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):