INTRODUCTION

Southern Stingrays (Dasyatis americana), are benthic elasmobranchs that are common to the Bahamas (Chapman et al., 2003).  Due to their foraging behavior, Dasyatis sp. has numerous gel-filled electrosensory pores located around the ventrum (Adair et al. 1998; Raschi and Mackanos, 2005).  These electrosensory pores are known as the Ampullae of Lorenzini.  The location of these pores allow D. americana to forage and detect minute organisms under the substrate due to the weak electrical impulses (e.g. heartbeat) given off by the organisms.  As the ray passes over an area containing an organism, the electric potential of this organism will vary from the electric potential of the ampullae.  This potential difference is then detected by the sensory cells that line the ampullae.  Once the voltage differential is recognized, the information is transmitted to the brain via afferent neurons (Adair et al. 1998).
 

We subjected three D. americana to a pen that contained uniformly arranged magnets throughout one quarter of the pen.  With this experimental setup, one could assume that these rays will avoid the area containing the magnets and spend a significantly greater amount of time within the remaining seventy-five percent of the-pen.  Lastly, a natural diurnal behavior of D. americana is to bury beneath the sand.  This is a resting behavior and it can be assumed that this behavior will occur more frequently within the control quadrant since the electric fields created by the magnets will deter the rays away from the magnetic area.
 

MATERIALS AND METHODS

All animals were collected from the shallow mangroves around South Bimini, Bahamas.  Once the animals were spotted from a boat, two students surrounded the D. americana using a seine net.  After the D. Americana became trapped within this seine net, one student captured the ray using a dip net.  The ray was then transported to a 6 meter diameter holding pen.  Each ray was fed a diet of Mangrove Snapper (Lutjanus griseus) and Yellowfin Mojarra (Gerres cinereus).  One 15 cm fish was fed to each ray once every 3 days.

For our second experiment, we captured one 67 cm female  and two 38 cm male southern stingrays.  The two 38 cm rays were differentiated by their distinctive tails (one short and one long).  For the experiment, each stingray was taken from the holding pen and then placed in the same experimental setup as the three G. cirratum .  Similar to the first experiment, we would place twenty magnets throughout one of the randomly chosen quadrants per day.   In another randomly chosen quadrant, we placed plastic pieces that would serve as our control.  Both the control plastic and the stimulus magnets were buried to avoid visual cues that could potentially affect the rays behavior.  It is important that the magnets  and the control were placed beneath the sand prior to their transport into the experimental pen.  Each day, we would randomly choose a new quadrant for both the control and the magnets based on the random number generator on Microsoft Excel.  Each trial would last a duration of 3 hours, excluding the one-hour acclimation period.

The rays were observed and their behaviors were recorded for a period of 3 hours per trial.  When each ray entered either the control or the stimulus region, a stopwatch was used to record the duration of time each ray spent in that region.  Also, behaviors such as 90 and 180 turns and accelerations away from the quadrants were noted.  Lastly, an important behavior we thought would be beneficial to note is the amount of times and in what quadrant the rays buried themselves.

PRELIMINARY RESULTS (OCTOBER 2006)

After conducting 6 x 3 hr trials, one can assume that the ceramic magnets had a large impact on the behavior of these three D. americana.  These rays were observed to be avoiding the magnetic quadrant and displayed a variety of avoidance behaviors:  90 and 180 turns away from the magnets and accelerations away from the magnets (fig. 1).  Also, it was found that each ray was more likely to bury itself within the control quadrants rather than the treatment quadrants (Figure 2). 

DISCUSSION OF RESULTS (OCTOBER 2006)

When compiling all quadrants together for comparison, each ray spent a greater amount of time in the control and empty quadrants as compared to the treatment quadrants.  Although the duration of time spent by each ray within the control and empty quadrants do not exactly match, they are relatively close in value (Figures 3-5).  Lastly, a sufficient amount of trials were conducted to make individual comparisons of treatment versus control within each quadrant for each ray.  One may notice that the rays spend a greater amount of time in a particular quadrant when there was a control in that quadrant, as opposed to the treatment (Figures 6-8).  But, there is an instance where ray 2 spends a slightly larger amount of time in the western quadrant when there is a treatment present as opposed to a control.  This may have occurred due to environmental factors, such as tides and an odor plume.  At the time of the trial where the treatment was placed in the western quadrant, sharks were being fed in a neighboring pen.   Due to tidal fluxes and currents, the odor would have been pushed towards the western quadrant and may have induced this behavior by ray 2. 

Once again, this experiment has displayed that magnets do affect the behavior of D. americana.  In order to truly see the effect of the magnets, more trials must be conducted on a larger sample size.