Other Invertebrates
eg 1 Zebra Mussel
Phylum : Mollusca (Molluscs)
Class: Bivalvia (Bivalves)
Order: Veneroida
Family: Dreissenidae (Mussels)
Scientific name: Dreissena polumorpha
Originating from Eastern Europe through to Russia, Caspian Sea and Ural River regions, the Zebra Mussel gets its name from streaked patterns on their shells. They can grow up to 5cm in shell length, and is generally a species of temperate waters. They feed mainly on algae by filtering over one litre of water per day, and prefer calm waters with medium flow speeds not exceeding 2 metres per second, with water depth not usually exceeding 7m. This predominantly freshwater species have been found to tolerate low salinity of brackish shallows quite well, but they require well-oxygenated waters, preferably not with much alkalinity.
Prior to the first discovery of introduced Zebra Mussels in North America within Lake St Clair in 1988, the range of this Bivalve has already spread from the native Eastern European countries to as far West as Ireland In Europe, due to the extensive canal systems of Central Europe. It took only 2 years for the Mussels to colonize the whole of the Great Lakes region, and subsequently they found their way out of the Great Lakes Basin, and soon were found in the Hudson and Mississippi Rivers and the lower 20 States. This rapid introduction can be attributed to the ability of Zebra Mussels to attach themselves to ships that navigate the rivers, and that they could stay alive out of water in cool, humid conditions probably increased the rate of advance of the alien species in America.
Zebra mussel has damaged power plants, water treatment facilities, and other structures and significantly changed freshwater ecosystems. In Lake Erie, the population density has reached over 70000 per square metre, clearing up the water to allow more sunlight to penetrate into the depths, where algae can proliferate and compete with native aquatic life for oxygen. The Mussels are also attacking native freshwater shellfish, evicting them from a number of former habitats. Also, they have been found to bioaccumulate toxic chemicals 10 times more than the native clams, and thus are more deleterious towards shellfish predators, like fish and ichthyophagous birds. The algae-feeding Zebra Mussels also deprive the zooplankton of their phytoplankton food, and these microscopic organisms, which in turn supply food for larval fish, drop drastically in number so that many native fish species are starved to death. Prolific breeders, females can produce over 50000 eggs each year. Zebra Mussel numbers are controlled by diving ducks in Europe, but the predator-free environment of the Great Lakes has allowed them to show their maximum reproductive potential.
There are no efficient control methods as yet for the Zebra Mussel. Various chemicals such as chlorine have been used, but as these are also highly harmful to the ecosystem, scientists are seeking other specific methods to deal with these silent invaders.
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eg 2 Golden Apple Snail
Phylum : Mollusca (Molluscs)
Class: Bivalvia (Bivalves)
Order: Gastropoda
Family: Caenogastropoda
Scientific name: Pomacea canaliculata
The Apple Snail is a large freshwater Snail growing to 10cm in length. The shell is rather globular and is generally brownish-green in colour, but Snails that have been bred in captivity can become rich golden-yellow in colour. The presence of Apple Snails is often first indicated by the pink egg masses they lay on stones, logs or other appropriate solid surfaces usually 50cm above the water level. Echoing many other invasive species, the reproductive rate of the Apple Snail is staggering, and clutches averaging 200 to 300, reaching 1000 at times, are laid every few weeks whenever conditions allow. These eggs can manage to hatch in 15 days but tend to vary a little on this theme according to the ambient temperature. The longevity of these Snails reach 4 years.
A native of South America from the temperate plains in Argentina to the Southern Amazonian Basin, the Apple Snails are divided a certain number of poorly-defined species and there are taxonomical disputes regarding the true identity of the Apple snails that are introduced widely throughout the circumtropical regions including the Southern States of Texas and Florida, the Dominican Republic, east to the Philippines, Japan, Taiwan, Vietnam, Cambodia, Thailand, Laos, Korea, Sri Lanka, Indonesia, Malaysia, southern China, Singapore, and many pacific Islands such as Hawaii, Guam, and Papua New Guinea. There they are serious crop pests as although they normally feed on rotting vegetation and does not infest on healthy plants unless there is food shortage, in introduced areas they show preference for many water plants. Among the list are water lotus, taro, rice, and water chestnut, the latter of which are commercial food crops severely affected by the introduction of the Apple Snail. The Apple Snail is amphibious and feeds mainly during the night, and often skulks in bottom muddy layers in rivers and ponds, looking for food. The Apple Snail was introduced to South-east Asia in 1980 as a potential food source and exported gourmet delicacy, but the market for these Snails never was established.
Apple Snails are generalist feeders and are responsible for destroying wetland plants and crop plants, by which the natural habitat is modified. The exotic Apple Snails have been found to drive the Pila Apple Snails in the Philippines to decline, competing with them for food sources. Other native aquatic Snails are also affected.
It has proved exceedingly difficult to remove this adaptable species, although numerous measures have been introduced with the aim of eradicating this pest. Pesticides have been used as well as biological controls like fish and ducks, but prevention of its further spread seems to be the best strategy to date.
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eg 3 Fishhook Water Flea
Phylum: Arthropoda (Arthropods)
Class: Crustacea (Crustaceans)
Order: Cladocera
Family: Cercopagidae
Scientific name: Cercopagis pengoi
Water Fleas are plankton-size crustaceans. Cercopagids, a particular group of such, are largely native to the Caspian-Aral regions, while this species is one of the most wide-ranging ones, its range having spread to the Black and Azov Seas, as well as some coastal lakes with brackish water and high salinity. The Fishhook Water Flea is a euryhaline species, being more abundant at lower salinities (3 to 8 %) and has established permanent populations in freshwater lakes around the fringes of their range. In the Caspian Sea, the Cercopagid migrates vertically, staying in the deeper, cooler waters in the daytime and accumulating around the surface layers at night.
