The occurrence of Mustelus canis in New Jersey estuaries is closely related to seasonal movements in the Mid-Atlantic Bight. During the winter the adults are found off Virginia and North Carolina. In April and May they move north along the coast as far as Cape Cod. The pups are born in New Jersey estuaries in June and can be found through September. They exhibit pronounced nocturnal movements into subtidal march creeks to feed on crustaceans (Crangon septemspinosa, Palaemonetes vulgaris, Callinectes sapidus, Libinia spp.). Growth is fast, averaging 1.6-2.2 mm/day with no apparent differences between the sexes. By the time the pups leave the estuary in the fall most are 40-50 cm. The juveniles and adults then migrate south for the winter. The fast growth rates achieved in the estuary are indicative of the importance of estuaries as nursery areas for this species.
The Chesapeake Bay area is a major nursery ground for sandbar sharks along the eastern U.S. Atlantic coast. From 1973 to present VIMS personnel have sampled the shark population of this area using longlines. Although effort has varied by year, this collection provides an historical indication of utilization of the Chesapeake region. Given the recent rapid expansion of fishing effort for C. plumbeus, fluctuations in population structure may be reflected in these records. The data are analyzed for catch-per-unit-effort (CPUE) by size class on a spatial and temporal basis to determine the extent of any such fluctuations.
Although the identity of the major ovarian steroids of a number of elasmobranchs, includingS. acanthias, has been known for some 25 years, little is known of their effects on the reproductive tract. Our laboratory has been concerned for some time with the role of gonadal steroids in the regulation of myometrial activity and its response to peptide hormones in elasmobranchs. Of particular interest is the potential for modulation of myometrial activity associated with viviparity and the retention of developing young. In this paper we will present the results of recent studies in which we demonstrate that homologous Squalus relaxin produces a marked slowing of the spontaneous rhythmic contraction of the oviduct accompanied by a marked lengthening of the interval between contractions. In estrogen treated animals, these effects of relaxin are accentuated. In contrast, if progesterone is administered along with estradiol the effects of relaxin are not seen. These results are discussed in the context of the overall control to the myometrium of the dogfish. Supported by NSF Grant DCB 86-06344 to IPC.
The study of the thorny stingray Dasyatis centroura (Mitchill, 1815) caught from off the Tunisian coasts, mainly from off the Gulf of Tunis and Gabes allows to describe some aspects about the biology of this non-placental viviparous Selachian. The size at first sexual maturity of males is reached between 15 cm and 80 cm of disc width (DW). Females keep juvenile genital apparatus until they reach 65 cm DW, thereafter maturation proceeds between 66 cm and 100 cm DW. The first adult females are 100 cm DW. Females are larger than males. Gestation spreads about 4 to 5 months at a minimum, but it is impossible to know the number of reproductive cycles per year, vitellogenesis completes at the end of the gestation. Parturition and ovulation occur in June. Fecundity (s.1.) reaches from 2 to 8 individuals per litter.
The females of many species of sharks travel to specific, geographically discrete, nursery areas to lay their eggs or to give birth to their young. These nurseries are usually in shallow, coastal waters in highly productive zones, where the young find abundant prey and where they are free from predation by larger sharks. The newborn young remain in the nurseries for a few months in temperate latitudes or for a few years in tropical waters. Criterion for a nursery area is the presence of both gravid females carrying term pups and free swimming, newborn young in the area. The following species of sharks have nurseries along the east coast: Atlantic sharpnose, blacknose, blacktip, bull, dusky, finetooth, lemon, sandbar, sandtiger, scalloped hammerhead, and spinner,
Predation on large, energy-rich eggs occurs commonly in terrestrial and freshwater communities, with amphibians, reptiles, and birds figuring as prominent targets. We might predict that predation on large eggs would be widespread in marine communities, however there is little information for testing this prediction. For example, few instances of predation on elasmobranch eggs have been reported. We present evidence for the occurrence based on SCUBA observation, long-term incubations of caged egg capsules held in museum collections. The principle egg predators appear to be gastropods, though vertebrates contribute in some measure to mortality of embryonic elasmobranchs. As yet we can only speculate about the consequences of egg predation for populations of oviparous elasmobranchs, or about the impact their energy-rich eggs may have on marine communities.
Immunocytochemical studies have localized GnRH in neurons of the terminal nerve (TN), preoptic area (a few cells only) and midbrain tegmentum. The paucity of preoptic immunoreactive (ir) cells may relate to an unusual feature of the elasmobranch pituitary, i.e.a lack of portal control of the gonadotropin producing cells. TN and midbrain GnRH-ir neurons may be major sources of GnRH used to modulate or otherwise control both pituitary and brain cells via delivery through the systemic circulation. These ir-nuclei also appear to directly innervate CNS regions (the preoptic area, habenula and clasper control area of the spinal cord) involved in sexual functions. Important regulatory mechanisms, represented by interactions between GnRH pathways and sex-steroid concentrating neurons, are likely to occur in the preoptic area, habenula and midbrain tegmentum.
