(Muller, 1776)
Length 9-15 (in Antarctic waters more than 20) mm. Head subequal in length to first two pereonites combined. Flagellum of ANT II slender in females. ART 1 of mandibular palp much shorter, than ART 2, ART 2 shorter than ART 3. Outer lobes of MXP much longer than inner lobes. P I and II chelate, with numerous spines (Hyperia medusarum 1). Posterior margins of ART 6 and dactyls of P I and II serrate. ART 5 and 6 of P III and IV with long and short spines on posterior margins. P V-VII usually unarmed, but with cluster of spines on anterodistal corner of ART 2 in P VII. Posterodistal corners of all epimeral plates sharply pointed, posterior margin of epimera III only slightly convex. Telson twice as short as basipodite of UR III (Hyperia medusarum 2).
In the Northern Hemisphere Hyperia medusarum has two forms: typical forma medusarum with long spines on ART 6 of P I and II (some reaching beyond apex of dactyl), forma hystrix with shorter spines on ART 6 of P I and II, and relatively longer dactyls on these P. Only H. medusarum forma hystrix lives in the Southern Hemisphere. Associated with scyphomedusae, especially with Aurelia, Chrysaora, Cyanea and Rhizostoma. Species from southern hemisphere were first described as Hyperia gaudichaudii Milne-Edwards, 1840, which must be considered a junior synonym of H. medusarum (see Vinogradov et al., 1982).