Florida Museum of Natural History

Abstracts of AES Scientific Papers

American Elasmobranch Society 2001 Annual Meeting
State College, Pennsylvania
ABSTRACTS - Part 4: Rand-Merson through Wyffels


11517 S.W.64th Street #A, Green Marine Institute, Miami, FL 33173

Shark dive tourism: blessing or menace?

Shark dive tourism has increased dramatically in the last years and turned into a major income source for dive operations around the world. Along with this increasing popularity, questionable and unsafe habits have and led to heated debates over the meaning and dangerousness of this type of diving. A major source of problem is the cowboy mentality of many operations left unguided and not been backed up by scientific leadership. Shark diving has, if properly handled, a great potential in demystifying the wrongful image of sharks, and is a very important tool of applied conservation if mixed with correct information. However it is inevitable that this increasing boom must be guided and restricted by governmental agencies and other bodies to establish proper leadership, minimize risks of accidents, and maximize education. This paper focuses on the pros and cons of shark diving, its present problems caused by unqualified input, and currently established operations running a strictly scientific oriented dive operation. (Session 16, Sunday, July 8, Penn Stater, Room 207, 10:45)



11517 S.W.64th Street #A, Green Marine Institute, Miami, FL 33173

Exploratory behavior: a typical cause for accidents with white sharks, Carcharodon carcharias

Shark accidents have wrongfully been categorized by authorities and inductive generalizations, forming an erroneous picture of contact approaches by sharks. Shark accidents are multi-layered with much more potential causes than initially assumed. One of the initial reasons why some shark species bite humans is exploratory behavior. A lot of media attention is given to white shark (Carcharodon carcharias) interactions with surfers and spearfishermen. It is inevitable that shark accidents be analyzed from a deductive point of view, excluding anthropomorphism and approaches of pure theoretical nature. The goal of shark accident analysis is not the description of the outcome of an interaction the accident - but the reasons behind it that eventually led to such an accident. This papers uses the analysis of the first ever filmed white shark accident on a diver and compares it with 10 randomly chosen white shark incidents on file where comparable injuries were found. (Session 7, Saturday, July 7, Penn Stater, Room 207, 8:00)



(EKR) 11517 S.W.64th Street #A, Green Marine Institute, Miami, FL 33173; (IG) P.O. Box W356, Off The Edge Research, Sydney, Sydney 2100

Australia Human induced approach behaviour on the bull shark (Carcharhinus leucas)

Approach behaviours of sharks leading to attacks on humans have been categorized in earlier years from a mostly theoretical point of view. However different shark species possess different approach behaviours due to size, sex, environment, situational circumstances and others. It is inevitable to study species individually exposing them to standardized scenarios and examine them on a intra and interspecies level. One of the major species being involved in incidents with humans is the bull shark (Carcharhinus leucas). An overview of general approach behaviour and interpretation of this species during different scenarios is given. (Session 3, Friday, July 6, Penn Stater, Room 207, 3:45)



(RSR) Centro de Cincias Exatas e da Natureza, Universidade Federal da Paraiba, Joao Pessa, PB 58059-900 Brazil; (SHG) 9300 S. W. 99th Street, University of Miami, Miami, FL 33176; (AHDB) Centro de Cincias Exatas e da Natureza, Universidade Federal da Paraiba, Joao Pessoa, PB 58059-900 Brazil; (MCEK) Goetoeweg #3, Curcao Sea Aquarium, Curcao, Netherlands Antilles

Diet of young lemon sharks (Negaprion brevirostris) at an isolated seamount, Atol das Rocas, Brazil.

The diet of young lemon sharks was studied in Atol das Rocas, 144 nautical miles off Natal, Brazil, based on a non-destructive, stomach eversion, sampling technique. Sharks were captured with gillnets and dipnets in an embayment of the atoll's lagoon, and subsequently anesthetized with tricaine. Stomach contents obtained through gastric eversion were preserved in isopropanol for qualitative and quantitative analyses. Eighteen sharks, total lenghts between 64.0 and 69.2 cm were sampled in March 2000. The stomachs of eight individuals were empty. One had a stomach so full that it precluded eversion. All the other nine sharks had teleost fishes in their stomachs (Albula sp., Ichthyapus sp., Synodus sp. and Sphyraena sp., as well as unidentified scales and vertebrae). Four of the sharks had cephalopod remains in their stomachs, including a possibly undescribed species of octopus. Further sampling to be conducted through the year 2001 will provide additional data on the diet of the species and on its role as an apex predator in atoll's ecosystem. (Session 17, Sunday, July 8, Penn Stater, Room 206, 8:00)



