The Leeuwin Effect – when the Indian Ocean houses Pacific taxa

MSU's blog | Created 11 months ago

Earlier this year, we published a paper that highlighted a number of information gaps in Indo-Pacific phylogeographic studies (Putting the ‘Indo’ back into the Indo-Pacific: resolving marine phylogeographic gaps- Invert. Syst. 30:867-94). One of the things we discussed was the unusual phenomenon of typically Pacific taxa occurring in the north west of Western Australia (WA) in the Indian Ocean! This has amusingly led to some authors listing Western Australian localities as being in the Pacific. Most people agree that the boundary between the Pacific and the Indian Ocean occurs in the coral triangle region (see map below, CIA World Fact book, public domain). 

Extent of the Indian Ocean

As part of the world ocean’s circulation patterns, water moves from the Pacific to the Indian Ocean. As this water passes through the Coral Triangle, it’s termed the Indonesian Throughflow. Lots of other currents and gyre systems are involved; it can be quite complicated. Ultimately the water moving down the coast of Western Australia (via the Holloway Current) ends up feeding the well-known Leeuwin Current, which sweeps down the southern part of WA.

So Pacific taxa that occur in northern WA have likely been transported (either directly as larvae or their ancestors) in that water that comes through the Indonesian Throughflow. We termed this the Leeuwin Effect, not after the current per se, but a nod to the effect of being transported beyond their normal space. This makes the north west of WA a really interesting place where typical marine Pacific taxa can be found living instead of, or together with, their Indian Ocean relatives in the same place, instead of being oceans apart. Trying to figure out which species occur there is part of the work that we are carrying out in the Net Conservation Benefits project at the WA Museum.