The longspine squirrelfish (Holocentrus rufus), also known as the slender-tailed squirrelfish, is a reef fish recognizable by its red body, large eyes, and spiny fins. It lives in the tropical waters of the western Atlantic, particularly the Caribbean Sea, where it finds shelter in reef crevices.

Mainly active at night, it leaves its shelter to search for small animals to feed on. Its eyes, adapted to low-light conditions, help it locate prey as darkness falls over the reef.

The longspine squirrelfish has an elongated, relatively deep body that is compressed from side to side. Its pointed head features large eyes and a well-developed mouth. Its body and head are generally red, while the base of the tail and the caudal fin have whitish to yellowish hues. Its spiny dorsal fin is particularly distinctive, with white triangular markings near the tips of the spines.

The longspine squirrelfish generally averages around 10 inches in length, but the largest individuals can reach 14 to 17 inches.

The longspine squirrelfish inhabits tropical waters of the western Atlantic, from Bermuda and Florida to northern South America and Brazil. It is found in the Gulf of Mexico and the Caribbean Sea, particularly around the Bahamas, Cuba, Jamaica, Hispaniola, Puerto Rico, and the Lesser Antilles.

The longspine squirrelfish mainly inhabits coral and rocky reefs at depths of up to about 100 feet, where it shelters in cracks, crevices, cavities, and cave entrances. It generally remains hidden during the day and emerges mainly at dusk to search for food.

During its nocturnal movements, it may leave the rocky parts of the reef and venture onto nearby sandy bottoms and seagrass beds.

The longspine squirrelfish feeds on small marine animals, mainly small crustaceans such as crabs and shrimp, as well as small fish and gastropods. It searches for prey mostly at night, leaving its reef shelters to explore nearby seafloors.

The longspine squirrelfish is oviparous: the female lays eggs that are fertilized in the water. The eggs and larvae develop in open water before the young fish gradually move into the reef environment.

The longspine squirrelfish is listed as many other marine species within The IUCN Red List of threatened species. The longspine squirrelfish appears in the IUCN Red List since 2015 within the category Least Concern !