Invasive Marine Animal: Green Crab - Essay Example

Invasive Marine Animal: Green Crab Introduction An invasive species is an organism that encroaches on an ecosystem and causes damage to the environment, other organism, the economy, and the human population (NWF, 2015). Their main attributes of invasive species are their capabilities to reproduce at a quick rate and to spread within the invaded ecosystem. Invasive species have tolerance for a broad range of climatic conditions and survive on an extensive variety of food. Invasive species invade the native ecosystem from places that are geographically distant from the native ecosystem through various natural means. Humans have been responsible for introducing some invasive species in new ecosystems (NWF, 2015). Invasive marine animals are non-indigenous invasive species that occupy a marine ecosystem. The animals reproduce and thrive in the new marine ecosystem and affect other species, threaten biodiversity and disrupt human activities. The paper will examine the green crab species as an invasive marine animal. The paper will also explore the ecological, conservational, and economic impact of blue crabs in the invaded ecosystem. Green Crabs: Description The green crabs are a species that go by the scientific name Carcinus maenas (Klassen & Locke, 2007). They have a fine granular dorsal shell that is normally range in size between 2 and 4 inches in width. The shells are wider than they are long (USGS, 2015). The dorsal shells range in color from dark green to red and brown. Other crabs have been observed to be yellow or orange (WDFW, 2015). The color is affected by the prevalent environmental conditions and the molting cycle. The green crab’s most distinctive feature is the arrangement of five spines on either side of the eyes on the front side of the dorsal shell (Cohen, 2011). Other distinctive features of the green crab are three round undulations located between their eyes. The green crab has gonopods that curve outwards unlike most crab species. Green crabs have five limbs on either side of the shell. The limbs have the same color as the dorsal shell. The first two limbs, pereopods, are developed into pincers (Cohen, 2011). The second and third pairs of limbs are the longest and are always longer than the shell. The last limbs are the shortest of the pairs. They lack swimming paddles and are more flattened than the rest of the limbs. The crab uses all the limbs for walking or sometimes just the last four, excluding the pincers (WDFW, 2015). The crabs mostly have black spots on their dorsal shell with other spots ranging from yellow to white. The green crabs have hairs on the sides of their thorax. Green Crab: Reproduction Green crabs mate after females have completed the molting cycle. The crabs mate between the months of April and October but the timing can vary (WDFW, 2015). During the mating period, the males choose partners who they protect from other males. The males choose mating partners who are smaller. The crabs mate in temperatures of between 11°C and 25°C. The optimal salinity for reproduction is between 26ppt and 36ppt. Salinities of below 13ppt prevent the development of larvae. Females spawn an average of 200,000 eggs during one reproductive cycle (Neal & Pizzolla, 2014). The fertilized female lays the eggs in a hole in the sand then attaches the eggs to a pouch on the underside of the abdomen and migrates to deeper waters. The female carries around the eggs for several months. After the eggs hatch, the larvae become planktonic for a few weeks to some months. The planktonic period is affected by the temperature and salinity of the water with low temperatures and high salinity reducing the rate of development (Roman & Palumbi, 2004). The larvae migrate further away from the shore in their early development then move closer to the shore during later stages of development (Rivera, Hitchcock, Teck, Steves, Hines, & Ruiz, 2004). After several cycles of molting, larval development is complete and the larvae metamorphose into small crabs. The crabs mature for a period of between 1 to three years and have a lifespan of between 3 to 6 years. The growth of the crabs is affected by their diet and the environmental conditions prevalent in the ecosystem. Crabs mate up to three times during their lifespan. Green Crabs: Nutrition Habits Green crabs are classified as omnivorous predators meaning they feed on plants and animals. They feed on organisms from over 100 families and 150 genera (USGS, 2015). Their prey consists of mussels, clams, fish, juvenile crabs, bivalves, worms, isopods, snails, gastropods, and oysters. As cannibals, the green crabs sometimes prey on their own species. The main types of plants fed on by the green crabs are algae and spartina (Neal & Pizzolla, 2014). The pereopods of the crabs are crucial when hunting as they hold prey and open shells. Green crabs are frequently involved in disputes over prey with bigger crabs routinely dominating smaller ones. Therefore, plants make up larger portions of the diets of small crabs. Crabs learn prey-handling skills as they mature. Male crabs hunt for prey during high tide while females hunt during low tides. When foraging, the crabs can dig about 6 inches into the sand or mud. The temperature of the water and the availability of prey affect the rate of predation. Green crabs can survive under starvation for 10 to 12 weeks (Gollasch, 2009). Green Crabs: Natural Habitat Green crabs characteristically inhabit marine and estuarine habitats. The green crabs mainly inhabit protected habitats such as rocky