Lieutenant surgeonfish

Geographical Origin and Habitat: The lieutenant surgeonfish is widely distributed in the tropical Indian Ocean, ranging from the eastern coasts of Africa and Madagascar to Sri Lanka, the Maldives, and western Indonesia, particularly along the coast of Bali. It primarily inhabits seaward reefs and clear shallow lagoons at depths ranging from 1 to 25 meters. This fish prefers areas exposed to moderate or strong currents, characterized by abundant growth of benthic algae on rocky and rubble substrates, where it actively grazes during daylight hours.

Taxonomy and Morphology: Acanthurus tennentii belongs to the family Acanthuridae. Morphologically, it has an oval and strongly compressed lateral body, with a small terminal mouth equipped with comb-like teeth adapted for scraping algal turf. The species can reach a maximum total length of about 31 centimeters in the wild, though it usually remains slightly smaller in aquarium environments. Like all acanthurids, it has a sharp, retractable caudal spine (scalpel) on each side of the caudal peduncle.

Social Behavior: This is an active, diurnal, and constant swimmer. In the wild, it is often seen in small groups or mixed schools grazing with other herbivorous fish, though adults can show solitary and highly territorial behavior. In an aquarium, it displays strong territoriality and intraspecific aggression toward other members of the genus Acanthurus or species with similar shapes and coloration, while remaining peaceful toward fish from other families that do not compete for food.

Coloration and Sexual Dimorphism: The base coloration of the body ranges from light bluish-grey to olive-brown, decorated with thin longitudinal wavy lines. The key distinguishing feature is the presence of two dark horseshoe-shaped markings (resembling a lieutenant's insignia) located right behind the operculum. The caudal fin is strongly lunate, with bluish-white margins and elongated lobes in mature males. There is no obvious chromatic sexual dimorphism, but males tend to grow slightly larger than females.

Aquarium Setup: It requires a very large aquarium with a minimum capacity of 500 liters to ensure sufficient open swimming space for this active fish. The aquascape should include a sturdy structure of live rocks with plenty of caves and crevices where the fish can hide at night or when startled. It is essential to install powerful water pumps to simulate the strong ocean currents of its native habitat and to keep the water highly oxygenated.

Diet and Feeding: Although it will accept meaty foods like mysis and brine shrimp, its diet must be predominantly herbivorous. It is crucial to feed it daily with dried seaweed (nori), spirulina flakes or pellets specific for herbivorous fish, and blanched fresh vegetables like spinach and zucchini. A constant supply of algae ensures the health of its long digestive tract and keeps its body coloration vibrant.

Water Quality: It needs extremely stable chemical and physical parameters and excellent water quality. The optimal values are a temperature between 24°C and 28°C, a stable pH between 8.1 and 8.4, specific gravity between 1.021 and 1.025, and nitrate and phosphate levels close to zero. An efficient protein skimmer and regular water changes are essential to prevent the accumulation of organic pollutants.

Compatibility and Cohabitation: Co-habitation with other surgeonfish, especially from the genus Acanthurus, is highly problematic and requires massive tanks with numerous visual barriers. It is advisable to add the lieutenant surgeonfish last to mitigate territorial aggression. It is compatible with most peaceful reef fish such as clownfish, damsels, and angelfish, and is considered reef safe as it does not harm corals.

Aquarium Breeding: Breeding in home aquariums is extremely difficult and is currently considered virtually impossible due to the pelagic spawning habits of the species. In the wild, surgeonfish gather in large groups, releasing eggs and sperm simultaneously into the open water column. The fertilized eggs float, and the larvae undergo a long planktonic phase before settling onto the reef.

Risks and Diseases: Like all surgeonfish, it has a very thin protective mucus layer and is highly susceptible to external parasitic skin infections such as Marine Ich (Cryptocaryon irritans) and Velvet (Oodinium). It is highly recommended to quarantine new specimens before introducing them to the display tank. It is also sensitive to Head and Lateral Line Erosion (HLLE), often caused by nutritional deficiencies or poor water quality.