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First published online May 1, 2009
Journal of Experimental Biology 212, 1544-1552 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.025247

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Electroreception in the euryhaline stingray, Dasyatis sabina

D. W. McGowan and S. M. Kajiura^*

Florida Atlantic University, Biological Sciences, Boca Raton, FL 33431, USA

View larger version (30K): [in this window] [in a new window]   Fig. 1. Map of the two sample areas: Lake Harney in the St Johns River system, located approximately 300 km upstream from its saltwater outlet in Jacksonville, FL, USA and the Indian River Lagoon (IRL) system along the east coast of FL, USA. For this study, stingrays were only collected from the southern half of the IRL, ranging from Vero Beach, FL, USA (27 deg.38'N, 80 deg.22'W) as the northern boundary to Jupiter, FL, USA (26 deg.58'N, 80 deg.05'W) as the southern boundary, and including the St Lucie River estuary to the west (north fork 27 deg.14.50'N, 80 deg.19.00'W and south fork 27 deg.10.00'N, 80 deg.15.25'W). The C-44 canal in the southwest corner of the St Lucie River connects Lake Okeechobee to the IRL and is a major source of freshwater inflow into the study area.

View larger version (11K): [in this window] [in a new window]   Fig. 2. The experimental apparatus used to measure the electrosensitivity of the stingrays. A 122x213 cm acrylic plate was placed at the bottom a 122x244x50 cm tank. Four dipoles were equally spaced 40 cm apart within a 1x1 m square near one end of the acrylic plate. The gap distance between each pair of dipole openings was 1 cm. A stimulus generator produced a DC field with an applied current of 6.9–8.3 µA to simulate a weak electric field of the stingray's natural prey. The response of the stingray was recorded by a digital video camera positioned directly over the center of the four dipoles, 1 m above the surface of the water.

View larger version (4K): [in this window] [in a new window]   Fig. 3. Electric field values for each stingray's best response to the prey-simulating dipole. Treatment classifications (IR0, IR15, IR35, SJ0, SJ15) are created by combining the stingray capture location, the Indian River Lagoon (IR) or St Johns River (SJ), with the salinity at which they were tested, freshwater (0 p.p.t.), brackish water (15 p.p.t.) or full-strength seawater (35 p.p.t.).

View larger version (8K): [in this window] [in a new window]   Fig. 4. Scatter plot of orientation distance vs angle to the dipole axis for the best response of each treatment. For all treatments, there is a slight but consistent trend for decreasing orientation distance at higher angles. Treatment classifications (IR0, IR15, IR35, SJ0, SJ15) are created by combining the stingray capture location, the Indian River Lagoon (IR) or St Johns River (SJ), with the salinity at which they were tested, freshwater (0 p.p.t.), brackish water (15 p.p.t.) or full-strength seawater (35 p.p.t.).

View larger version (23K): [in this window] [in a new window]   Fig. 5. Empirical orientation distances and derived detection ranges for brackish–saltwater (BSW) (left side) and freshwater (FW) (right side) groups. The center of the dipole that produced the prey-simulating electric field is located at the intersection of the horizontal and vertical axes. Both axes represent the distance from the dipole origin in cm (horizontal axis = distance in the plane of the dipole axis; vertical axis = distance in the plane normal to the dipole axis). (A) The spatial location of each stingray at which an orientation was initiated prior to a successful feeding response. The responses are depicted within a 90 deg. quadrant for both groups ( = all response points for BSW group; = best response points for each stingray in the BSW group; = all response points for FW group; • = best response points for each stingray in the FW group). (B) The derived detection range as the median detection distance (dashed line) and the maximum detection distance (dotted line). See text for further details.

View larger version (31K): [in this window] [in a new window]   Fig. 6. The distribution of voltage equipotentials produced by a prey-simulating electric dipole in an electrically conductive marine environment (left) and a resistive freshwater (FW) environment (right). Electric fields were modeled using the same parameters as in this study (i.e. 8 µA applied current, 1 cm dipole opening separation, seawater resistivity of 19 cm and FW resistivity of 2026 cm). In A, each equipotential represents an order of magnitude of the electric field, ranging from 10.0 µVcm–1 to 0.001 µVcm–1. The center of the dipole that produced the electric field is located at the intersection of the horizontal and vertical axes. Both axes represent the distance from the dipole origin in cm (horizontal axis = distance in the plane of the dipole axis; vertical axis = distance in the plane normal to the dipole axis). The inset in the lower right corner illustrates the full extent of the electric field in FW. (B) A representative slice through A at 0 deg. to the dipole axis. It depicts the rate at which an electric field decreases as distance from the dipole increases in seawater (19 cm – left side) and FW (2026 cm – right side) on a semi-log scale. Note that the electric field decreases much more dramatically with distance in seawater compared with FW.

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