Depth
Prostheceraeus · Flatworms

Pink Flatworm

Prostheceraeus roseus

0-40m
Depth Range
Variable
Max Size
Mediterranean
Distribution
0-40m
Depth Range
Variable
Max Size
Reef & Open
Habitat
Carnivore
Diet
Solitary
Social
LC
IUCN Status
Kingdom Animalia
Phylum Platyhelminthes
Genus Prostheceraeus
Species roseus

Flatworm

Identification & Biology

  • IdentificationA small, delicate marine flatworm with a thin, leaf-shaped body and vibrant pink to rose colouration with a fine white border around the edges. Moves with a graceful undulating motion across rock surfaces. Sometimes mistaken for a nudibranch.
  • SizeBody length up to 4 cm. Extremely thin and flat.
  • Social HabitSolitary. Glides smoothly over rock surfaces using cilia (tiny hair-like structures) on its underside. Can also swim with an undulating, ribbon-like motion when disturbed.
  • HabitatRocky reefs, under boulders, and on encrusted surfaces. Often found on or near colonial tunicates and bryozoans.
  • Depth2 to 40 m. Common on shallow to moderate rocky reefs.
  • Feeds OnColonial tunicates (sea squirts), bryozoans, and other small encrusting organisms. Feeds by covering its prey and secreting digestive enzymes.
  • DistributionMediterranean Sea and adjacent eastern Atlantic. Present in Maltese waters.
  • DescriptionProstheceraeus roseus is one of the most elegant small animals on Mediterranean reefs. Its vibrant pink body gliding across dark rock surfaces is a treat for observant divers. Flatworms are among the simplest bilateral animals, with no circulatory system, no respiratory organs, and a single opening that serves as both mouth and anus. Despite this simplicity, they are effective predators of colonial invertebrates.

Range

Habitat & Distribution

Found across the Mediterranean, inhabiting reef & open environments. Active from the surface down to 40m depth.

Where Malta divers have spotted this species

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Sightings are logged by divers using our What Did You See? dive-log tool. View the full live sightings map.

Detail

Preferred Environment

Reef & Open habitats, typically at depths of 0-40m. Most commonly encountered by divers at the shallower end of its range.

Encounter Tips

Best approached slowly and calmly. Avoid casting shadows directly over the animal. Neutral buoyancy and patience increase encounter success significantly.

Pink Flatworm species card

Biology

Behaviour & Diet

Feeding Strategy

Feeds on colonial ascidians and small encrusting invertebrates by extending its pharynx over prey and secreting digestive enzymes externally before absorbing the liquefied tissue.

External digester

Reproduction

A simultaneous hermaphrodite that reproduces through reciprocal mating. It lays flattened egg plates on the underside of rocks, where embryos develop directly without a larval stage.

Hermaphrodite

Behaviour & Defence

Swims away from threats with rapid undulations of its body margins, mimicking the movement of toxic nudibranchs. Its bright colouration may also serve as aposematic warning.

Undulating escape

Dive with diveshack

Where to Encounter in Malta

The following dive sites offer strong habitat match for Prostheceraeus roseus. All are accessible on a guided dive with diveshack.

5-30m
North Malta

Cirkewwa

Rocky reef with caves, arches and ledges hosting a rich diversity of reef species.

View dive site ›
5-50m
Gozo

Blue Hole

One of the Med's most famous reef dive sites. Rocky walls and archways covered in life.

View dive site ›
18-36m
South Malta

Um El Faroud

The wreck lies on a sandy bottom surrounded by rocky reef -- a diverse habitat attracting reef species.

View dive site ›
IUCN Red List Least Concern

Conservation Status

This species is assessed as Least Concern on the IUCN Red List. Mediterranean populations face ongoing pressure from fishing activity, habitat degradation, and climate-driven changes to prey availability.

Source: iNaturalist Guide #888
by Lesley Clements (CC BY-SA)
diveshack Mediterranean Marine Life guide