Red-eared Terrapin?

I appreciate this is an awful lot of grovelling for a potential lifer but stick with me!

Sifting through some old photos and noticed I've taken a plethora of photos of Red-eared Terrapin (Trachemys scripta) at Colchester Zoo, Essex. I regularly see them on more recent trips too.

Being a zoo, I've always presumed these are part of the animal collection however a look at the zoo website doesn't show the species on the animal list, and there is an accepted record at the zoo on the NBN Atlas map shown here - species.nbnatlas.org/species/NHMSYS0000080227#tab_mapView

The terrapins are always on the main lake with wild birds too, thus with all this info, would anyone else deem this as a countable species? 

Thanks, Ben

Hi Ben,
I think that there are a couple of Terrapin/Slider species that are regularly found in water bodies around the UK, many probably a result of releases of unwanted pets. I don't think that it's been proved that they are breeding, but they are relatively long-lived species, hence the reason, I think, why they are not included on the main species list.
Interestingly, here in Bedfordshire we have Prairie Dogs - no longer an 'exhibit' -  breeding in the grounds of Whipsnade Zoo (and sometimes leaving the confines of the zoo).  Black-tailed prairie dog : Whipsnade : 15 May 2016 - ZooChat I've always wondered whether they will spread and one day become tickable alongside the likes of Red-necked Wallaby!
Best wishes,
Stephen.

Hiya Ben, 
to echo what Stephen has said, the various terrapin species found across parts of Britain are merely dumped pets. They have, on rare occasion, been known to lay eggs but (so far) I don't believe any have ever hatched. Thus all terrapins, whether in your local duckpond or in a zoo lake are, in my opinion, rubbish and completely untickable. A harsh view, I agree, and obviously many folks disagree with this view seeing as they include them on their own PSL. Terrapins aren't breeding and they aren't naturalised. They are merely surviving discarded pets, much to the detriment of various amphibians, both native and introduced, which act as prey items.   

Hi Stephen / Seth,

Appreciate the responses/views, will leave this one away from my PSL - it's definitely an interesting topic as to when certain animals become 'wild' and tickable, and as Seth alluded to I'm sure everyone's rules are slightly different in terms of what's legit and what isn't, I guess that's the beauty of being your own boss! But yeah, ticking runaway pets is not for me. 

Thanks again for the input, much appreciated
Ben