Abstracts of AES Scientific Papers
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American Elasmobranch Society 1998 Annual Meeting
Guelph, Ontario, Canada
Editor's Note: The pages which follow list most of the papers presented at the
1998 meetings. Some abstracts were communicated after the initial deadline and have not
yet been added to this list. Certain special characters transmitted electronically are not
faithfully retained, and those corrections are being made. All corrections should be in
place by the end of September. Constructive critique is welcomed. (JCC)
ABSTRACTS - Part 1: Amesbury through Gruber
Amesbury, Elena, Crain, D. Andrew, Snelson, Franklin F., Jr., and Guillette,
Louis J., Jr.
Steroid hormones and uterine gland formation in the Atlantic stingray, Dasyatis
sabina.
Testosterone and 17-ß estradiol concentrations have been correlated with ovulation,
embryonic nutrition and parturition in various elasmobranch species. In this study, we
test the hypothesis that variations in serum concentrations of sex steroid hormones
correlate with reproductive changes occurring during gestation in the Atlantic stingray, Dasyatis
sabina. To test this hypothesis, blood and uterine samples were collected from rays
during the non-reproductive and reproductive seasons. Histological analysis revealed that
the uterus became glandular and highly vascular early in the reproductive season, when
females were carrying uterine encapsulated eggs. During early gestation, serum estradiol
concentrations were low, averaging 140 + 84 pg/ml. However, by mid-gestation, serum
concentrations were significantly elevated to 8293 + 4840 pg/ml. High estrogen
concentrations during mid- to late-gestation could be correlated with later gestational
events, not with gland formation and the onset of histotroph production. Testosterone
concentrations varied significantly through the cycle. Serum concentrations increased to
279 + 43 pg/ml during mid-gestation.
Araújo, M.L.
Reproductive strategy of a freshwater stingray Potamotrygon sp. from the
mid-Rio Negro Basin, Amazonas, Brazil.
An undescribed species of South American stingray, Potamotrygon, local name
"cururu," is endemic to the middle Rio Negro Basin. The reproductive biology,
migration, and the impact of its fishery have been investigated. Potamotrygon sp.
reach a maximum size of 300 mm disk width. It is a relatively small species for South
American freshwater stingray. They live in shallow floodplain and river margin with sandy
bottom and leaf litter. The water is usually warm, acidic and low in dissolved oxygen. The
reproductive cycle follows the river pulse, flood and drought, of the basin. Beginning in
July, when water level has peaked and started to descend, males and females are aggregated
in different areas; spermatogenesis starts earlier than vitellogenesis. After copulation
in August, ovulation may start three weeks later. The gestation period is about three
months and new born start to appear in November. The embryos are nourished by yolk sac and
trophonemata in the uterus. New born stay on top of female for 3-4 days before they assume
an independent life. During the reproductive season of 1996-97, females had one gestation
period. However, during the current El Niño year 1997-98, an earlier and prolonged
drought has occurred in the region, females have a second gestation.
Bodznick, D.
Electroreception: Mechanisms for detecting weak signals in noise
The elasmobranch electrosense is known for its extraordinary sensitivity which is
necessary for detecting the feeble, behaviorally-relevant electric fields in the sea. But
a cost of this sensitivity is vulnerability to electrical noise including that produced by
the fish's own behaviors. Behavioral, morphological and physiological specializations are
all necessary for extracting the important weak signals from noise. Each of these types of
specializations will be discussed and recent work on the physiological mechanisms for
extracting weak signals from noise will be reviewed. Physiological adaptations include
receptors that can operate over an extended voltage range without loss of sensitivity,
simple subtractive circuits in the hindbrain and sophisticated adaptive filter mechanisms
that involve the development of an expectation of the noise typically associated with each
of the fish's different behaviors. The end result of these mechanisms is that all traces
of the noise can be eliminated in the hindbrain without any loss of sensitivity to the
signals important for behavior.
Bonfil, Ramon
History and management of the dogfish fishery of British Columbia, Canada.
