Biology of Perna viridis
Perna viridis is a dioecious species with rare cases of hermaphrodites and while sexual organs are not distinguishable, sex can be determined by tissue color (Lee, 1988). The gonad is dispersed throughout the mantle lobes, mesosoma and intermixed between digestive glands (Rajagopal, 2006) leaving males appearing creamy white in color and females orange or brick red in appearance (Walter, 1982; Sreenivasan et al., 1989; Lee, 1986; Narasimham, 1981). During resting periods of gametogenesis this coloration is not as distinct leading to misidentification without histological confirmation, however in tropical to subtropical regions, reproductive activity frequently persists year round (Parulekar et al., 1982; Walter, 1982). Perna viridis are broadcast spawners, releasing egg and sperm into the water column for external fertilization and may be cued not only by environmental factors (temperature, salinity, food), but also from chemical cues (Widdows, 1991; Barber and Blake, 2006) which can be initiated by either sex (Stephen and Shetty, 1981). Larval densities have been recorded from approximately 20,000 larvae m-3 (Rajagopal et al., 1998a) to as high as 40,000 larvae m-3 (Rajagopal et al., 1998b), similar to that of Mytilus edulis, its cold water relative (Schram, 1970).
The life cycle begins with a pelagic larval phase reported to last from as short as 8 – 15 days (Tan, 1975a; Nair and Appukuttan, 2003) to 15-18 days (Sreenivasan et al., 1988; Nair and Appukutan, 2003) and as long as 24-35 days (Nair and Appukuttan, 2003; Laxmilatha et al., 2011) (Table 1). This wide range is due to variations in environmental conditions and substrate availability, which play a critical role in larval development and settlement (Sreenivasan et al., 1988; Nair and Appukutan, 2003). Marine mussels can delay metamorphosis for several weeks if suitable substrate is not encountered, however this delay results in decreased survival due to the interruption of feeding and growth leading to a prolonged pelagic phase accompanied by increased predation risks (Bayne, 1965; Widdows, 1991).
During the planktonic stage they are at the bottom of the food chain leading to increased predation pressure from plankton grazers and susceptibility to external stressors is at its high (Bayne, 1976). Food and temperature are the two of most important factors affecting growth and development of bivalve larvae (Widdows, 1991), however any environmental altercation outside the threshold will effect growth, development and survival, including temperature, salinity, pH, dissolved oxygen, food availability, HAB’s pollutants, etc. (Nair and Appukuttan, 2003; Bayne, 1965; Gilg et al., 2014). Nair and Appukutan (2003) observed significant difference in larval growth reared under different temperatures with a more narrow tolerance range than that of post metamorphosed mussels. Decreased embryogenesis, larval development and survival have been observed during exposure of local species of oysters, clams and scallops to K. brevis (Leverone et al., 2006; Rolton et al., 2014).
The niche of a new species is defined by several factors. Not only must the habitat fit the species regarding to physical attributes (substrate type and location, salinity, temperature, food availability), but the population must be able to successfully compete with local species for these resources and avoid predation. Green mussel populations are primarily found in habitats ranging from oceanic and high salinity estuarine waters favoring areas with high phytoplankton and / organic matter (Morton, 1987; Vakily, 1989; Wong and Cheung, 2001) and high current flow / flushing aiding in the removal of waste and continuous food supply (Rajagopal et al., 1998b; Buddo et al., 2003; Rajagopal et al., 2006). Green mussels commonly dominate biofouling communities on and near power plants where water temperatures remain high and flushing is extensive (Rajagopal et al, 1991a,b). High density settlement has caused damage to power plants in their native range due to extensive fouling of water intake pipes (Rajagopal et al, 1991a) and the water cooling systems of power plants in Tampa Bay are believed to be the first point of invasion creating a broodstock population allowing for the further spread throughout the bay (Benson et al., 2001).
