Jessie Riddle
Limnoperna fortunei in the Americas
INTRODUCTION
In 1993, the first specimens of Limnoperna fortunei in South America were discovered in the La Plata River basin of Argentina (Pastorino & Darrigan, 1993, ref. in Darrigan, 2000). This bivalve mussel species, originally from Southeast Asia, was released into the Río de la Plata estuary through the ballast water of ships (ref. Boltovskoy , Correa, Cataldo, & Sylvester, 2006). L. fortunei quickly expanded beyond the La Plata estuary into other inland waterways, proving remarkably successful in the South American aquatic environment. By the late 1990s, it was observed to be advancing along waterways at a speed of roughly 240 km/yr (Darrigan, 2000).
The impacts of this “Golden Mussel” have included a decline in some native species and “biofouling,” or an accumulation of organic material that corrodes the affected surface, of the local environment (Darrigan, 2002). Commercial, industrial, and public water supplies in South America have been affected, and L. fortunei is now classified as an invasive species (U.S.A.C.E.). According to the National Invasive Species Council, an invasive species is defined as “a species that is non-native to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health (I.S.A.C. 2006).” L. fortunei’s invasive behavior, however, may have had some positive impacts, including an increase in benthic (the bottom of an aquatic eco-system) density and a dramatic upswing in local fish harvesting.
L. fortunei’s impact on people and the environment and its invasive qualities have been compared to that of the Zebra mussel, a non-native bivalve species that has caused serious economic and ecological damage in North America (Karatayev, 2007). Although L. fortunei’s ultimate influence may or may not be analogous to that of the Zebra mussel, it appears that both mussels follow similar patterns of reproduction and invasion. It also seems likely that unless checked, L. fortunei will reach North America. L.Fortunei’s remarkable success in South America, coupled with its similarities to the Zebra mussel’s development, has caused speculation that it could supersede the Zebra mussel as an ecological hazard in North American waters.
PURPOSE
This paper will discuss the impacts of L. fortunei on South American aquatic ecosystems and local industries. It will also discuss the potential impact of an invasion into North America, and the most beneficial courses of action for affected areas. In order to evaluate the situation, this paper will first examine L. fortunei’s rapid progression through South America. It will then document the observed economic and environmental impacts of colonization and biofouling, including an increase in benthic densities, the growth or death of native species, and the corrosion, obstruction and pollution of waterways. Finally, this paper will review the likelihood of an invasion of L. fortunei into North America, and what steps can or should be taken to halt its advance in the Americas.
OBJECTIVE
It is the author’s opinion that L. fortunei would be very difficult to remove from the locations it has already colonized in South America. The documented impacts of this species do not appear to warrant the effort and expense a complete removal would require. However, they do appear to warrant quick and comprehensive efforts to stop the invasion of new areas. While L. fortunei has increased the numbers of certain freshwater fish, expanded benthic densities and had other positive influences, the observed negative outcomes appear to outweigh the potentially positive results of introduction. The negative consequences of observed invasions have included biofouling of important waterways, pollution of or damage to industrial and commercial water resources, and a loss of native species (thus a loss in overall biodiversity). Specific actions to stop the spread of L. fortunei could include stricter regulations on cleaning and inspection of boat traffic, in addition to the removal of any specimens discovered.
DISCUSSION
The spread of L. fortunei
L. fortunei was first observed in the Río de la Plata estuary in 1991. It has since colonized South American waterways to a surprising degree, and can now be found in five countries - Brazil, Paraguay, Uruguay, Bolivia and Argentina (Boltovsky 2006.) In 1995, four years after the species was first discovered, population densities in the original site of contact were about 150,000 individuals m² (Darrigan & Ezcurra de Drago, 2000). In 2004, thirteen years after its invasion, many places in the Paraná river delta (at the head of the Río de la Plata estuary) had consistent population densities of 200,000 individuals m² (Sylvester, Dorado, Boltovskoy, Juárez, & Cataldo, 2005). L. fortunei reached Brazil through the Jacuí canal soon after the year 2000. Current populations in this area, about eight years later, are at an average of 118,00 individuals m²(Bergonci, 2009). The fact that the populations in the Jacuí canal are still significant, but are smaller than those at the mouth of the Paraná river system, demonstrates that although the Golden mussel forms large colonies quickly, its success rate in colonization is not uniform.