The fishery for dogfish (Squalus acanthias) of British Columbia is one of the
oldest shark fisheries in the world. This paper describes the main features in the long
evolution of this fishery. It also analyses the current situation and the management
system of the fishery. Presently, this is a small fishery characterized by strong market
limitations. Although there is a reasonably modern management system for the fishery, the
apparent success to keep the dogfish stocks at healthy levels for the major part of the
125+ years of the fishery is more a product of the lack of strong demand for dogfish which
has offered a fortuitous protection to the stock. Some suggestions for improvements to the
stock assessment and possible ways to develop the fishery are given. Keywords: Squalus
acanthias, fisheries management, British Columbia.
Borucinska, J. D., Whiteley, H. E., Trettel, J., Jr. and Benz, G. W.
Corneal pathology associated with the attachment of Ommatokoita elongata
(Copepoda: Lernaepodidae) (Grant) to the cornea of Greenland sharks (Somniosus
microcephalus).
Eyes from six Greenland sharks, Somniosus microcephalus (Bloch and Schneider),
infected with the parasitic copepod Ommatokoita elongata (Grant) were collected
Arctic waters of Victor Bay, Northwest Territories, for study. Transformed adult female
copepods, one per eye, were firmly attached to the corneas by an anchoring structure
(i.e., the bulla), and each bulla was associated with an opaque area on the cornea. In
addition to opacities associated with copepods at the time of collection, there were
several randomly distributed, small, round to irregular, full-thickness corneal opacities
which were not associated with copepods. The sclera at the limbus of infected eyes was
thickened at the radius corresponding to the feeding range of the copepod. Histologically,
corneal epithelial ulceration and heterophilic keratitis, disruption, mineralization, and
detachment of Bowman's membrane, thinning, disorganization, mineralization, and fibrosis
of the corneal substantia propria, and focal thinning of Descement's membrane were
observed. Mild heterophilic and mononuclear scleritis at the limbus that was continuous
with anterior uveitis was also present.
Based on our observations, we conclude that parasitism by O. elongata could lead
to severe visual impairment in Greenland sharks. However, our current research is directed
at understanding the physiology of the normal and altered visual apparatus in this
species. We are examining retinal enzymes and analyzing the photopigment prosthetic group.
In addition we are investigating alterations in the density and localization of glutamate
and GABA transporters in the M=FCller cells. The results of this work could be used to
objectively quantify the extent of visual impairment caused by O. elongata in
Greenland sharks.
Bourdon, J.
Odontological variations in the dentition of the manta ray (Manta birostris)
The study of Neogene batoids requires an understanding of extant species. An in-depth
understanding of the ondontological characteristics of the living fish is needed because
fossil material is largely limited to teeth and dermal denticles. Although cursory
descriptions of dentitions are often provided, these descriptions usually lack the detail
required for paleontological purposes. I initiated a study of a Manta birostris
dentition in an effort to identify manta-like teeth from the Miocene of North Carolina.
These teeth, which are commonly referred to as "simple peg-like", were more
variable then anticipated and clearly reflect their mobulid origin.
Brewster-Giesz, Karyl K., and Miller, Thomas J.
A comparison of a stage-based model and a yield per recruit model of the sandbar shark
(Carcharhinus plumbeus).
A stage-based model of sandbar shark populations indicates that fishing mortality (F)
needs to be reduced to 0.08 to ensure sustainability of the population. This model assumes
the natural mortality (M) is 0.10. However, stage-based models do not consider the effect
size limits may have on the fishery. We have used a yield per recruit (YPR) analysis to
examine the effects of adopting different size limits in the fishery. We used various
growth parameter estimates, an age of recruitment of 1, and M = 0.10. Based on estimates
from Casey et. al (1985), peak YPR occurs at a combination of F = 0.40 and an age of first
capture of 10 years or approximately 144 cm fork length (fl). When growth parameter
estimates from Casey and Natanson (1992) are used, peak YPR occurs at a combination of F =
0.30 and an age of first capture of 10 years or approximately 99 cm fl. Maximum YPR for F
= 0.08, predicted by the stage-based model, is approximately 80% of the peak YPRs found
above, and occur at an age of first capture of approximately 6 years. With growth
parameters estimated for 1991 to 1992 from Sminkey and Musick (1995), peak YPR occurs at
an F = 0.45 and an age of first capture of 9 years or approximately 124 cm fl. In this
case, maximum YPR for F = 0.08 is approximately 75% of the peak YPR estimated and occurs
at an age of first capture of approximately 5 years. These results suggest that in order
to have a stable population, YPR for each set of growth parameter estimates must be 20 to
25% smaller than the peak YPR.