As an important aquaculture species, much attention has been given to production and environmental tolerances within their native range. In both native an invaded regions they are aggressive biofoulers and have been found coating hard substrate in densities as high as 4,000 – 35,000 individuals m-2 (Haung et al., 1983; Baker and Benson, 2002; Fajans and Baker, 2005) and clogging water intake pipes at biomasses exceeding 200 g m-2 (Rajagopal et al., 1991a). As a gregarious species, high growth rates and fecundity have allowed for the rapid colonization of new habitat and decreased native fauna through resource competition. Thriving in a range of tropical to subtropical waters, conditions frequently allow for year round gametogenesis and high food availability in productive bays and estuaries allows for continuous input to energy reserves and ample food supply for developing larvae.
Perna viridis populations are most commonly found on hard substrate including mangrove prop roots and submerged rock and shell, and other bivalves including oyster reefs, but also frequent artificial substrate such as pilings, piers, and floating objects such as buoys (Vakily, 1989; Buddo et al., 2003; Baker et al., 2007; 2012). Locally, Estero Bay comprises of soft bottom sediments with little hard substrate available leaving intertidal oyster reefs and artificial structure as the only available substrate. However, green mussel populations have been observed on the soft bottom sediments within seagrass beds in Tampa Bay and Hillsborough Bay Florida covering areas as large as 2300 m2 (Johansson and Avery, 2004) and in Kingston Harbor Jamaica (Buddo et al., 2003) often attaching to shell fragments, filamentous root structure, and / or other bivalves (Ingrao et al., 2001).
Although often referred to as an intertidal species (Shafee, 1978; Vakily, 1989) P. viridis is primarily found on subtidal substrate or areas with minimal emersion time (Tan, 1975b; Rajagopal et al., 1998b). Juvenile recruitment is often observed in the intertidal regions, however these individuals rarely survive to maturity (Baker et al., 2012; personal observation) and dense populations are primarily found subtidally (Nair and Appukuttan 2003) with the highest growth observed at depths below the mean low tide mark (Sivalingam, 1977). Tan (1975b) found the highest population densities at depths of 1.5 – 11.7 m below the high water spring tide mark and no living mussels were observed above the high tide mark. This distribution is likely due to desiccation stress from aerial exposure in which normal physiological requirements, (feeding, exchange of gases, and removal of waste) are inhibited. Green mussels found in the intertidal zone have been shown to be very sensitive to even short term extremes in air temperature (Urian et al., 2010; Baker et al., 2012; McFarland et al., 2014). Both field and lab observations suggest that P. viridis cannot tolerate aerial exposure at temperatures below 14˚C (Baker et al. 2012; Firth et al., 2011; Power et al., 2004) and Urian et al. (2010) observed 100% mortality at 3 ˚C
Problems with invasive species: Biofouling and potential harm to oysters and ecosystem disruption
Introducing non-native species to new environments is a rising problem world-wide. Increased boat traffic and international shipping have increased vectors for spread over wide geographic distances and increased urbanization and tourism in coastal areas has created new substrate for species including navigational structures, piers, and break walls (Bax et al., 2003). While Florida coastal waters already have a significant biofouling problem, consisting mainly of barnacles, tunicates and other native bivalves, none reach the same sizes and densities observed in P. viridis populations (Benson et al., 2001). Increased biofouling communities lead to increased costs for removal and can halt industrial production in water cooling systems. Pipes and pumps may become clogged leaving a reduced flow causing pumps to overheat and burn out, thereby increasing cost of both removal of the species and replacement of damaged infrastructure. Rajagopal et al. (1997) found dense populations of P. viridis coating water cooling systems of power plants in densities reaching 211 kg m-2 causing damage to equipment and clogging of pipes reducing water flow for the cooling system. Clogging is caused not only by dense settlement of mussels but also from byssal mats which can get sucked into the pumps and block protecting screens (Ingrao et al., 2001). While green mussels are only contributing to an existing biofouling problem not creating one, their fast growth and reproduction may increase this problem and frequency of treatment.