Characteristics of L. fortunei responsible for its expansion, and factors that limit its growth
The remarkable expansion of L. fortunei in South America and factors that have increased or inhibited its spread have been analyzed in careful studies. Several key elements have been found in the mussels’ successful colonization – (1) a lack of natural competitors for habitat, (2) an ability to colonize a variety of locations, (3) an amenability or adaptability to South American waters’ temperatures and composition, and (4) a short maturation and sexual reproduction cycle.
L. fortunei has little competition for its ecological niche. In the La Plata estuary, it is the only mussel species that attaches byssally to hard substrates, is epifaunal, and is well suited for freshwater regions (Darrigan, 2002). It has quickly become a major part of native fishes’ diets, and in general seems to have found an unfulfilled space in these aquatic ecosystems (Paolucci, Cataldo, Fuentes, & Boltovskoy, 2007). Although this lack of competition for habitat appears to have benefited the golden mussel, it is not necessarily vital for its development. This is illustrated by the fact that the Zebra mussel, which colonizes waterways in a similar manner to L. fortunei, has overtaken areas with and decimated pre-existing native mussel populations in North America (Karatayev, 2007).
The Golden Mussel is also successful because it can colonize a large variety of areas. L. fortunei produces free-swimming larva, which allows for a great amount of spreading nine months out of the year. After the initial, free-swimming stage, these larvae develop a “foot,” which can and will attach to any solid surface. This includes grains of clay or silt, the shells of other mussels, and industrial equipment such as pipes and condensers (Cataldo & Boltovsky, 2000).
The temperatures of most South American inland water bodies appear to be favorable for L. fortunei’s advance. This is important for the species’ success, as we can see from a 1995 study, which determined that the mussel’s growth reproductive cycle varies greatly according to temperature. Golden Mussels begin to reproduce in temperatures above 15-17°C (Cataldo & Boltovsky, 2000), and populations have been observed in water up to 32°C (Darrigan, 2002) A similar study in 2002 determined that L. fortunei’s filtering processes reach their peak between 20 and 25˚ C (Sylvester et al., 2005). Larvae are predicted to survive in temperatures from 16 to 28˚C, while adults can survive in water from 8 to 32˚C (U.S.A.C.E.). This suggests that the Golden mussel will be more abundant in the southern parts of North America, where the Zebra mussel, which has an ideal temperature of 12 to 24˚, is not as ecologically stable. Additionally, the golden mussel has been shown to be dependent on low levels of salinity, thus containing the species’ colonization to fresh or mostly freshwater areas. Specifically, L. fortunei is not likely to survive in water with a salinity of 2 ppt for greater than a year (Angonesi, da Rosa, & Bemvenuti, 2008). It should also be noted that L. fortunei’s umbonated larvae are extremely vulnerable to prolonged oxygenation or chlorination of their water. So far the mussel has not naturally encountered that obstacles in its colonization (Cataldo & Boltovsky, 2000).
L. fortunei typically mature sexually after one year (U.S.A.C.E.). They are a typically dioecious species, and larvae are fertilized externally. As mentioned above, the larvae are free swimming, and attach to almost all available hard substrates. Evidence of L. fortunei’s fast reproductive rate was given in a 1998-1999 study in the La Plata estuary, where a pipe wiped clean of all specimens by high summer temperatures will be covered with a dense growth of L. fortunei within six to eight months (Cataldo & Boltovsky, 2000).
Other factors related to the success of an invasive species that are possessed by L. fortunei were listed in Morton’s 1996 study as “high fecundity…gregarious behavior; some form of association with human activities…suspension feeding… [and] the ability to repopulate previously colonized habitats” (ref. Darrigan, 2002).
Impact in South America
The arrival of the Golden Mussel has had both positive and negative outcomes in the economies and environments of countries it has invaded. Argentina, Bolivia, Uruguay, Paraguay and Brazil have found a significant increase in the total annual freshwater fish harvest. Industries in these countries have also experienced difficulties and rising operational costs in using the affected waterways due to “macrofouling,” or the growth of life forms on non-living hard surfaces (Darrigan, 2002). Environmentally, the Golden mussel has increased benthic densities (Sylvester et al. 2007), become a major food source for at least one species of native fish (Paoulucci et al., 2007) and reduced populations of native invertebrates.
