Information and facts about flatworms


We already learnt about the nudibranches, which a lot of people think are relatives to the flatworms, but they are actually a completely different species with very different roots. What they do have in common though is that the come in a very broad variety of families and can be as colorful as the nudies too.


So What Are Flatworms?

Flatworms are soft bodied invertebrates and over half of them live as parasites. Some of them are very dangerous for humans such as the tapeworms, or more specifically, schistosomes. Another part of the flatworm family live on land but the ones we want to focus on are those lighting up our underwater world with their weird looks and vibrant colors.

Cannibalistic Vampires

This subgroup is called Turbellaria and consists of about 4,500 species ranging from 1 > 600mm. Most of the underwater living ones are predators or scavengers and feed of pretty much everything they can hunt down. They extend their mouth and suck the juices from their prey, which can be corpses left by other predators too and if food is short they would even turn cannibalistic and feed on their own species.

Diffusion System

Another specialty of the flatworms is that they do not have any circulatory or respiratory organs which allows them to be as flat as they are in the first place. Oxygen and nutrients are moved through their body by diffusion. This is why the bigger ones (which we are able to see in the water as most species are only microscopic) mostly have leaf or ribbon like shapes with their guts extending through the whole body so the nutrients can reach all parts.


The diffusion system makes them very vulnerable to dehydration which is why the majority lives in sea or freshwater, in wet soil or as parasites within other animals. The flatworms need to keep the concentration of their body fluids at a fairly constant level. Mostly they adjust this to the same concentration as the environment they live in.

While they have a network of special cells to push out the waste fluids they to have another set of cells that try to process still usable resources from this water and also retain as much liquid as necessary to keep a constant level.

In and Out Through The Same Hole

Most flatworms only have one opening (mouth) to feed which is then also used to dump processed food. This means that most of them can unlike me not eat continuously but have to feed and then wait until all the food is processed through their network of guts extending through their body. However some exceptions – usually found in the bigger worms – do exist and they do have one or more anuses to get rid of the processed foods.

The Turbellaria mostly have one pair of a primitive form of little eyes known as ocelli. Again there are some exceptions having multiple pairs, clusters or even eyes evenly spaced around their bodies. The ocelli give them an information on which direction light is coming from which they then try to avoid.


A few groups have a fluid filled chamber with a solid particle that acts as balance and accelerator sensor (before your smartphone did!), however how they sense movements and position of these particle is unknown.

They have touch sensors scattered all over their body and usually also feature tentacles that are extra sensitive. The tentacles are thought to have special cells that are used as smell sensors.

Most of the time flatworms crawl on the bottom (preferably sand) by using their muscles in combination with their body fluids to apply more pressure to their body. Some of them can use the same system to even swim freely in the water which is one of the most amazing sightings.

A Fencing Dual With A Difference

Now how do the flatworms reproduce? Again there is very diverse strategies. The bigger species which are the ones we observe underwater mate by penis fencing. They are born as males but also have female reproductive cells.

The two males duel each other trying to impregnate each other while loser will adapt the female role and develops the eggs. This means it will have a higher demand on nutrients and lower chances of survival in the ocean.

In most species, when the eggs hatch “miniature males” emerge and go on their quest for reproduction.

Author: Tobias Ebnoether (PADI MSDT # 364457)

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