Bruner, John Clay
Tooth replacement rate of the Great White Shark, Carcharodon carcharias
(Linnaeus, 1758)
Analysis of 31 jaws of Carcharodon carcharias, from specimens ranging in total
length from 143.5 to 460.9 cm, finds a range in tooth replacement rate of the Second
Lateral tooth family of (Upper/Lower) 106.24/113.59 days for young individuals to
225.90/242.18 days for old individuals using the Strasburg Plot Method. The Second Lateral
tooth family of the upper and lower jaws gives the best estimates of tooth replacement
rate. Upper and lower jaws demonstrate fluctuating asymmetry within the number of tooth
positions. The range in dental formulae of the upper/lower jaw for this sample is 11 to 15
- 0 - 12 to 14 / 11 to 16 - 0 - 11 to 15. One male (NOAA-NMFS Tag #610, TL 149.5 cm) taken
off New Jersey in the Northwest Atlantic, has a very minute parasymphysial tooth in the
upper right jaw. This is the first report of a parasymphysial tooth present in a white
shark and is considered here as an atavistic character. Using the tooth replacement rate
of 106.24 days/tooth, and previous reports of near term white shark embryos with 3 to 4
full tooth sets present in their alimentary canals, an estimate of when tooth replacement
begins prior to parturition and therefore a minimum estimate of gestation period for the
white shark can be calculated as 425 days.
KEYWORDS: Carcharodon carcharias, tooth replacement rate, gestation period,
fluctuating asymmetry, parasymphysial tooth, dental formulae
Caira, J. N.
Tapeworms as indicators of elasmobranch feeding biology
Tapeworms are potentially valuable indicators of elasmobranch feeding biology because
their life cycles require more than one host species and transmission between these host
species is accomplished through predation. Thus, tapeworms enter their elasmobranch hosts
when the intermediate hosts in which they reside as larvae are consumed by elasmobranchs.
The lack of life cycle data and lack of information on degree of specificity for their
final intermediate hosts currently limits the specific predictive value of tapeworms.
However, some generalizations can be made from the complete tapeworm assemblage observed
in a host. Inconsistencies in the tapeworm assemblages of related elasmobranch species can
provide indirect evidence of differences in food habit. For example, the tapeworms of
megamouth are fully consistent with this shark's status as a filter feeder rather than an
apex predator. Changes in the tapeworm fauna of a host species over time may reflect age
related changes in diet. For example, the differences in the tapeworm fauna between small
and large individuals of the Atlantic nurse shark may be indicative of a shift in food
habit. Depending on their specificity for their intermediate hosts, tapeworms may act as
indicators of specific prey species consumed by a host species or individuals.
Carlson, J.K. and G. R. Parsons
Estimates of daily ration and a bioenergetic model for the blacknose shark, Carcharhinus
acronotus.
Daily ration estimates and bioenergetic models for blacknose sharks, Carcharhinus
acronotus, were developed from laboratory experiments on metabolism and growth at 28"2.0°C. Size-specific metabolic
rates were determined by closed-system respirometry and growth estimated through sharks
held in captivity. Daily ration was estimated for sharks 0.5-0.8 kg, 1.5 kg, and 3.5 kg
using the equation of Winberg (1956). Daily ration (%body weight/day) decreased with body
size and was estimated as 1.56 for smallest sharks and 0.87 for largest sharks based on a
diet of menhaden, Brevoortia spp. General energy budgets were constructed for each size
class. No estimation of reproductive investment was made because all individuals were
determined to be immature. Routine metabolism generally accounted for the largest
percentage of the energy budget but differed among size classes. Metabolism increased with
body size from 60% of the energy budget in small sharks (0.5-0.8 kg) to 71% for the
largest sharks. In contrast, growth of somatic tissue was highest in small sharks
(mean=9.9%) but was 1.5% for the larger sharks.
Carrier, J.C., F.L. Murru, R.L. Davis, M.T. Walsh, S. Dover, and H.L.
