Author Archives: cfalvo

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.