Dicyema japonicum

On this blog, we like to revel in the obscure and bizarre, and it doesn't get much more obscure and bizarre than dicyemids - a group of parasites/symbionts which live exclusively in the kidneys of cephalopods like octopus, squid, and cuttlefish. But aside from its unusual habitat, the exact nature of dicyemids themselves has been a vexing mystery. Evolution has stripped them down to the bare minimum, they are bizarre animals with bodies that are composed only of 8-40 cells, with a significantly reduced genome. This also makes classifying them difficult, because they just don't have many of the usual features that scientists would use to work out an organism's evolutionary origin. There is some evidence to indicate that they might be relatives of flatworms (but perhaps not?), while other studies point to the so-called "jawed worms" as their relatives.

Left: Stained micrograph of Dicyema japnonicum, Right: East Asian Common octopus
Dicyema and Octopus photos taken by Dr. Hidedaka Furuya, used under Creative Commons (CC BY-SA 4.0 and 2.5) license

While dicyemids have very simple bodies, their life cycle is rather complicated. They have worm-like "vermiform" stages that live in the host's kidneys, and proliferate by giving birth to clones of themselves. Once their population reaches above a critical threshold, they get frisky and switch to sexual reproduction, producing stubby, ciliate-covered larvae called "infusoriform" that swim out to sea in search of a new host. Because of the different roles those stages play in the parasite's life cycle, they also exhibit very different behaviours.

A group of researchers in Japan investigated the behaviour of dicyemids from the East Asian common octopus (Octopus sinensis), a well-known and commercially-fished octopus species. Using octopuses obtained from commercial catches, the researchers were able to isolate both the vermiform and infusoriform stages of the parasite, and put them through a series of tests. The parasites were exposed to various different stimuli including light, drops of octopus fluids, pieces of coconut gel to mimic the interior of an octopus' kidneys, and flowing artificial sea water in a serological pippette to simulate the flow of urine through the kidneys.

They found that the wormy vermiform stages responded to flowing water by swimming against the current, like fish in a stream, and upon contact with the coconut gel, they stick their mushroom-shaped front end (called a "calotte") into the gel and hung on tight, as they would to the walls of an octopus' kidneys. In contrast, the infusoriform stage seems to swim around in a random manner, and tend to ignore or stayed away from the coconut gel. And even though they are supposed to go and infect a new host, they were not particularly attracted to any of the octopus fluids that the researchers presented them with, in fact, they were actually repelled by octopus blood.

The behavioural contrast between the two stages reflects their respective role in the parasite's life cycle. The vermiform is the reproductive stage that exists to populate the host's kidneys and give birth to the next generation. There is a lot of urine flowing through that part of the body, so if you actually want to stay there, you need to either keep swimming in place, or hang on tight. Because if you just go with the flow, you'll be on a one-way journey to the outside world. That's why in addition to swimming against the flow, when the vermiform stage touches something that feels like the interior of an octopus' kidney, it clings on for dear life. 

In contrast, the infusoriform is the dispersal stage which is meant to leave the octopus, so just going with the flow is a good way to achieve that end. But if their goal is to reach an octopus, why do they have an aversion to their host's blood? The researchers behind this study suggested that free flowing blood might be a sign of an injured or dying host which is not worth entering. This also tells us that these tiny parasites don't reach the kidneys through entering the host's circulatory system. 

But if that's the case, how do they reach the octopus' kidneys then? Also, if they are so indifferent to the scent of host material in the water, how do they even find their hosts? This is particularly puzzling as dicyemids are very picky about the host species they infect. But just as puzzling is how they are able to sense changes in their surrounding environment in the first place, since no sensory organs have been identified on these parasites. Dicyemids may seem like simple animals, but there's clearly a lot more to them than meets the eye.

Reference:

Hisayama, N., Takeuchi, Y., & Furuya, H. (2024). Taxes of Dicyemids (Phylum Dicyemida). Journal of Parasitology 110: 506-515.