The sixgill shark (Hexanchus griseus) and the sevengill shark (Notorynchus cepedianus), both members of the family Hexanchidae, employ similar reproductive strategies to enhance survivorship of their young. Both species give birth during the spring months and appear to be site specific as to where they pup. Sixgill and sevengill sharks give birth in areas of high primary productivity. Energetically, this is advantageous for the newborns to be placed in an area with an abundant food source. The rapid growth rates of sixgill and sevengill sharks over the first year would enhance their survivorship since neither species has many predators.
Patterns of enzyme expression, including number of presumptive loci and tissue specificity of expression (tissues surveyed included brain, heart, muscle, and liver) were determined for a number of enzyme systems in Prionace glauca using starch gel electrophoresis. Patterns of enzyme expression in the blue shark are compared to those reported in other vertebrate taxa.
Clasper siphons of 17 species, representing 10 families of sharks have been examined. Siphon size ranges from simple sacs situated beneath the ventral skin of the clasper (Chlamydoselachus and Heterodontus) to siphons of modest size (Squalus andGinglymostoma) to siphons that extend cranially beneath the belly skin to the coracoid bar (Carcharhinus, Negaprion, Galeocerdo, Sphyrna, Mustelus, Isurus, Hemiscyllium, Cephaloscyllium). The growth and action of the claspers and siphons in Squalus andMustelus has been described in detail by Gilbert and Heath (1972). Here attention is called to the lining epithelium of the siphons of Squalus which is richly supplied with large goblet cells. Their copious secretions not only lubricate the clasper during copulation, and furnish a medium, along with sea water, for sperm transport, but may play a role in sperm activation and survival. The lining epithelium of sharks whose siphons extend cranially to the coracoid bar are more sparsely supplied with secretory cells and here the siphons serve primarily for the storage of sea water prior to copulation.
The capture of recently inseminated or pregnant specimens of Odontaspis taurus, Isurus paucas, I. Oxyrinchus, Alopias superciliosus, and A. vulpinus has allowed new information to be obtained on the reproductive biology of these species. Oophagy and embryonic cannibalism has been documented in O. taurus, only oophagy in other lamnoid species. The occurrence of up to nine embryos of similar size per uterus in Isurus and no indication of functional erect teeth in embryos leaves considerable doubt that embryophagy occurs in this genus. Considerable data has been collected on Odontaspis taurus which allows a lamnoid reproductive model to be developed and tested in spite of the obvious differences between the reproductive biology of this species and other lamnoids. From ovarian development, fertilization, early embryonic differentiation to parturition, reproduction in O. taurus and other lamnoids differs significantly from other elasmobranchs. The functional significance of these developmental patterns will be examined in a series of experiments conducted with captive neonatal specimens of O. taurus and representative carcharhinoids.
Pre-mating behaviors were observed on two occasions in a captive colony of Grey Nurse Sharks (Eugomphodus taurus) ate Manly Oceanarium. These behaviors occurred fourteen rather than twelve months apart. Temperature and photoperiod were very similar to that of local coastal conditions where these animals occur naturally. The information recorded in both these cases was almost identical, giving an insight into a very complex social structure in this essentially colonial animal. Dominance displays occurred between both mature and immature males in addition to aggression towards other objects (for example small requiem sharks). Most interesting were the interactions between the males and females, especially those which implicate the possible use of phermones. Copulation occurred in both instances but was not filmed.
The Atlantic sharpnose shark is a viviparous anamniote that develops an epitheliochorial yolk sac placenta. Initially, the contents of the yolk sac nourish the embryos. Yolk is partially digested in the yolk syncytium and transferred to the vitelline circulation and also transported to the fetal gut by the yolk stalk ductus. When embryos are 4.0 cm in length, vascular ridges. termed appendiculae, develop on the yolk stalk. As yolk stores are depleted, the uterus begins to differentiate into a secretory and transport organ. The epithelium produces metachromatic secretion that is positive by the PAS and alcian blue methods. When the embryos are 7-10 cm, appendiculae are elongate and branched and are populated by microvillar and granulated cells. At term the yolk sac is a functional placenta and the yolk stalk an elongate umbilical cord, uterine capillaries are continuous and the placenta and the surface epithelium is active both in secretion and transport. Characteristics of the placenta include endocytotic activity, crystalline-like cytoplasmic inclusions and fenestrated capillaries. Fetal and maternal components of the uteroplacental complex are separated by the egg envelope. The interface zone is positive by the PAS and alcian blue methods. The Golgi region of granulated appendicular cells is also positive as are microvilli of adjacent absorptive cells.