West Virginia State College Institute, WV 25112

The peculiar cestode fauna of alopiid sharks

Alopias pelagicus and A. superciliosus are parasitized by species of Litobothrium, a morphologically unique genus of tapeworms found exclusively in selected lamniform shark species. These shark species are also host to tapeworms of the genera Marsupiobothrium and Paraorygmatobothrium. In addition, a cestode of unknown taxonomic affinity has been collected from A. pelagicus. The cestode fauna of A. vulpinus differs from the former two species. Litobothrium is absent from A. vulpinus, and immature specimens of cestode genera commonly parasitic in lamnid sharks and carcharhinid sharks have been collected from Alopias vulpinus. Paraorygmatobothrium exiguum has been found in A. vulpinus from the Atlantic and Pacific oceans. A species each from the genera Pithophorus and Paraorygmatobothrium have also been described from A. vulpinus, collected from the Pacific ocean. Molecular phylogenetic analyses of the cestode genera Litobothrium, Marsupiobothrium, Paraorygmatobothrium, the unknown cestode, and a number of other elasmobranch tapeworm species indicate the following: 1. the ªnative cestode fauna of alopiid sharks may be relictual in nature, and 2. other species clearly have been borrowed several times from a cestode clade evolutionarily linked to carharhiniform sharks. (Session 10, Saturday, July 7, Penn Stater, Room 207, 2:45)



(DS) 250 Tequesta Dr., Suite 304, Perry Institute for Marine Science/Caribbean Marine Research Center, Tequesta, FL 33469; (PJM) 4202 E Fowler Ave, SCA 110, University of South Florida, Tampa, FL 33620

The excavation kinematics and feeding-ventilatory coupling of the Atlantic cownose ray, Rhinoptera bonasus.

This study explores the feeding behavior and mechanics of a myliobatid, the Atlantic cownose ray Rhinoptera bonasus. High-speed videography was used to determine the prey excavation and capture kinematics of Rhinoptera bonasus. Videography and dye extraction were used to determine feeding behavior and water flow patterns during prey capture in the lab and in situ. Prey excavation involves repeated jaw opening and closing movements resulting in fluidization of the sand to uncover the benthic prey. Inertial suction prey capture during the preparatory phase is characterized by depression of the subrostral lobes and a slight closure of the jaws. The expansive phase begins with the closure ofthe spiracle, followed by depression of the mandible and protrusion of the palatoquadrate and nasal cartilages. During the compressive phase, the mandible is elevated toward the palatoquadrate and nasal cartilages grasping the prey. In the recovery phase, the jaws are brought back to a resting position and the spiracle is reopened. Thus, the feeding and ventilatory movements are coupled in time such that the ray only brings in water through the mouth during food capture. (Session P-14, Saturday, July 7, Penn Stater, Deans Hall)



Fisheries and Illinois Aquaculture Center, Southern Illinois University, Carbondale, IL 62901

Genetic Structure Analysis of the Shortfin Mako (Isurus oxyrinchus) from Multiple Microsatellite Loci.

We investigated genetic population structure in the shortfin mako shark using multiple microsatellite markers. A previous study, which utilized restriction fragment length polymorphism of mitochondrial DNA, found significant structure. We developed several microsatellite loci by screening a sub-genomic library for di-, tri-, and tetra-nucleotide repeats. Positive clones were sequenced and primers for the polymerase chain reaction were designed. Primers that amplified polymorphic loci were used to genotype samples from four oceans (N. Atlantic, S. Atlantic, N. Pacific, and S. Pacific) via polyacrylamide gel electrophoresis. Analyses of Molecular Variance (AMOVA) results show lower levels of genetic divergence than expected. Primers developed were tested for possible cross-species amplification in several other species of sharks. The markers may be useful for studies in species including white sharks (Carcharodon carcharias), thresher sharks (Alopias spp.) and porbeagle (Lamna nasus). (Session 3, Friday, July 6, Penn Stater, Room 207, 4:15



