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Genetic analysis helps separate species of billfish

By Staff | Aug 16, 2018

The “Billfish of the World” poster released by marine artist Craig Smith with the collaboration of Dr. Julian Pepperell shows only eight billfish instead of the current number of 13. This raised the issue in my mind of a long-standing debate among anglers and scientists alike: do we really know how many species of billfish there are in all the oceans of the world?

As recently as the 1920s or ’30s, the number of billfish species was quite uncertain. In those days, the idea that such fish might be capable of mixing across whole ocean basins was regarded with suspicion. Therefore, different populations of a given billfish species, on either sides of oceans, would be described as distinctly different species.

This confusion continued right up until very recently. However, with the advent of molecular genetics, including DNA fingerprinting, scientists are now generally agreed about the taxonomy of the billfishes.

There are now considered to be only four species of marlin worldwide. These are the striped and black, both occurring throughout the tropical Pacific and Indian oceans; the white marlin only found in the Atlantic and Mediterranean; and the blue marlin found in all three major oceans.

For many years, Atlantic and Indo-Pacific blue marlin were separated at the species level – being dubbed Makaira nigricans and Makaira mazara, respectively – mainly on basis of a rather obscure feature: the shape of the lateral line. And while the IGFA still maintains separate records for blue marlin, genetic results clearly indicate that blue marlin are one species, Makaira nigricans.

Various other marlins have been thought to be new species quite recently. For example, d’Ombrain’s marlin of Eastern Australia, for a while considered a separate species, is now known to be a melanistic or highly pigmented black marlin.

Spearfish do present some real problems for billfish taxonomists. On the one hand, the two most common and widespread species are quite clear-cut. These are the shortbill spearfish, the only spearfish found in the Indian and Pacific oceans; and the longbill spearfish that is widespread and readily recognized in the Atlantic.

On the other hand, an obscure species of spearfish, first dubbed the roundscale spearfish in 1840, had also been described from specimens that were found off Portugal in the 1960s. A few more specimens in the early 1970s proved that the roundscale spearfish was, in fact, a real species.

Similarly, another “new” Atlantic marlin species, the hatchet marlin, fueled debate among anglers and scientists alike for more than 30 years. Different molecular genetic markers showed that the hatchet marlin is a distinct species from white marlin.

During the fourth International Billfish Symposium on Catalina Island, California, in late 2005, genetic analyses demonstrated that the roundscale spearfish is a valid species, distinct from white marlin and longbill spearfish.

Because hatchet marlin and roundscale spearfish are relatively rare, using molecular markers, roundscale spearfish samples were genetically similar to hatchet marlin samples, while hatchet marlin samples typed out as roundscale spearfish. Hatchet marlin and roundscale spearfish are one and the same, and they stand as a valid species of billfish now recognized as roundscale spearfish, Tetrapturus georgii.

Finally, there does appear to be one other genuine spearfish species: the Mediterranean spearfish. Quite distinct from the longbill, this species is rarely seen, but nevertheless is recognized by science.

Sailfish occur in all three major oceans. The variable shape of their massive dorsal fins led early naturalists to split them into many species. In fact, it seemed that wherever sailfish were found, new scientific names were bestowed. Pacific and Atlantic sailfish were still officially divided into species until very recently – this time on the basis of the length of the pectoral fins of juveniles.

Again, however, genetic analysis has clearly shown that sailfish are one species worldwide, even though there are some odd pockets of isolated populations which, given plenty of time, could perhaps evolve into new species.