Pratt Jr.
Mating, Gestation, and Birth in Nurse Sharks (Ginglymostoma cirratum): The
Complete Story.
Shark mating activities were filmed in the wild and females recorded as participants in
the June mating events were subsequently captured. Each shark was examined in the field
using ultrasonography to determine the extent of the presence of eggs, and two animals
were selected from the reproductively active population and transferred to holding
facilities at Sea World of Florida (Orlando). Serial blood samples were taken from both
animals during the captive period. One animal was examined monthly using ultrasound and,
in August and October, by intrauterine endoscopy to assess the progress of the presumed
pregnancy. The endoscopic procedures verified the presence of numerous eggs and revealed
the appearance of young that had emerged from egg cases. Ultrasonic imagery was used to
further confirm the presence of hatchlings. Both animals eventually carried young to term.
The first births occurred after 3.5 months, and these earliest births occurred with
remnants of the yolk sac in place. The longest surviving offspring was born at the end of
October, approximately 4.5 months following observations of mating in the field. This time
frame differs from previous estimates of six months that were based solely on observations
from necropsy. A total of 15 offspring were born, five from one animal and ten from the
second animal. Only one newborn survived. Both of the females and the surviving offspring
were tagged for return to the site of capture. The value and restrictions of the
evaluation methodology and the limitations of the capture and captive elements are
discussed.
Childs, J.N.
Seasonal Habitat Use of the Flower Garden Banks National Marine Sanctuary by Pelagic
Elasmobranchs
The Flower Garden Banks National Marine Sanctuary consists of three topographic highs
in the northwestern Gulf of Mexico, that support the northernmost coral reefs on the North
American continental shelf. These banks are seasonally inhabitated by a variety of pelagic
elasmobranchs, which often occur in considerable numbers. To date, 20 elasmobranch species
have been reported or recorded in the Sanctuary. Seasonal occurrence, size, and social
organization will be presented for several of the pelagic elasmobranchs known to frequent
the Sanctuary.
Clermont, S.M. and S.H. Gruber
EXPERIMENTAL HOMING IN JUVENILE LEMON SHARKS, Negaprion brevirostris, AT
BIMINI, BAHAMAS
This study represents the first experimental trials on an elasmobranch's ability to
return home from an unfamiliar area. Ten juvenile lemon sharks (66.0 to 96.0 cm total
lengths) were tagged with ultrasonic transmitters and their home ranges determined over a
minimum of 3 days of tracking. Thereafter they were recaptured and experimentally
displaced 4 km off shore, south of the island. From late October 1997 to February 1998,
seven displacements during daylight and three displacements at night were performed.
Sharks were tracked for between 2 and 14 hours, and in all ten cases successfully returned
to their home ranges. The behavioral pattern of the sharks at the time of release was
characterized by circling and vertical migrations suggesting that they were attempting to
orient to sensory cues in a way similar to homing pigeons. Further, sharks released at
night appeared to change course to a more direct route home at sunrise, suggesting that
visual cues may play an important role in the final stages of homing. Therefore, we are
testing the hypothesis that sharks are able to use the position of the sun as a cue for
orientation. Finally, we are carefully documenting the behavior of sharks at the time of
release to give insight into possible mechanisms of orientation. Supported by the Florida
State Department of Education with a grant to SHG.
Cortes, E.
Standardized diet compositions and trophic levels of sharks.
Sharks are marine consumers believed to occupy top positions in marine food webs. But
surprisingly few quantitative estimates of trophic levels (TL) exist for this group. With
the hope to better define the ecological role of sharks in marine communities, this paper
presents standardized diet compositions and trophic levels for a suite of species. Dietary
compositions were calculated from published quantitative studies using a weighted average
index. TL values of the eleven food types used to characterize the diet were obtained from
published accounts and used to calculate fractional trophic levels for 151 species
representing seven orders and 21 families. Sharks as a group are tertiary consumers
(TL>4), and significant differences were found among six orders compared, attributable
mainly to differences between orectolobiformes and all other orders. Among four families
of carcharhiniform sharks, carcharhinids had significantly higher TL than triakids and
scyliorhinids, but not sphyrnids. When compared to TL for other top predators of marine
communities obtained from the literature, mean TL for sharks was significantly higher than
for seabirds, but not for marine mammals. Trophic level and body size were positively
correlated, with the fit increasing when the three predominantly planktivorous species
were omitted and when considering carcharhinids only.
