The remarkable sunfishes

A recent stranding of a giant ocean sunfish (Mola mola) in South Australia reminds us of these quite remarkable fish, some of which are amongst the largest of all bony fishes.

A smallish Mola mola, rare in Australian waters, recently stranded and died on South Australia's Coorong coast. Photo: Linette Grzelak, Facebook

Marine sunfishes belong in the family Molidae, which together with the pufferfishes, boxfishes and relatives comprise the Order Tetraodontiformes. Despite their vastly different body size and shape, the relationships of the tetraodonts are obvious on close inspection, especially of their teeth.

The sunfishes (curiously, also called moonfishes, and in German delightfully called Schwimmender Kopf, or "swimming head") are morphologically remarkable, and the German name says it all. They have no tail, a large, deep and rounded body, and propel themselves through the oceans using a remarkable and unique action of two large wing-like fins on the upper and lower side of the body. The efficient propulsion system of sunfish is the inspiration for some equally unique underwater self-powered robots.

And if the adults have a remarkable body plan, the fry (hatchlings) are even stranger

A sunfish fry. Image source: Australian Museum

The Molidae is a small family, with only five species, the most recent of which was discovered and named by Australian taxonomists in 2017. Only three species are found in Australian waters.

You can read more about sunfishes at:

https://australianmuseum.net.au/learn/animals/fishes/ocean-sunfish-mola-mola/