According to the literature Raja teevani and R. floridana are distinguished from each other by few characters, most commonly by conjectural differences in widths at mid-tail. Resulting identifications have, therefore, proven to be highly ambiguous. Examinations of a large sample of both species identified by W.C. Schroeder as R. teevani or R. floridanawere made for morphometric, meristic, and other more qualitative anatomical characteristics to elucidate the taxonomic status of these two species. Results indicate that no morphological, meristic, or anatomical differences exist between them and they should be synonymized as Raja teevani Bigelow and Schroeder, 1951.
Weak electric organs along the lateral aspects of the tail are considered to be a unique derived character of skates although they have been described for a relatively small number of skate species. Specimens from 24 of the 29 supraspecific taxa (genera and subgenera) were examined macroscopically and histologically to estimate the extent of variation in electric organ morphology and cytology within skates. Variation was estimated both within and among supraspecific taxa. Variation in electric organs was compared with a working hypothesis of skate phylogeny to both test and refine the working hypothesis.
The structure and functional of the extragonadal genital ducts of the male Port Jackson shark, Heterodontus portusjacksoni, has been studies to assess the role of the ducts in the maturation and storage of sperm, and to relate the processes in sharks and higher vertebrates. It is concluded that the extragonadal genital ducts in the sharks play an important role in sperm maturation and storage. However, due to differences in metabolic rate and the arrangement of spermatogenesis (cystic vs tubular) some of the functions of the ducts in, for example, sharks and mammals are quite different.
Sclerotization of the skate egg capsule occurs after secretion of capsule precursors from the shell gland and involves a form of quinone tanning in which catechols are introduced in uteroand subsequently oxidized to quinones by catechol oxidase. A latent form of enzyme is incorporated in the capsular matrix during secretion. Oxidase activity increases concomitantly with increasing catechol and quinone contents. Six major proteins ranging in size from 95 kDa to 20 kDa comprise the skate egg capsule, all of which contain elevated levels of glycine, serine, tyrosine and phenylalanine. Hydroxyproline occurs in all but one protein, however, none has an amino acid composition typical of collagen. Solubilization of two proteins from pre-tanned capsule requires reducing agents indicating that an early event leading to matrix stabilization is mediated by disulfide bonds. Stabilization of the other proteins along with the disulfide bonded proteins directly correlates with increasing catechol content, catechol oxidase activity and quinone formation.
The elasmobranchs represent a fascinating evolutionary continuum with regard to maternal support for developing embryos. In oviparous species, eggs are encapsulated in a tough, fibrous case. Some authors have suggested that this egg case is impermeable to solutes and therefore protects the embryo ionically and osmotically until it can regulate on its own. However, more recent evidence suggests that the case if highly permeable to ions and urea and that the embryo in bathed in a solution ionically similar to sea water. The embryo must osmoregulate at the time the egg is laid. In a more advanced species (e.g. the primitively viviparous Squalus acanthias) eggs hatch in utero and are bathed in a solution osmotically similar to maternal plasma. Yet just several months into the 22 month gestation period the embryos are capable of independent iono- and osmoregulation in a uterine solution resembling sea water. Data are sparse concerning more advanced viviparous species. Embryos of Mustelus canis develop in a solution which is ionically and osmotically similar to maternal plasma. Thus, while iono- and osmoregulation in the embryo of this species would appear to be unnecessary, there is no direct either for or against the embryo's ability to regulate. It is clear, however, that in at least some of the most primitive species of elasmobranchs, the ability of the embryo to regulate salts and urea is present at the earliest stage of development. Thus the ability of the embryo to regulate salts and urea would allow the evolution of a diverse array of reproductive strategies in the elasmobranchs.
Ruckert (1890-1892) was the only author who observed fertilization in Chondricthyes. he stated that yolk syncytial nuclei derived from supernumerary spermatozoa. Instead, we observed that unused sperm nuclei quickly disappeared. Yolk syncytial nuclei originated from the deepest, open cells beneath the blastodisk, as in teleosts. They migrated around the yolk as the external yolk sac was formed (epiboly). But at that time they appeared highly compressed and pycnotic, so they may be replaced by another generation of yolk syncytial nuclei. The EYS does not take part in yolk digestion, except in a specialized dorsal area during the end phase of development. Yolk utilization will be described.