de Silva, J.A., R.E. Condrey, and B.A. Thompson
Sharks associated with the gulf menhaden Brevoortia patronus fishery and their
relavance to shark populations of the Atlantic and Gulf of Mexico
We describe the shark bycatch in the U.S. gulf menhaden fishery their species
composition, fates, distribution and estimate the annual number of sharks caught by the
fishery. Ten species of sharks were identified, with blacktip sharks Carcharhinus
limbatus accounting for 50% of those observed. Approximately 75% of the sharks
encountered died, 12% were released disoriented and 8% were released healthy. Using
loglinear and logit models we determined spatial and temporal patterns in the shark
bycatch. Contrasts revealed the odds of observing a fishing set with shark bycatch was
significantly greater in June-August than September-October. The odds of observing shark
bycatch during April-May was also significantly different from September-October. However,
these differences were only apparent east of 93 degrees W (central Louisiana). For the
1994 and 1995 fishing seasons we estimated an annual take of approximately 30,000 sharks.
Stomach analyses on sharks indicated that the menhaden schools were functioning as a
foraging base for those sharks captured. While detailed age and growth information were
not collected, comparisons to length distributions in the literature indicated that the
fishery appears to be impacting the summer nursery grounds of sharks in the northern Gulf
of Mexico.
Ebert, David A.
Dietary changes associated with life history and sex for the sevengill shark:
implications for intraspecific competition and resource partitioning.
Dietary changes associated with life history stage and sex for the sevengill shark are
analysized and discussed. Data collected over a 15 year period were used to analyze the
sevengill shark's dietary habits. The size categories were grouped so as to approximate
this shark's life history stage; juvenile, adolescent, and adult. In addition, the diet of
male and female sharks of each life history stage was compared to note any differences
between the sexes. Dietary changes associated with an elasmobranchs life history and
sexual stage of development may explain how food resources within a given territory are
partitioned such that competiton between conspecifics at different stages of life may be
reduced. A reduction in competition between conspecifics may have evolved to ensure that
resource partitioning and cannibalism would be reduced between conspecifics at different
stages of life. Thus increasing survivorship for the species during the critical early
stages of life. The implications of dietary changes as related to life history and sexual
stage of development may be complex and far reaching, yet may play a critical role in the
success of elasmobranchs as a group.
Ellis and S.J. Rogers
The feeding ecology of elasmobranch fishes from the British Isles.
The diets of inshore elasmobranchs from the waters of the British Isles are reviewed
from historical and contemporary data. The feeding habits of ten of these species (Scyliorhinus
canicula, Scyliorhinus stellaris, Mustelus asterias, Galeorhinus galeus, Squatina
squatina, Raja brachyura, R.clavata, R.microocellata, R.montagui and R.naevus)
are discussed in relation to their comparative feeding ecology, their distributions in the
Irish Sea and English Channel, the demersal faunal assemblages to which they belong and
their dentition.
Ertan, B.A., L. Park, M.J. Stanhope, and M.S. Shivji
Characterization of blue shark (P. glauca) mitochondrial genome control region
sequences for population genetic studies.
Blue sharks are highly vagile elasmobranchs with a circumglobal temperate and tropical
distribution. This species displays complex reproductive and migratory behavior, making it
difficult to realize a clear picture of genetic stock structure. We report here our
examination of the utility of mitochondrial (mt) DNA control region sequences for
assessing blue shark genetic structure on a global scale. Complete control region (1069 bp
in size) sequences were obtained from 16 blue shark individuals collected from 5 globally
distributed sites. Contrary to the case in most vertebrates, this locus is remarkably
conserved in blue sharks. The 11 haplotypes observed differed from each other by 5 or less
single nucleotide substitutions or indels over the entire control region. Most of the
genetic variation (8 of 12 variable sites) was restricted to the 350 bp region adjacent to
the tRNA-proline gene, with no variation observed in the central 400 bp section of this
locus. Phylogenetic analysis revealed no indication of geographic population structuring.
