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If it is Erpobdella octoculata it usually has evidence of dark pigmentation on the dorsal surface, and this distinguishes it from E. testacea. If the leech was uniform in colour and reddish, check to see if there are at least 4.5 annuli between the genital pores on the underside of the leech near the head end, to confirm it is E. testacea. Pleasantly surprised to see an aquatic invertebrate on iSpot, as I'm an Environment Agency macroinvertebrate analyst!
Is it Erpobdella or Herpobdella? I can find Interweb matches for both!
All the taxonomic keys we use including the latest one from the FBA say Erpobdella, so I'm going with that - not heard of the added H before!
This probably because you are young! I am using an olde reference by Helen Mellanby
Lat/Lng: 51.428, 0.016
OS grid ref: TQ402718