Rhonciscus pauco

Rhonciscus pauco, commonly called the opalescent grunt, is a marine ray-finned fish in the Haemulidae family, which includes grunts and sweetlips. It was first scientifically described in 2022 based on specimens collected by fishermen off the western coast of Puerto Rico.

The species was formally named in 1922 by Jose Tavera, Michelle T. Schärer-Umpierre, and Arturo Acero Pizarro. Its type locality is Tres Cerros, Rincón, Puerto Rico, at coordinates 18.34433°N, 67.28343°W, taken from a depth of 280 meters (920 feet). It belongs to the genus Rhonciscus within the subfamily Haemulinae, part of the family Haemulidae. The Haemulidae also includes the sweetlips of the subfamily Plectorhinchinae, and the entire family is placed in the order Acanthuriformes.

The genus name Rhonciscus is a diminutive of rhencus, the Greek word for "snore." This refers to the grunting sounds haemulids produce by grinding their pharyngeal teeth, amplified by the swim bladder, especially when removed from water. The specific name pauco honors Pauco, the nickname of fisherman Edwin Font, who caught the type specimens and shared traditional knowledge about the fish’s ecology.

The dorsal fin has 12 spines and 12 soft rays, while the anal fin has 3 spines and 7 soft rays. The body is somewhat elongate, with a depth just over a third of its standard length.

The dorsal profile before the dorsal fin is convex, the eyes are large, and the snout is shorter than the eye’s diameter. The body is silvery-white with some opalescence and golden spots. The maximum published standard length is 25 centimeters (9.8 inches).

Rhonciscus pauco is known only from a small area of the Caribbean Sea off western Puerto Rico, in deep water with fine sandy sediments between Rincón and Mayagüez, at depths greater than 250 meters (820 feet).