The sea has always been a mystery and source of many legends. Sea cows were believed to be magical mermaids and the unicorn leech, Spiculum natabundus, was subject of many stories as well.

Because of their shape (see picture above), sailors called these worm-like creatures with a stinger in the front end "Arrows of Neptune". They were thought to be living weapons shot by Neptune, the god of the sea, against his enemies. Due to their poisonous stinger sailors feared them and invented all kinds of terrifying stories about them, although it is most unusual for them to attack humans.

Besides this historical background, the unicorn leech serves marine biologists and neurobiologists nowadays as a valuable subject of research. The following is an overview about this fascinating creature, its anatomy and behavior and especially focuses on its neurological features.

The unicorn leech is a member of the phylum of annelids and a close relative to the common leech. Adult individuals are in average two to three inch long and half an inch in diameter, although individuals up to seven / eight inch in length have been recorded. Even over a foot long specimens are mentioned in tales about them, but their existence still needs to be proven.
The unicorn leech lives in medium to deep waters. When not foraging, it rests in a burrow, which it dug with its stinger.
The other times it swims around by moving its tail like an eel. It is attracted to possible prey due to its fish-like lateral line, recognizing waves typical for the swimming patterns of its favored prey. This is supported by the chemoreceptors all over the skin of the unicorn leech, recognizing urine and other chemicals released by its prey, most of the time small fishes.

After swimming closer and closer to the fish, the unicorn leech moves closer to the ventral side of its victim, then attaching itself with the suction caps on its back. Before the fish has a chance to do something against its new occupant, the unicorn leech bends its head down a little, then swings it upwards, thus penetrating the skin of its victim with its stinger.
As soon as the stinger is inside of the fish the unicorn leech starts injecting its typical mixture of paralyzing poison and digesting enzymes while steadily inserting the stinger more and more. When the stinger is progressed so far that the mouth on the lower side of the stinger touches the skin of the fish, the unicorn leech starts biting movements, thereby making an even bigger hole into the fish, through which it can bring its whole head inside of the fish.

When this happens, the stinger stops moving forward and rather moves in a circular pattern, thus distributing the steadily ejected venomous liquid into a bigger area. This ejection stops only, when the mouth is completely surrounded by digested fish-tissue as noticed by the taste-sensors all around the mouth-area. Then the animal starts to suck this nutritious fluid into its storage cavity. The sucking continues until it has consumed about five times its body mass, which is recognized by internal stretch receptors.

After that, the unicorn leech retreats from the dying fish and heads back to its burrow, where it stays in a state of quiescence for a period up to three months, living off the stored, pre-digested fish-meat. The only movement at this point is a slight beating of the tail to keep the blood flowing and moving the nutriment through the colon.