Scientific Name: Berthella martensi
Common Name: Martens’ Side-Gilled Slug

Adult Size: 20 – 60 mm
Depth Range: 1 – 25 m; most often in shallow, sheltered reefs and under coral slabs
Biotope / Habitat: Under rocks, rubble, and dead coral covered with filamentous algae; active mainly at night
Diet: Grazes on encrusting sponges and thin layers of cyanobacteria; stores noxious acids in skin glands as chemical defence
Reproduction: Simultaneous hermaphrodite; partners align head-to-tail, then lay a broad, gelatinous, cream-white ribbon of eggs on the underside of rocks. Veliger larvae drift in the plankton before settling.
Fun Fact: When disturbed, B. martensi emits a milky-white acidic secretion that smells like citrus—effective at deterring curious fish and even persistent photographers!

Flat, oval, and translucent amber to brown, Berthella martensi hides by day and forages by night. A single plume-like gill is tucked along its right side (hence “side-gilled”), and a thin internal shell can be seen through its mantle—two key features that distinguish it from true nudibranchs.