A sexual cycle study was carried out on basis of 191 specimens caught on Maranhao Coastal waters between June 1984 and August 1986, using drift nets. No relationship was established between ovarian fecundity and female size. Uterine fecundity varied between 3 and 23 embryos or eggs increasing with female size. Ovulation took place mainly from August to October, when fully developed oocytes were found. Females bearing full-term embryo showed pre-vitellogenic oocytes. Fecundation occurred from August to October and embryonic occurred between August and April; parturition takes place from February to April. Atrophied embryos/eggs represented 8% of the total number of observed embryos. The complete reproduction cycle takes one year. It is considered a well-defined cycle.
The retina of the Atlantic stingray, Dasyatis sabina has been examined under light microscopy to determine the types of photoreceptors present. Based upon morphological evidence it has been determined that both rod and cone receptor elements are present in the retina of D. sabina. The distribution of these cones was also investigated and it was observed that the density of the cones, relative to the rods, would vary in the retina. The variation in cone density followed a consistent pattern in all observed specimens of D. sabina. Throughout the retina the cone density was greater nearer to the center of the eye than at the periphery as is the typical vertebrate pattern. The center of the eye also possessed a longitudinal area in which cones were exceptionally abundant. This structure was shown to extend from the optic disc and into both the rostral and caudal aspects of the eye. Due to its high cone density, location, and appearance the structure has been termed a visual streak.
Embryonic growth in viviparous matrotrophic sharks is facilitated by adaptations for maternal-embryonic gas exchange and nutrient transfer. We identify and discuss the evolution of structures that facilitate transplacental exchange in sharks through a comparative analysis of extant forms. The major embryonic structural adaptations for gas exchange and nutrient transfer include the yolk sac, umbilical stalk,and their derivatives. These structures vary greatly in size, cell/tissue organization, and function. The prototypic umbilical stalk, as found in extant aplacental lecithotrophs, consisted of a short vascularized tube connecting the yolk sac to the embryo. During the evolution of placentation where there is adhesion of yolk sac surfaces to the uterine lining, retention of a short umbilical stalk may have restricted the mobility, orientation, and growth potential of embryos within the uterine lumen. Axial elongation of the umbilical stalk removed these restrictions and offered selective advantages. Once elongated, the umbilical stalk became further modified as a site for exchange of gasses, nutrients, and other metabolites. In some species, radial amplification of umbilical stalk surfaces led to the evolution of appendiculae. Diversity in appendicular morphology involved differential modification of the surface epithelium, connective tissues, and pattern of vascularization. Embryonic adaptations for nutrient transfer and gas exchange in the genera Mustelus, Rhizoprionodon, Scoliodon, Sphyrna,and Squalus are compared.
Sixteen tissues were biopsied from two nurse sharks (Ginglymostoma cirratum) which had been anesthetized weekly for three years with tricaine methane sulfonate (MS-222). Tissues were analyzed histologically to evaluate any chronic effects apparent when compared with similar tissues from a previously unanesthetized control nurse shark of the same age and time in captivity. Tissues biopsied included externally exposed areas (eye, olfactory epithelium, and gill), visceral organs (liver, spleen, pancreas, stomach, intestine, and rectal gland), urogenital organs (testis and kidney), muscle (skeletal and cardiac), brain regions (cerebellum and optic tectum), and thyroid gland. Most tissues appeared normal following the chronic treatment, although minor alterations included microscopic crystallization of material in the kidney, thickened gill filaments with enlarged surface cells, and mucous cell hyperplasia in the gill, olfactory epithelium and rectal gland.
Clearnose skates (Raja eglanteria) have been bred in captivity at Mote Marine Laboratory (MML) during every winter breeding season since 1981. Adults will breed in large tanks in the presence of other skates, or as isolated pairs in tanks as small as 3 x 4 ft in area with about 1 ft of water depth. Following copulation, sperm are stored in the nidamental gland of the female, and fertile eggs, encapsulated within protective egg cases, are laid in pairs every 4 to 5 days from January through May. When eggs are maintained in filtered seawater at a constant temperature (22 to 22C and photoperiod (12 h light, 12 h dark), embryos will develop and fully formed offspring will hatch after an average incubation of about 12 wk (85 + 6 d; range, 74 to 94 d). Newly hatched offspring will feed on finely chopped shrimp or squid, and can be raised in captivity provided sufficient surface area is available on the tank bottom to accommodate their essentially "two dimensional" growth. As experimental animals, controlled populations of skates are currently in use at MML as embryos, as juveniles of known age, and as sexually mature individuals.
