Glassfish

How do you identify glassfish?

The glassfish on these pages belong to the family Pempheridae, also known as sweepers, which has two genera. Their body is compressed and deep, shaped like a hatchet, with large eyes, a short dorsal fin and a long anal fin. They measure 5 to 30 centimeters depending on the species. The smallest, the Parapriacanthus, grow no longer than 10 to 12 centimeters. Their body is translucent in juveniles, hence the name glassfish. The western Pacific adult, Parapriacanthus ransonneti, turns copper, with an almost glowing yellow-green head. In the Red Sea, the small wreck glassfish is Parapriacanthus guentheri, a species found only in that sea and long confused with its Pacific cousin. The Pempheris, the other genus, are larger, with muted colors.

How do glassfish live?

Glassfish inhabit the shallow waters of reefs and rocky coasts in the Indo-Pacific and the western Atlantic. They are night fish. By day, they pack into dense schools in shelter: in caves, under overhangs, beneath table corals or inside wrecks. At nightfall, they spread out above the bottom to feed on zooplankton, mostly crustacean larvae and small worms. Their schools feed the reef's hunters. In the Red Sea, small groupers of the genus Cephalopholis hunt them, and at the dive sites we describe, lionfish and trevallies plunge into their clouds.

Where and when can you see glassfish?

Glassfish can be seen all year round, and the Red Sea is where they are most spectacular. They fill the corridors and hold 2 of the Thistlegorm. Inside the hull of the Dunraven, around the boilers, they form a cloud that opens and closes as divers pass. On the Salem Express, their clouds fill the space under the decks and the open corridors, crossed by lionfish. At Abu Nuhas, together with Pempheris sweepers, they fill the engine room of the Giannis D. Away from the wrecks, the chimney at Erg Somaya, in the Giftun Islands, overflows with them, the caves of Sha'ab Claudio and St. John's Caves are full of them, and so is the Canyon, the shore crevice next to the Dahab Blue Hole. In the Maldives, the glassfish at Fish Head move in clouds through the cavities, between feather stars and black coral, as the trevallies approach.

Stolen light

Parapriacanthus are bioluminescent. Light organs under the belly light up their silhouette from below and erase it from the view of predators looking up from the bottom. What makes them special is where this light comes from. The enzyme that produces it, luciferase, does not come from them but from their food: tiny glowing crustaceans called ostracods, which they eat and whose protein they keep. This is called a kleptoprotein, a stolen protein.