Asterias amurensis: A mariculture pest in Australia.

Introduction
Globalization of the world economy has substantially increased the rate of biological invasions, as most harmful species arrive as hitchhikers on the vectors of international trade (2). To make it worst, the advances in world-wide shipping and transportation have accelerated the rate of biological invasions, through ocean-going vessel that regularly transit between continents, resulting in up to hundreds of thousands of gallons of plankton-laden ballast water introduced into new environment unintentionally each time (3). In another word, as international trade expands the rate of unintentional introduction of non-native species increases as well.
Many of the deliberate introduced species often do not cause any problem as humans knowingly choose a suitable environment to fill an open ecological niche (3). However, on average, those unintentional introduced species often become pest by spreading widely or quickly, and cause harm to the environment, human health, other valued resources or the economy (2). As many of the accidental introduced species lack all or most their native, presumably coevolved, natural competitor or predator to control their growth proportion (3, 8).
An example of the many accidental introduced pest species brought about by human activity in coastal zones worldwide which led to the disturbances of marine and estuarine ecosystem in Australia is Asterias amurensis, commonly known as Northern Pacific seastar, and it has been declared as animal pests under the Inland Fisheries Act 1995 (4).

Introduction of A. amurensis and its population density
The Northern Pacific seastar which is native to the coasts of northern China, Korea, Russia and Japan was first introduced to Australia (South Eastern Tasmania) in the early 1980s (5). The introduction was believed to be either as larvae through the released with discharged ballast water or by hull fouling, or other cargo ships arriving from Japan (4). Although other invertebrate predators similar to A. amurensis are present in the Derwent Estuary, such as the native seastar Coscinasterias muricata, but none of them are able to attain the density of A. amurensis (1) which results in competition of resources among them. The unmatchable density of A. amurensis in the Derwent estuary is mainly due to their capacity to form massive aggregations, high spawning capacity (capable of producing 10-25 million eggs per year) and lack of predator and parasite, hence result in a density greater than their density in native ground such as Japan. Initially A. amurensis was only found in the Derwent estuary; however they have since spread naturally to other parts of Tasmania as shown in Figure 1 (1, 4).
Growth, nutrient storage and gonad development (and thus sexual reproduction) depend on the availability of food supply. With NP seastar competiting directly with C. muricata for resources, the sexually reproducing rate declines. Figure 1: Distribution of the Northern Pacific Seastar in Tasmania. Box indicating the site of first introduction, and the shaded area indicates the distribution as noted in 2002. This image is taken from Ross J.D. et al. research paper done in 2006 (4).

The natural spread of A. amurensis to other parts of Tasmania is believed to have been restricted by the local pattern of estuarine circulation; however, because of seaport trading, A. amurensis has ‘port-hopped’ to Port Phillip Bay (~600 km away), and other major port of State of Victoria's which was believed to had occurred during 1995 (1, 4, 6, 8). Based on the genetic test done on the isolated A. amurensis in studies, the results has indicated that the transfer across Bass Strait was most likely from the Tasmanian population via hull fouling or ballast water (7, 8). Similar to the population in Derwent estuary, the population density of A. amurensis in Port Phillip Bay (Victoria) had reached approximately 30 million in a short period of time and it is estimated that within two years, the total Australian population of A. amurensis could be as high as 120 million (7).

Effect of A. amurensis on economy and native communities
Introduced predators such as A. amurensis, are likely to have large impacts on native communities this is because, besides being a direct competitor for resources to native seastar species, it is a voracious generalist predator in soft-sediment habitats which consumes a wide variety of prey according to the ecological availability of prey (1, 8). However, as A. amurensis have a clear food preference for bivalves that live on or just below the sediment surface, this greatly impact on native bivalves as well as commercial shellfishes such as scallops, mussels and oysters (1, 7). This is of particular concern to the small but growing number of Australian commercial operations harvesting wild populations of bivalves that live near the sediment surface. For instance, due to the introduction of A. amurensis in Tasmania, it has caused the loss of commercial scallop (P. fumatus) spat over a settlement season reported to be as high as 50% (7). This high percentage loss is possibly because P. fumatus did not co-evolved with A. amurensis, hence results in the failure to recognize A. amurensis as a predator. It is shown in study that when contact occurs between P. fumatus and native seastars, C. muricata characteristic of escape behavior was elicited; however, it is absent when P. fumatus come in contact with the introduced seastar Asterias amurensis (7).
Additionally, studies has also found that A. amurensis is largely responsible for the recent decline and subsequent rarity of large bivalves in its current distribution in Tasmania (1, 8). As evidence such as the shells of large adults of several species was present in the surface sediments suggesting that this has not always been the case, and live bivalves which are >5 to 10 mm are rare in areas where the seastar is now abundant (1). With the diet preference of A. amurensis, their predation on juveniles as found in Ross JD et al study (2002), their findings suggest that it may be preventing the subsequent establishment and potential recovering of adult populations in the estuary which will directly affect the commercial bivalves harvest and indirectly cause the extinction of native bivalves species as well as others such as Brachionichthys hirsutus (1, 2, 6, 9).

Impact on native endangered species
Brachionichthys hirsutus which is endemic to south eastern Tasmania in areas of the lower Derwent River estuary, which is currently listed as critically endangered species, is another native species threaten by the introduction of A. amurensis (9). B. hirustus was once regarded as the most common of the inshore handfish, however, it undergone a substantial decline in abundance around the same time as the introduction of A. amurensis (9). Additionally, many areas where A. amurensis is abundant now, were areas where B. hirustus used to be common (9). The impact of A. amurensis has caused B. hirustus to be displaced from their habit is one of the possible reasons for the decline in abundance, as B. hirustus have little to no ability to decolonize areas which they have been displaced from due to their modified pectoral and pelvic fins (9). Another possible reason is the predation on egg mass by A. amurensis, preventing the recovery of the population size (9). Hence it is likely to drive B. hirustus to extinction if nothing is done to eradicate A. amurensis.

A. amurensis in native environment
However, when A. amurensis is in its native environment, their population density is kept in checked by its native predator, competitions and parasites (3, 5). For instance, in its native Japan, Solaster paillatus (a sunstar) has been noted as a predator of A. amurensis (5). Similarly in Korea, Charonia species (Trumpet shell) was observed to prefer A. amurensis as their prey over other seastars (5). This prevents the seastar from being over populated or changes its native environment ecological system.

Conclusion
Based on the clear pattern of A. amurensis diet, it shows that they eat the most accessible prey first, and shift to other less accessible prey only when availability of more accessible prey declines. Therefore, it shows that A. amurensis has the potential to impact a large variety of taxa while having a wider effects on soft sediment communities (1). In addition, the high densities of A. amurensis that can occur without its native predator or competitors in the new environment, native assemblages, including commercial species are likely to be affected directly. Thus resulting A. amurensis to be a major mariculture pest in Australia, additionally, a huge amount of cost will be required in order to eradicate the established A. amurensis.

Reference

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[9] "Threatened fishes of the world: Brachionichthys hirsutus," Environmental Biology of Fishes, vol. 52, p. 418, 1998.