Friday, 2 September 2011

Aquarium tour: The Picasso Triggerfish

One of the most colourful marine fish we have on show is or Picasso Triggerfish, Rhinecanthus aculeatus. Triggerfish belong to the same order as the pufferfish, the Tetraodontiformes, but have a different means of self defence. Their skin is covered with very tough scales, and in the dorsdal and anal fins there are locking spines. When erected, these spines make the fish hard to swallow, and they are also used for wedging the fish into crevices in the reef rock, which is where they spend the night.

Different species of triggerfish are found in tropical and subtropical waters all over thw orld. One species, the Grey triggerfish Balistes capriscus, even reaches as far north as southern England and Ireland, at least in the summer, although whether they actually breed in British waters is unlikely. Most triggers grow to a fair size, although at around 20cm the Picasso Trigger is one of the smaller species. Picasso Triggers (they get their name from their colour pattern resembling a Cubist painting) live on reef slopes and lagoons, usually retreating to deeper water at low tide. As lagoon waters vary in salinity, temperature, and dissolved organics more than the open sea, triggers of this species tend to be less stressed by the varying conditions of a home aquarium than some other species, especially those from deeper waters, which means they are fairly popular with home aquarists.

In the wild, some Picasso triggers have been known to live and maintain a territory for eight years, and their maximum lifespan is probably in the 10-15 year range. Very few captive individuals live that long, and the capture of wild marine fish for the aquarium trade is a serious animal welfare issue. Most freshwater tropical fish today are bred on fish farms, and are mostly small species with a 1-2 year life expectancy anyway, but most of the marine fish for sale are wild caught. Some species are now being captive bred, notably clownfish, but with breeding other species aquaculture is still in its infancy. The key problem is raising the newly hatched fry, which spend anything from days to months in the plankton until they metamorphose into the adult form and settle on a reef. For example, although closely related to clownfish, few if any of the damselfish are captive bred, because unlike clownfish their larvae appear to be specialist predators on the larvae of marine copepods (according to one species account anyway), and culturing the correct species of prey is proving difficult.

Having said all that, at least one species of trigger fish has been successfully raised in captivity, so other species may also be in the near future. This raises another issue though, the actual environment a captive fish is kept in, and this is where I am somewhat concerned with trigger fish and similar large species. Whereas a clownfish in the wild has a home anemone and never moves more than a meter from it at the most, a trigger fish is a very wide ranging animal.

The exact social structure in most species has not been studied, but in Picasso triggers they appear to have a large home range of perhaps a hectare of reef, with a male’s territory overlapping with that of several females. When they spawn, the female will viciously defend the nest site for the few hours it takes for the eggs to hatch against all other intruders, even divers (divers rate trigger fish as among the more dangerous species on a reef because of this). While roaming the reef, they search for crustaceans, sea urchins, and similar hard-shelled prey, which they can easily crunch up with their strong bony beaks. They will often manipulate their environment to get at the prey, blowing strong water jets to blow away rocks or covering sediment.
Triggerfish on reef

In view of their natural behaviour, I think that despite their general hardiness and beauty they are not good subjects for a home aquarium as the captive environment, unless in a truly gigantic tank, is simply not roomy enough for them to fully express their natural behaviour. They are often inclined to take bites out of other fish in the aquarium, so many species are hard to keep with other fish in the tank, and although their diet of crustaceans and shellfish is easy enough to provide, their unique hunting strategy is not exercised when food is simply dropped in the tank by the owner.

I wonder whether puzzle toys could be created to allow them to use their natural behaviour to obtain food. They certainly seem to be intelligent animals, and this would be a useful means of exercising their minds. That might make an interesting study project – puzzles have been used in studies of octopus behaviour and a toy that released food when a suitably directed jet of water from the fish was applied should be fairly straightforward to design – perhaps an ecology student could design one?

(images from wikipedia)

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