Double-margined Flatworm

Overview

The Double-margined Flatworm (Pseudoceros bimarginatus) is a striking marine flatworm known for its vibrant, warning-like coloration. It is a conspicuous inhabitant of tropical reef environments, often observed moving across coral rubble during the day.

Distribution & habitat

This species is widely distributed across the Indo-Pacific region, including the Red Sea, Indian Ocean, and parts of the Western Pacific such as Australia, Indonesia, and the Philippines. It is typically found in shallow reef-associated waters, including lagoons and reef crests, often sheltering under rubble or rocks.

Appearance

This flatworm features an elongated, thin, oval-shaped body with a base color ranging from cream-white to yellowish or brownish. Its most distinctive characteristic is its complex marginal banding, which typically includes narrow black and orange bands, often finished with a yellow or white rim. A faint, narrow white line often runs medially down the back, starting behind the cerebral eyespot. It can be easily confused with similar species, such as Pseudoceros confusus, which may possess slightly different band arrangements.

Biology & behavior

Pseudoceros bimarginatus is generally solitary and is considered a rare find in many survey areas. Its brilliant color pattern is believed to be aposematic, serving as a warning to potential predators of its toxicity or unpalatability. As a carnivore, it feeds on organisms such as sea squirts and bryozoans.