Relatively speaking

Dr Julian Pepperell
January 2007

Relatively speaking

The classification of fish might seem to be a pretty dull subject to the average angler, but knowing which fish is which, and how they are related, is critical to our understanding of their evolution and overall biology. So it goes without saying that I am very excited with the recent announcement of a major breakthrough in the classification of the billfishes. Just over a year since it was held, the Fourth International Billfish Symposium proceedings have hit the streets containing many new and exciting research papers. And among this learned tome is one of the most important papers on billfish classification to see the light of day for many years. With the rather short title: “Phylogeny of recent billfishes (Xiphioidei)”, by Bruce Collette of the US National Marine Fisheries Service together with Jan McDowell and John Graves, both of the Virginia Institute of Marine Science, this paper has rewritten what we now understand about billfish evolution and relationships, both within the group itself and with other groups of oceanic fishes.

The history of classification of the billfishes is not all that long, but it is strewn with errors and controversy. Apart from the broadbill swordfish, which is a single, obvious species, the marlins, sailfish and spearfishes have caused much confusion as to how many species there are and what their relationships are to each other. From the late 1800s to as late as the 1980s, many species of marlin, sailfish and spearfish had been nominated, often simply on the basis of widely separated locations of capture, or, especially in the case of sailfish, on the basis of real or imagined slight variations in the shape of the dorsal fin. The other area of debate has been just how closely (or distantly) related are the billfishes (families Istiophoridae and Xiphiidae) with the tunas and their cousins (family Scombridae). Some have suggested very close relationships, but others have suggested that similarities in body form etc may be the result of convergent evolution rather than reflecting genuine kinship.

With the rapid advance of molecular genetics in recent years, the whole field of the classification of life on earth has gone through a major revolution. Molecular genetics, more popularly known in a loose sense as DNA profiling or fingerprinting, allows the real, rather than imagined ancestry and relationships of living organisms to be determined. The closer the DNA, the closer the relationship and vice versa. So at last, the true relationships among the billfishes, as well as between the tuna and billfishes, have become possible to dissect under the microscope – literally.

Without going into the details of the complicated genetics in this paper (phew, got out of that one neatly), the results of comparing DNA samples from hundreds of specimens collected all over the world (many from game fishing tournaments, by the way) can be summarized as follows:

Firstly, the authors found that there was no genetic justification to support the recognition of separate species of either sailfish or blue marlin in the Atlantic or Indo-pacific ocean basins. In other words, there is only one world wide species of sailfish, and one world wide species of blue marlin. Secondly, they found that the very closely related striped and white marlins were sufficiently separate from the four species of spearfishes (with which they shared their first Latin name, Tetrapturus) to warrant changing the first name of the striped and white marlin to Kajikia. Lastly, they found the black marlin to be sufficiently different from the other billfishes to separate it from the blue marlin (with which it shared its first, or generic name, Makaira) and to give it back its old name, Istiompax. So to make that perfectly clear, based on the latest molecular genetics, the scientific name of the black marlin should now be Istiompax indica while the names of the striped and white marlin respectively should be Kajikia audax and Kajikia albida. To steal a phrase: I’m excited!

Now that this renaming of at least three marlin species has been recommended in print, is it now compulsory to use these names from now on, under pain of academic death? Not being an expert in the protocols of naming animals and plants (technically, ‘nomenclature’) – and believe me, these can be very strict – I took the easy course of contacting the lead author of the paper, Bruce Collette. Now Bruce is one of the all-time gurus of fish classification and nomenclature, especially of the tunas and mackerels. His response was that the new names are strongly suggested to be used from now on, but that it is up to the scientific community to accept them or not, hopefully based on their reading of the paper. Those who continue to use the old names will not be excommunicated from either the world union of taxonomists or the church of how’s the fishin’?, but I for one have seen the light. So from now on, if you catch me still calling the black marlin Makaira indica, or the striped marlin Tetrapturus audax, give me a good, hard slap on the wrist will you?


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