We infer from these data that either the blue shark mt control region may be too conserved
to be useful for population genetic studies, or that dispersal over a global scale is
occurring, or has occurred in the recent past.
KEY WORDS: Blue shark, Prionace glauca, genetic variation, mitochondrial DNA,
control region.
Edmonds, Margaret A. and Motta, Philip J.
Modulation of the feeding behavior of the horn shark, Heterodontus francisci
Many fishes are capable of modulating prey capture behavior according to prey size or
elusivity. The physical attributes of the prey appear to affect attack behavior, but
factors such as position or location of prey may also be important. This study tests the
hypothesis that capture behavior can be modulated according to prey accessibility. Using a
high-speed video kinematic analysis, prey capture events are examined for the horn shark, Heterodontus
francisci. Each individual encounters three feeding situations: (1) squid pieces
resting freely on a platform; (2) attached to a platform; and (3) fitted snugly in a tube.
Using suction behavior, H. francisci successfully captures prey in each feeding
situation with a common series of kinematic events. However, repeated bites with
palatoquadrate protrusion occur during attached feedings, and the labial cartilages extend
to form a seal around the tube opening during tube feedings. Average bite duration for
three individuals is approximately 124 ms, although significant inter-individual variation
is observed. Preliminary analyses show that H. francisci is capable of modulating
the duration of some kinematic events during capture, such as mandible elevation and
palatoquadrate protrusion/retraction, according to the accessibility of the prey.
Ferry-Graham, L.A.
Mechanics and modulation of prey capture in generalists vs. specialist predators: a
kinematic comparison using two carcharhiniform sharks.
Recent work on teleosts suggests that attack kinematics (= behaviors) may be modified
by a predator based on specific attributes of the prey item Sharks are generally presumed
to be highly visual predators and, thus, it is reasonable to expect that they are also
capable of such modulation. Work on swellsharks, Cephaloscyllium ventriosum,
however, indicated that prey size did not induce modulation. To determine the generality
of this finding, I investigated the effect of both prey size and elusivity on prey capture
kinematics in a species with a more generalized diet, the leopard shark, Triakis
semifasciata. Sharks were filmed using high speed video feeding on two sizes of the
same prey (shrimp pieces) and two more elusive prey (earthworms and live mud shrimp). In
leopard sharks, size effects were noted for kinematic parameters related to buccal cavity
expansion. Elusivity effects were not detected for any parameters, but a performance
consequence was also not detected with increasingly elusive prey. These studies
cumulatively suggest that rather than possessing a single attack behavior, generalists are
capable of kinematic modulation. They also suggest that the phrase "master of
none" that is typically used to describe a "jack of all trades" generalist
may not apply.
Ferry-Graham, L.A.
Mechanics of ventilation in the swellshark, Cephaloscyllium ventriosum
(Scyliorhinidae).
Most fishes are faced with one of two options for gill ventilation: physically moving
their gills through the medium ("ram" ventilation), or actively pumping the
medium over their gills via "buccal pumping". The basic pattern of mechanical
movements during buccal pumping was studied over 40 years ago. However, unique patterns of
water flow through the head of the swellshark during feeding suggested that the generation
of respiratory currents might not be as simple as once thought. Using pressure transducers
implanted into the buccal and gill cavities of actively respiring swellsharks, it was
discovered that pressure traces vary within and among individuals. These data, combined
with both video and electrical impedance data for the displacement of the lower jaw and
gill septum, suggest that although the classic pattern of pressure generation regularly
occurs, at least one novel pattern of pressure generation exists. During this novel
pattern, pressure reversals exist over a much longer duration than previously thought
possible, and at much larger magnitudes. It is questionable whether the classic
alternating "suction" and "pressure" pumps are always used for moving
water over the gills unidirectionally, and whether continuous water flow over the gills
exists for large intervals of time in this species.
Forlano, P.M., Maruska, K.P., King, J.A., Sower, S.A., and Tricas, T.C.
Distribution, evolution, and functional significance of gonadotropin-releasing hormone
in the elasmobranch brain.
