The Smallest British Carabid

Now don't go quoting me on this, but I reckon the beetle I found today is the smallest carabid on the British list. If not, it must be pretty damned close (it's probably about the size of the largest carabid's egg, perhaps smaller).

I took myself down to the shore this morning, partly by way of a change from undertaking yet another shomble through Uig Wood, but mostly because I've always managed to find Aepus marinus here whenever I've searched for it - and today the search was on!

It took me until my second rock to find them, about four or five on the underside, I then went on to find several more beneath the other two rocks that I checked. They are an absolute dead cert here if you ever want to come up and see some (he confidently claims...)

First things first - the habitat shot

The astute amongst you may well be wondering why on earth I'd be looking for beetles beneath a seaweed-covered rock. Well, these are rather specialised beetles and they share this habitat with several other rather specialised air-breathing land animals. How? Well it all depends on the size of the substrate beneath certain shaped rocks. I shall explain.

The rock in the image above is probably two feet across at its widest and about a foot in height. Importantly, it has a large, essentially flat, lower surface which is shallowly embedded in the grit (we don't really have sand here in Uig, coarse volcanic grit is a better term) and this grit is noticeably fine. Not too fine, you understand, but not too coarse either. It has to be 'just right'. When the tide rises and the rock is submerged, a labyrinth of tiny interlinked air pockets are formed between the grit and the rock allowing small air-breathing creatures to hide out without fear of drowning. It seems a grim existence, but these rocks are absolutely teeming with air-breathing invertebrates! Not just Aepus marinus, but several species of staphylinids, whole knots of centipedes, masses of mites, lots of springtails (the presumed prey of Aepus) and even a few species of spiders. Clearly it is a niche but profitable habitat in which to eke out a living. 

It's a terrible clip, but here is Aepus marinus doing a runner across the underside of an overturned rock. Bear in mind that this beetle is between 2 and 3mm in length, I was struggling to focus!

I popped one into a tube for microscopic examination, just to absolutely confirm that it really is Aepus marinus and not Aepus robinii living on the Uig shoreline, though the habitat would strongly suggest the former (robinii is more often found deep inside silted cracks and crevices in shore rocks). I also potted up two small staphylinids from beneath the same rock. It's a very specific habitat, I suspect they too will prove to be a strongly littoral species. 

Here's a pic of the Aepus after I'd carded it, length is approx 2.7mm

Not great carding, I know

Aepus marinus differs from A.robinii in several ways. Firstly the terminal segment of all palpi are cylindrical rather than somewhat conical (subtle!), the eyes are very small as opposed to small (ideally I'd need both species together to say for certain, but they seemed very small to me). Thankfully there are a couple of straightforward features to check that aren't at all subjective. Firstly Aepus marinus has a truncate apex to the elytra (kind of abruptly squared off rather than tapering to a rounded end) and has sparse pubescence across the elytra. Aepus robinii on the other hand, has a lobate apex to each elytron (each being strongly recurved well before clearing the abdomen) and the elytral pubescence is restricted to just two dorsal punctures in each wing case. 

I hope you appreciate this next image, it took me bloody ages to get the angle, background and lighting just right so that it shows the elytral pubescence. Very shallow depth of field though, hence the poor overall focus 

So there you have it, lots of pubescence, truncated apex to the elytra and very small eyes. Through the microscope I could see that the tips to the palpi were cylindrical, but I was buggered if I could get the camera to focus on them rather than the top of the head, so just take my word for it. Aepus marinus is now officially on the 2020 Vision List. 

Not so this bad boy. Not yet, at least (later that night - I now believe this is Micralymma marinum)

Yet more squiffy carding. I know, I know...

I haven't attempted to figure this beast yet. It looks a good match for Micralymma marinum, a seashore specialist, but that's pure guesswork for now. Certainly it is known from the west coast of Scotland and has been found on adjacent Raasay underneath seaweed and rocks below the high water mark, as well as in pitfall traps on the shore (Moore, 2012). 

10pm update - using this RES key I worked my way through a couple of dead ends before finally noticing that the specimen really did have a pair of ocelli located approximately midway between the rear edges of the eyes (I missed seeing them several times - not easy!), from then onwards it keyed beautifully to Omaliinae and from there straight to Micralymma. There is only one species of the genus in Britain, occurring "in fissures of rock, under seaweed on rocks, etc., often below high water level; widely distributed  round the coast". It goes on to say that, "the shape of the elytra and abdomen will serve to distinguish the species from all other British Staphylinidae"  - so this is one for me to learn asap. Nice, very nice in fact. I managed a staphy key and (I hope!) came out at the right place. Micralymma marinus is a lifer for me, my first of the year. It all seems to fit, I'll await a grown up's reply below...

Aepus marinus - beetle number two on my 2020 Vision List

Tonight's musical choice is not one I'd ordinarily choose to listen to, but Underwater Rimes seemed appropriate for this post and it's actually pretty damn cool (I've had the album playing for the last twenty minutes and I'm enjoying it so far. Their track Danger Zone is truly amazing). The big question though...how well does Skev know them, and has he ever seen them play live?