Halgerda labyrinthus Donohoo & Gosliner, 2023

Location: Horse Shoes, Okinawa Island (Onna and Yomitan area), Okinawa, Japan
Date: 2017/02/15
Length: 120mm
Depth: 25.0m
Water temperature: 21.0℃

Description

A large dorid reaching 84–95 mm in length. The dorsum is bluish white, marked by thin black lines interspersed with yellow lines that interconnect to form a continuous, maze-like pattern. Low yellow to orange ridging runs across the back, and a yellow submarginal band rims the mantle, accompanied by perpendicular black and yellow lines along the edge. The gill consists of four tripinnate branchial leaves, each further divided into six large branches; the leaves are translucent yellow with numerous small black spots. The rhinophores are tapered, translucent white with a posterior black stripe, lateral stripes, and small black spots.

Distribution

The type locality is Horseshoe Cliffs, Onna Village, Okinawa Island, Ryukyu Islands, Japan, at 51.8 m depth. Additional records come from the Kerama Islands, the Izu Peninsula, and Taiwan. Found on steep reef faces and coral rubble between 20 and 55 m.

Etymology

The specific epithet labyrinthus is Latin for "labyrinth," referring to the maze-like pattern formed by the numerous fine lines decorating the dorsum.

Remarks

Specimens of this species were previously misidentified as Halgerda willeyi or treated as an unidentified Halgerda sp. before Donohoo & Gosliner 2023 recognised it as a distinct species through molecular phylogenetics. It is similar in pattern to Halgerda willeyi (western Pacific) and Halgerda paulayi (Red Sea) but is geographically isolated in the Ryukyu region and Taiwan, and is distinguished from both by external and genetic differences. Compared to the sympatric Halgerda batangas (max 40 mm), H. labyrinthus is much larger and shows continuous black lines across the dorsum that H. batangas lacks.