229 Arts Building, The Pennsylvania State University, University Park, PA 16802

Images of rays and sharks in seventeenth-century European art

From the 1570s to the close of the seventeenth century, artists in Italy and the Low Countries produced a substantial number of still life paintings representing the bounties of the ocean. Whether within the context of fish-stalls at local markets, kitchen interiors, or heaping piles of specimens carefully arranged on shore, the demand for and profusion of these images in early modern Europe raises a host of cultural and anthropological questions. Marine paintings in which rays and, less frequently, sharks figure prominently have been interpreted as lavish displays of abundance and wealth. Working from Neapolitan, Dutch and Flemish examples, this paper sheds light on the significance of sea animals as commodities in seventeenth-century economies. Perhaps more revealingly, my study examines images of these creatures as emblems of abundance in the context of European seaport and fishing centers where they functioned as important elements of the cult of the sea. (Session 26, Monday, July 9, Penn Stater, Room 207, 10:30)



(CAS) 1600 Ken Thompson Parkway, Mote Marine Laboratory, Sarasota, FL 34236; (GB) PO Box 117800, University of Florida, Gainesville, FL 32611

Status of the Atlantic sharpnose shark (Rhizoprionodon terraenovae) US waters: results of preliminary analysis using an age-structured model.

Landings of small coastal sharks, and especially the Atlantic sharpnose shark

(Rhizoprionodon terraenovae), have increased in US waters as populations of large coastal species have declined. We are assessing the impact of fishing on R. terraenovae using an age-structured population model. A Bayesian approach to the assessment is used to incorporate both prior information about this and other species, and uncertainty about the data and population processes. Rhizoprionodon terraenovae is caught in a wide variety of fisheries, including the shark longline fishery, drift gillnet fishery, menhaden purse-seine fishery, shrimp trawl fishery, coastal gillnet fisheries, recreational fisheries, head boat fisheries, and many others. This diversity of fisheries makes the assessment more difficult. Data on the biology, gear selectivity, landing and discard practices, and population structure are included in the model. We present the preliminary results from the assessment for R. terraenovae. Difficulties with the assessment are discussed, and avenues for future research to improve the assessment are identified. (Session 3, Friday, July 6, Penn Stater, Room 207, 2:00)



(CAS) 1600 Ken Thompson Parkway, Mote Marine Laboratory, Sarasota, FL 34236; (JIC) Mote Marine Laboratory, National Marine Fisheries Service, Sarasota, FL 34236

Habitat issues in the conservation of smalltooth sawfish (Pristis pectinata)

Smalltooth sawfish (Pristis pectinata) are critically endangered in the western Atlantic, and are currently being considered for inclusion on the US Endangered Species List. Recovery of the sawfish population will depend upon reducing fishing mortality and providing sufficient suitable habitat. Information on the current distribution of P. pectinata was gathered from the public and research surveys. The results of this study have identified several human-mediated habitat issues in the conservation of sawfish. These include the use of canal developments as alternative habitat, and the use of power station outflows as thermal refuges during winter. The importance of these issues is discussed and future research directions suggested to further understand their impact. (Session 16, Sunday, July 8, Penn Stater, Room 207, 9:45)



(GBS) P.O. Box 68, Massachusetts Division of Marine Fisheries, Vineyard Haven, MA 02568; (GWB) One Broad St., Tennessee Aquarium, Chattanooga, TN 37401

Sharks under Arctic ice: acoustic tracking of Greenland sharks, Somniosus microcephalus

In May 1999, six Greenland sharks (Somniosus microcephalus) were tracked off northern Baffin Island, Nunavut, Canada to determine if vertical and horizontal movements were indicative of seal hunting behavior. Six Greenland sharks in the size range of 190-355 cm fork length were caught on lines baited with seal meat, then tagged with depth sensitive individually coded ultrasonic transmitters and tracked for 5.5-31.4 h. Horizontal and vertical movements were determined from data collected by manual tracking with a directional hydrophone and by five remote receivers set in a listening array. Horizontal rates of movement were found to differ significantly (p< 0.01) among sharks and ranged from 0.015 to 0.463 m s - 1, with a mean of 0.217 m s - 1. Rates of descent for the six tracked sharks were significantly (p<0.01) higher than rates of ascent. The sharks exhibited no apparent depth or temperature preference and during 31%, 26%, and 42% of the total aggregate tracking time, they were 0-70 m, 70-170 m, and 170-280 m deep, respectively. Pooled data for the six sharks indicated nocturnal movement into shallower depths. This study demonstrated that Greenland sharks are not exclusively benthic sharks and may actively hunt ringed seals just under landfast sea ice. (Session 10, Saturday, July 7, Penn Stater, Room 207, 11:00)



(ALS) 141B Kersey Rd, University of Rhode Island, Wakefield, RI 02879; (NK) NMFS Laboratory, National Marine Fisheries/NOAA, Narragansett, RI 02882; (GS, RG) DMF, MA Division of Marine Fisheries, Vineyard Haven, MA 02568

The physiological effects of angling on post-release survivorship in juvenile sandbar sharks (Carcharhinus plumbeus).

