Behaviours of juvenile batfish and their importance.

Two juvenile longfin batfish (Platax teira) peform a drifting swimming behaviour close to the surface, where their behaviour and morphological features which closely mimic floating debris keeps them out of danger from predators lurking beneath.

A common highlight of a dive, often encountered when exploring some of the deeper isolated pinnacles located offshore are the sweeping schools of Platax teira, commonly known as longfin batfish. The compelling bands of silver and black amongst large schools which seem to effortlessly glide through currents are particularly eye catching for photographers, but it is the bold and curious behaviours of those swimming alone or in pairs that often becomes the subject of choice.

Longfin batfish have a wide distribution throughout the tropics, occupying an omnivorous role in marine ecosystems during their post-larval life stages. They are found living in a variety of habitats including sea grass beds, and coral reefs where they feed on a combination of algae and sessile invertebrates.  Herbivorous feeding performed in both of these environments is important, both in the removal of deleterious epiphytes (organisms that grow on the surface of plants and other organisms) from sea grass blades, and in preventing algal overgrowth amongst coral reefs. These roles are of particular importance in the event of eutrophication (nutrient overload).

Interesting nocturnal dietary shifts have been observed amongst the closely related juvenile Orbicular batfish (Platax orbicularis), helping us better understand the omnivorous nature of Platax species. During the night, carnivorous habits are developed as zooplankton are actively hunted, during which time several different feeding behaviours are adopted. These observations have revealed batfish species to have a much wider ranging diet than previously thought, and is of significanct relevance when evaluating their ecological throughout marine ecosystems.

Despite adults often being found living in deeper environments, juveniles across the Platax genus including P.teira often inhabit very shallow waters, spending the majority of their time close to the surface. During their juvenile stage, batfish are cryptic experts, developing a morphology which closely mimics floating leaves. Juveniles are commonly seen perfoming a drifting swimming behaviour amongst leaves scattered on the surface, around marine debris or near mooring lines, and have been observed flattening their bodies against the surface – a deceptive behaviour that has likely evolved as a means of surface object mimicry.

Another morphological feature that helps longfin batfish juveniles avoid being detected is their profound colouration, which provides exceptional contrast when living in sea grass beds, and amongst floating debris. Juveniles of other Platax species have also developed striking appearances, such as the orange and brown patterns that develop on P.orbicularis, resembling a dead leaf with astonishing likeness.

It is because of their striking morphology and appearance as juveniles that certain batfish species have become targeted as ornamental fish for use in aquaria. Adults have also attracted significant interest from commercial industries and are caught and used as fish food. In more recent years, they have also become an important species for aquaculture in certain parts of Asia and French Polynesia where they are reared for the production of human food.

As an important omnivore that resides amongst coral reefs, batfish often earn a valued position on species lists used as part of reef monitoring programs, including those conducted by the Roctopus ecoTrust where population trends are closely monitered, and may serve as a useful tool for identifying threats to coral reef ecosystems.

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