The Water Flea usually reproduces asexually in the summer months to take advantage of the abundant food sources, while sexual reproduction occurs later on in the year when water temperature drops. Dormant eggs laid in late autumn helps the dispersal of the species and enables the species to survive the inhospitable conditions of the winter.
Recently C. Pengoi has infested the Baltic Sea in 1992, and subsequently the Great Lakes in 1998, expanding its range into Lake Michigan in 1999. The Water Fleas are believed to have taken advantage of the ballast water of ships sailing across the Atlantic and traveled in it.
Water Fleas disturb the exotic freshwater and marine ecosystems as they uproot the food webs at the base - they are predators of zooplankton, the basic food supply of many juvenile fish and other invertebrates. Parthenogenetically reproducing Cercopagids certainly have the capability of fast colonization, as they do not need to invest energy and time into finding mates. The reduction the basic food supply for many predators, even the higher invertebrates, is remarkably severe when the species exist in high densities as it so often does. Each Water Flea can consume 16 preys per day. These Crustaceans also adhere to fishing lines and forms a paste on fishing nets, leaving the fishing equipment biofouled, causing economic losses in fish farms in the Gulf of Finland in particular. The ecological impact of Water Fleas, however, is partly speculative and it is not known whether larger fish will successfully control its population by taking them as prey.
eg 4 North Pacific Sea Star
Phylum: Echinoderma (Echinoderms)
Class: Asteroidea (Starfish)
Family: Asteridae
Scientific name: Asterias amurensis
Large Starfish coloured yellow and purple, the North Pacific Sea Star, also known as the Japanese Starfish, are native to the coasts of Japan, Korea, China, Russia, and east across the Bering Strait into Alaskan waters. They can reach 50cm in diameter, and have five arms that are upturned at the tips. Exhibiting high temperature tolerance, they have adapted to tropical water conditions in Australia of up to 22ºC while in their native temperate waters, ambient temperature ranges between 7 and 10ºC; however they are sensitive to salinity changes, and in areas of fluctuating salinity they are usually absent. Calm intertidal or sublittoral zones down to 25 metres in depth is the favoured habitat, and they tend to shun coral reefs and areas of highly dynamic water actions; some Pacific Sea Star have been found, though, down to 200m. Capable of parthenogenesis, a complete body of an individual can be developed from remnants of an arm and a portion of the central disc.
In the 1980s the Sea Star was introduced into the South Australia island of Tasmania from Japan, though the initial discovery of the invasion dated as late as 1992. The Sea Star has since then spread around the coast to Victoria, discovered there in 1995, and is numbering in the millions currently --- 30 million are residing in a single estuary in Tasmania.
The reproductive method of the Asteroid is highly dispersive. Larvae of the Sea Star exist as planktotrophic zooplankton and suspends in the water for up to 6 months during the colder winter months. The fact that in the non-native range of Southern Hemisphere the Sea Star shows a spawning time 6 months later than the populations in the Northern Hemisphere indicates a phase shift of 6 months in their reproductive cycle, suggesting that temperature or length of daylight is the regulatory factor. The climatic belt around Tasmania offers similar conditions that of the North Pacific, and the Sea Star may disperse in times of food abundance all around the Southern coast of Australia. Spawning occurs when the water temperature is around 10 to 12ºC, and juvenile Sea Stars grow rapidly. Each female may produce 20 million eggs, and dispersal is largely self-propelled.
The Sea Star, benign-looking, eats whatever it can capture, and is solely predatory, especially harmful to the marine invertebrate fauna including sea squirts, other echinoderms (Starfish, Sea Urchins), Molluscs such as Clams and Mussels, Crustaceans (Crabs and Barnacles) and even members of their own species in times of food shortage. They can detect prey from a distance and is able to excavate shallow sand to dig out submerged prey. They damage shellfish farms in this way and also reduces the marine biodiversity in places. Apart from usurping the place as dominant invertebrate predators, the Sea Star also causes decline in Spotted Handfish (Brachyionichthys hirsutus) populations as they eat the eggs of the Fish, and the spawning substrate - Sea Squirts - as well.
North Pacific Sea Stars seem to have arrived at Australia in the ballast water of vessels traveling from Japanese waters to South Australian ports, which carry their planktonic larvae. There they are dispersed by local ships either as larvae again or in the adult form, which adheres to ship hulls. Oyster seed trays, Mussel ropes and Salmon cages have also been found to contain the Sea Star and as these are translocated, the Sea Star is also dispersed.
Natural fluctuations of the Sea Star populations in North Pacific have been found to follow stochastic cycles of several years' durations and in the Australian waters Starfish has shown the potential to attain huge surges in populations, reaching 12 million near Port Philip in two years from very low numbers. Eradication measures have removed thousands but are not effective at controlling the whole introduced population. Fisheries in New South Wales, in fear of the spread of Sea Stars into waters where mariculture is important economically, has been encouraging vigilance and pest reporting. The invasions may be curbed when the populations are low; if allow to proliferate, however, the Sea Star can be hard to remove. Volunteering campaigns that have successfully removed 3 tonnes of Sea Star at times through physical removal have had little impact on the Starfish population, and scientists are currently researching on.