Bivalve Superpower: The Global Invasion of Corbiculid Clams

Corbiculidae is a family of clams which has the capability of invading habitats. Particularly, there are three genera (Batissa, Polymesoda, Corbicula) which are widely distributed all over the world. The genus Batissa territories are the tropical India and Indo-Pacific Region. Polymesoda species are tropical colonizers while Corbicula conquered all continents except Antarctica. Dispersal of the bivalves may take place by different media which includes animal, human and environmental phenomena. C. fluminea is the most successful invader with widest scope of distribution, worldwide. The invasion of corbiculid clams may bring apparent instability in the environment. Controlling the invasion is necessary to maintain the balance of nature.


INTRODUCTION
Corbiculidae clams are burrowing in soft bottoms of shallow fresh or brackish water areas. They are suspension filter-feeders which are commonly found in marshes and mangroves. The name "Corbicula" (little basket) refers to their characteristic concentrically ribbed shells and genera shape. Freshwater corbiculids are ovoviviparous, brooding internally fertilized eggs

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in brood pouches [1]. Development via a free swimming larva is typically an attribute of brackish water corbiculids. Some corbiculid clams exhibit hermaphroditism [2,3] while others have separate sexes [4,5].
Members of the family Corbiculidae are capable of successfully invading aquatic environments [6,7]. These clams are characterized as resilient bivalve species because they possess biological attributes that allow them to overcome adverse environmental conditions or migrate and subsequently adapt in new environments [8][9][10]. Accordingly, one can consider the corbiculid clams as a "superpower" of the bivalve world.
The invasion of non-indigenous species is an imminent danger to the stability of aquatic biomes. Such species may modify the aquatic ecosystem structure and functioning affecting baseline abiotic conditions and the resident biota. Many bivalves are known to be successful invaders of aquatic habitats [11][12][13][14]. Bivalve invasions may considerably alter pre-existing populations by displacing native species through competitive interactions [11,15,16]. The undesirable effect of exotic macrofouling bivalves in different kinds of industrial facilities had been documented in molluscan studies [17,18,19]. Controlling of biological invasions is important to maintain the balance in the ecosystem. This paper will review the biogeographical invasion of the corbiculid clams. In particular, this discusses the biology, global distribution, dispersal mechanisms and the bioinvasion controlling schemes of the family Corbiculidae.

THE CORBICULID AXIS POWER: THE BIOLOGY OF Batissa, Polymesoda AND Corbicula
There are three genera within the family Corbiculidae which are distributed worldwide, namely: Batissa, Polymesoda and Corbicula. These clams invaded aquatic ecosystems such as estuaries and rivers all over the world. Truly, they are the "Axis Power" of the family Corbiculidae. Their biological mechanisms serve as the battle gear in their successful invasion of habitats.
The genus Batissa is represented by the species B. violacea (Fig. 1

Polymesoda: Tropical Colonizer
Based on literatures, there are at least eight species of Polymesoda globally, namely: P. bengalensis, P. caroliniana, P. coaxans, P. erosa, P. expansa, P. inflata, P. radiata and P. solida. The distribution of these species appears being restricted to the tropical Central America and Indo-Pacific regions (Fig. 5)
Based on the distribution of the three corbiculid genera, it appeared that the Indo-Pacific region has the most diversity of clams (Fig. 7). This suggests that the said region is where the corbiculid clams originate. This also implies that the area has the ideal environmental condition for the three genera to coexist. Nonetheless, the distribution of these genera only shows the capability of the clams to invade habitats.

THE MAKING OF AN EMPIRE: DISPERSAL MECHANISMS
An invasive species should have several characteristics to be successful in the new environment: A short life; rapid growth, rapid sexual maturity; high fecundity; euryoecious; eurytopic; gregarious behavior; some form of association with human activities; wide genetic variability and phylogenetic plasticity; suspension feeding; and ability to repopulate previously colonized habitat [56]. Anthropogenic transportation is the most likely explanation for the global dispersal of these clams. Foster et al. [58] reported that C. fluminea first entered the United States as a food item.
Counts [59] accounted the spread of corbiculid clams primarily by human activities such as bait bucket introductions and accidental introductions associated with imported aquaculture species.
Other human-induced dispersals are by ballast water transport, aquarium releases, transport of juveniles and/or adults as a tourist curiosity and the juvenile byssal attachment to boat hulls [60].
Flooding and the flow of water current are the environmental phenomena responsible for the dispersal of corbiculid clams. McMahon [33] attributed the extraordinary dispersal capacities of corbiculid pediveliger and juvenile to their small size and mass. This allows them to remain suspended for long distances in even minimally turbulent water. Kraemer [61] reported that juvenile corbiculid clams are transported by water currents using their single long byssal thread which acts as a drag line. According to McMahon [33], channelization of waterways for flood control or navigation increases flow velocities and turbulence, this will improve the condition for dispersal of C. fluminea.
Hoyer et al. [62] developed a model to investigate the passive dispersal of planktonic larvae. The model was a three-dimensional (3D) Lagrangian, individual-based dispersion model. This larval dispersion model highlighted the importance of waves and currents in the dispersal and colonization of invasive species. The model was applied in the dispersion of C. fluminea larvae in a large, deep lake [63]. C. fluminea larvae dispersal was determined by the magnitude and timing of strong wind events.
Larvae are carried away from the original areas along a discrete number of dispersal pathways. Colonization of new habitats outside the larvae origin is low and sensitive to the larvae settling velocity.

