Despite Australia being considered a biodiverse continent, we have relatively few freshwater fish species. The newest explanation for this phenomenon is that we actually do have a diverse fish fauna. What we don’t have is the scientific effort required to discover and describe them.
The most recent field guide for Australian freshwater fish lists 209 species. This compares with 713 in continental temperate USA. Most scholars have explained this discrepancy as the consequence of relative differences in things like habitat availability, rainfall reliability, degree of isolation, etc. But how complete is the list?
To understand why the list may be incomplete we need to go back to how organsims are named and classified (taxonomy).
So, if you’re out collecting fish how do you know you may have a new species? One of the key ways is by what the fish looks like. Characteristics such as size, shape and colour. However, what if species look very similar? Another approach is to analyse the molecular make-up of fish such as their DNA.
An example of Australian fish that have multiple species of similar appearance (also known as cryptic species) is the genus Mogurnda (Purple-spotted gudgeons). Prior to 1999, the mainstream view was that there were only two Australian species, namely Mogurnda adspersa in eastern drainages and Mogurnda mogurnda across the north of the continent and central Australia*.
A revision in 1999 led to the number of species increasing from two to six. Each of the new species displaying quite restricted geographic distributions. This revision was prompted in part by the author’s prior knowledge of unpublished molecular systematic data clearly demonstrating populations of an individual species showing levels of divergence well beyond those expected.
The latest research* used the data from 1999 backed up by two additional molecular datasets for key sites unavailable to the original study.
Results indicated that M. adspersa actually consists of three species taking the total number of Mogurnda species from six to eight. Populations of M. adspersa, north and south of the Burdekin River are different species and a third was found in north-eastern Queensland (originally thought to be M. mogurnda).
Finding that even a well studied group such as Mogurnda has additional species has implications for biodiversity assessment across the entire Australian freshwater fish fauna. Some fish groups clearly harbour cryptic biodiversity.
The reasons for so many undiagnosed species among the Australian freshwater fishes fall into two broad categories. Namely lack of voucher collection and lack of taxonomic attention.
A voucher collection contains confirmed specimens of the organisms of interest – think the pinned insect collections in museums. It provides a point of reference if you are unsure about what species you have.
For example compared with the USA, Australian freshwater fishes have been hugely under-collected. The number of freshwater vouchers in the major Australian museums is three orders of magnitude less than the US (this is like 5 compared with 5000). This can’t be explained by fewer people living in Australia (roughly 23 million compared with 313 million in the US).
The implications of this situation are stark. Australian waterways are rapidly declining in quality and extent, and so are the freshwater fishes they contain. We also have a high degree of endemism in our fish fauna (ie species restricted to few locations). By failing to recognize the existence of geographically restricted species within the range of an apparently widespread single species we will likely see some species go extinct even before they are described.
*The full article on which this post is based is:
A molecular assessment of species boundaries and phylogenetic affinities in Mogurnda (Eleotridae): a case study of cryptic biodiversity in the Australian freshwater fishes. By M. Adams , T.J. Page, D.A. Hurwood and J.M. Hughes. Marine and Freshwater Research, 2013, 64, 920–931.