Food glorious Food - unusual feeding strategies in fish

Gone are the days when the home aquarist believed that all he or she had to do in order to keep his pet happy was to bung in a handful of dried ants eggs once a week and hope for the best. Nowadays the feeding of animals in the home aquarium is a high-tech business, and a whole industry has built up around the piscine equivalents of Jamie Oliver. Anyone who takes their fish keeping even slightly seriously will have a whole battery of dried food, live food, frozen food, and various supplements for their finny friends. And this is just for the fish species most commonly kept in home aquaria.

A friend of ours (a non fish keeper) once commented, when seeing our array of haute cuisine aimed at giving our fishy pals the best possible diet, "Fussy little bleeders ain't they?" He was amazed when we told him about some of the stranger ways that fish in the wild catch their prey.

The megamouth shark, for example. One of the biggest fish in the sea, it was discovered as recently as 1976 and has often been cited by cryptozoologists as an example of how new large species of animal are still lurking out there awaiting discovery. It's feeding strategies, however are almost as extraordinary. Like many other species of large shark, it is a plankton feeder. It has weird prehensile lips and an enormous mouth which give it its name. However, unlike other plankton feeding sharks such as the whale shark or the basking shark, recent research has suggested that its teeth exhibit a weird bioluminescence which attracts the tiny sea creatures on which it feeds and lures them - like a moth to a light bulb - into its enormous maw. were warned

I have always wondered why the more radical feminists in society have not decided to adopt some of the deeper sea angler fish as an icon. In these species, whilst the female is a large and complex creature, the male - essentially just a swimming bag of sperm - attaches itself to the side of the female and has lives a parasitic existence whose only biological function is to fertilise the eggs and then die. Jon moonlights as a freelance designer and would be haappy to come up with a picture of Valerie Solanas. For the purposes of this article, however, it is not the breeding strategies of these fishes which are of interest - but the way they feed.

Anglerfish get their name from their use of a specialized fleshy appendage - a modified ray of the dorsal fin - which it may wiggle like a fishing lure to assist in attracting prey. Angler Fish are found in many parts of the ocean - from relatively shallow waters to the abyssal deeps. The deeper the water, the stranger they become. Below 1,000 metres you enter the dark zone and an alien world. The hairy angler is the size of a beach ball and its body is covered in long antennae designed to pick out the movements of any prey foolish enough to venture close to its terrifying teeth.

Another species is even found in British coastal waters.It has a large flattened head with a wide semicircualr mouth and inward curving, pointed teeth. Behind the head the body tapers to a short, thickset tail. Like so many other fish whose appearance is such that the consumer would find it unappetising, when it is caught and filleted for sale it is renamed "rock salmon" before it reaches the supermarket shelves.

Possibly the strangest strategy employed by any fish species in order to catch and subdue its prey is found in those species which use electricity. Electric rays of the families Torpedinidae, Narkidae, Hyphnidae, and an extralimital family, are unique in having two large, kidney-shaped electric organs in the disc on either side of the head. These organs are capable of generating strong electric shock which are administered at will. The powerful electric organs derive from branchial muscles. The presence of electric organs require a good conducting medium; it is thus not surprising to find such adaptations only among aqueous organisms. Electric signals are produced by several other species of fishes and used for quite a different task. In contrast with the weakly electric fishes - like the elephant fish - that use electrical fields for navigation and signaling these fish produce a powerful discharge of current to stun enemies and prey.

Whilst torpedo rays - like angler fish and megamouth sharks - are not suitable subjects for the home aquarium, at least one species - the misnamed electric eel ('cos whilst it is certainly electric it ain't an Eel, but is closely related to knifefishes), sometimes turns up on dealers lists and has been kept in the larger home aquarium for many years. The eel has a dark grey/brown back with a yellow under-body, and a smooth skin covered in tiny scales. Its vital organs are found at the front of the body, leaving the rest of the "tail" for the electric organ, this can take up to 4/5 of the entire length. Despite the fact that the eel has gills, if submerged for longer than fifteen minutes at a time, it will suffocate through lack of oxygen, hence it surfaces at regular intervals. An adult eel can reach any size from five to nine feet in length - this will give an indication of the potential voltage it produces when in attack or defence. A large adult can produce a potential difference in excess of 600 volts (between its head and tail), and combined with a current of up to 1 ampere, this can be fatal if delivered to a human in the near vicinity of the eel. The eels thick skin acts as protection against its own electric shocks. Injuries inflicted upon the eel can result in the eel electrocuting itself.

