Opechona olssoni

Jellyfish are one of the most common inhabitants of the sea and while they are soft and squishy, their venomous stinging cells (call cnidocytes) can act as a deterrent for many potential predators. But not all predator though - jellies are on the menu for a wide range of different marine animals including sea turtles and various species of fish - this is a fact which has been exploited by some species of parasitic flukes (digenean trematodes) as a mean of completing their life-cycles.

Clockwise from top left: Fluke larvae embedded in sea jelly tissue, close up of fluke larvae, Opechona olssoni from the gut of a butterfish, a juvenile butterfish collected from the Seto Inland Sea. Image modified from Figure 2, 9, 10 of the paper

Flukes have a rather complicated life-cycle which involves multiple different hosts. While the adult stage lives in vertebrate animals, they lay eggs that hatch into larvae which infect a "first intermediate host" (usually some kind of snail) where the parasite undergo asexual proliferation, producing a different type of clonal larval stages that infect a "second intermediate host" which serves as a vehicle to reach the vertebrate host. This is usually accomplished through the vertebrate host eating the second intermediate host. For some species of flukes, such as Opechona olssoni, the second intermediate hosts are sea jellies, and it is not alone - some of its fellow travellers include Leptotrema clavatum and Cephalolepidapedon saba - all flukes from the Lepocreadiidae family which is commonly known to use gelatinous animals as their second host.

In this study, a team of researchers in Japan set out to examine the seasonal pattern in the abundance of parasitic fluke larvae found in some of the jellyfish inhabiting the Seto Inland Sea. Over the course of two and a half years from March 2010 to September 2012, they collected three different species of jellyfish: the moon jelly (Aurelia aurita), the Japanese sea nettle (Chrysaora pacifica). and the ghost jellyfish (Cyanae nozakii).

They noticed that while the fluke larvae were consistent present in the jellyfishes for most the year, their abundance varied over different seasons. Sea jellies were most heavily infected during the middle of the year when it is the warmest. Since the free-swimming clonal larvae of digenean fluke use temperature as their trigger to leave the first intermediate host, when things start heating up, they come streaming out of their host in large numbers.

Out of the three different species jellyfish species they sampled, the ghost jellyfish was the host with the most. While the moon jelly and the sea nettle on average harboured about 28 and 8 fluke cysts respectively, the ghost jellyfish had on average almost 150 fluke larvae, with some individuals carrying over 400 cysts in their tissue. But why are the ghost jellyfish so much more heavily infected than the other jellies? It is only slightly bigger than those other species, so size alone cannot be the answer.

Well, the ghost jellyfish is also known to prey on other gelatinous animals, and by doing so it could have been accumulating an impressive collection of fluke larvae second-hand (so to speak). Due to its jelly-eating ways, the ghost jellyfish earns itself the dubious distinction of serving as a hub for all these flukes to come together as they wait for a final host to come along and gobble them up.

But if the sea jellies are the second intermediate host for O. olssoni and friends, then what are the final hosts where the parasites mature into their adult form? Flukes from the Lepocreadiidae usually use fish as their final host, so the researchers also collected some fish species which are known to either associated with and/or have jellyfish on their menus, since they are the most likely final host for those parasites. They examined their gut content for bits of half-digested jellyfish and the adult form of the jelly-infecting fluke species.

One of the fish which they found to harbour maturing and/or adult flukes is the Japanese butterfish. Juvenile butterfish are known to hangout around the tentacles of jellies, using them for food and shelter, taking a few nibbles here and there whenever they get peckish.  Alongside the butterfish, another species that hosts those flukes is a filefish known as the black scraper. It eats a variety of different marine invertebrates, but is known to partake in some jellies now and then.

For these small fish, hanging out around jellyfishes is a pretty good deal - the curtain of tentacles act as a shelter from their predators, and the shelter itself is edible too. But because the jellyfish's tissue is also laced with fluke larvae, it means that every mouthful of jellies those fish swallow also goes down with a bunch of parasite larvae. As with everything in life, there is always a catch

Reference:
Kondo, Y. et al. (2016). Seasonal changes in infection with trematode species utilizing jellyfish as hosts: evidence of transmission to definitive host fish via medusivory. Parasite, 23: 16

P.S. Some of you might know through my activities on Twitter (@The_Episiarch) that when I'm not writing these blog posts about new scientific papers about various parasites, I also do illustrations. Some of my drawings are about and/or inspired by parasites, however some may find my more recent parasite-inspired pieces to be slightly unusual - I am of course talking about Parasite Monster Girls...