Today’s amazing facts from Twitter: marine mites and woodlouse flies

September 13, 2013 • 4:22 am

by Matthew Cobb

Last night I and a group of arthropodologists (?) had a brief chat of Twitter, starting with Chris Buddle asking that fascinating question about why there are no insects in the sea, which we’ve explored here (NB the comments are good, as usual). Then the really interesting stuff took off, which led me to make two discoveries: there are mites living in the sea, and there are flies that parasitise crustaceans!

Here’s the exchange following Chris’s enquiry, and below there’s some science, all of which I discovered in the last few hours!


So – first with the marine mites, the Halacaroidea. Why am I so amazed by this? Well the basic story of arthropod evolution is that from around 400 million years ago (very roughly), a number of arthropods made it onto the land. These were myriapods (the ancestors of today’s millipedes and centipedes), insects (which we now know are really just an odd kind of crustacean), chelicerates (spiders, harvestmen, mites and scorpions)and . The sea was left to most of the crustaceans and some chelicerates (‘sea scorpions’ – now extinct, ‘sea spiders’ and horseshoe ‘crabs’; the scare quotes are there because they are not scorpions, spiders or crabs…)

There are lots of reasons why there may be no insects in the sea (which was the question that started the conversation), all of which may be true, but one probable answer is that the sea is full of crustaceans, so any insect going back into the sea would found itself either outcompeted, or eaten or both. Never mind the fact that the insect would have to reverse engineer its respiration and physiology to cope with the high saline levels etc.

So that’s why I was so amazed when Wayne just threw in the Halacaroidea. These chelicerates must have gone back into the sea. And they don’t just bumble around at the edge of the shore, or in brackish tidal pools, like some insects. They can live in the abyssal depths. This 1967 paper recorded marine mites from 3680-4100 metres down in the Pacific. This group of mites can be found on land, on the sea edge, and way way down and has varied life styles – some eat plants, some are predators, others just eat crap.

But the big question is, how do they breathe if they live in the sea? Well the answer seems to be – like their chelicerate cousins, the ‘sea spiders’ or pycnogonids. They simply absorb oxygen through their body wall. But while pycnogonids do this by having a massive surface area to volume ratio (they are basically all legs), the Halacaroidea are simply very small (a millimeter or so).

Here’s a picture of one of these bottom-dwelling mites, from here.

What I haven’t been able to find out from any molecular phylogenies is when these mites went back into the sea… And as to why, that is anyone’s guess. Maybe they just fell in, didn’t get eaten, and then carried on…

So the second question we raised was whether there were any insects that parasitised or preyed upon their crustacean brethren. My point was that there aren’t many terrestrial crustaceans (basically woodlice and some weird crabs). But of course, if there’s an opportunity to eat, then evolution finds a way. And while you might think there’s not much meat on a woodlouse, it looks rather different if you are a fly.

As Morgan Jackson pointed out, rhinophorid flies are parasitoids of woodlice (amongst other things, including spiders and snails, but not puppy d*g tails). Their rather catholic appetite suggests that woodlice simply became a target of the flies when they were rummaging around on the floor and under stones looking for prey to lay their eggs in.

A lot of these flies look like their blowfly relatives (generic dark flies), but this one (don’t know the species) is quite pretty (from here):

I haven’t been able to find any gruesome pictures of a woodlouse seething with rhinophorid maggots, but someone may be able to find one.

And finally, as Morgan pointed out, the great biologist Hampton Carson discovered a Drosophilid that lives on Christmas Island that lays its eggs on terrestrial crabs, including the monstrous robber crab, which is the biggest terrestrial crab. Here’s old-style Attenborough on the crab but not the fly (apologies to aspect-ratio fiends who will hate this):

21 thoughts on “Today’s amazing facts from Twitter: marine mites and woodlouse flies

  1. I am wondering if the mite shown has hemoglobin to help retain oxygen. Hemoglobin is used some in other arthropods for the same reason. Daphnia comes to mind.

    1. Vertebrates use haemoglobin as a transport chemcal to get oxygen from their respiratory organs to their body cells. Crustaceans on the other hand use a copper-based chemical (“haemocyanin”? ; I’m on a tablet, and it’s not so easy to shoot out to Google) which is notorious for changing between shades of blue as it binds/ releases the oxygen.
      Given that background, I don’t know what mites (spiders and scorpions too, probably) use for an oxygen transporting compound, but it’s a pretty safe bet that (1) there is one ; (2) it has a transition metal atom in multiple coordination in a “haem-” structure (is it 4-fold tetrahedral coordination, or 6-fold octahedral coordination? Or is that the oxygen uptake/ release process?

  2. Good science for the day.

    Or perhaps a good day for science: Pioneer has (NASA officially) left the Solar System and entered the interstellar medium. [Well, it did back in April -12. The science of it was complicated. But interesting.]

