Invertebrates
Marine Flatworm
Polycladida · also called polyclad flatworms
Gliding across a coral reef in a wave of vivid colour, a marine flatworm looks close enough to a nudibranch that even an experienced diver might need a second look, despite the two animals sharing no real evolutionary relationship at all.
Overview
The marine flatworm, order Polycladida, is a vividly coloured, extremely thin-bodied invertebrate found across reef and coastal habitat worldwide. Its complete lack of a circulatory or respiratory system, combined with striking colouration that convincingly mimics unrelated nudibranchs, make it one of the more visually deceptive and anatomically minimalist animals found on any reef.
A body simple enough to skip organs entirely
By staying thin enough for diffusion alone to handle oxygen and nutrient exchange, the marine flatworm has sidestepped the need for a circulatory or respiratory system altogether, proving just how far a genuinely simple body plan can still be pushed.
Status
Not Evaluated — common across suitable reef and coastal habitat worldwide, with no documented widespread decline.
Marine Flatworm: common questions
Why is the marine flatworm sometimes confused with a nudibranch?
A marine flatworm often displays vivid, boldly patterned colouration similar in general appearance to a nudibranch, a case of convergent evolution in which two unrelated groups of soft-bodied marine animals independently arrived at similarly bright warning colouration, making the two genuinely easy to confuse at a glance despite belonging to entirely different animal phyla.
How does a marine flatworm breathe without lungs or gills?
A marine flatworm has no circulatory or respiratory system at all, relying entirely on diffusion across its extremely thin, flattened body to exchange oxygen and nutrients directly with the surrounding water, a simple but effective solution that works specifically because the body is thin enough for diffusion alone to reach every cell.
Can a marine flatworm really regrow from a piece of itself?
Yes, some marine flatworm species are capable of regenerating a complete individual from a comparatively small fragment of the original body, a remarkable regenerative ability shared with several other flatworm relatives, including freshwater planarians.
Where does the marine flatworm live?
Marine flatworm species are found across tropical and temperate reef and rocky coastal habitat worldwide, generally gliding along surfaces using cilia or occasionally swimming short distances with an undulating body motion.