Cichlids no longer percifoms?


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Geneticists have for some time been re-examining the higher levels of systematics in acanthomorph (spiny-finned) fishes, as there are indications from morphology (physical features) that some Orders might contain more than one lineage. The rules of systematics require that all taxa should have a common ancestor.

Hitherto the Order Perciformes (perch-like fishes), which includes cichlids, has been defined on the basis of shared morphological characteristics that suggest a common ancestor.  

But recent genetic research discounts these physical similarities as the result of convergent evolution producing the same response to the same environmental requirements.  

Cichlids, we are told, are not perciforms after all but belong – together with a number of other fish groups – in a new Order Stiassnyformes, named in honour of Dr Melanie Stiassny, Curator of Ichthyology at the American Museum of Natural History.  

Itself unusual, as Orders are normally named for an important family they contain, as with the Perciformes, so that a ready-made popular name (perch-like fishes) is available. Despite her copious work on cichlids, Dr Stiassny does not resemble one!   

Leading geneticists will tell you that molecular DNA study for phylogenetic purposes (determining evolutionary relationships) is highly experimental, and that while it is a useful tool, it should be regarded as credible only if it confirms what conventional (e.g. morphological) methods suggest.  

In other words, if you have two similar-looking fishes with similar internal structure and behaviour, and DNA says they aren’t related, DNA is probably wrong.

Historically a number of revolutionary discoveries made by genetic study have already quietly disappeared into oblivion. The researchers responsible for the Stiassnyformes are at pains to explain and justify their methods, but the message that comes across is not too convincing. Terms such as "Carefully selected markers" (those that produce the desired result?) and "pruning" (of anything that doesn’t fit the desired result from the carefully selected markers?) do not inspire confidence.  

Perhaps mindful of the need for morphological evidence to back their findings, the researchers looked for a morphological link between the families they had re-classified as Stiassnyformes. They found none in the fishes themselves, but they did find that most of them had a shared feature in their eggs - sticky threads around the opening where sperm enters. Only most of them, though, not all!  And there are a few other fish groups, not included in the Stiassnyformes, which have the same feature; these are put down to convergent evolution!  

Systematics is largely a matter of consensus – if everybody, or the majority of workers in the field, agrees with a suggestion, then it is accepted.  More than a year after publication of the name, if you check out Stiassnyformes on FishBase you will draw a blank...

For further information see:  Li, B., Dettaï, A., Cruaud, C., Couloux, A., Desoutter-Meniger, M., & Lecointre, G. (2009): RNF213, a new nuclear marker for acanthomorph phylogeny. Molecular Phylogenetics and Evolution, 50: 345–363.