The hitchhiker's guide to the oceans

Shipping practices rewrote the travel guides for many marine organisms because many of them are well suited to producing stowaways, forming exotic populations far beyond their normal ranges. With increased shipping to Australia, marine creatures that have never lived here before are arriving as dispersive spores or larvae in ships' ballast water or directly as adults attached to their hulls.

Luckily, not all introduced marine species are harmful and only half a dozen or so are true pests. They range from several toxic dinoflagellates (planktonic creatures secreting poisons) to tube worms, macroalgae and sea stars. Many pests successfully compete for substrate or food used by native creatures. For example, the European fan worm (Sabella spallanzanii) fouls hard surfaces and commercial oyster beds, competing with native animals for food in the water column. The northern Pacific sea star (Asterias amurensis) is a voracious predator, eating many native animals including commercial and non-commercial shellfish.

How did these creatures become pests? The answer is quite simple. The Earth's oceans have two temperate regions, in the Northern and Southern Hemispheres, divided by warm equatorial waters. The warm water around the equator is a natural barrier to migration. First, it is warm and may exceed the temperature tolerances of the cool-temperate organisms and secondly the surface ocean currents are restricted to each hemisphere with limited exchange between them.