Whale Sharks (Rhincodon typus) Respond to Krill and DMS Odor Plumes
Matthew A.
1,2
Foretich ,
Marc J.
2
Weissburg
, Alistair
2,3
Dove
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
A
A
Introduction:
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 twosided fisher exact test comparing response ratios of pooled data (n=197) are shown.
Pre-Krill: p << .01
Post-Krill: p << .01
Methods:
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 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.
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.
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.
Conclusion/Discussion
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. typus has 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.
Acknowledgements:
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.
References:
Martin, R. A. (2007). A review of behavioral ecology of whale
sharks (Rhincodon typus). Fisheries Research 84: 10-16
Nevitt, G., Reid, K., and Trathan, P. (2004). Testing olfactory
foraging strategies in an Antarctic seabird assemblage. The
Journal of Experimental Biology 207: 3537-3544
Steven, J. D. (2007). Whale shark (Rhincodon typus) biology
and ecology: A review of the primary literature. Fisheries
Research 84: 4-9