Limnoperna fortunei larvae were found to be the most important food source, in terms of biomass and nutrient content, for multiple taxa of larval fish in main channels of the Paraná river. L. fortunei was also found to be the second most important food source for a variety of larval fish in the marginal lagoons San Nícolas and El Saco, which are attached to the Paraná river system. The fish species that consumed the highest percentages of L. fortunei were also the most abundant local species (Paolucci et al., 2007). The vital role of the mussel in local fish’s diets is also visible in the Río de la Plata, where L. fortunei constitutes the main food source of the native “boga” fish (Penchaszadeh et al. 2000). Between 1992-93 and 2000-2001, the total amount of freshwater fish harvested multiplied 2.4-fold (Boltovskoy et al., 2006). Although this is not necessarily a direct result of l. fortunei’s introduction, the mussel’s integral role in native fish diets seems to indicate a correlation.
“Macrofouling” of indisustrial pipes and waterways in Argentina has caused problems for Hydroelectric power stations, nuclear plans, water treatment stations, distilleries and refineries (Boltovskoy et al. 2006). Macrofouling, or the deleterious growth of large organic material on a substrate, occurs in several parts of the Golden Mussels’ life cycle. First, when L. fortunei forms a colony on a hard surface such as pipes or filters, the water intake lowers and the water pressure may be reduced. The water in the pipes may become polluted, and the pipes themselves are often corroded. When the colonies die en masse due to temperature or water content, corrosion and pollution are compounded by large quantities of empty shells. These processes result in expenses to companies for cleaning, repairs, and lowered productivity (Boltovskoy et al. 2006, Darrigan 2002).
L. fortunei increases benthic densities through filtering of the surrounding water. After mussels filter nutrients from the water, they will precipitate organic material onto the surfaces they are attached to (or ‘substratum,’). This material settles in the benthic, or bottom, layer of the body of water, which increases the complexity of the substrata by providing new microhabitats on, under and in between shells. The presence of mussels has been found to increase numbers, biomass, and diversity of invertebrate species associated with the mussel bed. This growth may be related to increased sedimentation, but is almost certainly related to the new microhabitat (Sylvester et al., 2007.) However, in another study L. fortunei was found to displace native species of mollusks, and other potential displacements were suggested (Darrigan 2002).
Potential advance into North America
Although L. fortunei can be transported through natural spawning, attachment to recreational vessels and other various techniques, the most important mechanism of transport is commercial vessels. The likelihood of invasion in new areas is determined by the probability of introduction and environment suitability (Boltovskoy et al. 2006). As the number of ports that have been invaded grows, so does the likelihood that the Golden mussel will be transported to North America through the ballast water of international trade ships.
In terms of ecological suitability, L. fortunei shares key habitation characteristics with the Zebra mussel (byssal attachment to hard substrata), which has successfully colonized much of North America. Significantly, L. fortunei not only tolerates warmer temperatures, lower levels of oxygen and calcium, and higher levels of pollution than the Zebra mussel, but also spawns for nine months out of the year, while the Zebra spawns for three (Boltovskoy et al. 2006). These factors indicate that the economic damage caused by the Zebra mussel could be less significant than the problems created by L. fortunei. It is important to note, however, that the environmental impact of the Golden mussel may or may not be similarly damaging – as mentioned, it has been found to assimilate much better than the Zebra mussel into the diet of native species (Paolucci et al. 2002), but it has also been found to displace native species (Darrigan 2002).
Conclusion
Although the effects of Limnoperna fortunei on South American ecosystems and economies has been profitable in some areas, the remarkable ability of this species to spread out from original points of invasion and its unusual compatibility with American water systems makes the advance of this species undesirable. Techniques that would effectively prevent this advance include stringent examination of commercial ballast water and possibly private vessels as well as careful observance of waterways with international traffic. Temporarily oxygenating or otherwise disrupting the water supply in order to annihilate populations would most likely prove difficult, expensive and harmful to other species, removing individuals before they can get into the water supply or colonize substrata would appear to be the most effective strategy.
References
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