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Matthew A. Foretich 1,2 , Marc J. Weissburg 2 , Alistair Dove 2,3.
1Odum School of Ecology, 140 E. Green St., The University of Georgia, Athens, Georgia 30602, 2School of Biology, 310 Ferst Dr., Georgia Institute of Technology, Atlanta, Georgia 30332, 3Georgia Aquarium Research Center, Georgia Aquarium, 225 Baker Street NW, Atlanta, GA 30313
The whale shark is the world’s largest fish and a versatile filter-feeder. Their large nostrils and olfactory capsules suggest they may be good at detecting dissolved chemicals (Martin 2007). Their global distribution and capacity for long distance travel to specific areas where food sources are most abundant (Stevens 2007) suggest that whale sharks navigate in response to environmental cues.We hypothesize that the species may use chemical odor cues such as pyrazine or dimethyl sulfide to locate their prey, as has been documented in several other species (Nevitt et al 2004).
Figure 2. Krill plumes elicit feeding behavior. The number of cases where animals do not respond (A) is greater for Pre- and Post-Krill plumes, whereas animals respond to Krill plumes with feeding behaviors including open-mouthed cruising (B), gulping (C), and assuming a tail-down posture to bring their mouth to the surface (D). P-values of a two-sided fisher exact test comparing response ratios of pooled data (n=197) are shown.
Pre-Krill: p << .01
Post-Krill: p << .01
We observed the responses of 3 sharks at the Georgia Aquarium in Atlanta, GA to introduced odor plumes and plumes of dyed seawater (figure 1). The first odor stimulus was 1L of blended, filtered krill solution diluted with 9L of tank water. The second odor stimulus was a 10 mM solution of dimethyl sulfide (DMS) in tank water. Fluorescein dye was added to both treatments and controls at a concentration of 0.5g per 10L. We video recorded responses to these different treatments, examined the frequency of various behaviors associated with feeding, and examined the kinematics of animals exposed to krill (K) or DMS plumes (DMS), as well as to control plumes prior to (Pre-), and after (Post-) the stimulus plumes. We also analyzed the frequency of visitations to the area where we created the plumes for these same conditions.
Figure 4. Krill and DMS plumes increase visitation rate. All animals visited the site of plume introduction significantly more often in response to Krill (A) and DMS (B) plumes according to pairwise t-tests (p-values displayed) of the pooled data (n=18 for krill, n=21 for DMS). One standard error is depicted.
Figure 3 . DMS plumes elicit feeding behavior. The number of cases where animals do not respond (A) is greater for Pre- and Post DMS plumes. An open-mouthed cruising response (A) was a common response to DMS plumes, and other behaviors, including gulping (C) and a tail-down posture (D) were only weakly elicited by DMS. P-values of a two-sided fisher exact test comparing response ratios of pooled data (n=111) are shown.
Krill and DMS plumes elicited feeding behaviors in all animals tested, as well as increased the frequency of visits to the site of the plume. In addition, we documented significant changes in swimming speed and direction changes for all animals in response to krill plumes (data not shown). The types and directions of these changes varied between animals and may suggest preferences for different foraging strategies. We conclude that R. typushas the capacity to detect odor and associates the odor of krill and DMS with food, as evidenced by the high frequency of gulping and tail–down behaviors. Krill odor may also function as an immediate attractant as shown by the dramatic behavioral response and increased visitation rate in response to krill juice plumes. DMS produced less dramatic feeding-type responses, but strong changes in visitation rates similar to that evoked by krill odor. This suggests that DMS plays a role similar to that seen in other animals foraging for krill, and functions primarily as a distance attractant.
We would like to thank Kristen Jolley, Lauren Fuess, and Juliette Fry for assisting with videography, everyone at the Georgia Aquarium for allowing us to use their space and collection for the study, and the National Science Foundation for providing the funds and opportunity via the REU site grant awarded to the GT School of Biology.
Martin, R. A. (2007). A review of behavioral ecology of whale sharks (Rhincodon typus). Fisheries Research84: 10-16
Nevitt, G., Reid, K., and Trathan, P. (2004). Testing olfactory foraging strategies in an Antarctic seabird assemblage. The Journal of Experimental Biology207: 3537-3544
Steven, J. D. (2007). Whale shark (Rhincodon typus) biology and ecology: A review of the primary literature. Fisheries Research84: 4-9
Figure 1. Plume Introduction Method. We constructed an apparatus which allowed us to control the size, location, and depth of the odor plumes. Plumes were only introduced at one location of the tank.