They say "if you've got it, flaunt it". And this marine flatworm certainly has it ...

Scuba diver instructor Gregor Irvine was lucky enough to film this ribbon-like creature cruising in clear waters around Indonesia's Lembeh Strait late last year. Flatworms are soft-bodied invertebrates that are found pretty much anywhere there is moisture. With no body cavity or specialised respiratory or circulatory organs, flatworms rely on diffusion to acquire their oxygen. Their flattened shape helps allow more nutrients and oxygen to pass through their bodies.

Flatworms are a diverse group. There are over 20,000 known species, from tapeworms that can infect humans to planarians capable of regrowing their entire bodies should they be unlucky enough to get chopped in half. The skilled swimmer in this clip belongs to the genus Pseudobiceros. Like other marine flatworms, it can propel itself through the water by undulating the edges of its body in a wave-like motion. 

Although they lack a respiratory or circulatory system, flatworms do have a digestive tract of sorts. Larger species have a branch-like gut that allows nutrients to be dispersed throughout their bodies. Most species lack an anus, so excreting undigested food is done through the mouth (unless you're a long flatworm, in which case you may be gifted with multiple anuses).

Most flatworms are hermaphroditic, meaning each individual possesses functional reproductive organs of both sexes. Which brings us to penis fencing. For some species of flatworm, mating involves a strange wrestling match. The flatworms rear up with their two-headed, dagger-like penises exposed and attempt to forcefully pierce each other's epidermis in order to inject sperm. 

Motherhood is physically demanding, so the goal for flatworms is to live the bachelor life for as long as possible. The worm that gets stabbed in these bouts of penis fencing will go on to produce young, while the victor can continue to do male flatworm stuff (we're guessing all-night reef parties). In some cases, both individuals will be injected with sperm. And then there are those species that do away with a mate altogether and simply reproduce asexually. 

Still, they look pretty when they swim right?

Header image: Jens Petersen