Trematode Biomass = Beetle Biomass = True Bug Biomass = Odonate Biomass

One big interest in parasite ecology right now is the quantification of parasite biomass in ecosystems.  Ecologists have spent a lot of time looking at energy flow through ecosystems and quantifying how much of the total biomass in ecosystems can be found in each trophic level and taxonomic group.  Until recently though, no one had quantified parasite biomass.  But since parasites eat a little bit of just about everyone, they might add up to a big chunk of the total biomass, right?

The initial work regarding parasite biomass in ecosystems came from three California estuaries.  There are several cool papers that have already stemmed from that work, and if you’re interested, you should check out the publications from the UCSB Ecological Parasitology Lab.  Lately, the Johnson Lab has been tackling the parasite biomass question in ponds by studying trematode parasites, specifically.

Remember when I said that parasites eat a little bit of everyone?  Well, according to Preston et al. (2013), if you’re a first intermediate infected rams horn snail (genus Helisoma), you’re actually 25% trematode larvae, on average.  That’s NUTS.  That’s like a human having an entire leg made out of parasites.  Because ~30% of snails were first intermediate trematode infected in ponds, roughly 2-4% of the total biomass for that snail species was actually trematode parasites.  NEAT!

Overall, the dry trematode biomass in the ponds was ~0.1 g per meter squared.  That was roughly equal to the dry biomass of beetles..and the dry biomass of odonate larvae (dragonflies and damselflies)…and the dry biomass of hemipterans.  In other words, there were a lot of parasites!  Preston et al. (2013) estimated that ~0.1 g of cercariae (just one trematode stage) would be produced per meter squared per summer.  Like I mentioned in a previous post about how cercariae get neglected from inclusion in zooplankton studies, all those cercariae make a great food resource for hungry aquatic predators!


Larval trematode biomass is equal to that of odonate larvae in ponds. (I know what you’re thinking. You’re thinking that I spent <5 minutes making this picture. You are correct.)

Ok, one last neat thing.  Preston et al. (2013) found that there was a negative relationship between the size of the cercariae (which varies by trematode species) and the number of cercariae that were released by a snail within 24 hours.  Preston et al. (2013) weren’t the first to discover this, but I still think it’s a very cool trematode example of evolutionary trade-offs between producing many offspring and big (high quality) offspring.  If you’re going to produce relatively small cercariae, you can produce a lot of them.  But if you want to produce big, energy rich cercariae, you can only crank out a few per day.

So far, we only have data for systems where aquatic trematodes (in snails) are a huge component of the parasite community.  What ecosystem should we quantify parasite biomass in next?


Preston, D.L., S.A. Orlofske, J.P. Lambden,  P.T.J. Johnson. 2013. Biomass and productivity of trematode parasites in pond ecosystems. Journal of Animal Ecology.

Bazillions of Parasites in Your Pond: Delicious!

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.


Interview with a neglected Ribeiroia cercaria.

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.

Zygocercous cercariae making a wiggling wheel of deliciousness. Check out the “beaks!” Photo from here.

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. 


Interview, continued.

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.