A study of correlation between the total body length and the somites length was carried out in a population of Hyale media (Dana, 1857), in order to know which somite or group of somites has the highest correlation index with the total body length. As the sum of the length of the first to fourth pereonites showed the highest linear correlation index (Y=0.0764+0.2736X; r=0.9723), this meristic parameter was chosen to describe the population structure of the species. The following aspects were treated: distribution of the body size classes in the various phytals, population composition, seasonal fluctuation of population density. relative frequency of the ovigerous females and correlation between the body length and the number of eggs inside the marsupium of the ovigerous females. The amphipods were obtained from the seasonal collections of six phytals from a rocky seashore of Caiobá, Paraná State: Pterosiphonia pennata (Roth) Falkenberg. Gymrogongrus griffithsiae (Turner) Martius, Pterocladia capillacea (Gmelin) Bornet & Thured, Sargassum cymosum Garth, Gelidium sp and Ulva fasciata Delile; they did not occurred in Padina gymnospora (Kútsing) Vickers and Porphyra atropurpurea (Olivi) De Toni. The air temperature oscillated from 16ºC (winter and autumn) to 23ºC (summer), the surface water temperature from 17ºC (winter) to 25ºC (summer) and the surface water salinity, from 29.3‰ (autumn) to 32.8‰ (winter). The density oi Hyale media varied from 0.20 ind.g-1 (in Ulva) to 26.37 ind.g-1 (in Pterosiphonia) of alga-substratum weigth, and the population was distributed mainly in branched algae. It was determined three size classes in the population, within a range from 0.01 to 2.99mm of pereonits 1-4 length. Small amphipods prefer finely branched algae like Gymnogongrusand Pterosiphonia, whereas broad-thallii or less branched algae such as Sargassum, Pterocladia, Gelidium and Ulva harbour proporcionally high number of large individuais. The life cycle of Hyale media takes place wholly in the phytals and the species reproduces continually all year round: males, ovigerous females and juveniles are present every season. The highest femalc reproductive activity occurs in winter and the juveniles are more numerous in summer. The number of eggs inside the marsupium and the pereonites 1-4 length has a linear correlation (Y=-9,9682+12,0729X; r=0.8024).
Hyale media; population; phytal; correlation; density; Matinhos; Brazil