A post release survivorship study of juvenile sandbar sharks (Carcharhinus plumbeus) was conducted in Delaware Bay during 1999-2000. A total of 104 sharks were captured and sampled for changes in blood chemistry after exposure to exhaustive exercise. Of these, 24 sharks were angled in the field, sampled, tagged, and released. The remaining 80 sharks were transported to a holding tank, allowed to recover, and half were angled to exhaustion. To quantify recovery, these fish were sampled at 0,1.5, 3, 6, 10, 14 and 24 hours, then tagged and released. Blood was obtained by caudal venipuncture and analyzed immediately for blood gasses and glucose. Serum samples were sent to a commercial laboratory for the determination of blood metabolites, proteins, and electrolytes. Blood levels of lactate, PCO[-2], glucose, potassium, Ca, Mg, and CK were elevated during stress, while pH and HCO[-3,+-] levels declined. Most metabolites returned to normal within 6-10 hours. Moreover, 5 sharks were recaptured 0.03-12 months after release over the course of the study. These preliminary data indicate that sandbar sharks are able to physiologically recover after the exhaustive exercise associated with rod and reel angling and therefore, catch and release fishing may not severely impact neonatal and juvenile sandbar sharks in important nursery areas. (Session 20, Sunday, July 8, Penn Stater, Room 207, 11:30)



Feldbergstrasse 22, Basel, Switzerland, Basel, Basel 4057 Switzerland

The activity pattern of the two carcharhinid shark species bull shark, Carcharhinus leucas and blacktip shark, Carcharhinus limbatus, in the estuary of the Ten Thousand Islands, Florida

Ultrasonic telemetry was used to determine the movement pattern of juvenile bull sharks Carcharhinus leucas and blacktip sharks, Carcharhinus limbatus in the estuary of the Ten Thousand Islands. A total of 11 bull sharks and 17 blacktip sharks were tracked between 4 and 23 hours. The sharks were tracked manually using a small boat with a trawling engine. The study was conducted during May through October 1999 and 2000. All bull sharks except one remained within 2000 meters of the capture cite and they never left the estuary while tracking. Bull sharks movement patterns did not appear to be effected by the tide. Blacktip sharks left the area with the outgoing tide to then return on almost the same path with the incoming tide. Blacktip sharks swam as far as 7000 meters away from the capture site and up to 3000 meters away from the shoreline. Only one bull shark displayed no tendency towards a specific core area where all the others showed places with significantly higher occupation rates, especially during daytime. All except three blacktip sharks occupied core areas. There was no significant difference in the swimming speed of the two shark species. (Session 10, Saturday, July 7, Penn Stater, Room 207, 11:30)



400 Sumter, University of South Carolina, Columbia, SC 29208

Phylogeography and conservation genetics of an endangered marine fish, the barndoor skate, Dipturus laevis

Analyses of long-term research surveys on the continental shelf between the Grand Banks of Newfoundland and southern New England suggest that the barndoor skate is at risk of extinction. This flattened, bottom-dwelling fish, which can grow up to 5 feet long, was one of the more abundant fish in the Northwest Atlantic as recently as the 1950s. But since then, heavy fishing pressures reduced the numbers of barndoor skates so precipitously that it is now the subject of a petition for listing under the U. S. Endangered Species Act. To help in the development of a management strategy to preserve the barndoor skate, we undertook a survey that consisted of the genetic typing of individual barndoor skates using mitochondrial markers. The sites where these individuals were collected were selected such that they would span a large portion of the barndoor skates' geographic range. Statistical analysis of the data generated by this survey provided estimates of genetic diversity, degree of population subdivision, effective population size, and evolutionary relationship among sampled populations. The implication of these statistical findings are discussed with respect to the future management of barndoor skate populations, particularly regarding maintenance of genetic variation, reintroduction strategies, site selection of marine preserves, and the setting of future conservation priorities. (Session 38, Tuesday, July 10, Penn Stater, Room 207, 2:30)



(APS) 321 Steinhaus Hall, UC Irvine, Irvine, CA 92697; (LH) Department of Biology, Wesleyan University, Middletown, CT 06459

Why are cartilaginous fishes so big?

