Biology

Light for predation

The threadfin dragonfish has a short tentacle beneath its lower jaw known as a barbel which has an illuminated tip.

The illuminated barbel attracts small prey animals towards the mouth.

Chasing after small lights is a necessary gamble for many deep sea creatures - usually a small glowing object means something to eat, but occasionally, the opposite is true!

Unusually for a dragonfish, the structure of the barbel varies within the species and in young fish is quite different to older specimens. The barbel is much more elaborate in younger specimens.

As the fish grows, it is possible its diet changes and it uses a different arrangement of lights to attract different kinds of prey.

Light for camouflage

Rows of light organs along the belly of the fish act as camouflage.

During the day, light can penetrate down to around 1,000m in the clearest parts of the ocean. Although this light would be far too dim for our eyes to make out, it is used by midwater predators to find prey.

To avoid being eaten by larger animals, the threadfin dragonfish lights up its underside very faintly to blend in with any light coming down. This eliminates its silhouette, and the fish becomes invisible to any animal looking upwards. To something looking downwards from above, objects are invisible against the blackness below.

Light for courtship?

Behind the eye of the threadfin dragonfish is a light called the post-orbital photophore. We don’t know exactly what this light is used for, but it is interesting that it is sexually dimorphic - it is different in male and female animals.

The light is much bigger in male E. barbatum than in females and it’s possible it could be involved in courtship behaviour, such as the male using this light to attract a female.

As it is almost impossible to watch this fish behaving naturally at depth, we may never know how these lights are used.

Print version