From an ecosystem perspective, invasive species may cause alterations in ecosystem services including nutrient cycling; habitat modification (or loss); competition and displacement of native species, which may include aquaculture facilities and fisheries; act as vectors for disease, viruses, parasites, bacteria, and harmful algae (Bax et al., 2003; Hégaret et al., 2008). Bivalves may transport and introduce parasites or diseases from their native range not normally found in the new ecosystem or may be immune to parasites in the new ecosystem leaving themselves free of parasites giving them a leg up on the competition (Branch and Steffani, 2004). Competition for resources, such as food and settling space, can lead to displacement and reduction in numbers of native species (Dulvy et al., 2003; Karatayev et al., 2007). As ecosystem engineers, bivalve invasions have been shown to drastically alter ecosystems. For example the zebra mussel Dreissena polymorphia in the Great Lakes resulted in alterations of the phytoplankton community, reduction of native species and an estimated cost of $1 billion per year in biofouling damage and control (Pimentel et al., 2005; Karatayev et al., 2007). Similarly, the Mediterranean mussel, Mytilys galloprovincialis reduced native bivalves and limpets by out competing for space on the coast of South Africa (Branch and Steffani, 2004) and P. viridis invasion in Venezuela has resulted in heavy competition and a population decrease of the brown mussel Perna Perna due to substrate competition (Segnini de Bravo et al., 1998; Rylander et al., 1996). In Tampa Bay Florida, dead oyster shell was found under green mussel populations on bridge pilings and the subtidal, outer crest of a reef system (Baker et al., 2007).
Through rapid growth and vigorous reproductive activity, P. viridis is an aggressive marine invader rapidly reaching high densities allowing for heavy competition with other bivalves for hard substrate. The primary concern surrounding green mussel population spread in the southeastern United States is competition with the native oyster Crassostrea virginica (Fig. 3). Oysters are a keystone species in the soft bottom bays of south Florida where they form permanent three-dimensional habitat which creates a refuge and / or foraging grounds for over 300 species of fish and crab (Wells, 1961) including several recreational and commercially important species (Henderson and O’Neil, 2003; Tolley et al., 2005). Many estuarine species spend their entire lives on the oyster reefs while others utilize the intricate reef system as a safe hiding place to lay their eggs and nursery for juveniles (Tolley and Volety, 2005). Many offshore fish and crab species enter the estuary for breeding where their young use oyster reefs as a refuge (Beck et al., 2011). This refuge for early life stages and small organisms creates an extensive feeding grounds for larger species, both estuarine and oceanic (Peterson et al., 2003; Grabowski et al., 2005). Peterson et al. (2003) found that oyster reefs enhanced growth and increased the abundance of 19 species of fish and large mobile crustaceans, with an estimated contribution to fish production of 2.57 kg 10m-2 yr-1.
Table of contents :
Chapter1: General Introduction
History, origin and transport of green mussels
Study site: Estero Bay, Florida, USA
Biology of P. viridis
Problems with invasive species: Biofouling and potential harm to oysters and ecosystem
Understanding population dynamics through applying the Dynamic Energy Budget
Theory to predict and prevent spread
Chapter 2: Uptake and elimination of brevetoxin in the invasive green mussel, Perna viridis, during natural Karenia brevis blooms in southwest Florida
Chapter 3: Potential impacts on growth, survival and juvenile recruitment of the green mussel Perna viridis during blooms of the toxic dinoflagellate Karenia brevis in southwest Florida
Chapter 4: Seasonal variation in gametogenesis and energy storage of the invasive green mussel, Perna viridis, in southwest Florida
Chapter 5: Application of the Dynamic Energy Budget theory to model growth and reproduction of established population of the non-native green mussel Perna viridis in southwest
Florida coastal waters
Chapter 6: General Discussion
Field observations: Effects on juvenile and adult mussels
Field observations: Effects on early life stages
DEB model: Metabolic acceleration
DEB model: Spawning in bivalves
Ecological implications and concerns for P. viridis spread.
Trophic transfer of brevetoxins
Effects of climate change on habitat expansion
Competition for space with oysters
Publications and Communications