No abstract

Elasmobranch fishes use electroreception to detect electric fields in the environment, particularly minute bioelectric fields of potential prey. A single family of obligate freshwater stingrays, Potamotrygonidae, endemic to the Amazon River, demonstrates morphological adaptations of their electrosensory system due to characteristics of a high impedance freshwater environment. Little work has investigated whether the reduced morphology translates to reduced sensitivity because of the electrical properties of freshwater, or because of a marine-tuned sensory system attempting to function in freshwater. The objective of the present study was to measure electric potential from prey of Potamotrygon motoro and replicate the measurements in a behavioural assay to quantify P. motoro electrosensitivity. Median orientation distance to prey-simulating electric fields was 2.73 cm, and the median voltage gradient detected was 0.20 mV cm À1 . This sensitivity is greatly reduced compared with marine batoids. A euryhaline species with marine-type ampullary morphology was previously tested in freshwater and demonstrated reduced sensitivity compared with when it was tested in seawater (0.2 mV cm À1 v. 0.6 nV cm À1 ). When the data were adjusted with a modified ideal dipole equation, sensitivity was comparable to P. motoro. This suggests that the conductivity of the medium, more so than ampullary morphology, dictates the sensitivity of elasmobranch electroreception.

ABSTRACT:Studies of islands have emerged as a unique and vital focus of research over the last couple decades. Works like Hau'ofa's 1994 'Our Sea of Islands' have moved us quite systematically towards the study of islands, underlining the dynamic connectedness between terrestrial and marine environments, and between individual islands and elsewhere. By tracing the many and varied ways that salmon (and other actants) connect oceans, islands, and other land forms in an ongoing inter-species dialogue, we can move the discourse one step further, and dissolve islands into a multispecies dialogue made in movement. Such a strategy opens up some insights on the inter-connectedness of islands and others.