Predaceous diving beetle larva, with hydra hitch-hikers

2010 October 30

At the beginning of June, we all went down to the little pond on Springbrook Lane looking for tadpoles to raise into frogs. We didn’t find any, but we did find some other stuff, including this rather aggressive-looking individual:

It’s almost an inch long, and is evidently the larva of one of the Predaceous Diving Beetles, family Dytiscidae. As you might guess from the mandibles, they are quite ferocious predators. This larva appears to have gills on the tip of its abdomen, suggesting it can extract oxygen directly from water. But at the same time, it tended to rest with its body curved and its abdomen tip up close to the surface, which would be consistent with it having an air snorkel. Although, it could be that there is just more oxygen dissolved in the water near the surface, and so it was trying to keep its gills up where there was the most oxygen.

It tended to flex and rear quite a bit, this probably gives it a good striking pose to snatch up whatever prey item might happen to come by (in this case, it looks like it might have had its eye on that midge larva floating near its head).

Those jaws are obviously no joke, and could probably handle a small minnow if necessary. And the eyes are interesting, it looks like they are a lot of individual eye elements that are almost, but not quite, organized into a full compound eye.

And . . . wait a minute. There’s something on its head. What is that?

It’s something stuck on with a stalk, and tentacles. It’s not a bug, it looks more like . . .

Ha! It’s a Hydra! I’ve heard of these, and I understand they are really common, but this is the first time I’ve ever seen one in the flesh. Mainly because I never actually went out looking for them, I suppose. They are related to jellyfish, coral, sea anemones, and other Cnidarians[1], but the hydras are one of only a very few cnidarians that have made the jump to living in fresh water. If you basically think of them as being tiny freshwater sea anemones, you won’t be far wrong.

Now, one hydra stuck on a beetle larva, as odd as that might be, could just be a coincidence. But looking a bit further, there is another one on the larva’s side.

And, zooming in on part of the very first picture on this page, I see there was even one that was free-floating (and may have been just knocked off of the beetle larva):

The high concentration of hydras on and around the beetle larva suggests several possibilities:

(1) There may just have been a heck of a lot of hydras in the water at that time, and they were stuck to every available surface;
(2) Hydras might have a specific affinity for sticking onto creatures that are going to crawl around. Hydras are generally nearly-immobile polyps (although they don’t attach permanently to surfaces, and can move slowly by a kind of somersaulting motion), so it might be to their advantage to ride around on other things to find new food sources; or
(3) A combination of (1) and (2).

And the beetle larva probably doesn’t care one way or another. The hydra isn’t big enough to hurt it, and the hydra’s food is smaller than the beetle larva’s food. So, they can get along together, so long as the hydras attach onto spots where the beetle can’t reach them and make a snack of them.

It is even possible that hydras could be of some small benefit to the beetle larva. They might catch and eat some of the water mites and other small parasites that could otherwise infest the larva.

[1] These used to be called Coelenterates, along with the Ctenophores (“comb jellies”) until it was (fairly recently) decided that the cnidarians and ctenophores weren’t actually as closely related as all that and should be considered separate phylums. Just in case anyone was under the delusion that taxonomy is rigid, static, and unchanging.

12 Responses
  1. October 30, 2010

    Nice observation! I’ve seen many Dytiscus larvae, but never one with Hydra attached. A few years ago I did find some Belosoma with freshwater limpets attached:

    Water Bug with Limpets Attached

  2. October 30, 2010

    This post is oozing awesome.

  3. Carole permalink
    October 30, 2010

    What an interesting world. Thank you for inviting us in.

  4. October 31, 2010

    Thanks, everybody! This one was a surprise to me, too. I didn’t see the hydras while taking the pictures, I only spotted them while prepping the photos for posting. I like Mike’s water bug with the limpets, and now I kind of wonder just how many different kinds of things are likely to attach to bugs in the water.

  5. November 1, 2010

    Fantastic photography. I’m jealous! The post made me wonder about the life cycles of the two creatures and how short they must be. I think your assertion that there must have been lots of hydras in the water is the best. Imagine how short a time they, floating through the water, have to find that particular beetle larvae. That several found him or her indicates they must have been pretty dense in the water.

  6. November 1, 2010

    As always, totally impressed by the photography!

  7. November 2, 2010

    Regarding the hydra lifecycle, on the one hand they do reproduce pretty fast (they bud off new hydras when food is plentiful), so they can get numerous quite quickly. On the other hand, they can live for quite a long time (until recently, it was believed that they were potentially immortal, although there is now some argument about this). So, they would have pretty much the whole summer to hang around and maybe hitch rides. Of course, their nervous system is so simple that, if it were any simpler, they wouldn’t *have* a nervous system, so it’s probably a safe bet that they aren’t going to do anything too complex.

  8. November 22, 2010

    Great childhood memories: I often had Dytiscus larvae and hydras in my aquarium together, probably because hydras are one of the few living things the larvae didn’t devour. No wonder you didn’t find tadpoles

  9. November 25, 2010

    Margarethe: I hadn’t thought of that, but you’re right – that probably *is* why we didn’t find any tadpoles. Last year, the same pond mainly had lots of damselfly nymphs, and was completely lacking in mosquito larvae. So, I guess the fauna in the pond probably varies quite a lot depending on which predator is there that year.

  10. November 25, 2010

    And an interesting note: if one does a search for “hydras in aquarium”, about half the sites returned seem to be people who want to try raising them in their aquariums, and the other half are people trying to get rid of them.

  11. February 24, 2011

    can sombody help me with my question. if

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