Last week I gave you this little critter to have a go at identifying:

This particular shrimpy specimen arrived as an enquiry, after it was found in some Irish drinking water.

Prawn cocktails may be a thing, but most of us don’t think of shrimps (or shrimp-like decapod crustaceans) as an ideal addition to a beverage.

According to WHO these critters aren’t actually a health problem if they get into the water supply in temperate regions, where they don’t carry significant parasites or pathogens – but it’s a different story in the tropics.

Knowing which species this is could help in working out how it might have entered the water supply. I’m not an expert on crustaceans by any means, but there are useful keys out there [links to pdf] for working out this kind of information and I’m very fortunate in having to hand the expertise of my predecessor at the Dead Zoo, Mark Holmes, who specialises in crustaceans and is still often at the Museum doing research. Of course, I also have all of you lovely people!

My first thought on seeing this was that it was one of the Gammeridae, based mainly on my exposure to many photos of Gammarus shrimps infected by microsporidian parasites that change them from males into females (which is some fascinating biology). I was therefore delighted when so many of you put forth suggestions in that same area.

By working through the key of Irish shrimp I narrowed it down to Gammarus pulex (Linnaeus, 1758), which was also suggested on Twitter by @RobertsZivtins and @DianeBarlee. It could be G. tigrinus or perhaps G. lacustris – or of course a species not previously recorded from Ireland that doesn’t appear on the key.

However, I got it fresh and there were no stripes and the uropods and telson (taily-bit) looks more pulexy to my eye.

I am now eagerly waiting to hear what Mark thinks – I will update this post as soon as I do!

I have now checked with Mark and he identified this as G. lacustris so it looks like the taily bit isn’t pulexy after all. Thanks Mark!