Friday, 19 August 2011

Aquarium Tour: Archer Fish

The next tank along contains a selection of species from mangrove or estuarine habitats. The most distinctive of these are the Archer fish, Toxotes sp. There are at least seven different species of Toxotes, but the fish we have are probably T.jaculatrix or T.chatareus, which are the species most commonly seen in the aquarium trade.

Archer fish have a unique way of catching the insects which make up most of their diet. By flicking their tongues, they can eject water as much as 1m into the air (more in large individuals). They use this to shoot down flies, small land crabs, and similar prey so that they can grab them. The technique is especially effective when several fire at the same time, which often happens because they tend to live in loose schools.

Clearly, the habits of these fish make specialised aquaria essential for their proper care. As well as shooting prey into the water, they will readily jump to catch food, so the ideal aquarium is a deep tank that is only half full. Depth of water is not essential, as they spend almost all their time at the water surface. In the wild they tend to lurk amongst mangrove roots and under overhanging vegetation, only occasionally venturing into open water. They are quite large fish, some species growing to 20cm, so a fairly large aquarium is required. They can be mixed with other species, but will prey on surface-dwelling fish, and they can tackle fairly large prey. Ours share their tank with scats, Scatophagus sp, and Monos, Monodactylus sp.

Although Archer fish will take food from the water surface, they much prefer to hunt live prey, and here at Bristol we have a feeder rigged for them. This is a plastic pot with some holes drilled in the side and some artificial plants attached. Live crickets are placed inside, and every so often one will crawl out into view of the Archers and be shot down. This keeps them occupied in feeding throughout the day, just as they would in the wild. This is an example of an issue that is rarely considered in aquarium fish keeping, what is referred to as “enrichment” activities. For land animals, especially mammals and parrots, a lot of keepers time is spent on ensuring they spend as much of their day as possible exercising skills they would need in the wild, but as soon as other groups are considered very little, if anything, tends to be done. Partly this is because it is assumed that non-endothermic, especially aquatic, animals are too stupid to need anything, and partly because no one is sure how to provide it. Scatter feeds are useful for animals that spend a lot of time searching for small food items, and in an aquarium live food has a similar effect, but it would be a great help if home aquarists would spend some time in considering the natural behaviour and habitat of their pets and see what they can do about it. If you have any ideas, do leave a comment.

One other point that would repay research is reproduction of Archer fish. Some Toxotes sp are purely fresh water animals, living hundreds of kilometres inland, but the species commonly seen in the aquarium trade are believed to breed out on the reef rather than in the mangroves. Very few aquarium spawnings are reported, probably because they tend to be kept singly, and a breeding tank for Archer fish would need to be very large. They are probably seasonal spawners as well, timing reproduction to the monsoon, possibly also with the phase of the moon or tide levels. The eggs themselves are apparently scattered at the water surface, and hatch in about 12 hours, but this may not be the case for all species.

For a video of Archer fish feeding, see here:

(images from Wikipedia)

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