The independent origin and evolution of secondary sexual dimorphism has had a profound effect on sexual lineages of Paleozoic Chondrichthyes. One of these clades, the Chimaeriformes, persists to this day. Secondary sexual dimorphism in the modern Chimaeriformes involves a body size differential between females and males, and the presence of a median frontal clasper and prepelvic tenaculae in males that are not present in females. Secondary sexual dimorphism in modern sharks (Euselachii) is limited to a body size differential between females and males, and to significantly different thicknesses of the dermis. Among skates and rays, secondary sexual differences in dentition and in scale distribution and size have been noted. The precise developmental mechanisms for the switch from sexually monomorphic to dimorphic structures remain unclear, but cell receptors are clearly related to the processes of differentiation and morphogenesis. Changes in the type of timing of certain cell receptors can be hypothesized to be the primary mechanism responsible for switching structures from one developmental fate to another. Examples from the fossil record are presented and considered.
Sharks are often described as entirely opportunistic, unselective feeders. To test this hypothesis, a series of three sampling procedures were carried out between July, 1987 and January, 1989 to assess the role of selectivity in the feeding of young lemon sharks. The composition of prey available was determined by casting specially designed nets into the shallow-water habitat of the young sharks. Thus 14 bottom and 19 pelagic samples yielded quantitative data on 24 families of potential prey. During the same period, 35 sets of gill nets of approximately 8 hours each captured 170 lemon sharks of which 131 or 71% had identifiable food items in their stomachs. A total of 20 families of actual prey were identified and these samples were treated quantitatively. Actual and potential prey were then compared using four models of selectivity indices. Results demonstrated that feeding depends to some extent upon prey density but showed that young lemon sharks cannot be described as opportunistic feeders. The selectivity indices showed that the sharks definitely select their prey and that items selected change over time and space. The importance of this prey selection is briefly discussed in terms of trophic dynamics and optimal foraging. Supported by NSF-OCE 884325 and ONR N00014-87-J-1116.
Previous experience with M type dart tags suggested that implantation of the stainless steel dart caused trauma leading to chronic infection, necrosis, and ultimately to retarded growth. To test this hypothesis, a group of 72 juvenile lemon sharks was marked with tiny (2mm x 11mm), intramuscularly implanted, passive integrated transponder (PIT) tags and released into the field. A second group was marked with the M type dart tags. Both groups were tagged between May, 1985 and October, 1989 and released into waters around Big Pine Key in Florida Bay. A third group of juvenile lemon sharks wa marked with multiple external tags and released in various locales in the Florida Keys between 1980 and 1983. Growth of 30 recaptured sharks of the group with multiple marks averaged 7.8 cm/year (sd = + 4.53) precaudal length (PCL) while that of the 9 recaptured with the M type tag averaged 9.5 cm/year (sd = + 2.66). Ten recaptures of the PIT group grew an average of 15.7 cm/year (sd = + 3.51). A t-test revealed that growth of the PIT group was significantly different (P<.001) from the other two groups. Von Bertalanffy parameters for the PIT group closely corresponded with those of an independent analysis of lemon sharks growth based on aging structures in the vertebral centra. Hypotheses are advanced for explaining why external tags retard growth and a strong recommendation is made to develop another method for tagging young sharks. Supported by NSF-OCE 884325 and ONR N00014-87-J-1116.
A total of 65,000 spiny dogfish were tagged in north Pacific waters off the west coast of Canada over the period 1978 to 1987. All of these were tagged with a modified Petersen disc tag. As of december 31, 1989, 1855 have been recovered. recoveries were made throughout the range of spiny dogfish in north temperate waters including Baja California, the Gulf of Alaska and northern Japan. Variation in seasonal migrations and dispersion rates are examined by size and sex. Information on growth rates as indicated by tagging are also included.
The site fidelity, patterns of distribution, burying behavior, orientation, and abundance of skates off outer Cape Cod were observed in situ. The population consisted mainly of winter skates with a few little skates also present. Site fidelity was tested by tagging and an elimination experiment. Specimens collected during the elimination study provided information about species composition, sex ratio, and maturity states of these skates. Abundance and distribution patterns were investigated and comparisons for different times of the day were made. Special attention was paid to interactions between individuals and the behaviors correlated with such interactions. Burying behaviors were examined at different times of the day. The resting orientation of these skates was recorded and possible relationships with physical factors, such as current direction, were investigated.
Most vertebrates confine their movements to specific areas. Properties of these activity spaces (e.g., size, location) should have adaptive significance. Twenty-three juvenile lemon sharks were fitted with ultrasonic transmitters and manually tracked in the North Sound, Bimini, Bahamas. These young sharks maintain a very restricted activity space, usually close to the mangrove-fringed shoreline with averages 0.69 km2 in area. Daily activity spaces of each individual and total activity spaces of neighboring sharks overlap broadly (indices of site fixity and site defense are 0.45 and 0.55 respectively). Hence these juvenile lemon sharks are both home ranging and social. Preliminary evidence suggests that home range area increases and changes location during ontogeny. Swimming speeds are highly variable (0-1.2 bl/s) and seem to be highest during crepuscular periods.
