).
Manipulative studies by Manoj Nair and Appukuttan (2003) confirm the thermal optimum for Perna viridis resides very close to the upper lethal limits of the species; optimum larval development, growth and survival occurred at 31 ºC, but total mortality was reported after 24 hours of exposure at 33 ºC and 35 ºC.
Salinity: The green mussel is euryhaline, able to tolerate both hypersaline conditions (80 ppt) and reduced salinities, e.g., 12 ppt (Sivalingam 1977, Chatterji et al. 1984, Morton, 1987). An optimal salinity range has been reported as 27-33 ppt (Power 2004).
Trophic Mode: Like other mussels, Perna viridis is a cilliary-mucus suspension feeder. Firmly attached to a substrate, immobile mussels use their incurrent siphon to draw in food-laden water that is then carried by ciliary action to the branchial chamber. Water is discharged through the excurrent siphon while appropriately sized food particles are funneled by the labial palps into the mouth for ingestion and digestion.
Associated Species: The likely nature of the association between green mussels and co-occurring fouling organisms will be as spatial and/or food resource competitors.
Invasion History: Several aspects of the life history of Perna viridis are responsible for its success as an invasive animal. These include broad temperature, salinity, turbidity, and pollution tolerances, rapid growth and onset of maturity, and broadcast dispersal of planktonic larvae.
The first reported Caribbean appearance of the species dates to the 1990s in Trinidad (Agard et al. 1992, Power et al. 2004). Rylander et al. (1996) suggest that current-mediated larval dispersal from this population resulted in successive introductions in Venezuela. Green mussels have also been reported in Jamaican waters (Benson et al. 2002, Buddo et al. 2003).
The first occurrence of P. viridis in U.S. coastal waters apparently occurred as a result of accidental release into Tampa Bay on the Gulf coast of Florida in 1999 (Ingrao et al. 2001, Benson et al. 2002). The most likely pathway for this invasion has been identified as larval release in ballast water (Power et al. 2004). Current-facilitated dispersal of larvae originating from in situ reproduction of the Tampa Bay population has subsequently led to southward range expansion along western peninsular Florida to Boca Grande outside of Charlotte Harbor (Benson et al. 2002, Power et al. 2004).
A subsequent Florida occurrence of P. viridis was reported near St. Augustine on the northeast coast in 2002. The most likely vector for accidental introduction in this case is overland transport between Florida coasts, perhaps occurring on or in recreational or fishing boats, or possibly gear that was not cleaned properly (Power et al. 2004).
The green mussel population introduced to St. Augustine waters is the likely source of larvae that dispersed northward via currents along the Atlantic coast to Jacksonville and then into Georgia. As of 2003 the Atlantic distribution of this invasive species had been extended northward through the entirety of coastal Georgia. This represents the northernmost U.S. occurrence of the species (Power et al 2004).
In 2006, small numbers of P. viridis began to be uncovered within Mosquito Lagoon within a couple miles of Ponce Inlet (Linda Walters, pers. comm.). These individuals also likely originated from the previously detected St. Augustine population.
Potential to Compete With Natives: Juvenile settlement densities of several thousand individuals per square meter in Tampa Bay suggest Perna viridis is a formidable spatial competitor (Power 2004). Large non-native green mussel populations may also represent a significant source of competition for planktonic food resources.
Baker and Benson (2002) report that oyster reefs composed of native eastern oysters (Crassostrea virginica) have been displaced by invading green mussels in Tampa Bay.
Possible Economic Consequences of Invasion: Although economically important fisheries and aquaculture industries based green mussels exist within their native range, utilization as a food resource in areas into which they have been introduced is uncommon. Consumption of introduced green mussels taken from polluted waters is discouraged as they are known to accumulate some toxic substances.
Green mussels are common nuisance organisms fouling manmade structures such as intake and outfall pipes, buoys, bridges, pilings, and seawalls. Power et al. (2004) suggest the species may eventually become the marine equivalent of the highly invasive freshwater Asian zebra mussel (Dreissena polymorpha). Zebra mussels currently occur in more than 40% of inland US waterways and their partial-control in this country cost nearly $1 billion over a span of 15 years.
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