Unusual stable morphotype of acroperus harpae (Baird, 1834) from Lake Mỳvatn, Iceland (Cladocera: Anomopoda: Chydoridae) revealed by palaeolimnological studies

A new unusual morphotype of Acroperus harpae (Baird, 1834) was found in Lake Mỳvatn and its neighbouring Lake Budlungaflói in Iceland. This form, A. harpae f. multidentata is characterized by a very large head keel and by 5-9 denticles on the posterovental angle of the valves instead of 1-3 common for A. harpae. The new form did not differ from typical A. harpae in the morphology of the postabdomen, head pores or any appendages. A study of Lake Mỳvatn sediments reveled that this form existed in the lake for at least two thousand years.