The largest fishes in the sea have cartilaginous skeletons. There are a number of hypotheses as to why this might be true, though none explain the diversity and number of very large cartilaginous fishes. There is a functional limitation on size that may be a general explanation for the skewed size distribution. As a fish grows, its weight and the negative buoyancy of its skeleton grows as the third power of length. At the same time the thrust and lift generated by the fins grow with the square of length. At some length, there will be insufficient lift to counter the sinking force of the skeleton. A cartilaginous skeleton weighs less than a bony skeleton of the same length, so this theoretical maximal size is larger for the cartilaginous fish. This may provide an explanation for why the eight largest fishes are cartilaginous and the two largest bony fishes have cartilaginous skeletons. We show that cartilaginous fishes do have lighter skeletons per unit length than do bony fishes and that the heaviest cartilaginous skeletons are from benthic fishes. (Session 29, Monday, July 9, Penn Stater, Room 105, 8:00)



(APS) 321 Steinhaus, UC Irvine, Irvine, CA 92697; (RK, TR) CT scan Facility, UT Austin, Austin, TX 78712

The ontogeny of the hornshark chndrocranium and the evolution of trabecular cartilage.

The skeleton of the chondrichthian fishes is composed of cartilage, some of which is calcified. The vertebrae are completely calcified, while the remainder of the skeleton has one or more thin layers of calcified blocks covering a core of uncalcified cartilage. In a clade of stingrays containing hard-prey specialists an internal network of calcified struts augments the surface calcification. This strut-reinforced tissue is called trabecular cartilage. Some sharks also eat hard prey. We examined a series of Heterodontus francisci with a high-resolution CT scanner for evidence of trabecular cartilage. The CT scans and dissection revealed no trace of trabeculae. The jaws are the mechanical antithesis of those of hard-prey crushing stingrays. Rays have a well-calcified symphysis and the jaws meet at a joint with a large uncalcified region. In contrast heterodontids have a loose, uncalcified, ligamentous symphysis and the jaw joint is a well-calcified hinge. Dissection of other shark taxa that are reported to eat hard prey, Sphyrna tiburo and Orectolobus ornatus, did not reveal any trabecular cartilage. Trabecular cartilage appears to be an evolutionary innovation, confined to the myliobatid stingrays that allows them to eat harder prey than other cartilaginous fishes. (Session P-14, Saturday, July 7, Penn Stater, Deans Hall)



P.O. Box 8874, AES, Ketchikan, AK 99901

The True Tale of How Paleozoic Sharks Came to Rule my Life and Art

I will be presenting an illustrated informal talk about how fish came to dominate my artwork since moving to Alaska 20 years ago and more specifically how Paleozoic Chondrichthyans have recently dominated my illustration work.

This work includes various museum installations I have worked on, Discovery Channel films, and magazine assignments. I have worked for the past year on a suite of new shark drawings that I will debut in my slide talk. The drawings have been produced for a book to be published early next year. For more information on my work look at www.trollart.com (Session 26, Monday, July 9, Penn Stater, Room 207, 11:30)



1600 Ken Thompson Parkway, Mote Marine Laboratory, Sarasota, FL 34236

Growth rates of bonnethead sharks, Sphyrna tiburo, from the west coast of Florida estimated from tag-recapture data.

The growth of bonnethead sharks off the west coast of Florida was investigated using tag-release-recapture methods. Juvenile and adult bonnetheads were tagged with external nylon-barbed tags and released along Florida's Gulf coast beginning in November of 1991. Growth data were obtained from 113 tag recaptures, 70 of which were recaptured by Mote Marine Laboratory biologists. The time at liberty ranged from 1 to 2029 days while the measured growth increments ranged from -5.5 to 30.5 cm. A maximum likelihood approach was employed to analyze these growth increment data. In addition to von Bertalanffy parameters, this method allows for an estimation of measurement error, growth variability, and treats data outliers objectively. A bootstrapping method was utilized to estimate confidence limits of the parameters. When combining all usable samples from all and K were estimated at 104.3 (cm STL) and 0.28 year - 1, respectively. A areas, Linf subset of female-only recaptures from Tampa Bay produced comparable growth parameters (Linf= 105.4 and K= 0.28 year - 1). The results will be discussed and compared with published results from age-at-length data for bonnetheads inhabiting similar regions in Florida. (Session 10, Saturday, July 7, Penn Stater, Room 207, 3:00)



