Mud dragons (Phylum Kinorhyncha)
Mud dragons are miniscule, worm-like marine invertebrates that are small enough to live between grains of sand and gravel on the seafloor, rarely reach 1 mm in length. Some extinct species from the Cambrian Period were much larger.
They are elongate, have a retractable feeding apparatus (an introvert) at the head end, a trunk with exactly 11 body segments when mature, and many spine-like appendages especially around the introvert. The body is often naturally curved. When the introvert is retracted a series of plate-like shields cover the opening.
Mud dragons move through sediment by anchoring the head using its backward-pointing spines, contracting the body to bring it forwards, then pushing the head forward to anchor again.
Mud dragons form part of the rich ecosystem in sea floor sediments, eating single-celled organisms and other tiny morsels and in turn being eaten by worms, nematodes and other larger organisms. They are likely to play an important ecological role where they occur, helping recycle nutrients and grazing detritus and micro-organisms.
Australian mud dragons have been collected around much of the coastline, in both tropical and temperate waters. One species, Echinoderes cavernus, is only known from Jim's Cave off the NSW coast. It is possible this cave-dwelling species is more closely related to ancient deep-sea mud dragons than other shallow-water species, suggesting a deep evolutionary lineage. Other animals found in the same rich cave ecosystem include roundworms, gastrotrichs, bristle-worms, water bears, various crustaceans, shell-less molluscs, and loriciferans.
Mud dragons are poorly collected throughout the world, including in Australian waters. Of 180 species known worldwide, only 8 have been reported in Australia. It is hard to know whether this scarcity reflects low abundance or low sampling opportunity. Elsewhere in the world species have been collected from abyssal depths to 5000m, and it is likely that they also occur in deep waters in Australia.
Thanks to Robin Hare for helping prepare this article.