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I think this is most likely a large spider crab abdomen in this other observation:
My comment above about Hyas araneus having a carapace width of up to 31mm is inaccurate- it was based on the following quote from 'A key to the crabs and crab-like animals of British inshore waters' by Crothers, 1988.
"Hyas araneus: male. Drawn from a specimen in the British museum(Natural History). Carapace 31mm across."
This text is beside a line drawing of the crab and the size can only refer to that specific individual because both specimens above, male and female have carapace widths which much exceed 31mm. From memory, these 2 crabs have carapaces of approx 5cm wide.
Yes, shape, size and rostrum all fit this and MARLIN says the carapace can be up to 8cms wide http://www.marlin.ac.uk/speciesinformation.php?speciesID=3532
Nick Upton, naturalist and photographer.
This was really only put up as its abdomen was so similar to the unidentified object in the other ob. I would love to see one at 8cm wide, thatd be quite a beast. I once got an amazing look at one on a dead still winters night, it was sitting up high on kelp, only inches from the surface. Zero water movement. I was just about to take a pic when it moved away. Its amazing what you see sometimes when you shine a light into the sea.
Yes, the sea at night can be magical. Top memories for me are showers of sparks splashing on a Scottish beach when the sea was full of luminous dinoflagellates, finding luminous jellyfish washed up on mudflats in Norfolk while mist-netting wading birds at night, and snorkelling among a vast shoal of millions swimming crabs (no really - a mass spawning event) lit by a powerful spotlight from above and then being confronted by a fur seal darting past me repeatedly - quite scary at first - (off the Galapagos...).
Magical experiences Nick, the seal could have been a bit unnerving I'd say. I wasnt aware of bioluminescence(?) in our waters here until I was about 20, it came as a bit of a surprise- no one had ever told me about it and I thought I was experiencing radiation first hand as I moved piles of salmon net at night in an unlit shed to try to find some big edible crabs which I knew were in there.
The net had only been lifted from the sea that evening and was still full of live animals and plankton, it hadnt been cleaned yet. With no light I was working in the dark and the net glowed as I shifted it. Full of tiny green lights. Ive seen it a few times since that. I'd say I am describing the same thing you saw on the beach in Scotland?
I had great fun a couple of years ago trying to convince a friend that I was seeing luminous plankton in the waters at our feet, every time he looked it was gone, eventually he was actually watching when the movement of the water disturbed the plankton below him and the little green lights came on for a moment- he thought I was winding him up.
Yes, it's quite a sight isn't it! I knew about it before I ever saw it but was amazed how bright it can be. As far as I know the commonest bioluminescent planktonic organism in UK waters the dinoflagellate Noctiluca scintillans. http://en.wikipedia.org/wiki/Noctiluca_scintillans It's found worldwide, in warm and cool waters, but I've only seen it in late autumn in Scotland and in the Wash in Norfolk. There, I was wading through water flooding over mudflats and every step threw up showers of sparks - they luminesce more when disturbed. I'm not sure what the jellyfish was - but it was a small blob like one an inch across. Apparently Piddocks and some scaleworms can be luminescent as well... and there are some real wonders in tropical waters... I should have mentioned seeing Bermuda fireworms spawning... on the darkest nights of the mont the females swim to the see surface and circle around luminescing. Males swim towards them emitting flashes and then when they meet there is a revolving explosion of light. I worked with a cameraman using a specialist low light camera system and it made a stunning sequence.
That was the peculiar thing with the net I was moving, it was black dark when I went into the shed, zero lights, when I began to shift armfuls of wet net it glowed with green lights. I began to think of Chernobyl. Since that I saw the net glowing at night as it fished in the water, this is June and July. The plankton within the algal growth on the netting (we called that glit) glowed as the flow of the tide through the netting caused it to be disturbed. The glow was subtle until you gave the net a yank and then it got brighter. It didnt happen every year.
It sounds like youve had some interesting travels, the Bermuda fireworms sound very good, that wouldnt be viewable anywhere?
I must gather some scale worms and have a look at them in the dark, thanks for that, I hope the ones I find are the right type.
The fireworms went into a David Attenborough series "The Trials of Life" way back in 1992 in the "Talking to Strangers" episode. I was lucky enough to get around a lot working on that series (as Series Researcher) and for other programmes I've produced since. It sold a lot on VHS back in the day, and it may be out on DVD now, but there's no neat Youtube clip from it that I'm aware of. I'm not sure which scaleworms luminesce, some Polynoe species I think, I just remember reading about it with the explanation being that it may be an anti predator strategy: shed a flashing scale and creep off in darkness while a distracted crab or fish concentrates on the scale (like a lizard shedding a wriggly tail when attacked). The luminous net effect sounds pretty!
You both might be interested in http://www.nhm.ac.uk/natureplus/message/40072
(bioluminescent worms in Scotland)
Sounds like they had an extra special hogmany there on Vatersay. They hadnt anticipated a bit of bioluminescence on the beach.
I dont think Ive seen it on worms, just plankton. The last time I saw it was in September, just over the gunnel of the boat when fishing into the dusk.
Interesting link… Defo looks like a polychaete of some kind. I've heard of bioluminescent polychaetes in the UK before e.g. Polynoid scale worms e.g. Harmothoe imbrictata a H. glabra, but this looks more like a Nereid or a Syllid to me. Syllidae is one of the polychaete families where it's reported see: http://digital.csic.es/bitstream/10261/39786/3/2009%20Bioluminescent%20w... (e.g. the Bermuda firework and the Canadian Odontosyllis phosphorea.