We all have the odd unsavoury habit. At least, that’s what I told myself yesterday as I knelt on the bank of the River Purwell to take an appraising sniff of a sausage-shaped poo, an excremental exhibit displayed in the middle of a grassy mound as if it were a crown on a ceremonial cushion.
The aroma was not unpleasant. Faintly fishy with a hint of animal musk. The placement and scent of the scat suggested only one thing – an otter had passed by recently and left its calling card: a poo with a view.
This morning I’ve returned to the same spot – there’s a fresh offering, this one smaller and smellier, a stew of unidentifiable lumps and pieces of shattered shell. The otter has been dining on signal crayfish: an American species that has supplanted the native white-clawed crayfish in many UK waterways, including my local chalk streams. As I lean in closer with my hand lens, I have the disconcerting feeling that the spraint is staring back at me. From between shards of orange exoskeleton, a beady eye regards me, blankly.
I poke about with a dried grass stem and out rolls a chalky-white pebble. It is less than a centimetre across, pitted on the surface and concave on one side. The structure looks rough but intentional, like a chunky bowl thrown on a miniature pottery wheel.
Intrigued, I search the internet. It seems the eye is a gastrolith or “stomach stone” – one of a pair of disc-like structures produced by crayfish to store calcium from their old exoskeletons while they moult. These bizarre objects surface from time to time in otter spraint and have been recorded in the nests of lesser black-backed gulls.
I know just how aggressive signal crayfish can be, having once watched a lengthy duel between a lesser black-backed gull and the 15cm-long crustacean with hefty claws it had just plucked out of the River Lee. But I can’t imagine any crayfish would stand a chance against an otter, especially during moulting when its calcareous defences are cached in two tiny buttons, one of which now eyeballs me from the centre of my palm.
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