Flatworms:Minute Predators of the Rock Pool

One of the easiest ways to find the smallest animals that inhabit rock pools at the seaside is to collect fronds of red seaweed and examine them in a shallow dish of sea water under a low power microscope. I discovered this little flatworm this afternoon amongst the seaweed fronds from a rock pool on the Northumberland coast at Low Newton. It’s about two millimetres long and the images here were taken at magnifications of x40 (whole animal), x100 (head) and x400 (eye). Free-living flatworms, some tropical examples of which are over 30cm. long, belong to a very large phylum of animals called the Platyhelminthes that includes notorious parasites like liver flukes. These minute marine flatworms are predators on even smaller marine organisms. Their mouth is in the middle of the underside of the flattened body and opens into a highly branched digestive tract that you can see clearly as the dark brown network in the low power image of the whole animal (top). Flatworms are equipped with a pair of very simple eyes that are efficient at detecting the direction of light but don’t form images. They open via a ‘pinhole’ (just visible in the bottom photograph) and are lined with just a few light receptors attached to nerve endings. Flatworms glide around with the aid of thousands of rhythmically-beating cilia (see video) that cover a surface layer of large brick-like cells that you can see around the edge of the animal in the middle photograph. Freshwater flatworms that are very similar to this marine species, but are black rather than translucent, are present in every garden pond and can often be seen gliding around under the surface film.