I read a cool review paper today. It was a 2012 Hydrobiologia paper – “Cercariae (Platyhelminthes: Trematoda) as neglected components of zooplankton communities in freshwater habitats.”
Morley (2012) argued that cercariae (cute little trematode larvae) have been neglected from zooplankton surveys for decades. That is just the saddest thing! Back in the day (early 1900s), people like Wesenberg-Lund considered cercariae to be important, but people rarely consider them part of the zooplankton community anymore. As a shout out to all those neglected cercariae, I’m going to tell you some cool stuff that Morley (2012) reviewed in the paper.
First, let us revel in the awesomeness that is/are zygocercous cercariae. The person who does the Parasite of the Day blog covered this before, so I recommend checking that out here and here. Instead of leaving the snail host single-file and braving the world alone, zygocercous cercariae join forces before leaving the snail. They attach their tails together and then venture out into the world in groups (holding hands!). Together, they wiggle enticingly until a fish (the next host) is duped into eating them. Evolution is pretty much the coolest thing ever.
Cercariae don’t eat (=lecithotrophic) because they are short-lived organisms whose only goal is to get from one host to the next. (They don’t have sex either.) They rely on their glycogen stores for energy, and when those stores are used up, they die. The time it takes to use up that energy varies by species, so some cercariae live for just a few hours, and others live for weeks!
There are quadrillions of bazillions of cercariae in the world. (Seriously.) Individual infected snails can shed hundreds to thousands of cercariae per day, depending on the trematode species. There are estimates that this results in 40 kg of cercariae per hectare per year in some ecosystems – for just one cercariae species (see the Kuris 2008 paper in Morley 2012)! These cercariae are so small (typically <1mm) that you can hardly see individuals with your naked eye, and yet they add up to be a huge chunk of the animal biomass in aquatic systems.
So, we have millions of juicy, energy-rich cercariae entering aquatic systems every day. Do all of them find their next hosts? Not all of them. You see, as they’re swimming around looking for their next hosts, they’re also getting gobbled up by other organisms like fish, dragonfly larvae, and symbiotic oligochaetes of snails. Cercariae are therefore important parts of aquatic foodwebs, and they really shouldn’t be ignored.
Why do they get ignored, anyway? Morley (2012) suggested that typical zooplankton sampling methods are probably too rough for soft-bodied cercariae. And even when cercariae aren’t destroyed by zooplankton nets, there aren’t many good identification keys for limnologists to use to identify them.
Limnologists and parasitologists, unite! End cercariae neglect! Rah, rah!
Morley, N.J. 2012. Cercariae (Platyhelminthes: Trematoda) as neglected components of zooplankton communities in freshwater habitats. Hydrobiologia 691: 7-19.