The Northern Pacific seastar, also known as Asterias amurensis or the Japanese seastar, is a globally recognized invasive marine species. It has established itself far beyond its native range, causing widespread concern in marine environments. Its presence poses significant challenges to marine ecosystems and commercial industries.
Identification and Biology
The Northern Pacific seastar features a central disc from which five arms radiate, tapering to pointed, upturned tips. Its coloration varies, often appearing as a uniform pale yellow with purple arm tips, or mottled yellow-purple, and can also display shades of orange to yellow with red and purple markings on its upper side. Fully grown individuals can reach up to 50 centimeters in diameter, although arms commonly extend to about 16 centimeters. Small, jagged spines are unevenly distributed along its arms, lining the groove where its tube feet are located and converging near its mouth.
Reproduction in Asterias amurensis involves a high capacity for egg production, with females capable of releasing between 10 to 25 million eggs annually. Gametes are externally fertilized in seawater, developing into free-swimming larvae. Spawning occurs seasonally, from July to October in Australian waters, and January to April in its native Japanese waters.
The larval stage of the seastar floats as plankton for 41 to 120 days. This prolonged larval duration significantly contributes to its ability to disperse over vast distances via ocean currents. After this planktonic phase, larvae settle and metamorphose into juvenile seastars.
Native and Invasive Distribution
The Northern Pacific seastar is native to coastal waters of the far North Pacific, including northern China, the Korean Peninsula, far eastern Russia, Japan, the Aleutian Islands, and British Columbia in Canada. It is commonly found in shallow seas and estuaries, where it is a natural component of the marine fauna.
Beyond its native habitat, Asterias amurensis has become a widespread invasive species, particularly in southern Australia. Populations have established in Tasmania, notably the Derwent River estuary, and in Victoria, including Port Phillip Bay. It was first detected in Tasmania in 1985 and Victoria by 1998, and has also been noted in parts of Europe and Maine in the United States.
Its global spread primarily involves human activity, predominantly through commercial shipping. Planktonic larvae are transported in ship ballast water, taken up in one port and discharged in another, colonizing new areas. Hull fouling (adult or juvenile seastars attaching to ship hulls) and accidental translocation via aquaculture or fishing gear also contribute to its dispersal.
Ecological and Economic Impacts
Asterias amurensis is an effective predator in its introduced environments, with a broad diet. While it shows a preference for large bivalve molluscs such as scallops, mussels, and clams, it also consumes gastropods, barnacles, crabs, worms, other echinoderms, and even carrion or other seastars if preferred food sources become scarce. It feeds by everting its stomach over its prey for external digestion.
The seastar’s predatory nature leads to ecological disruption in invaded habitats. It can outcompete native predators for food resources, causing declines in local populations of commercially important bivalves like Fulvia tenuicostata and Katelysia rhytiphora in the Derwent River Estuary. The presence of Asterias amurensis has also been linked to the decline of the endangered spotted handfish in the Derwent Estuary, demonstrating its broader impact on native species and food webs.
The economic consequences of this invasion are significant, especially for aquaculture and shellfish industries. Seastar predation results in losses for farms cultivating scallops, mussels, and oysters. For instance, populations in Port Phillip Bay reached an estimated 165 million individuals by 2000, severely affecting local fisheries. Outbreaks in its native Japan have also caused financial damage to mariculture operations.
Management and Control Strategies
Managing Asterias amurensis invasions involves various strategies, though complete eradication is often difficult. Physical removal methods, such as diver-led culling and trapping, are employed. Community efforts in Hobart have removed over 200,000 seastars, with another group in Victoria removing over 60,000 within months. However, these manual efforts are labor-intensive and challenging to sustain given its high reproductive capacity.
Research continues into biological and genetic approaches. Scientists are exploring biological control agents, such as native predators like the starfish Coscinasterias muricata, which consumes Asterias amurensis in laboratory trials. Investigations into parasitic organisms are also ongoing, though concerns exist regarding host specificity.
Genetic research aims to identify pheromones that trigger mass congregation during mating, to develop targeted traps for efficient mass removal. Genomic studies are also underway to understand population outbreaks and genetic adaptations facilitating its invasive success. Preventative measures, such as ballast water management during spawning seasons and hull cleaning regulations, are also implemented to limit further spread.