Differently sourced colourants may affect ornamental fish activity

Carotenoids are nature’s vivid colourants. They give characteristic visual tinges to carrots, corn and canaries, as well as many other brightly hued plants and animals. At this time of year, their effects are particularly obvious as they are responsible for the brilliant yellows and oranges that tint dying autumn leaves.

These naturally occurring chemicals are routinely incorporated into ornamental fish diets to ensure vibrant colours are maintained. However, they don’t just affect a fish’s appearance; they can also influence its behaviour. They are often associated with sexual colouring, and consequently can affect sexual behaviour, particularly in males.

Nature’s colourants

In the past synthetic carotenoids have been most commonly incorporated into fish foods. These have routinely been observed to affect behaviour under certain circumstances. However, there is now a growing public demand for food additives to be derived from natural sources. Of these astaxanthin is probably the most well-known carotenoid added to fish feed. It’s produced by marine algae and bacteria and then passed up the food chain. It is the principal carotenoid responsible for the red flesh of salmon and also accumulates in the shells of crustaceans like shrimps, crabs and lobsters. However, there has been very little research into whether natural carotenoids alter fish behaviour in a similar way to synthetic carotenoids.

Now a new study, recently published in the journal Applied Animal Behaviour Science, attempts for the first time to establish whether behavioural changes typically associated with increased carotenoid consumption differed according to source in a popular breed for tanks: the cherry barb.
These reddish fish are native to Sri Lanka and very popular with aquarists. Males can often be seen harassing females to mate, and generally develop a much deeper colour.

Redder, but calmer

The fish were fed foods with natural or synthetic carotenoids and compared with control populations for differences in behaviour.

The researchers found that, even though males fed both the synthetic and natural diets displayed enhanced red colouration, they were less likely to attack their own mirror image. This is a standard test for aggressive behaviour in tank fish, and one might have expected this to rise. This is because increased colour intensity is often used by fish to signal increased competitive ability.
But the researchers did find a difference between natural and artificial carotenoids when it came to seeking out females. Males spent more time with females fed on food with synthetic additives compared to a similar concentration of natural ones.

So there is clearly an effect on behaviour, but it is unclear at this stage just how the different forms of carotenoid bringing this about. Future studies are likely to explore the phenomenon further, hopefully leading to a greater understanding of the relationship between diet and behaviour in ornamental fish.