Flatworm

Finding marine flatworms requires a keen eye for macro life around sheltered reef zones. Divers should inspect shallow coral rubble, shaded ledges, coastal bays, and quiet lagoon slopes where encrusting organisms and ascidians thrive. Flatworms are often seen creeping smoothly over substrate or resting in rubble patches.

It is vital to distinguish flatworms from nudibranchs (sea slugs). Flatworms are paper-thin and flat, possessing pseudotentacles formed by folded front margins, whereas nudibranchs have distinct sensory rhinophores and tufted external gills on their backs.

Because flatworms have soft, unsegmented bodies without protective shells, strict approach etiquette is required. Maintain excellent buoyancy and keep fins well elevated to avoid stirring up sand or creating water turbulence that could sweep these delicate creatures away. Never touch, handle, or attempt to lift a flatworm. If you witness a flatworm swimming or engaged in penis fencing, stay still at a respectful distance and avoid shining intense dive lights directly on them for prolonged periods.

Because marine flatworms reside in tropical and subtropical waters across the Indo-Pacific and Indian Ocean, encounters are generally possible year-round without strict seasonal restrictions. Their presence is dictated more by localized environmental conditions than specific calendar months.

Flatworms strongly prefer calm, sheltered aquatic environments such as shallow lagoons, sheltered back reefs, and protected coastal bays. Severe wave exposure and strong surge disrupt their ability to crawl across substrate or swim effectively, so diving during calm sea conditions yields the best observations.

When planning dives to search for macro flatworms, target periods of minimal water movement, such as slacks between tides or neap tides. Milder currents make searching through coral rubble and along reef ledges significantly easier, while also increasing the likelihood of observing flatworms actively swimming across open patches between reef structures.

Free-living marine flatworms inhabit a wide geographical range across the tropical Indo-Pacific and Western Indian Ocean. Documented populations occur off Kenya, Indonesia, Malaysia, Thailand, Myanmar, the Philippines, Palau, Micronesia, the Northern Mariana Islands, the Solomon Islands, and Australia.

Their primary habitats include shallow back reefs, coastal bays, sheltered lagoons, and areas covered with coral rubble and ledges. Microscopic species, such as *Macrostomum lignano*, live as meiofauna interstitially within the upper 5 to 10 millimetres of moist sand in intertidal zones and protected beaches.

Marine polyclads typically occupy shallow reef zones where light supports vibrant benthic communities. They are commonly found crawling over coral debris, hidden under rock overhangs, or moving across surfaces encrusted with ascidians and sponges.

Free-living marine flatworms are active carnivorous predators and scavengers. Larger polyclad species, such as *Pseudobiceros bedfordi*, feed primarily on ascidians (sea squirts) and small crustaceans that are small enough to be swallowed whole. Microscopic interstitial species feed mainly on diatoms, small invertebrates, and occasionally eggs.

Flatworm anatomy features a digestive cavity with only a single opening for both nutrient ingestion and waste egestion. Because of this structure, food cannot be processed continuously; intake and waste removal must occur through the same opening.

To feed, flatworms use a muscular pharynx that can extend to capture or ingest prey items. Foraging typically occurs as flatworms glide across the substrate, using chemical sensory cues to locate sessile prey like ascidians or small invertebrates sheltering in rubble.