shores, sandflats, lagoons, muddy beaches, and tidal marshes (USGS, 2015). Green crabs have also been observed to live in semi-protected habitats. The crabs choose these habitats because they offer protection from predators and have high availability of food. Crabs primarily occupy depths ranging from high tide to 10 meters but a few crabs have been observed to live up to 50 meters. The crabs can survive in marine waters with salinities ranging from 4% to 54% (Gollasch, 2009). The crabs can subsist in water of temperatures ranging between 0°C and 33°C. During winter, green crabs migrate to areas with deeper water where salinity levels are higher and temperatures are more stable (Rivera, Hitchcock, Teck, Steves, Hines, & Ruiz, 2004). The native habitat of green crabs is the European shores of the Atlantic Ocean along the coastlines of Spain, Norway, Morocco, Belgium, Iceland, France, Ireland and the United Kingdom. The green crabs also inhabit the Baltic Sea on the shores of Poland, Sweden, Denmark, Finland, and Estonia. Invasion of Indigenous Habitats Green crabs are able to occupy diverse marine habitats because of their feeding habits and ability to tolerate an extensive range of climatic conditions such as high salinity and low temperature. The crabs have the ability to increase their range of habitat to over 700 kilometers in less than a decade (USGS, 2015). In current times, the crabs have invaded coastlines on the East and West coasts of North America, South America, Southern Africa, Australia, North Africa, and Japan (WDFW, 2015). There have been sightings of green crabs on the shores of countries with temperate climates. The crabs are introduced to new ecosystems through various ways such as transference of larvae in ballast water and packaging materials used in shipments of marine animals. The crabs are moved through ship hull fouling, ocean currents, movement of submerged plants, and rafting. Human activities such as aquaculture and the trade of live food have enabled the dispersion of green crabs (Roman & Palumbi, 2004). When introduced to new habitats, the crabs reproduce quickly and disperse geographically. Impact of the Introduction of Green Crabs to Non-Indigenous Habitats The introduction of green crabs in non-indigenous habitats has wide-ranging consequences on the ecology, conservation efforts, and human activities. Green crabs disrupt the populations of the ecosystem through their excessive feeding habits. Their feeding practices lead to the reduction of the biomass and diversity of other marine ecosystems (Molnar, Gamboa, Revenga, & Spalding, 2008). Green crabs have reduced the populations of organisms such as mussels, oysters, clams, and native crabs in the invaded shores. The reduction in these organisms leads to an imbalance in the ecosystem. The reduced populations lead to the disruption of the pre-existing food chain. The crabs feed on the larvae of other crab species hence stagnating their growth. Native crab populations of invaded areas record significant declines. Green crabs compete with fish and birds for food thereby causing a strain on resources. Some species have had their numbers reduced so considerably that they have been classified under endangered. The endangered species include the soft-shell clams, Olympia oysters, Pinto abalone, butter clams, littorine snails, and Dungeness crabs (Molnar, Gamboa, Revenga, & Spalding, 2008). Green crabs are hosts for a many parasitic worms that infect crab-eating birds causing a drop in their numbers. The invasion and spread of green crabs in native habitats displaces the populations of crabs and various mollusk species to other habitats. The burrowing activities of green crabs when hunting for food disrupt the lives of organisms that inhabit mud and sand (USGS, 2015). The burrowing activities modify the habitats and make it difficult for the organisms to survive. Burrowing exposes the animals to other predators, which affects their population. Burrowing activities cause damages to plant habitats such as eelgrass by damaging the roots of the plants (DFO, 2011). The reduction in the vegetative cover of eelgrass poses a problem for organisms that inhabit in the grass. The destruction of the eelgrass reduces food supplies for organisms that feed on the plant. Eelgrass is also a shelter and nursery areas for other crabs such as the Dungeness crabs and rock crabs (NCEE, 2008). Green crabs prey on the juveniles of crabs in the native habitat. The depletion of the juvenile population leads to a reduced growth rate for various indigenous crabs, which affects the biomass of the ecosystem and affects biological diversity. Marine agencies in invaded countries spend millions of dollars annually trying to eliminate the green crabs for their shores. A lot more is used in restoring the invaded areas and maintaining biodiversity. Green crabs have a substantial impact on aquaculture activities. The crabs have been reported to invade oyster and shellfish farms (NCEE, 2008). The invasion of the aquaculture farms leads to heavy financial losses to the farmers by reducing the output of the farms and by the expensive crab control strategies. Shellfish industries on the East coasts of USA and Canada have collapsed due to the invasion of green crabs. The collapse of the industries led to losses of income for many people and losses in revenue to the respective governments. In the US, the green crabs are estimated to cause losses of over $20 million to local fisheries (Gollasch, 2009). Invasion of green crabs has led to the reduction of Dungeness crab populations by preying on the native crabs and