    And to get back to the arthropods that we can’t do without:

    Also today is an interesting result in Current Biology on what the authors claim is the first estimate of evolution rates, of the representative arthropod clade (then 40 % of species), under the Cambrian Explosion:

    “The 4- and 5.5-fold increases in phenotypic and molecular evolutionary rates respectively provide quantitative support for the widespread view that evolutionary rates were elevated during the Cambrian explosion. Notably, both the patterns and magnitude of the rate elevations are strikingly similar for two very different suites of characters (one set dominated by anatomical traits, the other consisting of protein-coding nuclear genes).”

    “Rather, the pattern is consistent with many Cambrian lineages exhibiting accelerated—yet plausible—rates of morphological and molecular evolution. Typical directional selection can increase phenotypic evolutionary rates by orders of magnitude over short timescales [39], and even conserved genomic regions can exhibit 10-fold differences in evolutionary rates in living sister lineages [40]. More specifically, in arthropods, data sets of first- and second-position codons alone often exhibit 2-fold, and occasionally 5-fold, differences between closely related taxa (Figure S2D).”

    [ ]

    The result is robust for telescoping into a “long fuse” time horizon, and also has interesting consequences for potential drivers. Apparently it is consistent with an earlier model paper that showed how such elevated rates could resolve the CE.

    If this passes peer review, I feel like when the standard cosmology and the standard particles were resolved – a major itch has been settled. Good both for biology (I assume as a layman) and against creationists.

    1. Mea culpa. Not one of the Pioneers obviously!

      It was Voyager 1 that became our first interstellar craft.

      1. Arguably, it became our first interstellar craft when it swung past Saturn and achieved solar escape speed. August 2012 is when it actually arrived in interstellar space.

  3. “So the second question we raised was whether there were any insects that parasitised or preyed upon their crustacean brethren. My point was that there aren’t many terrestrial crustaceans (basically woodlice and some weird crabs).”

    It may not just be a few “weird” crabs that live on land. When I worked in Costa Rica in the 90s the forest floor was swarming with crabs of multiple species. Big colorful ones. This was in coastal lowlands in a fairly wet area. The crabs seemed to be a major ecological force — devouring many tree seedlings, for example. The southern Nicoya Peninsula would be a good place to look for insect parasites of land crabs — if they exist at all,they probably exist there.

  4. The Rhinophoridae has similar red/black colours to a Tachinidae I photographed recently – I wonder why?

    Some of those flies are hyperparasites – amazing to think within a caterpillar, the fly parasites might be themselves infected by another fly parasite! Darwin would have marvelled at that…

    1. Oh dear – it seems I mean parasitoid as they KILL or sterilise the host – parasites just live off the host without killing it, so the Alien of film fame is a protelean parasitod!

    2. At the risk of this appearing twice – it seems I mean parasitoid NOT Parasite. A parasitoid KILLS or sterilises its host while a parasite lives off the host – a bit like a teenager! So the Alien of film fame would be a parasitoid.

      1. “Protelean organisms are those that begin the growing phase of their lives as parasites, and in particular, typically as internal parasites” – see wikipedia (where else…)

  5. I just realized those mites reminds me of dust mites. Maybe it was here I read it, but here goes:

    When they sequenced dust mites IIRC they were smack dab in a parasitic clade, nearest relatives living on humans. Ordinarily parasites would have a hard time adapting to free living I hear.

    But it seems these mites just dropped off and continued (as here) when the environment was survivable enough. They were living on organic scraps anyway.

    Sounds like a mite fantastic arthropod!

  6. Oedoparena flies might qualify: they are partly marine and parasitoids of marine crustaceans. The female adult fly lays an egg on the outside of a barnacle shell at low tide, the first instar maggot hatches from the egg, jumps inside the shell and eats the barnacle (living in the barnacle shell over several tide cycles – so in that sense it is a marine maggot). After moulting a couple of times, the maggot then emerges from the dead barnacle shell and looks for a second barnacle to eat, waiting for low tide to move from the dead husk to the next live barnacle, and slipping inside when the barnacle opens it’s opercular plates to feed on the incoming tide. The third instar pupates inside the second dead barnacle shell (they can apparently overwinter as pupae inside the barnacle shell, which would make them marine for many months), and then emerge as adult flies.

    CDG Harley & JP Lopez (2003) The natural history, thermal physiology, and ecological impacts of intertidal mesopredators, Oedoparena spp. (Diptera : Dryomyzidae). INVERTEBRATE BIOLOGY 122: 61-73.

  7. The black specks in the photo below are actually very tiny marine mites, whose name I’ve forgotten. Most of the mites are under the aquarium side’s silicone seal, but on the one under the flatworm (Convolutriloba retrogemma) legs can just be made out. From my old SW nano tank:

    (These were taken in situ through a 10X loupe; wish I’d thought to digiscope the mites when I viewed them through my dissecting scope.)

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