The Northern Pacific Seastar, Asterias amurensis, is a marine invertebrate native to the coastal waters of the North Pacific, including areas around Japan, Korea, and Russia. It is considered one of the world’s most damaging invasive marine organisms due to its voracious appetite for native shellfish and its ability to rapidly colonize new habitats. The successful global proliferation of this seastar to distant regions, such as southern Australia, is not a natural phenomenon. Instead, it is the result of several distinct human-mediated transport vectors that allowed the species to cross vast oceanic distances and establish self-sustaining populations.
Long-Distance Spread Through Shipping
The primary mechanism responsible for the intercontinental spread of the Northern Pacific Seastar is its unintentional transport via global shipping, specifically through the use of ballast water. Commercial vessels take on enormous volumes of seawater in one port to maintain stability, a process known as ballasting, and later discharge this water in a distant port. This practice creates a direct pathway for the movement of marine life across oceans.
The seastar’s free-swimming planktonic larvae are perfectly suited to survive this long-distance journey within a ship’s ballast tanks. Larvae can persist in the water column for up to 120 days, which is ample time for a trans-oceanic voyage. When a ship reaches its destination port, the ballast water is discharged, releasing the viable larvae directly into a new environment.
The initial major invasions of this species have been directly linked to high-volume shipping ports. For instance, the first confirmed sighting in Australia, in the Derwent Estuary in Tasmania, was traced back to the arrival of commercial vessels, likely from Japan. The continuous flow of shipping traffic ensures a repeated inoculation of larvae, increasing the probability of successful establishment. Larvae are often drawn into the tanks when ships are in port during the seastar’s spawning season, which occurs between July and October in the southern hemisphere.
Movement via Commercial Fisheries
Beyond the trans-oceanic spread facilitated by ballast water, commercial marine activities, particularly fisheries and aquaculture, serve as a secondary vector for the seastar’s movement over shorter distances. The transfer of live marine products, such as commercial shellfish, can inadvertently carry the seastar into new harbors and waterways. The seastar can hitchhike either as small juvenile individuals or as attached egg masses.
Commercial shellfish, including oysters, scallops, and mussels, are often moved between different marine farms or trade locations for seeding or market preparation. Juvenile seastars easily attach themselves to the shells of these bivalves or to aquaculture equipment like spat bags and longlines, which are then transported. This vector facilitates the translocation of the seastar from an already-invaded port to other coastal areas, expanding the local distribution.
The movement of fishing gear, equipment, and the live fish trade also contributes to this dispersal. The seastar’s ability to survive out of water for short periods or cling to submerged surfaces makes it a challenging contaminant to manage. This human-assisted movement bypasses geographical barriers, allowing the seastar to jump quickly from one harbor to the next along a coastline.
Local Colonization and Dispersal
Once the Northern Pacific Seastar has been introduced, its biology enables rapid local colonization. Female seastars exhibit extremely high fecundity, with a single individual capable of releasing between 10 million and 25 million eggs during a single spawning season. This massive output of gametes leads to an enormous number of planktonic larvae, fueling exponential population growth following a successful introduction.
The planktonic larvae are subject to larval drift, the natural dispersal mechanism where ocean currents carry the tiny organisms away from the spawning site. Since the larvae can remain suspended for up to four months, they can be transported substantial distances along a coastline or within a large bay. This process allows the seastar to quickly colonize new bays, estuaries, and sheltered coastlines far from the initial point of introduction.
The seastar also reaches sexual maturity quickly, often within its first year, at an arm length of around 3.6 to 5.5 centimeters. This short generation time accelerates the reproductive cycle, ensuring that an established population produces a new wave of millions of larvae annually.