Unidentified water spider

[rich3245]

Hi, Any ideas what this spider is?



Found today in a dyke on the edge of a Norfolk Broads nature reserve. One side of ditch grazed meadow, other species rich fen. Specifically found, between boards on a drop-board sluice just above water level, but had been seen last week walking on and, once, submerging in water.

Very good water quality, pH likely to be around neutral or slightly alkaline.

Size - 30mm leg span, 10mm body length. White strips on each side of thorax obvious on photo, much feinter strips on either side of abdomen can be seen when zoom in on pic, also paler brown stripe down centre of abdomen.

Thankyou

[GuyF]

Hi Rich, welcome to the site.
Your spider is one of the Raft Spiders - Dolomedes sp. There are two species in Britain, the commonest being Dolomedes fimbriatus, however in your area the much rarer Fen Raft Spider - Dolomedes plantarius is a possibility. Judging by the habitat it may be the rarer species, as the common Raft Spider seems to prefer more acidic boggy habitats, however I'm not sure whether the species can be identified with certainty from the photo,
Guy

[No.9 Spider]

Have a look at this website.

Dolomedes home page


No.9 Spider

[rich3245]

Hi,

Thanks alot for your replies,

When I posted the image I was very excited as I thought it was a dolomedes species, and based on habitat thought it was most likely to be the very rare d.plantarius.

However I found that same website in my searching and came across the 13th image down on the picture Gallery page and saw a spider identical to mine - pirata hygrophila. Im not sure if you were linking to this image to point this out No.9 spider or if you had been fooled like myself!

I also cant find any other good images of pirata or good desriptions so dont want to give up now just incase this is a mistake on the part of the website, so if anyone can give 100% confirmation then that would be great.

Thanks,

Rich

[GuyF]

I'm not a spider expert (although I have spent quite a lot of time watching Raft Spiders), but I'm 99% sure that yours is a Raft Spider of some kind, the proportions and shape are fairly distinctive.
Here's a shot that I got recently of D.fimbriatus which looks very similar to yours.

Guy

[Venger]

Size wise puts it more likely to be Dolomedes spp rather than a Pirata hygrophilus (size 4.5 - 5.5mm).
Though thats not a confirmation

[rich3245]

Thanks for all your help,

I have now showed the photo to an entomologist colleague and he suggests, pirata piscatorious - the largest of the pirata.

Can any of you confirm the normal size of p.piscatorius and if this could indeed be correct?

The internet is not proving much use either way, but the more pictures I see the more confident I am that it is not a dolomedes sp.

I have also found out that p.piscatorius was recorded on a survey of the site in 2007.

If I cant be 100% sure, I would like to be as close to that as possible!

Thanks,
Rich

[No.9 Spider]

Hi Rich, welcome to WAB

My link to the Dolomedes site was to give you a comparison.
I am not familiar with Dolomedes, but I was not convinced by the white margins on the carapace of your specimen. Dolomedes appear to have these inset from the edges of the carapace whereas yours has them along the actual edge of the carapace.
Pirata piscatorius could be a possibility - body sizes are for the female 5 to 10mm, male 4.5 to 8.5mm and it does have white margins to the carapace (and paired white spots on the abdomen). As Venger has said, the leg span is a bit on the large size for a Pirata species and more in keeping with Dolomedes.
You could send your photo to the Dolomedes website for confirmation.
It is often very difficult to i.d. from a photo and gets harder the more you look at it.


No.9 Spider