The basic structure of gonadotropin-releasing hormone (GnRH) includes at least nine
different forms found across the vertebrates. Some forms are believed to serve
extrapituitary functions, as indicated by their wide distribution throughout the brain.
Chicken II GnRH (cII) is considered the most evolutionarily conserved variant because it
is generally found to coexist with at least one other form in all the jawed vertebrate
classes, while dogfish GnRH (df) is found exclusively in elasmobranch fishes. This study
examined the distribution of df and cII GnRH-immunoreactive (ir) cell bodies and fibers in
the brain of the Atlantic stingray, Dasyatis sabina, by immunocytochemical
methods as a first step to correlate anatomical distribution with possible functionality
of the different forms in an elasmobranch. Dogfish GnRH-ir cells and fibers were localized
in the ganglia of the terminal nerve and throughout the ventro-caudal telencephalon and
preoptic area, while a large, discrete cII GnRH-ir cell nucleus was found in the midbrain
tegmentum with main projections to the diencephalon and hindbrain. This is the first study
to show differential distribution of cII and df GnRH in the elasmobranch brain, and
supports the hypothesis of divergent function of GnRH variants such as gonadotropin
control and neuromodulation of sensory function. Keywords: stingray, brain, endocrinology,
sensory biology
Gelsleichter, J. and J. A. Musick
Hormonal regulation of growth in vertebral cartilage of the clearnose skate, Raja
eglanteria.
In most elasmobranchs, growth of vertebral cartilage proceeds incrementally and forms
periodic marks that are extensively used in age and growth studies. Such research is
essential for the management of these fishes, yet is often limited by poor understanding
of the physiological processes that regulate vertebral growth. Since skeletal growth in
most vertebrates is under strict control of the endocrine system, the goal of this study
was to investigate the hormonal regulation of vertebral growth in the clearnose skate, Raja
eglanteria. Synthesis of cartilage matrix glycosaminoglycans (GAG) was used as a
marker for vertebral growth using an in vitro culture system. The effects of growth
hormone (GH), insulin-like growth factor-I (IGF-I), tri-iodo-L-thyronine (T3), and
corticosteroids on GAG synthesis were assessed, due to their well-documented roles in
cartilage growth. The effect of calcitonin (CT) on vertebral growth was also investigated,
due to its involvement in the skeletal physiology and mineral homeostasis of terrestrial
vertebrates. Finally, the effect of parathyroid hormone-related protein (PTHrp) on GAG
synthesis was examined, because of its critical role in mammalian skeletogenesis. PTHrp
has recently been detected in the perichondrium of elasmobranch vertebral cartilage, thus
it may play a role in chondrocyte proliferation and differentiation. The present study is
the first investigation on the hormonal regulation of elasmobranch skeletal physiology.
Gruber, S.H. and R.E. Hueter
SENSORY BIOLOGY OF VISION IN THE ELASMOBRANCHS.
The visual system of elasmobranchs is reviewed with respect to structure, function and
behavior. As a general review, the visual systems of elasmobranchs and teleosts are
compared and contrasted. Similar visual structures and functions of phylogenetically
unrelated elasmobranchs are shown to occur in species occupying similar habitats and often
displaying comparable behaviors. For example, the vertical slit pupil is found in active,
pelagic and epibenthic elasmobranchs as widely divergent as carcharhinid sharks and
myliobatid rays. Likewise, a strong correlation is found between depth of occurrence and
position of the peak spectral absorption of the visual pigment, regardless of systematic
affiliation. Performance testing of the visual system also reveals features adaptive to
behavior and environment. Since the majority of information on visual performance has been
gleaned from the lemon shark (Negaprion brevirostris), that visual system is
reviewed. Correlates between visual structure, function and behavior show that the lemon
shark has an ecologically broad period of activity and is able to carry out its normal
behavior throughout the entire diel period. The review ends with an unexpected finding
relating the lemon shark's visual pigments to habitat changes with aging. The final
conclusion is that an understanding of how elasmobranch visual structure and function are
correlated with behavior and ecology is barely at a beginning stage, and the field of
elasmobranch vision, even after extensive study in recent decades, remains wide open.
Supported by a grant from the Florida Department of Education to SHG and NOAA/NMFS grants
to REH.
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