Weird fish

There are almost 28,000 known species of fish in the Earth’s rivers and oceans.

They come in various shapes and sizes with different behaviours and life cycles but the one thing many of them do have in common is that, to our mammalian eyes, they are weird. Here is just a small selection of the weirdest.

Blobfish

The blobfish is not only weird, it is also ugly. In fact, in 2013 it was voted the world’s ugliest animal. But some good may come of this maligning, as it is now the mascot of the Ugly Animal Preservation Society, an initiative ‘dedicated to raising the profile of some of Mother Nature’s more aesthetically challenged children.’ 

The blobfish is seen by humans when it is dredged up dead in nets as by-catch, as the fish usually lives at depths of up to 900 metres. At this depth, the pressure is between 60 and 120 times greater than that at the sea surface. To avoid being squished the blobfish is mainly comprised of gelatinous flesh; because this is less dense than water, it can floats along the bottom of the ocean quite happily. However, with very little muscle, actively hunting prey is impossible. Instead it sits and waits for small crustaceans and other edible matter to float into its mouth.

The blobfish has never been photographed underwater, but scientists believe that at the ocean floor it looks more like a normal fish, with a large head, tapered body and a flat tail, and spikes instead of scales. It is only when brought (dead) to the surface, due to a change in pressure, that it changes shape to look like, well, a blob.

Jawfish

The jawfish family comprises around 80 different species, native to warmer parts of the Atlantic, Pacific and Indian Oceans. They are small is size, mostly no bigger than 10 cm, but with heads, mouths and eyes that are very large relative to the size of their bodies.

They typically reside in burrows that they construct in sandy substrate—they will stuff sand into their mouths and spit it out elsewhere, slowly creating a tunnel. But this is not the only thing they put in their mouths, because (apart from the blue-spotted jawfish) they are mouthbrooders. After mating, the female jawfish gives hundreds of eggs to the male, who then holds them in his mouth for days or even weeks. They hatch in his mouth, where the fry are protected from predators.

While he has the eggs in his mouth, the male cannot eat—his only activity is juggling the eggs in order to aerate them. Why it is the male who looks after the eggs is not fully understood, but it is believed to enable the female (who invests heavily in egg production) to be able to rest, and have the ability to reproduce more often.

Frogfish

The many species of bizarre looking frogfish are found in almost all tropical and subtropical oceans and seas from around the world, except the Mediterranean Sea. They are small, stocky and sometimes covered in spinules and other appendages. These aid camouflage—some species resemble stones, coral, sponges or sea squirts. Other species can change colour, and some are covered with other organisms such as algae or hydrozoa.

The camouflage enables them to hide from predators and to lure prey, by lying in wait and striking extremely rapidly (in as little as six milliseconds). But the way they move across the seafloor is extremely weird, at least it is for fish, because they walk. By using their pectoral and pelvic fins, they clamber along using one of two ‘gaits’.

One is to propel themselves forward by alternatively moving their pectoral fins. The other is by moving in something resembling a slow gallop, simultaneously moving their pectoral fins back and forward, transferring their weight to the pelvic fins while moving the pectoral fins forward.

Goblin shark

The goblin shark is a rare, deep-sea shark that is known as a ‘living fossil’ because it is the only extant member of the Mitsukurinidae family, which has a lineage dating back 125 million years. It was first described in 1898, after a specimen was caught in Japanese waters, where it no doubt surprised its captures with its odd appearance. It has a long, flat and very pointed snout, with jaws that can project open quickly to catch prey such as squid, extending almost to the end of the snout itself.

Its large, oily liver makes the shark neutrally buoyant, enabling it to drift towards prey slowly with minimal movement to avoid detection, even though it can grow to over three metres long. The purpose of the goblin shark’s long snout was once thought to be to stir up prey from the sea bottom but its lack of rigidity made this proposal unlikely.

It is now known that the snout contains Ampullae of Lorenzi—electroreceptors which are specialised sensing organs forming a network of jelly-filled pores. They allow the shark to detect temperature gradients in the water, as well as the electric fields produced by other animals.

Deep sea dragonfish

The deep sea dragonfish is a ferocious predator, living at depths of up to 1,500 metres. It has extremely large, sharp teeth compared to the rest of its body, which measures only about 15cm in size. It is one of the many deep sea fish that can produce its own light, through a chemical process known as bioluminescence.

The light is produced by a special organ called a photophore. Bioluminescent fish use flashing lights to signal to potential mates, as well as to attract prey. The deep sea dragonfish has a long protrusion known as a barbel attached to its chin, with a light-producing photophore at its tip. This, it uses as a fishing lure, waving it back and forth and flashing it on and off.

Once any small fish or crustaceans get to close, they are snapped up in the dragonfish’s powerful jaws. In the deep, dark water, many prey fish also produce light, so the dragonfish has had to evolve a way to remain hidden after it has eaten—the walls of its stomach are black to keep the lights of the prey fish concealed while it is digesting its meal.