British Wildlife of the Week

Sunfish

Large, lazy and very strange-looking – meet the ocean sunfish. This extraordinary animal certainly has very peculiar proportions. Tall but vertically flattened, with a pale, circular body that seems to end abruptly behind its huge dorsal and anal fins, the sunfish almost looks as though it is simply a massive severed head with a short, frilly tail attached.

Some sunfish have been recorded at 3.3 metres in length, as tall as they are long, and weighing as much as 2,300 kg – the same weight as an adult hippopotamus. This easily makes it the largest and heaviest bony fish in the world (only a few cartilaginous fish, such as the whale shark, attain greater sizes). The sunfish can reach these huge proportions because it casually drifts around the ocean, using very little energy as it does so. It moves forward by flapping those tall, triangular fins from side to side, while its tail functions as a rudder, steering the fish as it swims.

The sunfish is a gentle giant. Possessing only a small mouth and a single, beak-like tooth (which consists of smaller teeth that are fused together), it nibbles off pieces of jellyfish and other drifting, soft-bodied animals as it swims gently along. During the summer months, it visits the waters around the British Isles, particularly in the south and west, to feast on the huge swarms of jellyfish that form there.

Despite being classified as a bony fish, the sunfish doesn’t really have many bones. Its skeleton largely consists of cartilaginous tissue, which is lighter than bone, allowing it to grow to sizes impractical for other bony fish. It also has the fewest vertebrae of any fish – a mere 16 – and its spinal column is extremely short.

Though the sunfish spends a lot of time in the deep sea, it occasionally ventures up to the surface to warm up. Its English name comes from its tendency to bask in warm, sunlit waters by turning on its side and exposing a greater area of skin towards the sun. It may look almost helpless or even distressed while on its side in this way, especially when it flaps its tiny pectoral fins as if trying to right itself, but the sunfish is just ‘thermally recharging’ – warming its body after returning from deep, cold dives. Confusingly, in many other languages (including French, Spanish, German and Russian), the sunfish is referred to as the moonfish because of its rounded shape and pale grey colouration.

Occasionally, a sunfish may turn onto its side not only to bask but to attend a ‘sunfish spa’. Sunfish commonly become infested with parasitic crustaceans that suck their blood, and by floating sideways on the surface they are advertising to oceanic birds such as albatrosses and gulls to come and clean them. The birds land on the water and pick even the most stubborn parasites off the sunfish’s thick, rubbery, scaleless skin, simultaneously earning a quick, easy meal and relieving the fish of its unwanted passengers.

But if everything that we’ve already mentioned wasn’t proof enough that the sunfish is a truly amazing animal, then how about this? Female sunfish can produce more eggs than any other backboned animal – up to 300 million at a time. Newly-hatched larvae are only 2.5 mm in length and weigh less than a gram. To reach adult proportions, they must grow 60 million times their original size, which is possibly the most extreme growth of any vertebrate in the world.

In the next British Wildlife of the Week, we’ll be looking at a rodent who is a serious sleeper – the hazel dormouse.