Nuidbranches and Flat Worms

So last week we looked at Trigger Fish in our big 5 count down..this week we are celebrating the mighty Nuidbranches and Flat Worms!

Completely different animals, about as close relatives as humans and sea urchins (or even farther apart!) and both equally amazing so I had to include both! The more I have read up on these gems the more determined I have become to seek them out whilst diving!

So what is a Nudibranch you may be wondering? Well in fact they are actually sea slugs that are members of the class Gastropoda in the Phylum Mollusca, which also includes snails, slugs, limpets and sea hairs. All nudibranchs are sea slugs, but not all sea slugs are nudibranchs. They can be as small as 0.5cm and can grow up to about 60cm. They are beautifully coloured little creatures. They are shell-less and boneless.

Over 3,000 species of nudibranchs exist, and most live in shallow, tropical waters. They can be anywhere from a quarter of an inch to 12 inches long and can weigh up to 3.3 pounds. These sea slugs spend their time sliding on their bellies around their habitat in search of snacks. The animals have a set of curved teeth, which they use to eat coral, sponges, and fish eggs off the ocean floor. Nudibranchs use tentacles on their heads to poke around for grub. The nudibranch’s meals don’t just satisfy its hunger—the food also gives the animal its colouring. When the sea slug eats, it absorbs and displays its prey’s pigment—the substance that gives the prey its colour. Some nudibranchs also absorb toxins from certain prey and secrete the poison from their own skin. This allows them to fend off predators such as fish.

So the Nudibranch is stunning, resourceful, and it recycles? This slug sounds far from sluggish! flatworm

FUN FACT! It has been known that some humans eat these creatures, although when the experience is likened to "chewing an eraser" it's not high on the list of delicacies I would like to try!

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 they come in a very broad variety of families and can be as colourful as the nudies too. 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 colours

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. 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.

Now how do the flatworms reproduce? Again there are some 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 the loser will adapt the female role and develop 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.

You can easily tell which is which by observing three simple characteristics:
1- Flatworms are "flat", thin, while nudibranchs show some more thickness
2- Nudibranchs have their main sensory organs, the rhinophores (the two "antennae" on the head)
3- Flatworms move a lot faster than nudibranchs.

Another important aspect of flatworms is they have no circulatory nor respiratory systems... so there are no gills! Pretty cool stuff! You can see them everywhere around Koh Tao but keep your eyes peeled for flat worms swimming, it is truly a memorizing experience.