Triggerfish

Triggerfish are species of shallow-water marine fishes found in tropical and subtropical seas worldwide. They belong to the family Balistidae and are found in abundance in the Indo-Pacific region. They live in shallow, coastal regions, especially the coral reefs, but a few species like the oceanic triggerfish (Canthidermis maculata), are pelagic (freshwater triggerfish) and live in the open waters of the ocean. 

Several species of this fish are popular in the marine aquarium trade but they are known to be notorious and aggressive. Triggerfishes are colorful with deep bodies covered with large scales. They have high-set eyes and small mouths. They are called triggerfish because of the triggering mechanism that occurs in the first two out of three dorsal fin spines. The first spine is erected and locked in place by the second, which is the trigger, and it must be withdrawn before the first spine is released. The first spine is large and strong and it can hold the fish in protective crevices when locked upright by the second spine.

Triggerfish are found among marine plants and reefs. They can grow about 60 cm (2 feet) long in size. Common triggerfish species include the Queen triggerfish (Balistes vetula), which is a tropical Atlantic fish covered with bright blue stripes, and Rhinecanthus aculeatus, which is a grayish, Indo-Pacific fish having patterns of blue, orange, black, and white bands.

Anatomy and Appearance of Triggerfish

There are about 40 species of triggerfish in the world. The Clown triggerfish is one of the most popular aquarium fish because of its beautiful colors. Stone triggerfish (Pseudobalistes naufragium) which is the largest member of the family, reaches up to 1 m (3.3 ft) in size but most triggerfish species grow up to a length of 20 and 50 cm (8–20 in).

Triggerfish have a compressed oval-shaped body with a large head. They have a small but strong-jawed mouth with sharp teeth that are adapted for crushing shells. The eyes are small and set at the top of the head, far back from the mouth. The anterior dorsal fin is made up of a set of three spines that are generally retracted into a groove. The first spine is stout and the longest amongst all three. The anal and posterior dorsal fins can undulate from side to side for providing slow movement and it is the primary mode of propulsion for triggerfish. This is a distinct characteristic of the species Tetraodontiformes. The sickle-shaped caudal fin of triggerfish helps them in escaping predators.

Most of the two pelvic fins are overlaid by the skin and they are fused to form a single spine. It is terminated by very short rays that are the only external evidence of these fins. Triggerfish also have gill plates (opercula), but since they are overlaid by tough skin, they are not visible. The skin is covered with colorful, rough, and rhomboid scales that create stout armor for their bodies. The only gill opening is in the form of a vertical slit that is directly above the pectoral fins. Along with triggerfish, several members of the Tetraodontidae family have this distinct covering of the gill plates. Each jaw has a row of four teeth on either side and the upper jaw has an extra set of six plate-like pharyngeal teeth.

The triggerfish erect the first two dorsal spines as a protection strategy against predators: The first (anterior) spine gets locked in place by the erection of the second shorter spine. It can be unlocked only when the second (trigger ) spine is depressed. This phenomenon gives the fish its family name “triggerfish''. Except for a few species from the genus Xanthichthys, fish from all other species in the triggerfish family are similar in appearance. 

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The Behavior of a Triggerfish 

The anatomy of a triggerfish is related to its die that includes slow-moving and bottom-dwelling crustaceans, sea urchins, mollusks, and other echinoderms that are creatures with protective spines and shells. Some triggerfish also feed on small fishes; others like the members of the genus Melichthys, feed on algae. The Red-toothed triggerfish (Odonus niger), feeds primarily on plankton. Triggerfish are bottom dwellers who dig out their prey, such as worms and crabs by using their fins to flap away debris. They sandblast their prey by squirting water from their mouths. 

A triggerfish has sharp teeth and jaws that help them in taking on sea urchins. Once the sea urchins are flipped over, the triggerfish attack their bellies which are armed with fewer spines. Triggerfish are often followed by smaller fish who feast on the leftovers of the reef dwellers attacked by it.

They display a high level of intelligence for a fish and can learn from previous experiences. The Titan triggerfish is strong enough to move relatively large rocks at the time of feeding and is often followed by smaller fish such as Moorish idol and Orange-lined triggerfish that feed on leftovers.

