Mystery shrimp-like creature in small pond

[frogplate]

We have a year-old very small pond made out of a trug where we keep the tadpoles safe from the denizens of the main pond. The trug pond has regular water changes with water from the main pond and so has a variety of small wildlife in addition to many hundreds of tadpoles.

Twice now I've seen what appears to be a glass shrimp swim by in the trug pond. It is about a cm long, silver grey, but not transparent and moves with a fast steady speed. It's a thin straight torpedo shape, widest about a third of the way back. It appears to be unsegmented but has a diamond shaped plate on its back at the widest point.

I couldn't see any antennae or eyes, legs or paddles - but it moves fast. Each time it has been passing across the pond just under the surface and then dived on reaching the vertical edge of the pond. I keep shrimp in our tropical aquarium and the movement is very reminiscent of a shrimp, but I don't think it is fairy shrimp.

Any ideas what it could be? Thanks in advance,

Jonathan

[The Woodman]

I wonder if you have a backswimmer - Notonecta glauca in your trug?

Here's an image http://www.jonathanlatimer.com/img/i...cta_glauca.jpg

You may not see the legs as it's swimming and the folded wings may also look diamond shaped.

[frogplate]

Good call, but I don't think it was a Backswimmer. The body was narrower - perhaps only half the relative width of Notonecta glauca. The diamond shape plate on its back was about a third of the way from the front and was the same colour as the rest of the body - in fact there were no noticeable colour markings.

Would have replied earlier, but every time I start looking at Jonathan Latimer's illustrations a lot of time suddenly disappears...

[Wetnwild]

To be honest we really do need a pic - there are so many possibilities.

[frogplate]

I found this small shrimp-like creature in the trug pond today:





It's about 6mm long, slightly green in some lights - though that may have been food in its gut. It swam upside down in open water, but the right way round when in shallow water. It's movements when upright are very similar to my mystery creature, though the colour and length are little different.

Can any one identify it? Could it be Crangonyx pseudogracilis or similar. This was in a one year-old trug pond, but the water is supplied from a decade-old mature pond.

[Jim Ford]

Freshwater shrimp - Gammarus pulex?

Biodiversity Invs

Jim

[frogplate]

Yes it could well be Gammarus pulex - that would explain the one I saw that was about a centimetre long. One thing I hadn't mentioned before is that we have watercress in the main pond. I understand that Gammarus pulex is common in watercress fields, so that would explain how it got into the pond.

Sounds like the Gammarus pulex young will be good food for the tadpoles too!

Thanks,

Jonathan

[The Woodman]

Good call, Jim, I'd go along with Gammarus. An indicator of good quality water.