THE AFTERMATH: IMPACTS OF INVASION
The impact of corbiculid clams on the endemic bivalve inhabitants transpire mainly when major die-offs occur. In most cases, the non-indigenous corbiculids and other indigenous bivalves coexist [64]. However, several adverse effects have been reported. Clarke [65] reported negative impacts of Corbicula on native unionids in the Tar River system, Virginia. Buchanan [66] reported high mortality of endemic unionid mussels after die-offs of Corbicula in Bourbeuse and Meramec rivers, Missouri. Scheller [67] attributed die-offs of native clams population crashes of Corbicula, which during putrefaction release ammonia and reduce dissolved oxygen in the sediment. In a two-year ecological study, Sickel [68] reported that corbiculid clams did not appear to be affecting adult native clams directly; however, they may be excluding the juveniles, which may result in the loss of many endemic species.
The corbiculids have high rates of filtration and their abundances make them a major consumer of phytoplankton [30]. These clams decreased phytoplankton and chlorophyll a concentrations in the Potomac River by 20-75% [69]. Similar seston reductions by Corbicula fluminea occurred in the Chowan River [70], Savannah River [71], and other lotic systems [72]. The removal of seston by corbiculid clams has changed the zooplankton population by increasing the density of copepods and decreasing that of the rotifers in some systems [64]. Hass et al. [73] claimed that the mass invasion by Corbicula sp. strongly affected ecosystem functions by linking pelagic and benthic processes by their intense filter feeding activity and thereby enhance the capacity for self-purification of the river system. Hakenkamp and Palmer [55] found that Corbicula consumed significant quantities of organic material in the streambed when conditions favored pedal feeding but increased buried organic matter stores when filter feeding promoted deposition of organic matter as feces and pseudofeces. These clams contributed considerably to total benthic community respiration, and used pedal feeding on benthic organic material to grow at a faster rate than that possible by filter feeding alone [6].
The corbiculid clam's greatest economic impact has been macrofouling of raw water systems in fossil-fueled or nuclear power stations [74,75]. Even though the planktonic stage is short-lived, the non-swimming and settling pediveligers and juveniles enter the facility through the intakes, then through traveling screen and strainers [76]. Accumulations of adult shells also reduce flow velocities below the threshold for settlement, promoting accumulations of silt and sand in which clams burrow [77].
Positive effects of corbiculid clams were also observed. Empty shells of C. fluminea serve as shelter and substrate for other organisms [78,79]. These clams are also food resource for other pelagic and benthic species [80]. Members of family Corbiculidae were also reported as good bioindicator species for ecotoxicological studies [81,82].

CONQUERING THE CONQUERORS: CONTROLLING BIOINVASIONS
Mackie and Claudi [83] discussed some mitigation for controlling the bioinvasions of corbiculid clams. Prevention is still the best method to control the invasion of the clams. To eliminate the source of many introductions, navigation and dredging activities should be regulated. Thermal regulation is one technique to eliminate corbiculids in water pipes. In this method, water in the pipes is heated to temperatures exceeding 37°C. There are also mechanical methods that can be employed. For example, corbiculids may be removed from piping by passing wads under pressure. One can also use screens and shell traps to reduce impacts. Chemical methods are also options that may be considered in the elimination of invasive corbiculids. There are oxidizing molluscicides which may significantly reduce the corbiculid invasions if applied with water at 40°C. Oxygen depletion is also a chemical technique in controlling the invasion of corbiculids.

CONCLUSION
The biogeography of corbiculid clams showed worldwide distribution. This only implies the ability of these bivalves to adapt and invade different types of environment. Their resiliency to different environmental conditions is due to their biological characteristics. The invasions of these clams affect the community structure of the native inhabitants which may lead to the loss of biodiversity. Preventing introductions of these bivalves is the best method of controlling the invasions.