Despite indications to the contrary, this post is not about googly-eyed spaghetti people from beyond our galaxy. It’s actually about fish, eventually.
Many species live on continents separated by thousands of miles of ocean. Since one species cannot evolve into being in two separate places at the same time, it falls to biogeographers (not that they mind) to explain where the species originally came from and how it got to be where it lives today. For example, humans live on practically every parcel of land on Earth and it is now well-accepted that we originated in Africa 200,000 years ago and have since spread across land and sea to pretty much everywhere.
But, what about species that are not so ubiquitous and lack our ingenuity to build boats, coats, and airplanes? If a group of animals is old enough, there may have been no need to find transport across an ocean to explain a modern disjoint distribution because 200 million years ago the continents were all connected! The ancestors of today’s species could have ridden the continents as they moved apart from one another- gradually separately evolving new groups of species.
This is what biogeographers had long thought happened with the Galaxiid fish- a group of unremarkable-looking fish that live in the cold lakes and streams of southern Australia, South America, South Africa, and New Zealand. As freshwater fish, most species of Galaxiids cannot survive in the ocean because of its high salt content, so it was a bit of a mystery how these closely related fish came to live on such widely separated continents. However, about 180 million years ago, Australia, New Zealand, South America, Africa and Antarctica used to be united in a big southern continent called ‘Gondwana’. If the ancestor of the Galaxiids lived on Gondwana before it broke apart, this could explain their wide distribution.
In the most recent issue of the Journal of Biogeography, a group of researchers from Australia and New Zealand investigated whether the dates when the continents broke apart matches up with the dates when Galaxiid species lichen on separate continents last shared an ancestor. Surprisingly they found several cases where species living on Australia and New Zealand had last shared an ancestor as recently as 5-24 million years ago. Australia and New Zealand were last connected about 80 million years ago, so this means that the species must have crossed the ocean. Even more amazing is Galaxia maculatus, a single species that live in New Zealand, Tasmania, and South America.
The key to the mystery is the fact that a few of the modern Galaxiids, including Galaxia maculatus, have a special ability called ‘diadromy’, where they are able to live in both freshwater and saltwater (like salmon). The researchers proposed that the ancestors of some of these fish that now live on separate continents were diadromous allowing them to actually swim between the continents. Once they got there, most eventually lost the ability to live in the sea. As for Galaxias maculatus, it probably got swirled around Antarctica from Australia to South America by the WestWind Drift. Known less romantically as the Antarctic Circumpolar Current, this westward flow of water circles Antarctica and is implicated in the dispersal routes of several other plants and animals.
Not all ancestral Galaxids had to swim to get to their current homes. The research revealed that some of the ancestors of the modern Galaxiids may have ridden the continents as Gondwana was breaking up. But, contrary to previous belief, these were probably diadromous species that had been able to reach wide distributions across the continent due to their ability to swim across shallow seas. Biogeographers had thought that species restricted to freshwater (i.e. not diadromous) would be the ones more likely to have arisen from the Gondwanan break-up, since they are less able to disperse.
Although the title of this post suggests otherwise, Galaxids are not, in fact ‘invaders’- their dispersal to their current homes millions of years ago makes them integral parts of native ecosystems in the southern hemisphere. The actual “invaders” are a northern group- the salmonids (including the diadromous salmon), which have been introduced by people into many southern hemisphere waterways because they are tasty and fun to catch. Land-locked life no longer need rely on the long-term movement of continents to get around; humans are a new and important route by which animals and plants can travel the world.
You can find this paper at:
Burridge, C., McDowall, R., Craw, D., Wilson, M., & Waters, J. (2012). Marine dispersal as a pre-requisite for Gondwanan vicariance among elements of the galaxiid fish fauna Journal of Biogeography, 39 (2), 306-321 DOI: 10.1111/j.1365-2699.2011.02600.x