However, there is one species commonly kept in home aquaria whose strategy for gathering its food is so peculiar that it almost defies belief. In mangrove swamps and brackish rivers across the Indopacific region a deadly hunter stalks its prey. With the precision of a sniper with a high powered rifle, it carefully manoeuvres itself into the optimum position and then fires its weapon. A "bullet" of water shoots through the air and knocks a flying insect off balance and into the water below. The archer fish has claimed another victim.

There are six species of archerfish which live across the Indopacific region from Burma and northern India as far south as Australia and New Guinea. They are strikingly marked, silver and black fish found both singly and in shoals. The markings are excellent camouflage for small fishes which swim amongst the roots of the tidal mangrove plants, but they have excellent eyesight (with eyes that are capable of being moved in all directions except downwards) and it is also thought that they are able to recognise members of their own species from these markings. Archerfish are so adept at manoeuvring through the mangroves that they can even swim backwards! They control populations of their prey and are also important food for others higher up on the food chain.

Archer Fishes are skilled underwater hunters. They feed off small invertebrates which they catch using a specialised hunting technique which has earned them their name. They are unusual amongst fish in hunting creatures that live out of the water. They spit out carefully aimed jets of water droplets to knock their intended prey off balance and into the water. Archer fish have a specially adapted groove in the roof of their mouths. The fish presses its tongue against the groove to form a tube. Forcing water through their mouths with a powerful snap of the gills, the stream of water droplets can reach a distance of two or three metres although it is only accurate at a range of about a metre and a half. In order to take accurate aim the tiny fish swim along the surface of the water with their pointed snouths protruding out of the water.

Archer fishes have particularly large eyes and excellent three dimensional vision. However, because rays of light `bend` as they go through water, they have had to learn how to compensate for this and aim slightly below the prey for the best resultts. Although they have this unique ability to become a living water pistol, they actually prefer to hunt for food in more conventional ways. Archer fish in public aquaria, where there is an abundance of food actually seem to forget how to shoot water. Both in captivity and in the wild archer fish seem to prefer to leap up to 30 cm out of the water in order to snatch their prey in mid air. These fishes also hunt in a more conventional manner, and form shoals in deeper water where they hunt aquatic crustaceans and smaller fishes.

Like a surprisingly large number of species often seen in public and private aquaria, archerfish are not bred in captivity and little is known about their breeding behaviour. It is believed that it is mostly the juvenile specimens which are found in "shooting parties" in fresh and brackish water whereas the adults are more solitary and swim out to sea to breed in and around coastal coral reefs. Between 20,000-150,000 eggs are laid, but only a few of these will ever reach maturity a year or two later. The colour of the archerfish changes as they get older. Perhaps because when they are young they live in shoals which swim in muddy water, the younger specimens have flashes of a beautiful iridescent yellow on their sides. As the fish become older and more solitary, these flashes disappear, and even the distinctive black banding gets shorter. In the most mature specimens the black stripes are only found on the upper parts of the fish.

It is to be hoped that as the aquarium hobby progresses into the 21st century that we shall get to learn more about the biology of some of the animals that we keep. Whilst the larger fishes described in this article will never become commonly kept as household pets, it seems probable that as the technology of fish keeping progresses that the number of species were able to keep in captivity will widen further. It is our firm belief that as aquarists and as amateur naturalists we have a duty to study the animals in our care and to to the sum total of human knowledge about them. For it is only with such knowledge that mankind will learn how to protect and conserve the animals with whom we share this planet and insure that our children, and our childrens children will continue to gain the pleasure and joy that we do from observing the microcosms of the great oceans in the tanks in the corners of our living rooms.