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Strange tail of the sunfish

20 February 2006

The century-old mystery of what comprises the peculiar tail of one of the largest of all bony fishes, the ocean sunfish has been resolved.

The sunfish's tail evolved, not from an adaptation of the tail or caudal fin, but when the dorsal and anal fins merged.

Biggest bony fish

Ocean sunfishes, Mola mola , belong to the family Molidae and are amongst the largest of all bony fishes, with up to 1.5 tonnes in weight and 3m in length. Instead of a tail fin they have a rudder-like structure called the clavus .

Gesner’s illustration of the ocean sunfish from 1558

Gesner’s illustration of the ocean sunfish from 1558

Their large size and peculiar appearance has fascinated scientists since they were first illustrated as early as the mid-1500s.

Two theories

Dr Ralf Britz, fish researcher (ichthyologist) at the Natural History Museum , and his colleague Dr G David Johnson from the National Museum of Natural History, Smithsonian Institution, wanted to find out why the tail was so unlike those of any other fishes.

Stained larval stage, 7.2mm, of ocean sunfish Ranzania laevis © Ralf Britz

Stained larval stage, 7.2mm long, of ocean sunfish Ranzania laevis, to enable study of early clavus development © Ralf Britz

There are two previous explanations for how the sunfish tail evolved  - one was that the tail or caudal fin had been modified through evolution, the other highly unusual possibility was that the tail evolved from the merging of the dorsal and anal fins.

Looking at larvae

Britz and Johnson studied the tails of the ocean sunfish in detail by looking at the developing skeleton of the young, or larvae, under a microscope.

They compared its development with that of a less modified relative of the sunfish, the puffer fish.

Close-up of ocean sunfish tail showing clavus developing © Ralf Britz

Close-up of 4.2mm ocean sunfish tail showing how the unusual tail or clavus develops from fusion of the dorsal and anal fins, closing the gap inwards © Ralf Britz

They found no sign of the caudal fin at any stage of the development of the ocean sunfish and discovered that the dorsal and anal fins grow together to form the clavus. In evolutionary terms the dorsal and anal fins have completely replaced the tail fin.

Dr Britz said 'The colossal ocean sunfish, a pelagic fish (living in the open sea) with a wide distribution, has lost its tail fin, the main locomotory structure in all other fishes. This was a very surprising and unexpected result!'

The details of this research were published in the October issue of the Journal of Morphology .