Behavioral reports of free-swimming sharks or those held under semi-field conditions often contain a section on social activities among conspecifics and often equally pertinent information dealing with interspecific groups. Such inclusions suggest that requisite information is being transferred between individuals to achieve social functions. Such transfer could occur by visual signals, the latter being specific motor patterns and/or specific body markings. Many sharks possess body markings. Often, the latter are correlated with bottom habitats and likely serve as camouflage. Yet, open water sharks oftentimes also possess distinctive markings. It is one group of these enigmatic markings that forms the subject of this report... the distinctive fin markings of sharks within the families Carcharhinidae and Sphyrnidae. The report summarizes various functions that such markings could subserve. Focus then centers on the probable function in light of correlational data relating the intensity of such markings to the optical quality of water... species recognition.
Since to date only one developmental stage of a pristid has been described, the present study was undertaken to chronicle the developmental stages of a collection of 150 specimens from Lake Nicaragua. Thirty-one characters were measured and analyzed to describe growth relationships between structures. Major developmental events unique to pristids included the following: the rostrum, not formed in the smallest specimen, had elongated in a 33 mm fetus and could be distinguished as a developing saw; rostral teeth, the number of which is the main diagnostic character in separating the species of Pristis, were sufficiently developed to be counted in a 158 mm fetus, and pectorals were completely fused to the head by the time fetuses reached 92 mm in length.
A study of two populations of the bonnethead shark, Sphyrna tiburo was conducted in the Florida Keys and in Tampa Bay, Florida from September, 1982 to December, 1986. The maintenance of sharks in captivity at the Marine Science and Conservation Center in the Florida Keys, and the collection of sharks from widely separated geographical areas allowed the examination of latitudinal variation in reproduction. Several parameters were found to differ: (1) size and age at maturation (2) mating season (3) rate of embryonic development (4) size at birth (5) the energetic investment in producing offspring (6) gestation period and (7) the incidence of infertility. Litter size was not statistically different between the two populations. It is noteworthy that the average size of adult sharks in Tampa Bay is significantly greater than that of Florida Keys sharks. This size difference may be important in explaining the observed differences in reproductive strategy.
Among vertebrates, the general feature of the tetrapod hypothalamus-pars distalis axis is the presence of a portal system. oN the contrary, in Elasmobranch fishes there is general consensus indicating the absence of a vascular supply from the hypothalamus in the ventral lobe where gonadotropin activity is detectable. The hypothalamus contains GnRH immunoreactivity and GnRH acts increasing plasma gonadal steroids probably via a ventral lobe stimulation. Therefore a question arises: how GnRH reaches the pituitary. Since hydrophobic forms of GnRH have been shown to be present in the few species studies so far, we suggest that the plasma route might be practicable. This may allow the GnRH to reach the gonads and to act also by direct mechanisms. Intragonadal levels of steroids may have a paracrine and/or autocrine role in ovary and testis. Particularly, the morphology of the testis supports the concept of a diverse environment for different spermatogenic stages. Finally, gonadal steroids may feedback pituitary activity.
Age estimation in elasmobranchs is accomplished by counting calcified rings in vertebral centra without the knowledge and understanding of the importance of serum calcium. Blood from juvenile lemon sharks, Negaprion brevirostris, was collected, caudally via venipuncture, once weekly for a period of 19 weeks. Calcium levels ranged from 12.1 mg/dl to 22 mg/dl. Calcium levels in all sharks reached a peak during the ninth and tenth weeks of the study. This peak was immediately followed by a sudden decline in calcium levels. Autocorrelation showed no relationship of serum calcium to other blood parameters. Presently, deposition of vertebral rings is being correlated with fluctuations in serum calcium and environmental variables.
Storage of spermatozoa in the oviducal gland is used advantageously by several species of large Atlantic sharks. Using light microscopy I have observed females of: Alopias vulpinus, Lamna nasus, Carcharhinus obscurus, Prionace glauca, Sphyrna lewini, and S. tiburo. Details of sperm retention remain unresolved. Gravid females of the two carcharhinid and two sphyrnid species examined often possess oviducal spermatozoa. This suggests that multiple litters may be fertilized by one insemination. Male sharks also store spermatozoa in the ampulla ducts epididymis as sperm packets; variously termed spermatophores and spermozeugma. These packets disintegrate before reaching the oviducal gland.
