Kajiura, Stephen M.

Many marine species distributions have shifted poleward in response to global climate change. Many environmental characteristics will be affected by climate change including temperature and phytoplankton concentration; yet, photoperiod will remain the same. It is imperative to gather baseline distribution data on migratory species so that these shifts can be measured and mitigated. Sex-specific differences in reproductive strategies cause sexual segregation, sex-specific differences in spatial distribution. Female blacktip sharks exhibit a synchronous, biennial reproductive cycle in which one year of reproduction is followed by a resting year. Acoustic telemetry can be used in conjunction with collaborative networks to track migratory species over great distances. However, the irregular spacing of acoustic receivers often results in sporadic detection data, which can lead to skewed distribution information. This project developed and tested an analysis process to regularize sporadic acoustic detection data. Those regularized data were then applied to cluster analyses to determine the seasonal spatial distributions of blacktip sharks, Carcharhinus limbatus, off the United States East Coast and corresponding environmental correlates of latitudinal movement. Sexes of this population were investigated separately and in combination. Differences in distribution were evaluated between sexes, and within females, between reproductive states. These data showed that the U.S. East Coast blacktip shark population distributes from Palm Beach County, FL to Long Island, NY and exhibits sexual segregation, in which females display a more truncated migratory pattern than males.

To fully understand the function of the elasmobranch
electrosensory system it is necessary to examine
electrosensory nerves extending from the ampullae
of Lorenzini to the central nervous system. Studies
detailing the composition of sensory axons are rare,
but they have shown that ontogenetic and sexual
dimorphism exists in the anterior lateral line nerve
(ALLN) of numerous species. This study obtained a
count of the number of axons comprising the ALLN in
male vs. female and adult vs. juvenile yellow stingrays
(Urobatis jamaicensis). We hypothesized that
males have more axons than females, and that the
number of axons is ontogenetically constant. We
expect males to have 30% more axons in their ALLN,
and that the number of axons is ontogenetically constant
for both sexes. This study will provide unique
data about the electrosensory nerves of Yellow
stingrays that can be used in future studies to make
comparisons between other species.

Elasmobranchs (sharks, skates, and rays) migrate across a wide range of
spatiotemporal scales, display philopatry, seasonal residency, and maintain
home ranges. Many animals use the Earth’s magnetic field to orient and navigate
between habitats. The geomagnetic field provides a variety of sensory cues to
magnetically sensitive species, which could potentially use the polarity, or
intensity and inclination angle of the field, to derive a sense of direction, or
location, during migration. Magnetoreception has never been unequivocally
demonstrated in any elasmobranch species and the cognitive abilities of these
fishes are poorly studied. This project used behavioral conditioning assays that
paired magnetic and reinforcement stimuli in order to elicit behavioral responses.
The specific goals were to determine if the yellow stingray, Urobatis jamaicensis,
could detect magnetic fields, to quantify the nature of the magnetic stimuli it could
detect, and to quantify the learning and memory capabilities of this species. The results supported the original hypotheses and demonstrated that the yellow
stingray could: discriminate between magnetic and non-magnetic objects; detect
and discriminate between changes in geomagnetic field strength and inclination
angle; and use geomagnetic field polarity to solve a navigational task. The yellow
stingray learned behavioral tasks faster and retained the memories of learned
associations longer than any batoid (skate or ray) to date. The data also suggest
that this species can classify magnetic field stimuli into categories and learn
similar behavioral tasks with increased efficiency, which indicate behavioral
flexibility. These data support the idea that cartilaginous fishes use the
geomagnetic field as an environmental cue to derive a sense of location and
direction during migrations. Future studies should investigate the mechanism,
physiological threshold, and sensitivity range of the elasmobranch magnetic
sense in order to understand the effects of anthropogenic activities and
environmental change on the migratory ability of these fishes.

This study determined the electrosensitivity of a euryhaline elasmobranch,
the Atlantic stingray, Dasyatis sabina, throughout the range of salinities that it
would naturally encounter. It quantified the behavioral response of the stingrays to prey-simulating electric stimuli in freshwater, brackish, and full strength
seawater. The electroreceptive capability of stingrays from a permanent
freshwater population in the St. Johns River system was also compared with
stingrays that inhabit the tidally-dynamic Indian River Lagoon in east Florida.
This study demonstrated that D. sabina can detect prey-simulating electric fields
in freshwater, but the function of its electrosensory system is significantly
reduced. The SJR stingrays did not demonstrate an enhanced electrosensitivity
in freshwater, nor did they have reduced sensitivity when introduced to higher
salinities. The reduction in electrosensitivity and detection range in freshwater is
attributed to both an environmental factor (electrical resistivity of the water) and
the physiological limitations of the ampullary canals.

Sharks possess an electrosensory system which allows the detection of
electric fields . How this system varies among related taxa and among species
inhabiting different environments remains unexplored. Electroreceptor number
was quantified for representative species of related taxa (genera, families, orders)
from different environments (pelagic, coastal, deepwater) and taxa from similar
environments to determine potential phylogenetic constraint or evolutionary
convergence. Coastal open water sharks possess the greatest number of
electroreceptors; deepwater sharks the least. Pelagic and coastal benthic sharks
retain comparable electrosensory pore numbers despite inhabiting vastly different
environments. Electrosensory pores were primarily located in ventral
distributions, except among coastal open water sharks which possess roughly
even distributions around the head. Among related species and genera, pore
numbers and distribution are comparable, with greater variation among higher
taxa. Results implicate evolutionary convergence as the primary influence in
electroreceptor development, while phylogenetic constraint establishes similar
base values for number and distribution.

Crude oil causes both lethal and sublethal effects on marine organisms, but the
impact upon sensory function remains unexplored. Elasmobranchs rely upon the effective
functioning of their sensory systems for use in feeding, mating, and predator avoidance.
The objective of this study was to test the effect of crude oil upon the olfactory and
electroreceptive sensitivity of the Atlantic stingray, Dasyatis sabina. The magnitudes of
the electro-olfactogram (EOG) responses were significantly depressed by 26% (Glutamic
Acid) to 157% (Cysteine) for all amino acids when stingrays were exposed to crude oil.
The shapes of the EOG responses when exposed to oil were also significantly different,
exhibiting a more protracted response compared to un-exposed stingrays. Oil exposed
stingrays exhibited a significant decrease in orientation distance to prey-simulating
electric fields. This study is the first to quantify the effects of crude oil on olfactory and
electrosensory sensitivity of marine predators.