(CJW, JDT, CAL) 1600 Ken Thompson Parkway, Mote Marine Laboratory, Sarasota, FL 34236; (ABB) 131 Poole Agricultural Center, Clemson University, Clemson, SC 29634

Production of nitric oxide (NO) and inducible nitric oxide synthase (iNOS) by peripheral blood leukocytes from nurse sharks Ginglymostoma cirratum

Nitric oxide (NO) is a potent cytotoxic molecule that serves as an antipathogenic agent in cell-mediated immune responses of higher vertebrates. Its production in immune cells results from the enzymatic transformation of L-arginine to L-citrulline by the inducible isoform of nitric oxide synthase (iNOS). To establish whether elasmobranchs produce reactive nitrogen intermediates, NO production by nurse shark Ginglymostoma cirratum peripheral blood leukocytes (PBL) stimulated with bacterial lipopolysaccharide (LPS) was investigated. Nurse shark PBL (6.25x105, 1.25x106, and 2.5x106 cells/mL) were cultured for 24 to 96 h following stimulation with 0, 2.5, 5, 10, or 25 ug/mL LPS, in both serum supplemented (fetal bovine serum, FBS, and nurse shark serum, NSS) and serum-free culture conditions. Production of NO was measured indirectly via nitrite detection using the Griess reaction. Maximal NO production was observed after 72 h using a PBL concentration of 2.5x106 cells/mL cultured with 10% FBS and stimulated with 10 ug/mL LPS. When PBL were cultured with a specific inhibitor of iNOS, the arginine analog L-N6-(1-iminoethyl)lysine (L-NIL), production of NO was inhibited. Procedures to confirm the presence of iNOS, which rapidly degrades in cell lysates, are being developed for Western blot analysis. (Session 20, Sunday, July 8, Penn Stater, Room 207, 2:00)



224 Arts II, The Pennsylvania State University, University Park, PA 16802

Ancient Greece: Tall but deadly fish 'tails'

The wine dark sea of Homer still conveys the majesty and power of the seemingly limitless water surrounding Greece. This paper explores Greek and Roman values with regard to deadly encounters at sea, and aggressive attacks inspired from predators such as sharks. This investigation identifies the poet Homer as invaluable, stimulating the arts of his time (eighth century B.C.) and inspiring monumental works 700 years later in Greece and monumental acclaim by the first century A.D. In particular, murals for estates in Rome in the first century B.C. involved seascapes of Homer's Odyssey. This tale involved the arduous return home from ancient Troy, and sudden death in several places and at sea that wiley Odysseus barely escaped. Sea monsters include the Scylla who grew immeasurably in the arts by the first century A.D. to command the attention of the emperor Tiberius in an imperial estate with sea grotto at Sperlonga south of Rome. Rhodian artists used the Scylla as their signed centerpiece, and best conveyed the ferocity of the combined physical forms and aggression, while permitting the viewer, the emperor to live as dangerously as Odysseus. (Session 26, Monday, July 9, Penn Stater, Room 207, 8:30)



(JW, ABB) Clemson University, Clemson, SC 29634; (CAL, CJW) 1600 Ken Thompson Parkway, Mote Marine Laboratory, Sarasota, FL 34236

Acute radiation exposure in the clearnose skate, Raja eglanteria: A histological study of the thymus

Clearnose skate hatchlings (25g) were exposed to 0-75 Gray (1Gray = 100 rads) of ionizing radiation (n=122). The skates were sacrificed 10, 14, 20, 30 and 40 days post-irradiation. Thymii were fixed in 10% neutral buffered formalin and processed for routine histology. Fifty 3 micron cross-sections of thymus were collected at 50 micron intervals. Five thymus sections at 500 micron intervals were digitized for area analysis. The averaged areas were expressed as a percent of the control and treatments compared with ANOVA. Beginning at 1.5 Gray there was a significant logarithmic decline in thymus area as a function of radiation dose that became asymptotic after 15 Gray. Histologically, the medulla was infiltrated with large cysts containing apoptotic thymocytes and eosinophils. There was a decrease in cellularity of the cortex evident after only 1.5 Gray. This decline continued until radiation levels reached 15 Gray, when only stromal connective tissue remained. Repopulation of the thymus began at day 30 after 9 Gray but was not yet complete after 40 days. In skates exposed to 13.5 and 18 Gray the thymus began to recover its cellularity on day 40. There was no significant increase in thymic area over the 40 day recovery period. (Session 20, Sunday, July 8, Penn Stater, Room 207, 2:15)