The blue bastard fish previously known only through seafarers tales has been officially documented, with Plectorhinchus caeruleonothus described by Australian scientist Jeff Johnson. The species was named blue bastard "affectionately" by anglers because it was so hard to catch.

Blue bastard was described in the journal Zootaxa and in it Johnson, from Queensland Museum, notes how the species has long been confused with Plectorhinchus schotaf. He distinguished the fish with 17 specimens collected off the coasts of western and northern Australia – but the range of blue bastard is much farther, photographic evidence confirms.

P. caeruleonothus was identified through several features unique to it, such as a distinctive pattern on juveniles and minute nostrils. The scientists also used DNA barcoding to show significant genetic divergences between it and their closest sampled relatives.

blue bastard
Juvenile blue bastard Queensland Museum

However, the most telling sign of the species was its aggressive kissing behaviour.

"Some behavioural observations are also presented for the species treated, including aggressive interactions between individuals of the new species, the likes of which have not previously been recorded among species of Plectorhinchus," the study said.

Previously, people had described how blue bastards would kiss. This involves rival males defending their territory by rushing at one another and locking jaws for prolonged and violent struggles.

Johnson told Mashable: "The fisherman call it kissing, but it's anything but. They'll come to the surface and struggle away for minutes at a time... it's a behaviour that's unique to this species among sweetlips."

On the decision to officially name the fish blue bastard, he added: "Fisherman have been calling it this name for 20 odd years, so I thought what better name to use? It's quite an affectionate term, because the fisherman have to do quite a bit of fishing before they find one."