Rhynchactis

Rhynchactis is a genus of deep-sea anglerfish belonging to the family Gigantactinidae. Three species are recognized, and they occur across the world’s oceans at depths exceeding 400 meters.

What sets this genus apart from nearly all other deep-sea anglerfishes is the absence of a bioluminescent lure, or esca, at the tip of the illicium—the modified dorsal fin ray that functions like a fishing rod. Additionally, the mouth contains almost no teeth, and the inner surfaces of both jaws are lined with numerous white glands found nowhere else in the group. These features point to a highly specialized feeding strategy, though its exact nature remains unknown.

Biology and ecology

As in other deep-sea anglerfishes, sexual dimorphism is extreme: adult females reach a standard length of 11 to 13 centimeters, while males are much smaller and lack an illicium entirely. Unlike some anglerfish families, however, males do not appear to become permanently parasitic on females. Reproduction is oviparous, and the larvae have a rounded body shape with enlarged pectoral fins.

Taxonomy and phylogeny

The genus was first described by British ichthyologist Charles Tate Regan in 1925, based on a single metamorphosed female collected by the research vessel Dana in the western central Atlantic. He named the type species Rhynchactis leptonema, drawing the generic name from the Greek rhynchos (“snout”) and aktis (“ray”), and the specific epithet from leptos (“thin”) and nema (“thread”). In 1998, Erik Bertelsen and Theodore Pietsch revised the genus, recognizing two additional species: R. macrothrix (from Greek makros for “long” and thrix for “hair”) and R. microthrix (from mikros for “small” and thrix for “hair”), both names referring to features of the illicium. Within its family, Rhynchactis is considered the more derived genus, showing secondary reductions in bones, fin rays, and teeth.

Distribution and habitat

The three currently recognized species are R. leptonema, R. macrothrix, and R. microthrix. Specimens have been caught between 32°N and 12°S latitude in the Atlantic, Indian, and Pacific Oceans. R. leptonema is known from the western central Atlantic and off Hawaii; R. macrothrix has been collected near Bermuda and Madagascar; and the single known R. microthrix specimen came from the western Indian Ocean. The genus occurs from depths of at least 400 meters down to 900–1,000 meters.