Leo Demski suggested the development of a national inventory at the 1989 meeting of the American Elasmobranch Society in San Francisco. To that end, I have developed a semiannual regional inventory of live, captive sharks and rays for fourteen institutions (3 public aquariums, 10 zoos, and 1 university). The Great Lakes Region includes the states: MI, OH, KY, IN, IL, WI, MN, IA, and MO. The January 1990 edition of the Great Lakes Inventory showed 89 specimens of 16 shark species, and 62 specimens of 13 ray species. Coordinators are currently working on similar inventories for the West, Central, Southern, and Northeastern regions of the USA.
The innervation of the clasper has been studied in several batoids with emphasis onUrolophus halleri. Large myelinated nerves (ca. 10 at diameters of approx. .7mm; #56-65 counting from the vagus) innervate the clasper muscles and skin. Although the same nerves are present in females they are considerably smaller in cross sectional area. In males, low level electrical stimulation (20-150A) of the nerve evokes clasper movements including: rotation, elevation, medial and lateral extension and opening. Stimulation of the spinal cord in the area of the roots of the latter nerves also evoked the movements (<100A with best freq. 3-10 Hz). Filling of nerves 62 and 63 with retrograde tracers confirmed that motor neurons and sensory components of the nerves are at the levels indicated by stimulation. Axons immunoreactive for GnRH are present near the clasper motor neurons, suggesting a supraspinal peptidergic control of the system.
For several years we have been examining the levels of reproductively-related steroid hormones in carcharhinid sharks, unusual lower vertebrates possessing the unique attribute of placental viviparity. Our measurements of the serum levels of estradiol (E), testosterone (T), progesterone (P), dihydrotestosterone (D) and corticosterone (C) have established baseline data for these hormones including ranges and means. Our studies have encompassed both immature and adult males and females of 2 sphyrnid and 6 carcharhinid species, with the focal species being the lemon shark Negaprion brevirostris. We have focused on several aspects of reproduction including: 1. hormonal level changes during female and male maturation; 2. serum hormonal levels during the peak breeding season; and 3. hormonal levels during gestation, including levels in serial samples from several free-ranging pre- and post-partum lemon sharks. The data suggest that steroids are important in regulating reproduction in higher mammals are also essential in these cartilaginous fishes and may provide additional models for studying reproductive adaptations.
A total of 3,068 female dogfish were examined to determine maturity condition. The median age-at-maturity for females calculated using PROBIT analysis was 35.3 years with 95% confidence limits of 0.43 years. AS deterministic age-structures model incorporating information on fecundity, growth and reproduction was used to examine the reproductive strategy of the dogfish. The age-at-maturity reported corresponds to the level that theoretically maximizes lifetime reproductive output for a cohort of dogfish. The implications of these findings to the management of this species are examined.
During the past ten years we have isolated and sequenced relaxins from elasmobranch ovaries as well as from ovaries of other species including cetaceans. Structural and functional comparison of these relaxins revealed surprising differences and, very conspicuously, a lack of correlation with generally accepted phylogenetic branching patterns. Elasmobranch relaxins differ by the same number of residues from mammalian relaxins as the mammalian relaxins differ from each other. By comparison with the fossil record the evolutionary clock for relaxin therefore runs in error by 300 million years, more than 50% of the total time allotted to macroscopic evolution. Yet relaxins from Odontaspis taurus, Squalus acanthias, and Raja erinacea do effect the symphysis pubis of mice in a hormone-specific way. The structure of cetacean relaxins tops off an intriguing, non-Darwinian story of evolution that is at once telling us that protein structure-based geneology may be an illusion created by a paradigm.
The primary structure of gonadotropin-releasing hormone (GnRH) was determined for ratfish (Hydrolagus colliei). Amino acid composition and sequencing analysis in combination with mass spectrometry were used to show that this form of the peptide was identical to a form originally extracted from chicken brains. Both ratfish and dogfish ()Squalus acanthias) have a similar chromatographic and immunological profile suggesting that both species may possess the same form of GnRH. Using an antiserum (GF-4), which recognizes all known molecular species of GnRH, immunohistochemical analysis revealed that only a few positive fibers were found in the ratfish brain. In contrast, GnRH cell bodies occurred in the forebrain and hypothalamus in both dogfish and black skate (Bathyraja kincaidii). In the latter two species, an extensive GnRH fiber system radiated throughout the brain with the exception of the cerebellum. GnRH was detected in the blood of ratfish and big skate (Raja binoculate) suggesting that the transport of GnRH to the pituitary may be different from that of other vertebrates.
