Gilded triggerfish

Not to be confused with Bluethroat triggerfish.

The gilded triggerfish or blue-throated triggerfish (Xanthichthys auromarginatus) is a species in the triggerfish (Balistidae) family found in the Indo-Pacific region.

Gilded triggerfish
Male
Female
Conservation status
Least Concern  (IUCN 3.1)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Tetraodontiformes
Family:	Balistidae
Genus:	Xanthichthys
Species:	X. auromarginatus
Binomial name
Xanthichthys auromarginatus (E. T. Bennett, 1832)
Distribution of the gilded triggerfish

Description

The gilded triggerfish is a spotted brownish-gray triggerfish, sometimes showing a bluish or lavender tint, that resides off the coast of islands and reefs in the Indo-Pacific region.^[1] They have a laterally compressed body, with a small upturned mouth, and a truncated caudal fin which is emarginate (ending with a slight inward curve). The median paired fin swimming technique used by species in the Balistidae family involves the use of both the dorsal and anal fins for propulsion. This is known as balistiform swimming because it is one of the most distinctive traits of balistids. While maintaining body rigidity, median paired fin swimming involves oscillating the pectoral fins on the sides.^[2] It is most prevalent in triggerfish, but this type of swimming occurs also in flatfish and filefish.^[3]

The rear three-quarters of the gilded triggerfish’s body has scales with pale central white spots. It is a permanently sexually dichromatic fish, meaning males and females have different colorations.^[4] This is unusual in other triggerfish species. The gilded triggerfish males have distinctive blue cheek patches on the ventral (lower) part of their head in addition to yellow-bordered, white second dorsal, anal, and caudal fins. All other triggerfish species have red or reddish brown caudal fins. The female gilded triggerfish do not have the blue patches and yellow-bordered fins; instead, they have thin brown margins on the edges of their second dorsal, anal, and caudal fins. Both sexes have dark brown lips and gill membranes in addition to a small white spot near the nostrils (nares).^[5]

From the lower and rear corners of the mouth almost to the upper pectoral base and gill opening, there are either five or six longitudinal cheek grooves that divide the scale rows on the cheek. These grooves are closer together and slope upward posteriorly. Compared to other triggerfish, the cheek grooves on gilded triggerfish are less pronounced and lack pigmentation. Additionally, the ridges along the scale rows sit further forward on the body and are considerably rougher and firmer to the touch.^[1] On average, gilded triggerfish have 27 to 30 dorsal (soft) rays, 25 to 27 anal rays, and 12 to 14 pectoral rays, while having 42 to 47 body scale rows and 17 to 20 head scale rows.^[5]

Size

The genus of the gilded triggerfish, Xanthichthys, contains six species. The gilded triggerfish is the second smallest of these species,^[1] ranging from about 100–300 mm.^[5] Its anal rays (longest ray 1.69–1.92 mm), pectoral fins (longest ray 2.18–2.91 mm), and second posterior fin are all shorter than the first dorsal fin, which is by far the longest. Moreover, the caudal fin (1.31–1.82 mm) is shorter and hardly emarginate, curving slightly inward at the end (concavity: 4.10–8.20 mm). In order to search the reef for food and potential predators, they also possess small, high-set eyes (orbit diameter: 3.57–5.45 mm; interorbital width: 2.61–3.05 mm) that can move on their own.^[5][6] The depth of its body at the origin of the anal fin is 2.45-2.68 mm, the width of the body is 5.14–6.24 mm, head length is 2.79–3.29 mm, and snout length is 4.70–5.14 mm. Other measurements include the snout to origin of the anal fin as 1.44–1.54 mm, snout to origin of the first dorsal fin as 2.85–3.39 mm, and length of the gill opening as 3.72–4.80 mm.^[5]

Trigger mechanism

A male gilded triggerfish showcasing its erect first dorsal fin

When threatened, a triggerfish can erect its first and longest spine of their first dorsal fin. They use their second spine of the same fin to wedge themselves into holes or crevices. This is an excellent strategy to discourage predators because the fish virtually lock themselves within the cracks, preventing their predators from dragging them out. The fin will return flushed along their dorsal side when the threat has passed. People who catch triggerfish can depress the second spine, which in turn releases the mechanism, and the fin can be folded back down.^[6] In at least one species of the Balistidae family, this unique “trigger” mechanism is active early in the developmental stages and may provide a layer of protection from predators of larvae and adults.^[7]

Distribution and habitat

The gilded triggerfish occupies the waters in the Indo-Pacific region, spanning from eastern Africa in the western Indian Ocean to the Hawaiian islands and latitudinally between the Ryukyu Islands and New Caledonia. Other localities where specimens have been recorded include the Maldive Islands, Nicobar Islands, and the islands of Micronesia.^[4] The gilded triggerfish was first identified from the Marshall Islands at Kwajalein Atoll in 1978 as part of a revision of the triggerfish genus Xanthichthys.^[5][8] Similar to most triggerfishes, they can be located off of islands, reefs, or venture out to more open waters.^[9]

While their usual range is between 20 and 50 meters, they have been seen in a much larger span of 8 to 161 meters. They are typically found toward the deeper end of their usual range, with it being rarer to find them in depths above 20 meters. Despite their preference for deeper depths, scuba divers can frequently observe them at recreational diving depths.^[10]

Life history

Diet

A calanoid copepod

Through examining the stomach contents of the gilded triggerfish, it was determined that their diet consisted exclusively of zooplankton, primarily calanoid copepods.^[4] This species' body and fin morphology suggest they live in mostly pelagic environments and practice pelagic feeding habits as their bodies are more tapered, with pointed dorsal and anal fins and shorter heads with upturned mouths.

Reproduction and territory

Reproduction in all species of the triggerfish family is roughly the same through ovipary, consisting of a single male and female, with no group spawning having been reported. Within seconds, the couple touch their abdomens and produce the gametes. Both time and place are important when the pair chooses a spawning site. They tend to prefer sandy substrates on reefs, with timing narrowed down to the early morning. A single female can release as few as 55,500 or as many as 430,000 eggs during spawning. Eggs are attached to sand particles on the sea floor and are about 0.47–0.55 mm in diameter. Both maternal and biparental care have been documented in the triggerfish family. Hatching of the eggs typically occurs the same evening as the day they were laid. The distribution of food and breeding locations has been shown to be connected to the various mating strategies and parental care patterns among triggerfish.^[11]

Setting up a territory is typical of triggerfish males, but can differ slightly in some species, depending on whether sites are temporary or permanent. However, this differs slightly in females, as only some species establish territories of their own. During mating, some females just visit males’ territories while others reside there along with the male. Depending on certain ecological conditions and geographical locations, territoriality and mating systems may alter.^[11]

Hypoxia tolerance

A 2018 study on the hypoxia tolerance of five species of coral reef-dwelling triggerfishes, including the gilded triggerfish, examined the critical oxygen tension, the threshold below which oxygen uptake cannot be sustained to maintain the standard metabolic rate. In the context of this page, hypoxia refers to a body of water with significantly low oxygen levels, which makes it challenging or even impossible for aquatic life to exist. Gilded triggerfish were found to have an intermediate P_crit, about 3.7 kPa, roughly equivalent to the other species looked at during this study. The values gathered from the triggerfishes were at the lower end of other coral reef-dwelling species.^[12]

Conservation status

According to the IUCN Red List, the gilded triggerfish is considered a species of least concern as it is a widely distributed fish with no known major threats that could lead to a population decline. Henceforth, there are also no conservation measures taking place for this species.^[13]