Marine flatworms and sea slugs are so often confused that it is worth stating clearly at the start of any field guide: they are not the same kind of animal. They are not even the same kind of animal in a broad sense. Sea slugs are molluscs, closely related to snails and octopuses. Marine flatworms are members of the phylum Platyhelminthes, an entirely separate branch of the animal kingdom that split off from the ancestors of molluscs somewhere around six hundred million years ago. The similarity between the two groups is a textbook case of convergent evolution: two very distantly related lineages have arrived at similar answers to the same problem — how to live on a reef as a soft-bodied, chemically defended, colourful macro invertebrate — by independent routes.
The differences between flatworms and sea slugs are fundamental. Flatworms have no coelom (the fluid-filled body cavity present in most animal phyla), no gills, no heart, and no shell to lose. Their paper-thin body is so slim that no part of the animal is more than a fraction of a millimetre from the surrounding seawater, and gas exchange happens by simple diffusion across the body wall. Sea slugs, by contrast, are anatomically much more complex, with a well-developed body cavity, specialised gills or cerata for respiration, and internal organs organised in ways that reflect their gastropod ancestry.
The two groups also move differently, and this is often the fastest way to tell them apart underwater. Flatworms glide across the substrate using millions of microscopic cilia on their underside, beating in coordinated waves against a thin layer of secreted mucus. The result is a smooth, undulation-free motion, as if the animal were floating just above the surface. Sea slugs, by contrast, crawl on a muscular foot, and their body visibly ripples as the muscular waves pass along it. Once you have seen both, the difference is unmistakable.
Feeding differs too. Sea slugs use a radula — a ribbon of microscopic teeth — to scrape or pierce their food. Marine flatworms have no radula. Instead, they use a muscular pharynx that can be everted through the mouth on the underside of the body, fanned out across the prey, and used to secrete digestive enzymes externally. The partially digested food is then absorbed through the pharynx into a branched intestine that distributes nutrients throughout the body without any need for a circulatory system.
Even reproduction is different. Both groups are simultaneous hermaphrodites, but marine flatworms are famous for a courtship behaviour known as penis fencing: two flatworms engage in a literal fencing match with their stylet-tipped penises, each trying to inseminate the other while avoiding being inseminated in return. Sea slug mating is far more sedate, with reciprocal sperm exchange during a coordinated pairing.
For the diver, the practical upshot is straightforward. If it glides smoothly across the reef, has a paper-thin ruffled body, and shows no visible gills or cerata on its back, you are almost certainly looking at a marine flatworm. If it crawls with visible muscular waves and carries a plume of gills or a cluster of cerata on its dorsum, it is a sea slug. Knowing which is which is the first step toward accurate species identification.
