The Asian green mussel, Perna viridis, is a large (> 80 mm) bivalve, with a
smooth, elongate shell typical of several mytilids (but 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
Within the genus Perna, karyotypeing (counting chromosomes) reveals that P. viridis possesses 30
chromosomes while congeners such as the brown mussel the brown mussel (P. perna) only have 28 (Ahmed 1974).
HABITAT AND DISTRIBUTION
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).
Although the first Florida east coast reports of Perna virdis 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
LIFE HISTORY AND POPULATION BIOLOGY
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).
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 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).
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).
Green mussels occur in environments whose temperatures range from 10-35ºC and
exhibit optimal response at temperatures between 26ºC and 32ºC (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 31ºC, but
total mortality was reported after 24 h exposure at 33ºC and 35ºC.
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).
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.
The likely nature of the association between green mussels and co-occurring fouling organisms will be as spatial and/or food resource competitors.
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 occuirring 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 cost
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
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