Did you guess correctly?…… This is a worm, not a roundworm like you would find in your garden, but a Flatworm. It belongs to a group with a long name, Platyhelminthes. In this group, there are around 18500 species of flatworms but only about 3000 of these are free-living, the rest are parasites, living on or in a host.
As the name suggests these animals are flat and very thin, less than 1mm thick. We do not see most of them because they are less than 2mm long. They are usually oval or leaf-shaped with ruffled edges and on our beaches in the shades of brown or black. They have a definite head end with a pair of eyespots on top and a mouth below. Their eye spots can only distinguish the direction from which light is coming to allow the flatworm to avoid it.
Being such a thin animal means that they are very susceptible to water loss, so they must live in moist environments. When looking for them at the beach they will be under rocks moving in the space between the rock and the moist sand beneath.
The majority of flatworms are carnivores, hunting small invertebrates or feeding on dead remains. Food is drawn through the mouth opening into an internal cavity where it is digested. Flatworms don’t have teeth to chew food, instead, they released enzymes onto their food that break it into smaller manageable pieces. Undigested food is spat out the mouth because there is no anus.
Being so thin means that they do not need to have specialised organs for breathing or circulation. All the cells of a flatworm are close enough to the surface of the animal that they can exchange oxygen and carbon dioxide directly with the water surrounding them. Nutrients from its food pass directly from the internal cavity to the cells of the flatworm, so no circulatory system is required.
The flatworms pictured here are a uniform brown colour but tropical species can be very brightly coloured. Some species have the same colours and patterns as nudibranchs, trying to mimic them, to take advantage of the nudibranch’s reputation with predators as an unpleasant meal.
Flatworms are hermaphrodites, containing both male and female reproductive organs. After fertilisation, they will lay an egg mass containing hundreds of eggs on a solid surface. The eggs hatch into free-swimming larvae which after a few weeks change into the adult form.
This video shows a flatworm at a local beach moving about unhappy that it was exposed to sunlight.
If you would like to learn more about flatworms and the huge diversity of life that can be found in the shallows along our shorelines please contact Education Director Mandy Robertson on firstname.lastname@example.org