A marine invasive species is a foreign organism that has been introduced into a new environment and then threatens the biodiversity of the area by disturbing the normal on goings of that habitat. Whether this be through the predation or outcompeting of other native species, invasive species have the potential to critically damage species numbers and also put strain upon commercial operations (Bax et al, 2003).
Asterias amurensis (The northern Pacific seastar)
A prime example of one of the most invasive marine species is Asterias amurensis (the Nothern pacific seastar). A. amurensis is a benthic oceanic predator native to the northwest Pacific shoreline, with the ability to alter local marine ecosystems as well as effecting aquaculture industries (Ross et al, 2006). Throughout this review it will be determined how A. amurensis’ invasive impact effect the warmer waters of Australia.
A. amurensis is placing pressure on the biodiversity of Australian waters; although best suited to cooler waters of around 7-10 °C, it has adapted to live in the far warmer waters of Australia, which measure at around 22 °C. They are able to reproduce in large quantities, with estimations of population reaching up to 12 million after only two years in Port Philip Bay, where they were first detected. It is believed they first entered Australian waters via ballast water from central Japan, this being made possible by its larvae being able to survive in the water column for up to 120 days before settlement (Bruce et al, 1995). A. amurensis are able to consume any and all types of matter, but its main source of intake include muscles, scallops and clams, although they can consume other matter such as dead fish, crabs and many other marine organisms. In particular the seastar has had a major impact on the decline of the spotted handfish, which is now critically endangered (Global Invasive Species Database, 2018).
Invasive Impact of A. amurensis upon Australian Waters
A. amurensis is ranked alongside the most threatening invasive species in Australia and they pose a major threat to global biodiversity, according to the national priority pests report (Goggin, 1998). This is mainly due to its ability to adapt to environmental changes, in this case, being able to strive in the warmer temperatures of Australia. A study on the seastar generated a list of 150 possible genes that could be responsible for the adaptation of the seastar, giving us valuable information into the genetic basis of adaptive change and other features that help in a successful invasion (Richardson, 2015).
One explanation for A. amurensis’ destructive impact upon native species is due to them not recognising the predator and are thus unaware of its threat. Evidence of this behaviour can be seen in Australian scallops (Pecten fumatus and Chlamys asperrima) which exhibited an almost immediate escape response to the native species of seastar (Coscinasterias muricata) completely contrasting the response to A. amurensis in which very few of the scallops managed to escape. This reinforces that the migrant species were not recognised by the native scallops and again highlights the potential detrimental effect A. amurensis could have upon the marine life in Australian waters if their population continues to grow and spread and more of the native populations start to suffer from its voracious eating habits (Hutson et al, 2005). This predation of scallops, along with other bivalves has had a collateral impact upon commercial bivalve harvesting in Australia, with losses reaching up to 50% during settlement season, this further illustrating the damage that a reduction in biodiversity has on a wider spectrum, with it even effecting human activities.
A. amurensis has also been linked to the massive decline in Brachionichthys hirsutus (Spotted Hadfish), a fish native to Australian waters and that is now labelled as critically endangered. It is believed that A. amurensis feeds on the eggs of the organism along with the sea squirts in which they are attached to, this combined with the selective and spatially restricted areas in which the B. hirsutus occupy, has in theory led to its populations demise (Bruce et al, 1998). B. hirsutus experienced it first major decline in population numbers between 1980s and 1990s which coincided with the establishment of A. amurensis into southern Australian waters (Barrett, 1996) giving further evidence for the two events being strongly linked.
The continued reduction in these species above due to A. amurensis has the potential to dramatically change, and is somewhat already having a dramatic impact upon population structure and diversity in Australian waters, as seen by the critical endangerment of B. hirsutus, with many food chains being altered by the removal of these key species.
Responses to the invasion of A. amurensis
In attempt to prevent the waters of New Zealand from also becoming populated by A. amurensis along with other marine pests, a fluorescent in situ hybridisation assay is being used to detect if any viable larvae that is present in the area and further uses microscopy to calculate the specific number of the detected species present. (Mountfort et al, 2010). This means that A. amurensis can be monitored carefully and if they are detected in new regions the threat can be dealt with so more ecosystems and food chains aren’t disrupted and biodiversity remains in equilibrium.
As for B. hirsutus, in attempt to counter its decline and essentially prevent the species from becoming extinct, arrays of artificial spawning grounds have been established to replace the sea squirts in which the eggs would naturally be laid upon (Green et al, 2012). This is done in order to stop the eggs being consumed by A. amurensis when they graze upon the sea squirts.
In another attempt to control A.amurensis numbers a native species of seastar, Coscinasterias muricate, were artificially made more abundant in Australian waters. This native species of seastar both outcompetes and even consumes A. amurensis and acts as a biocontrol strategy in order to eradicate its growing population (Atalah et al, 2013). Although argued that the increase in C. muricate could also have a negative impact on native communities, it is believed that this is more of a temporary issue that would resolve itself over time and natural order to the area would once again be restored (Parry, 2017).
This being said, it is believed that some of the A. amurensis’ prey, in particular bivalves, are expected to adapt to recognise the threat due to the increased selection pressure and at a rapid rate (Remy et al, 1998). New evidence presented by Remy et al (1998) demonstrates that some molluscs have rapid responses to new pressures in their environment which could mean that the invasive impact of the A. amurensis may only be a temporary problem and natural selection will resolve the problems and food chains will once again become balanced.
Summary of A. amurensis’ invasive impact as an example of a marine invasive species
As presented above, marine invasive species can have huge influences upon the biodiversity of an ecosystem. A. amurensis stands as a perfect representation of how an invasive species can have a substantial impact upon the other organisms within an area. Reductions in populations, as seen in B. hirsutus, which is now labelled as critically endangered, thought to be primarily due to the introduction of A. amurensis into the Australian waters, along with multiple other species which have also experienced a decline in population due to a boom in numbers of the A. amurensis stands as a prime example of an impact of an invasive species upon biodiversity. Further to this, invasive species have a wider impact upon food webs, and consequently the commercial farming of some organisms due to a reduction in biodiversity and the deterioration of population structures.
However, counter to their impact it could also be said that natural selection will also take place in invaded areas and native species will develop adaptations to become accustomed to living alongside the invading species. For example, scallops recognising A. amurensis as a threat and therefore adopting an escape mechanism to avoid being ingested could be a possibility in the future that could lead to invasive species becoming just another trophic level in an ecosystem.
References;
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