out-competing them for food. Dungeness fishing is a valuable industry in North America and therefore, their reduction leads to massive losses of revenue. The green crabs feed on small and juvenile fish thereby reducing the populations of the fish and curtailing their growth. The decrease in the fish population creates an imbalance in marine and estuarine habitats. The reduction in the fish populations leads to the decline of the fishing industry and causing heavy financial losses (NWF, 2015). Green crabs also lead to the reduction in food supplies for human beings. Invasions of green crabs have resulted in the reduction of edible clams such as the hard clams and blue clams. The reduction of clam and oyster populations reduces the profitability of the recreational harvesting industry. Soft-shell clams help in the provision of plankton and detritus when feeding through filtration processes in their digestive system (DFO, 2011). Therefore, a reduction in the number of clams leads to the reduction of nutrients in the marine ecosystem, which affects the populations of marine species. The reduction in marine populations could lead to reduced fish supplies for human beings. The reduction in bird populations also affects leisure activities such as bird watching (NCEE, 2008). Prevention and Control of Green Crab Invasions There are a number of strategies that can be implemented to prevent the introduction of green crabs, to curb their spread and to mitigate the impact of the invasions. Local governments should prohibit the discharge of ballast water into marine habitats. Such water should be discharged at least 50 miles away from the shore (NCEE, 2008). Local governments should enact policies that prohibit the transfer of shellfish and between different habitats. The populations of green crabs can be reduced by sustained trapping efforts. Trapping has been noted to significantly reduce green crab populations and hence reduce their impact on marine ecosystems. Marine agencies should undertake regular monitoring of shores to detect any invasion of green crabs, which can help in early mitigation. Local authorities can fence areas that have been invaded by crabs to halt their spread. Some areas have used pesticides such as carbaryl to reduce green crab populations although the method affects other organisms in the habitat. Other control methods include educating aqua-farmers, inspecting boats, and introducing parasitic isopods and barnacles that reduce green crab populations (Gollasch, 2009). Conclusion An invasive species is an organism that encroaches on an ecosystem and adversely affects the environment, other organism, the economy, and the human population. Invasive marine animals reproduce and thrive in the new marine ecosystem and affect other species, threaten diversity and disrupt human activities. The green crab is a native of the European and North African shores bordering the Atlantic Ocean and the Baltic Sea. Green crabs are omnivorous predators that feed on a wide variety of animal and plant species. Green crabs are introduced into non-indigenous through natural means and human activities. Green crabs are able to occupy diverse marine habitats because of their feeding habits and ability to tolerate an extensive range of climatic conditions. Green crabs reduce the populations of other organisms, disrupt food chains, spread parasites, reduce food supplies, and cause financial losses. Green crabs can be controlled through methods such as traps, fences, pesticides and biological control methods. References Cohen, A. (2011). The Exotics Guide: Non-native Marine Species of the North American Pacific Coast. Center for Research on Aquatic Bioinvasions. Retrieved from: http://www.exoticsguide.org/carcinus_maenas DFO. (2011). Ecological Assessment of the Invasive European Green Crab (Carcinus maneas) in Newfoundland 2007-2009. St. Johns, NL: Centre for Science Advice. Gollasch, S. (2009). Carcinus maenas (crustacean). Global Invasive Species Database. Retrieved from: http://www.issg.org/database/species/ecology.asp?si=114 Klassen, G. J., & Locke, A. (2007). A biological synopsis of the European green crab, Carcinus maenas. Vancouver: Fisheries and Oceans Canada. Molnar, J., Gamboa, R. L., Revenga, C., & Spalding, M. D. (2008). Assessing the global threat of invasive species to marine biodiversity. Frontiers in Ecology and the Environment, 6(9), 485-492. NCEE. (2008). Ecological and Economic Impacts and Invasion Management Strategies for the European Green Crab. Cambridge, MA: National Center of Environmental Economics. Neal, K., & Pizzolla, P. (2014). BIOTIC Species Information for Carcinus maenas. Marlin. Retrieved from: http://www.marlin.ac.uk/biotic/browse.php?sp=4286 NWF. (2015). Invasive Species. National Wildlife Federation. Retrieved from: https://www.nwf.org/Wildlife/Threats-to-Wildlife/Invasive-Species.aspx Rivera, C., Hitchcock, N. G., Teck, S., Steves, B., Hines, T., & Ruiz, G. (2004). European Green Crab, Carcinus maenas. Smithsonia Environmental Research Center. Retrieved from: http://www.serc.si.edu/labs/marine_invasions/population_ecology/carcinus_larvae.aspx Roman, J., & Palumbi, S. (2004). A global invader at home: Population structure of the green crab, Carcinus maenas, in Europe. Molecular Ecology, 13(10), 2891-2898. USGS. (2015). Green Crab (Carcinus maenas). United States Geological Study. Retrieved from: http://nas.er.usgs.gov/queries/factsheet.aspx?SpeciesID=190 WDFW. (2015). Carcinus maenas (European Green crab). Washington Department of Fish & Wildlife. Retrieved from: http://wdfw.wa.gov/ais/carcinus_maenas/ Read More