Some triggerfish species display highly aggressive behavior when guarding their eggs. The Titan triggerfish (Balistoides viridescens) and the Picasso triggerfish (Rhinecanthus aculeatus) defend their nests aggressively against intruders like snorkelers, scuba divers, and other males that appear near their territory. The territory of this fish extends from the nest toward the surface in a cone shape, so if a diver is swimming upwards, it is potentially entering further inside the fishes' territory. When attacked by a defending triggerfish, it is better to swim horizontally, away from the nest site. The Titan triggerfish is more aggressive than the relatively smaller Picasso triggerfish. It can be dangerous for inattentive divers because of its large and powerful teeth.

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Male Territoriality

Before mating, triggerfish males migrate to their traditional spawning sites to establish their territories since it is essential for reproduction. Some male triggerfish species (i.e. Balistes carolinensis and Pseudobalistes flavimarginatus) build hollow nests for the eggs in their territories. These nests can hold tens of thousands of eggs at a time. The males guard their territories viciously as it has both the female and the eggs. It is where the spawning and parental care takes place. Most male territories are over a sandy sea bottom or on a rocky reef as triggerfish are demersal or groundfish. 

Sometimes there are more than one female in a single territory and the male mates with all the females residing in the territory or visiting it. The males keep a harem of females in their territory.  This characteristic is termed polygyny. A species of male Crosshatch triggerfish (Xanthichthys mento) in Hachijojima, Izu Islands, Japan has as many as three females in its territory at the same time. The male mates with them in pairs. A male Red-toothed triggerfish (Odonus niger) can mate with more than 10 females living in his territory on the same day. Yellow margin triggerfish (Pseudobalistes flavimarginatus) are also polygamous.

Mating in Triggerfish

The eggs of the Yellow margin triggerfish (Pseudobalistes flavimarginatus) and Crosshatch triggerfish (Xanthichthys mento) are spawned in the morning and they hatch on the same day after the sunset. After the embryos are hatched, the female Crosshatch triggerfish leaves the male fish’s territory. This kind of mating system is called male-territory-visiting polygamy. Other mating systems displayed by triggerfishes are nonterritorial-female (NTF) polygyny and territorial-female (TF) polygyny. In NTF polygyny, nonterritorial females stay inside the male's territory to reproduce. In TF polygyny, the female has its territory within a male's territory and reproduces in her territory. A triggerfish digs a small hole in the sea bed to lay its demersal eggs. Juveniles of some species of triggerfishes off Florida are found in floating Sargassum, which is a kind of seaweed. They feed on crabs, small shrimp, crabs, mollusks found in that region.

Spawning in Triggerfish

The spawning of triggerfish is timed according to lunar cycles, tides, and the changeover time of tides. Based on the lunar cycle, the eggs are laid 2–6 days before a full moon and 3–5 days before the new moon. When going according to the tides, the spawning of eggs takes place 1–5 days before the spring tide. Concerning the timing of tides, triggerfish eggs are laid on days when high tides occur around sunset.

Despite their large size, triggerfish are demersal spawners which means they live and feed on the sea bottom or near it. Male and female triggerfish indulge in pre-spawning behaviors that include blowing and touching. The male and female blow water on the sandy bottom, usually at the same spot and at the same time. This is done to set up their egg site. The abdomens of the male and the female touch on the bottom, which gives an impression of spawning. At the time of spawning, triggerfish lays eggs on the sandy sea bottom. These eggs get scattered and are attached to sand particles. The eggs of a triggerfish are usually very small with a diameter of 0.5–0.6 mm and they get easily spread by waves. 

Bi-parental Care

After the spawning of eggs, both the parents participate in taking care of the fertilized eggs. This is referred to as biparental egg care. The female stays close to the spawning ground, around 5 m off the bottom. She guards the eggs against intruders within her territory. Some common intruders for triggerfish include Zanclus cornutus, Parupeneus multifasciatus, Prionurus scalprum, and conspecifics. The females also fan, roll and blow water on the eggs to provide oxygen to the embryos and to induce hatching. This behavior of the female triggerfish is termed ‘tending’, and is rarely exhibited by the males. The male triggerfish stays farther above the eggs to guard all the females and the eggs in his territory. A male triggerfish displays aggressive behaviors in front of conspicuous males that appear near the boundaries of its territory. 

Triggerfish as Food

Some species such as the titan triggerfish, are mostly ciguatoxic and should be avoided as they may cause food poisoning. Variants like the Grey triggerfish (Balistes capriscus), are edible.