A sexual development study was carried out on the basis of 191 specimens of Sphyrna tiburo caught on the north coast of Brazil between 1984 and 1986. Results reveal that taking into account seminal ampullae fullness and claspers condition, the length class of 80.0 cm TL was considered to correspond to the full maturity. Males lever growth showed a discontinuity at the moment where they achieve maturity, which indicates that this organ may play an important role during the reproductive process. Vitellogenic activity starts for females longer than 70.0 cm TL, thereafter individuals with both yellow and white eggs were found. Pregnancy appeared in 90.0 cm TL females, longer specimens were pregnant or not. Liver growth process showed two different phases. A discontinuity was observed at the moment where pregnancy started.
During a study of stingrays near Bimini, we observed and filmed the pursuit, attack, and consumption of a southern stingray (Dasyatis americana) by a great hammerhead shark (Sphyrna mokarran). The sharks (ca. 3 m TL) intercepted the fleeing ray (ca. 93 cm DW) and used the underside of its broad snout to batter the ray to the bottom. As the ray resumed flight, the sharks again used the expanded rostrum to pin the ray to the bottom. This time, however, the shark pivoted while the ray was restrained and took a crescent-shaped bite out of the anterior margin of the left pectoral fin. When the ray resumed feeble flight, the shark again pinned it with its head and pivoted, this time taking a symmetrical bite from the right pectoral fin. The shark subsequently circles the incapacitated ray for 24 min. before returning to pick up and finish consuming the prey. Although the function of the expanded cephalic lobes of sphyrnid sharks has been the subject of much conjecture, this is the first observation of the use of the head in prey handling. Many authors have pointed out that both S. mokarran and S. zygaena consume large numbers of batoids. The behaviors involved in this predatory act, especially the use of the expanded head and the placement of the initial incapacitating bites, suggest an adaptive strategy for maximizing predatory efficiency while minimizing the risk of injury when feeding on rays.
Among the various electric fishes are the Torpediformes (electric rays), whose phylogenetic relationship to the rest of the Order Batoidea is obscured by the large electric organs that dominate the pectoral region, and demand skeletal adaptations. As a representative of this group Narcine brasiliensis was examined in order to characterize the ontogenetic relationship between the electric organs and the developing embryo. Research focused on development of body shape (by Morphometric Analysis) and skeletal structures (by clearing and staining). In the range of specimens examined, there was no significant shape change, while skeletal structures forming in this range show great accommodation to the electric organs. Results indicate that the Torpediform "shape" emerges early in ontogeny, even before the electric organs have fully formed; therefore, future research should focus on ontogeny in early stages in order to observe a more generalized state of development in this group.
Placental viviparity has evolved in S. laticaudus to a degree that rivals eutherian mammals. Its egg, the smallest of any sharks (1 mm, 0.065 mg), is nearly yolk free. Cleavage and gastrulation are modified. Implantation is early (at 3 mm) and gestation short (5-6 months). With a gestational weight increase of 10,000x, term embryos (130-150 mm, 600-900 g) are extreme matrotrophes. Hemotrophic placental nutrient transfer occurs across a unique uterine implantation site, termed the trophonematous cup, at which maternal blood bathes the embryonic yolk sac placenta. the latter is solid and filled with a stalk that contains the vitelline vessels but lacks a yolk duct. Its surface is amplified by many long villous appendiculae, each comprised of a vascular core that terminates in surface capillary beds invested by a simple squamous epithelium. Appendiculae are sites of metabolite transport and display design principles that are similar to those of the uterine trophonemata of matrotrophic rays, but are unlike the appendiculae of other placental sharks.
This study presents the result of reproductive biology of slender smooth hound, Gollum attenuatus. This species belongs to the family Proscylliidae, and can be distinguished other relative genera by slender body and bell-shaped snout. 740 specimens were caught with bottom longline from the seamount around New Zealand waters. The sharks ovulated about 100 ova at a diameter of 5-8 mm. One egg capsule had 50-80 ova. However, development of embryo in each egg capsules was only one. On the other hand, many other ova are mixed in the capsules and are probably used to nutrients of development of the embryo. The embryos has external yolk sac. Size at maturity are also discussed.
Reproductive biology of the guitarfish, Rhinobatos hynnicephalus from Xiamen coastal waters is described. Males have two functional testes. Spermatogenic cells in different seminiferous follicles are at different developmental stages while those in the same follicle are at the same stage. The development of claspers suggests that males mature at 380-400 mm TL. Females have two functional ovaries which attain maturity in May or June. Ovarian egg diameters show that females mature at 420-440 mm TL. Mature eggs of 22-24 mm in diameter are ovulated in July. Gestation takes 12 months and embryos are released at about 160 mm TL. Pregnancy takes place again soon after parturition. The number of embryos per uterus has a positive relation with the adult total length. The sex ratio of embryos is about 1:1.