Overview
The Three-lined Nudibranch (Pectenodoris trilineata) is a gem-like chromodorid sea slug that graces the shallow, sunlit coral reefs of the tropical Indo-West Pacific. Despite its miniature proportions—rarely exceeding half an inch in total length—this dazzling opisthobranch commands instant attention with its intense violet-magenta mantle accented by three luminous golden-yellow longitudinal racing stripes. It is famous in marine biology for an unusual behavioral display: it rapidly flaps its anterior mantle margins in a rhythmic, undulating flutter as it crawls.
Biology and Physical Characteristics
Pectenodoris trilineata features an elongate, oval body bordered by a wide, undulating mantle skirt. Its background coloration is a saturated violet-pink or magenta, traversed by three bold, golden-yellow stripes: a median stripe extending between the sensory rhinophores to the gills, flanked by two parallel submarginal lines. Its paired, lamellate rhinophores and posterior branchial gill plume are pure translucent white or pale violet with vibrant violet tips. Its mantle contains specialized mantle dermal formations (MDFs) that store concentrated spongian diterpenoid chemical defenses sequestered directly from its food.
Behavior and Ecology
A stenophagous spongivore, the three-lined nudibranch feeds exclusively on specific toxic, encrusting marine sponges growing on dead coral rock. Using a microscopic radula equipped with comb-like, denticulate teeth, it scrapes away sponge tissue while harmlessly concentrating the sponge’s secondary metabolites inside its own mantle edge glands. When a fish approaches, the nudibranch flaps its brightly colored mantle margins; this motion flashes its aposematic color warning, signaling noxious distaste and warding off predators without sustaining harm.
Conservation and Human Impact
Evaluated as Least Concern by the IUCN, Pectenodoris trilineata is common and widely distributed throughout the rich coral habitats of the Coral Triangle. However, like all obligate coral reef invertebrates, it is acutely susceptible to coral bleaching, ocean acidification, and sedimentation caused by coastal development. Preserving marine protected areas and maintaining resilient coral reef structures guarantees the persistence of this vibrant micro-carnivore.
Interesting Factoid
The Three-lined Nudibranch performs a tiny ‘wave’ as it walks: it rhythmically flaps the front edges of its bright magenta mantle up and down like miniature butterfly wings to warn passing fish that it tastes disgusting!
Last updated: October 2026