Mytella charruana is a native mussel species of South and Central America and is currently an invasive species to the coastline of Florida, Georgia, and South Carolina. M. charruana has been introduced in the St. John River estuary in Jacksonville, Florida, and other areas by the release of ballast water from cargo and cruise ships. A study done in 2007-2008 in our laboratory showed that M. charruana had the potential of displacing local species, including oysters and native mussels, posing both an ecological and economical threat to Florida, Georgia and South Carolina including maintenance of biodiversity of coastal ecosystems, commercial and recreational fishing, eco-tourism, power plants functionality by clogging seawater intakes pipes. In fact, M. charruana showed an opportunistic type of gametogenesis, a pelagic larval phase and, for the most part of the year, a female biased sex ratio in the non-native environment . Moreover, the adult individuals had the ability to switch sex in response to food availability. In the condition of starvation, most of the individuals changed sex from female to male, possibly as an energy saving mechanism, since spermatogenesis requires less energy than oogenesis. Another possibility is that the sex reversal, of part of the population to male, is a mechanism to reduce the next generation population size when food resources are limited.
In this study we tested the effect of mussel density on population sex ratio since density can have a direct effect on food availability. Mussels were c ollected in the St. John River estuary in Jacksonville, Florida, and maintained in aquarium tanks. We tested a density of 25 mussels per 19 L and 50 per 19 L, in conditions of optimal feeding or starvation.
Contrary to what observed in 2007-2008, the population sex ratios at the collection sites were not significantly different from 1:1 throughout the year. The experimental trials showed that the sex ratio was not significantly different from 1:1 in fed conditions at either mussel density. Instead, a male biased sex ratio was observed in th e starvation treatment at the lower mussel density, but not at the higher.
Our results suggests that, in conditions of limited food availability, mussel density might be a factor influencing M. charruana sex ratio. More studies will be needed to understand why , contrary to what expected, the lower density, and not the higher, triggered sex reversal to male. Moreover, the sex ratio at the collection sites was not significantly different from 1:1, similarly to what observed for most marine mussel species, including M. charruana in its native environment. The fact that in 2007-2008 M. charruana sex ratio at other collection sites, in Florida and Georgia, was widely variable throughout the year suggests that there might be other factors involved. For example, water quality, which can influence th e physiolog y of the mussels .