I give you the mud dragon…

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Well, it’s not much of a dragon, but it does live in mud. These animals, correctly known as kinorhynchs, are miniscule, usually much less than a millimetre long, but under the gaze of a microscope they are quite appealing little things.

The specimen posing in the photo below was found in a kelp holdfast taken from the rocky shore in southwest Wales. These micro-habitats are seething with animal life, but you could say this about just anywhere in the ocean where there are sediments. The animals that live on and between these sediment grains are known as meiofauna and they’re bewilderingly diverse. It is in these marine sediments where you find the most animal lineages, many of which are found nowhere else, e.g. the Kinorhyncha.

All you need to do to see the incredible diversity of animals in these habitats is to take a handful of marine mud or sand and shake it with water for a bit. Then, let the sediment settle and all the animals will be swirling around in the water. Finally, pour the water through a very fine filter and observe the miniature menagerie with a microscope.

What we know about the kinorhynchs and the other interstitial creatures could be written on their tiny undersides. For the most part they live out their lives out of sight and out of mind, but I’d quite happily wager that the finer details of what they get up to in the sediment is as fascinating as their appearance.

Below are two photographs of another kinorhynch collected by Martin V. Sørensen from sediment on the bottom of Ikka Fjord, Greenland. This is one of the larger kinorhynch species, but it is still much less than 1 mm long. The top photo is a dark-field image, while the second was taken with a normal microscope setting. The weight of the microscope slide on the body of the animal has squeezed out the introvert (right). This part of a kinorhynch’s body can be forced out under fluid pressure and retracted by muscles. It bears characteristic sensory structures (scalids – curved backwards in this image) and the stylets of the mouth cone. In the lower image the cuticular plates of the body are very obvious as are the contents of the gut that have been squeezed out of the mouth by the weight of the cover-slip.