Description. Rostrum long, slender, dorsal margin straight or slightly curved in the middle and pointed upwards, shorter, equal to, or longer than scaphocerite, 6.0–9.50, most often (84% of the examined individuals) 6.33 to 8.76, × as long as high. 18–27 (18–24 in 91% of the individuals) pre orbital teeth on dorsal margin arranged up to tip of rostrum. 0–2, predominantly (84%) 1–2, post-orbital teeth. 4–10 teeth, most often (87%) 5–8, arranged on ventral margin of rostrum (Fig. 7A). Carapace smooth with pterygostomial angle bluntly produced (Fig. 7B). Pleuron of fifth abdominal segment pointed with an acute posterior angle (Fig. 7D). Telson with 5–8, mostly (97%) 5–7, pairs of dorsal spines arranged in curved fashion (Fig. 7E). Distal border of telson with 8–12, mostly (86%) 8–10, spines (4–6 pairs) arranged in fork-like pattern. Outermost pair of spines shortest, similar to dorsal spines, adjacent pair stronger terminating beyond (or along with) the finely setulose inner pairs (Figs 7E–F). Basal segment of antennular peduncle with long stylocerite, with its tip reaching or overreaching the distal end of basal segment. Anterolateral lobe of basal segment short and round (Fig. 7H). Distal segment of antennular peduncle with 1–6, frequently (92%) 2–4, spines (Fig. 7G). Basal lower endite of maxilla densely covered with long simple setae arranged in 12–16 (13–15 in 80% of the individuals), oblique parallel rows. Endite of maxilla 1.84–2.24, mostly (93%) 1.89–2.05, × as long as basal lower endite (Fig. 8G). Basal endite of first maxilliped failing or reaching to distal end of exopod (Fig. 8F). Distal third of terminal segment of third maxilliped bearing 13–38 (19–30 in 88% of the individuals) mesial spines and one subdistal lateral spine near the base of larger terminal spine (Fig. 8H). Armature along flexor margin of dactylus of third and fourth pereiopod consisting of 6–9 (7–9 in 97% of the individuals) and 6–10 (7–9 in 97% of the individuals) spines respectively (Figs 8B, 8D). Merus of third and fourth pereiopod with 3–7 (4–6 in 93% of the individuals) and 2–6 (4–5 in 96% of the individuals) spines respectively (Figs 8A, 8C). Dactylus of fifth pereiopod with 28–43, usually (82%) 32–40, spines arranged in comb-like fashion on flexor margin (Fig. 8E). Endopod of first male pleopod expanded proximally and with a distal portion elongated (ribbon shaped) and tapering. Endopod with 14–21 spines arranged on a slightly or strongly curved inner margin and 12–18 setae arranged on outer margin (Fig. 8I). 172–465 eggs of 0.60–0.7 × 0.40–0.45 mm in size.
Atyaephyra thyamisensis sp. n. is found in fresh water habitats of North-west Greece as well as in the islands Corfu and Lefkada.