DavidHowdon's picture

Wet critter

Observed: 16th February 2013 By: DavidHowdon
Amateur Entomologists' SocietyLondon Natural History SocietySelborne Society
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Not really any good on under water stuff. Perhaps around 8-10mm long, although would not stay still long enough to be measured.

Species interactions

No interactions present.


D.M.H.'s picture

looks like the critters I

looks like the critters I find down the river Wey.


All given ID's are subject to error/ommissions. Please seek independent verification before acting on ANY advice given. BE SAFE =)

DavidHowdon's picture


The site is probably about three miles from the Thames and right next to the Grand Union Canal.

The stream in question feeds some ponds which then drain away, I don't think it connects directly into the Thames.

The stream is run off from a few nearby sites and so is probably less than half a mile long.

What features would distinguish it from G. duebeni?

nightfly's picture

Interesting post David. John

Interesting post David. John thats good to know in streams, G. pulex is the likeliest or only option. How many native species are found in still waters?

I know there was recent controversy over an new large invasive species from the continent but I'd like to know how many native gammarus are to be found in Britain/Ireland?

EDIT- Just chanced upon a plate of gammarus and the like in a small book. It says lake one is G. lacustris. I'd forgotten that name, but are there other natives also?


DavidHowdon's picture

This one

I think I will have to assume that G. duebeni cannot be ruled out unless someone can give me a stronger steer on the ID.

John Bratton's picture

G. duebeni is in weakly

G. duebeni is in weakly brackish waters so could come up the Thames. It is easy enough to separate from pulex under the microscope on the shape of hind corner of one of the lateral plates. Also on eye shape but I will have to check the key to remind myself of this difference. G. lacustris is a mainly northwestern species. I don't think you will get it in the Thames valley, depsite some published records from SE England in the London Naturalist. It is quite likely that where you have Gammarus in your area, you wil also have Crangonyx pseudogracilis and I wonder if the two paler specimens in your photos could be this sp.

DavidHowdon's picture

All the pictures

are of the same specimen, just different lighting conditions (the dark ones were taken under artificial lighting with on camera flash, the light ones in daylight). So the pales ones are the same as the dark one.

The water this is in is not brackish (nor is that in the neighbouring canal) so perhaps G. duebeni can be eliminated.

WIll be interested to hear what the eye shape characters are (although I doubt the photos will be good enough to show them).

John Bratton's picture

Eye shape fits pulex. I was

Eye shape fits pulex. I was wrong to say lateral plate shape differentiates duebeni. It is leg shape that does that. Anyway, have done some photos over the weekend and will post them now, to distinguish pulex, duebeni and Crangonyx pseudogracilis.

DavidHowdon's picture


I've looked at the pictures you have uploaded (http://www.ispotnature.org/node/388255, http://www.ispotnature.org/node/388252 and http://www.ispotnature.org/node/388253).

The eye on the specimen here seems quite round, not kidney shaped as in the two Gammarus you have illustrated.

I'll defer to your judgement but I don't feel confident myself that I can separate between G. pulex and C. psuedogracilis from the photos I have taken.

DavidHowdon's picture


John if you have time I'd appreciate you casting a glance at http://www.ispotnature.org/node/415600 to see if you think that is also pulex.


John Bratton's picture

In answer to Cathal

In Great Britain the two native species in standing freshwaters are Gammarus pulex and lacustris. G. pulex is more associated with running waters, and in still water is usually only in large lakes and spring-fed ponds. I'm less familiar with G. lacustris. The books say lakes but it is also in some of the Anglesey fen pools.

In GB G. duebeni is mainly brackish but can occur in freshwater, in small west coast streams isolated from the main river systems. In Ireland, duebeni is or was the common freshwater species but is now being edged out by introduced G. pulex. This is possibly what has happened in GB but before biological recording began. There are slight morphological differences between the brackish and freshwater duebeni populations, and in France the fw ones are given subspecies rank, celticus, but I think study of the British difference found it to be less pronounced so I don't think ssp. celticus is recognied in GB.

An amphipod in small silty still freshwaters such as cattle ponds is almost certain to be Crangonyx pseudogracilis. I haven't seen the new invasives yet but I understand them to be quite obvious - larger and patterned.

Then there are Orchestia spp., semi-terrestrial water-margin amphipods to take into account, and the subterranean ones if you are lucky. And several other species restricted to brackish water.