Triggerfish contains 40 species of often brightly coloured fish of the family Balistidae. Usually marked with spots and lines, they inhabit subtropical and tropical oceans throughout the world.
Most triggerfish are found in relatively shallow, coastal habitat like coral reefs. A few triggerfish such as oceanic triggerfish (Canthidermis maculate) are pelagic.
Several species from this family are often notoriously ill-tempered but are popular in the marine aquarium trade.
|Genera||Abalistes, Balistapus, Balistes, Balistoides, Canthidermis, Melichthys, Odonus, Pseudobalistes, Rhinecanthus, Sufflamen, Xanthichthys, Xenobalistes|
Appearance and anatomy
The stone triggerfish (Pseudobalistes naufragium) is the largest member of the family Balistidae, and it reaches 3.3 ft (1 m), but most species have a maximum length between 8 to 20 in (20 to 50 cm).
A triggerfish has an oval-shaped, highly compressed body. Its head is large, which terminates a small but strong-jawed mouth with teeth custom-built for crushing shells.
Its eyes are small, at the top of the head and set back from the mouth. It has a set of three spines which was reduced from the anterior dorsal fin. The first spine is stout and also the longest.
Characteristics of the order Tetraodontiformes are; the posterior and anal dorsal fins are capable of undulating from side to side to comprise their primary mode of propulsion and provide slow movement. The sickle-shaped caudal fin is used to escape from predators.
The two pelvic fins are overlaid by the skin and fused to form a single spine. Opercula (gill plates) are present but not visible, and overlaid by the skin. The skin is covered with rough, rhomboid scales that form a sturdy armour on their body.
The gill opening is a vertical slit, just above the pectoral fins. This trait is shared with other members of the Tetradontae. Both jaws contain a row of four teeth of each side, although, the upper jaw includes an extra set of six plate-like pharyngeal teeth.
Triggerfish erect the first two dorsal spines as a form of protection against predators. The anterior (first) spine is locked in by erection of the smaller (second) spine.
It can be unlocked only by releasing the second, trigger spine, hence the originated family name “triggerfish”. The sexes of all species under this family are similar in appearance, except for a few species under the genus Xanthichthys.
The rather strange anatomy of the triggerfishes reflects their diet of slow-moving, bottom-dwelling sea urchins, molluscs, crustaceans, and other echinoderms, generally creatures with spines and protective shells.
The members of the genus Melichthys are known to feed on algae, while species of triggerfish take small fishes in their diet. An example of a triggerfish species that feed algae is the red toothed triggerfish (Odonus niger), and it mainly feeds on plankton.
When guarding their eggs, some triggerfish species are known to express a form of aggression. Both titan triggerfish (Balistoides viridescens) and Picasso (Rhinecanthus aculeatus) viciously defend their nest against intruders, including snorkelers and scuba divers.
Dissimilar from the relatively small Picasso triggerfish, the titan triggerfish poses a serious threat to inattentive divers due to its powerful teeth and large size.
Biparental care and spawning
Triggerfish spawning is timed in relation to tides, time of changeover of tides, and lunar cycles. In relation to lunar cycles, eggs are observed 2 to 6 days before the full moon and 3 to 5 days before the new moon.
In relation to tides, spawning happens 1 to 5 days before the spring tide. In relation to timing of tides, on days when high tides take place around sunset, the eggs are observed.
Both sexes of triggerfish perform certain pre-spawning behaviours; touching and blowing. A male and female triggerfish blow water in the same spot (at the sandy bottom) at the same time and set up the area to spawn. During spawning, eggs are laid on the sandy sea bottom. Eggs are attached to sand particles but are scattered.
Triggerfish eggs are usually very small in size (diameter of 0.5 to 0.6 mm), and are easily spread by waves. After spawning, both sexes participate in caring for biparental egg care (the fertilized eggs).
The female triggerfish guards the spawning ground where the eggs were laid along with territory against intruders. Some common intruders include Zanclus cornutus, Parupeneus multifasciatus, conspecifics, and Prionurus scalprum.
Besides guarding, females fan, roll and blow water on eggs to provide oxygen to the developing embryos, which induces hatching. This behaviour of the female triggerfish is known as “tending”, and males rarely perform this behaviour.
A male triggerfish stays far above the eggs and guards all the eggs and females in his territory.
In yellow margin triggerfish (Pseudobalistes flavimarginatus) and crosshatch triggerfish (Xanthichthys mento), eggs that were spawned in the morning usually hatch after the sunset of the same day. After the embryos hatches, the female crosshatch triggerfish vacate the male’s territory.
Triggerfish practice a type of male territory visiting polygamy. The triggerfishes practice other types of mating systems, such as territorial female (TF) polygyny, nonterritorial female (NTF) polygyny.
In NTF polygyny, nonterritorial females stay and reproduce in the male’s territory. In TF polygyny, a female will own her territory inside a male’s territory and will spawn in her own share of the territory.