Species Description: Mytella charruana is a mussel with a shell mostly dark brown to black exhibiting a wavy dark pattern on a lighter background. A growth pattern of concentric semicircular rings is evident on the shell surface. The interior of the shell is iridescent purple and the exterior of the shell lacks radial ribs (TNC 2006).
Potentially Misidentified Species: M. charruana can potentially be mistaken for the more common IRL native mussel species Geukensia demissa (ribbed mussel) and Brachedontes exustis (scorched mussel). Both of these species exhibit distinct radial ribs on their shell surfaces that the charru mussel lacks. (TNC 2006).
Regional Occurrence: The charru mussel is a tropical bivalve whose native distribution extends from South and Central America northward through Mexico. Within the United States populations of this species have reportedly become established in Florida and Georgia (TNC/UCF 2006).
IRL Distribution: A population of charru mussels now appears to be established within the Mosquito Lagoon portion of the Indian River Lagoon system (TNC 2006)
Age, Size, Lifespan: Like other mussel species, M. charruana attaches to a variety of natural submerged and intertidal hard substrates such as oyster shells as well as man-made structures like water intake pipes, wood pilings, and driftwood.
Most individuals are less than 2 cm in length, but they can grow to twice that size.
Abundance: As of April 2007, charru mussels were still uncommon where they occurred in the upper Indian River Lagoon region. Focused efforts to locate and collect specimens had yielded only about 600 animals since the species was rediscovered in the region (Linda Walters, personal communication).
Reproduction: Reproduction is sexual with males and females occurring as separate individuals. Fertilization is external, taking place in the water column.
Embryology: A free-living planktonic larval stage precedes recruitment of settlement-age animals onto appropriate hard benthic substrata.
Temperature: M. charruana is a tropical species whose northern distribution limits are likely dictated by cold intolerance. The die-off of the Jacksonville power plant founder population in the winter of 1987 is attributed to thermal stress (Boudreaux and Walters 2006). Similarly, the failure of monthly surveys to locate charru mussels between February 2005 and August 2005 in the Mosquito Lagoon (Boudreaux and Walters 2006) is likely the result of significant seasonal die-back associated with colder winter water temperatures.
Salinity: M. charruana is listed as a euryhaline species by McCann et al. (1996).
Trophic Mode: Like other mussels, M. charruana 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 food particles of the appropriate size range are funneled by the labial palps into the mouth for ingestion and digestion.
Associated Species: The likely nature of the association between charru mussels and co-occurring fouling organisms will be as spatial and/or food resource competitors.
Invasion History: Lee (1987) notes that the first report of M. charruana on the east coast of Florida occurred in 1986 when large numbers were found in the seawater intake pipes of a power plant near Jacksonville's Blount Island on the St. Johns River. Venezuelan tankers that frequent a nearby port have been indicated as the putative source of this founder population (McCann et al. 1996), brought in either as hull fouling organisms or as larvae in ballast water. Despite early concerns about potential impacts, this exotic mussel failed to become established; the animals that were present in 1986 experienced a die-off in the winter of 1987 (Boudreaux and Walters 2006, TNC 2006). Carlton (1992) noted that no subsequent observations of this species in the area were made.
The charru mussel was again found in Florida in the Summer of 2004, this time within the Mosquito Lagoon basin of the IRL system (Boudreaux and Walters 2006, TNC/UCF 2006). Since this initial sighting report, many more charru mussels have been found and their numbers appear to be increasing. As of Spring 2006, nearly 600 individuals have been collected from the Mosquito Lagoon portion of the IRL system (TNC/UCF 2006).
Potential to Compete With Natives: The potential exists for a growing population of charru mussels to compete with native mussels, oysters and other organisms for food as well as for colonizable substrate to use as living space (TNC 2006).
Boudreaux and Walters (2006) suggest that in areas where populations become established, this species has the potential to reproduce rapidly and outcompete native mussel populations as well as populations of oysters already in decline.
Possible Economic Consequences of Invasion: Among the species the charru mussel may compete with is the commercially important native oyster Crassostrea virginica. As with other hard fouling organisms, negative economic impacts may also occur if large numbers of charru mussels settle into and clog municipal seawater intake pipes at power plants and other coastal utilities (Carlton and Rosenfield 1994).
Boudreaux and Walters (2006) suggest that a rapid response to this invader in Florida could prevent the sorts of ecological and economic impacts caused when other non-native mussels like the zebra mussel (Dreissena polymorpha) and green mussel (Perna viridis) have become established in new locations.
Boudreaux M.L., and L.J. Walters. 2006. Mytella charuanna along the Atlantic coast of Florida: A successful invasion? Poster. Society for Integrative and Comparative Biology 2006 Meeting. Orlando FL - January 4-8, 2006.
Carlton J.T. 1992. Introduced marine and estuarine mollusks of North America: An end-of-20th-century perspective. Journal of Shellfish Research 11:489-505.
Carlton J.T. and A. Rosenfield (eds.) 1994. Molluscan introductions and transfers: risk considerations and implications. Maryland Sea Grant, College Park, MD. 73 pp.
Lee H.G. 1987. Immigrant mussel settles in Northside generator. The Shell-O-Gram (Jacksonville Shell Club, Jacksonville, FL) 28:7-9.
McCann J.A., Arkin L.N., and J.D. Williams. 1996. Nonindigenous Aquatic and Selected Terrestrial Species of Florida: Status, Pathway and Time of Introduction, Present Distribution, and Significant Ecological and Economic Effects. Published on the Internet by the University of Florida, Center for Aquatic Plants.
The Nature Conservancy Florida Chapter. 2006. Invasive Mussel Alert: Mytella charruana found in Florida waters.
The Nature Conservancy and University of Central Florida Invasive Species Alert: Recurrence of Mytella charruana in Florida waters. September, 2006.