Triggered

Triggerfish are diamond shaped fish with a trigger-like dorsal fin, or actually dorsal spine, which they can raise and retract into a groove in their back. In case of danger this spine pops up and looks like the trigger of a revolver. At night the spine is also used to secure their body between rocks and corals. Triggerfish do most of their swimming by slowly rolling their broad dorsal and ventral fins, rather than by flexing their whole body like many other fish.

Fishes adapt to their aquatic environments; maximizing their efficiency for foraging, mating, and escaping predators (or: catching prey). Speed and acceleration are necessities in predator/prey interactions; i.e. manoeuvrability is king when you need to escape predator attacks. Fishes have produced a great diversity of forms that differ in their propulsive mechanisms using the body, caudal fin, pectoral fins, and median (dorsal and anal) fins. Several species have evolved into deep-bodied specialists using non-caudal propulsion. This adaptation may benefit in a number of ways: manoeuvrability in complex environments (like coral reefs) and at higher speeds, and specialization in acceleration to escape from predators.

Coral reefs are an environment where defensive armour (like the thick impenetrable skin of seahorses) has developed at the expense of speed. Moving around the complex habitat of coral reefs requires exact manoeuvring. Large predators (like sharks) lack the fine control to navigate within this environment. Reef fish must be able to situate their bodies in any number of directions, including the ability to swim backwards effectively and to turn in small radius circles. Triggerfish, the sole species of the family Balistidae and “type form” for balistiform locomotion, use their dorsal and anal fins as their sole method of propulsion at speeds up to half of their maximum swimming velocity. In order to swim backwards, triggerfish reverse the wave on their median fins providing a very efficient method of swimming backwards.

The first triggerfish to gain popularity for aquariums, and still one of the most popular triggerfish, is Rhinecanthus aculeatus, the Picasso triggerfish. The name refers to the fish’s striking coloration, its geometry and its prominent eyes, much like the cubistic art of the Spanish painter Pablo Picasso. The Picasso triggerfish is distinguished by the set of black and blue stripes between its independently working eyes over the top of the head and the vertical yellow band leading from the mouth to below the eye on its cream-colored body.

References
Loofbourrow, Hale (2009): Hydrodynamics of Balistiform swimming in the Picasso triggerfish, Rhinecanthus aculeatus. - Thesis, University of British Columbia (Vancouver): 1-81.

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