Tidal and diel movement patterns of the Atlantic stingray (Dasyatis sabina) along a stream-order gradient

The behaviour of the Atlantic stingray (Dasyatis sabina) is affected by environmental cues, including time of day and tide. The purpose of the present study was to determine whether these cycles and differences in creek geomorphology affected presence and movement patterns of Atlantic stingrays near Savannah, GA, USA. Forty-five stingrays were tagged with Vemco coded-acoustic transmitters and passively tracked in two creek systems between December 2012 and December 2014. The geomorphology of the creeks was classified using the Horton stream-order concept, with the smallest tributaries assigned to the first order. Stingrays moved most at dusk (0.26±0.09 movements day–1), followed by night (0.23±0.09 movements day–1). Stingrays were found in deeper water during the cold season than during the warm season. Patterns in creek usage across tidal stages were nearly consistent across seasons. However, the diel pattern varied; stingrays utilised all creek orders throughout the diel cycle in the warm seasons, but entered only shallow creeks during the day in the cold season. Because presence and movements of stingrays were affected by tidal and diel cycles, these environmental factors could also affect their foraging patterns and their influence as an ecosystem engineer.

Description of the mechanoreceptive lateral line and electroreceptive ampullary systems in the freshwater whipray, Himantura dalyensis

Mechanoreceptive and electroreceptive anatomical specialisations in freshwater elasmobranch fishes are largely unknown. The freshwater whipray, Himantura dalyensis, is one of a few Australian elasmobranch species that occur in low salinity (oligohaline) environments. The distribution and morphology of the mechanoreceptive lateral line and the electroreceptive ampullae of Lorenzini were investigated by dissection and compared with previous studies on related species. The distribution of the pit organs resembles that of a marine ray, Dasyatis sabina, although their orientation differs. The lateral line canals of H. dalyensis are distributed similarly compared with two marine relatives, H. gerrardi and D. sabina. However, convolutions of the ventral canals and proliferations of the infraorbital canal are more extensive in H. dalyensis than H. gerrardi. The intricate nature of the ventral, non-pored canals suggests a mechanotactile function, as previously demonstrated in D. sabina. The ampullary system of H. dalyensis is not typical of an obligate freshwater elasmobranch (i.e. H. signifer), and its morphology and pore distribution resembles those of marine dasyatids. These results suggest that H. dalyensis is euryhaline, with sensory systems adapted similarly to those described in marine and estuarine species.