Derwent Estuary, the Finest Sea Star Hotel

The world thrives on life’s diversity. Every natural space or ecosystem is filled with a variety of organisms that together work to keep the earth in its natural cycle. This natural cycle is rather delicate, and if parts of the puzzle are absent, the whole system does not work as well. One example of this is when a pivotal or keystone species is taken out of an ecosystem (by extinction for instance) and alters the ecosystem by disrupting the food web or natural order of the habitat. Species can be eliminated by natural and unnatural causes. Some unnatural causes include urbanization, human disturbances, hunting and poaching, and invasive species inhabitation.

Invasive species can come about as a result of human disturbance activity. These disturbance activities include people picking up and moving species to a different place, or animals attaching themselves to transportation vessels and traveling large distances, ending up in an environment they are not native to. One example of this is found in the Derwent Estuary in Tasmania, where millions of Northern-Pacific sea stars have moved into and overtaken the waterway. These sea stars are native to Northern Pacific areas such as Japan, and because ocean currents do not naturally carry them down to Australia, it is thought that some sea stars in larval stages were sucked up and carried by harboring ships to the Tasmanian shores, thus they are classified as an invasive species [1].

In a study published in The Journal of Applied Ecology, it was found that the feeding and breeding habits of these sea stars make them particularly successful in the Derwent Estuary [1]. The Northern-Pacific sea stars feed on bivalves (clams, mussels, scallops, etc.) and reproduce by external fertilization (expelling sperm and eggs out into the water to be fertilized). Due to the Derwent Estuary having an abundance of manmade docks, jetties and piers in the water, there is plenty of available substrate for bivalves to latch on to and filter feed. This provides the sea stars with an abundance of food in a very concentrated area. Not only do they thrive on the food source, but also the close vicinity of the docks provides a convenient space for the sea stars to expel their eggs and sperm into the water to be readily fertilized by other sea stars in the area. The overabundance of sea stars in their non-native environment significantly depletes the native species (assortment of bivalves) that are typically found there. In this way, the human construction of docking structures has facilitated an explosion of this invasive species population, and significantly decreased biodiversity.

With our knowledge of the causes and consequences of this problem, we have the opportunity to solve it. One approach would be reducing the availability of the food source for the sea stars by either displacing or blocking off the mussels and clams attached to docking areas. This could be done by fencing off the docking substrate (after the bivalves have established themselves) so the sea stars can’t reach the bivalves. Another potential solution would be changing the structure of the docks so that they are non-attachable for sea stars, or so they are more spread out, making external fertilization less effective to slow their reproduction. Another approach being tested is the implication of ciliates that parasitize the male gonads. Introducing these parasites may be effective in controlling the reproductive rates of the invasive sea stars. Research would have to be done to ensure that the ciliate species is specific to the targeted invasive species and would not be detrimental to other species in the ecosystem.

Basically any method to decrease the numbers of sea stars will help to keep the bivalve populations at normal levels and sustain the biodiversity in the ecosystem. It is very important for diversity to thrive in any ecosystem because, if the natural order of things is thrown off, it is very hard to recover it by natural or artificial means. Halting the cause of what sparks invasive species populations can help control them before they get bad, and ultimately help preserve biodiversity.

–Cristiana Falvo
[1] Ling, S. D., Johnson, C. R., Mundy, C. N., Morris, A. and Ross, D. J. (2012), Hotspots of exotic free-spawning sex: man-made environment facilitates success of an invasive seastar. Journal of Applied Ecology, 49: 733–741.

[2] Bouland, Catherine and Goggin, Louise. The ciliate Orchitophrya cf. stellarum and other parasites and commensals of the northern pacific seastar Asterias amurensis from Japan. International Journal for Parasitology, vol 27; p 1415-1418.

10 thoughts on “Derwent Estuary, the Finest Sea Star Hotel

  1. Lisa Angeloni

    I didn’t know about this story during my recent trip to Tasmania. I wish I had! I was at the Derwent estuary, and it would have been interesting to ask the locals whether they were familiar with the problem. Shipping is a huge problem for the introduction of invasive marine organisms. I wonder if measures are being adopted to stop the further spread of this sea star throughout Australian waters.

  2. reneev

    This was a good read, you very clearly explained the problem and a solution and I learned something I didn’t know before! I really like sea stars and sometimes I forget that they cause a lot of problems in certain parts of the world. I wonder if introducing one of those sea stars that preys on other sea stars might be a good idea, or if that would make the problem worse.

  3. amajor

    When I’ve read this, a thought has come to mind. If humans disappeared from that area and the man-made objects (docks, piers, etc.) that helped created the starfish to thrive were not maintained and were allowed to degrade immensely, it would be possible that the species will decline to the point that it would reach to a sort of equilibrium? I would have to agree that invasive species can and are bad for the environment, but there are some species that are successful only due to other forms of disruption that man has caused. But good job on this article, by the way.

  4. mplatt

    Very interesting post! Unfortunately it seems to be a very tricky problem to solve. Do you think blocking off the bivalves will have a negative effect on them?

  5. ssteele1

    This is a really interesting case of invasive species. I wonder if it is possible to relocate some of these sea stars to more depleted areas? I don’t know enough about the conservation issues for invertebrates. The parasite option seems valid, although it may completely wipe out the population in Tasmania. I don’t know if this ecosystem would benefit from the complete removal of the sea stars, but like you said a parasite could effect other species in the ecosystem so management would have to tread carefully and do their due diligence on research.

  6. leorah

    Are these sea stars expanding beyond this harbor? What negative effects are they having across the food web, outside of the bivalves? Could this population potentially burn itself out in this harbor and solve the problem? These are just things I thought of while reading this.

  7. mberne

    I have never heard of this issue before, its really interesting! I never knew that they cause such a problem. Are the docks being used? If some of them aren’t being used as frequently as others maybe they could be removed to help decrease the increasing population size. Are there natural predators in the area that could also help reduce their numbers?

  8. Savimay

    I really like your ideas for how to prevent the sea stars from over taking the estuary. I think it would be interesting to try Gabe’s idea of using humans to eradicate the population. They could try paying locals for bringing in star fish and see if that effectively lowers the population. Another option would be using a natural predator as long as the predator is a specialist on the star fish. Interesting article, thanks!

  9. Lacey Humphreys

    It sounds just like the zebra muscle problem we are having here in the US. We need to be checking ship hulls to make sure there are no species “hitching” a ride into places where they have no apex predators and thus high reproduction rates. The loss of biodiversity is a monumental threat to the plants and animals of our earth’s ecosystem. Also, how do ciliates “sterilize” males? Just curious…

  10. Amy D'Arcey

    Introducing non-native parasites or predators to try and wipe out an invasive species seems like it’s usually a bad idea. It’d be cool if a ciliate species could be found that was native and could still parasitize the starfish.

Comments are closed.