Population genetic structure of Earth's largest fish, the whale shark (Rhincodon typus).

Large pelagic vertebrates pose special conservation challenges because their movements generally exceed the boundaries of any single jurisdiction. To assess the population structure of whale sharks (Rhincodon typus), we sequenced complete mitochondrial DNA control regions from individuals collected across a global distribution. We observed 51 single site polymorphisms and 8 regions with indels comprising 44 haplotypes in 70 individuals, with high haplotype (h = 0.974 +/- 0.008) and nucleotide diversity (pi = 0.011 +/- 0.006). The control region has the largest length variation yet reported for an elasmobranch (1143-1332 bp). Phylogenetic analyses reveal no geographical clustering of lineages and the most common haplotype was distributed globally. The absence of population structure across the Indian and Pacific basins indicates that oceanic expanses and land barriers in Southeast Asia are not impediments to whale shark dispersal. We did, however, find significant haplotype frequency differences (AMOVA, Phi(ST) = 0.107, P < 0.001) principally between the Atlantic and Indo-Pacific populations. In contrast to other recent surveys of globally distributed sharks, we find much less population subdivision and no evidence for cryptic evolutionary partitions. Discovery of the mating and pupping areas of whale sharks is key to further population genetic studies. The global pattern of shared haplotypes in whale sharks provides a compelling argument for development of broad international approaches for management and conservation of Earth's largest fish.

Microsatellite and mitochondrial DNA analyses of the genetic structure of blacktip shark (Carcharhinus limbatus) nurseries in the northwestern Atlantic, Gulf of Mexico, and Caribbean Sea.

Abstract We investigated the genetic structure of blacktip shark (Carcharhinus limbatus) continental nurseries in the northwestern Atlantic Ocean, Gulf of Mexico, and Caribbean Sea using mitochondrial DNA control region sequences and eight nuclear microsatellite loci scored in neonate and young-of-the-year sharks. Significant structure was detected with both markers among nine nurseries (mitochondrial PhiST = 0.350, P < 0.001; nuclear PhiST = 0.007, P < 0.001) and sharks from the northwestern Atlantic, eastern Gulf of Mexico, western Gulf of Mexico, northern Yucatan, and Belize possessed significantly different mitochondrial DNA haplotype frequencies. Microsatellite differentiation was limited to comparisons involving northern Yucatan and Belize sharks with nuclear genetic homogeneity throughout the eastern Gulf of Mexico, western Gulf of Mexico, and northwestern Atlantic. Differences in the magnitude of maternal vs. biparental genetic differentiation support female philopatry to northwestern Atlantic, Gulf of Mexico, and Caribbean Sea natal nursery regions with higher levels of male-mediated gene flow. Philopatry has produced multiple reproductive stocks of this commercially important shark species throughout the range of this study.

Refractive state and accommodation in the eyes of free-swimming versus restrained juvenile lemon sharks (Negaprion brevirostris).

Optical measurements of the refractive state of the eyes of various shark species typically have depicted sharks as hyperopic (far-sighted) with little evidence of accommodation (i.e. the ability to change focus for visualizing objects at different distances from the eye). In this study, we used infrared video retinoscopy to measure the refractive state in juvenile lemon sharks (Negaprion brevirostris). This technique allows dynamic measurement of refractive state in free-swimming animals as they pass by an aquarium window. We found that unrestrained lemon sharks are focused emmetropically relative to a 1-m distant photorefractor for the lateral visual field. However, when restrained either right side up or upside down (the latter inducing tonic immobility), the sharks become increasingly hyperopic, an artifact also reported in some other vertebrates. In addition, unrestrained lemon sharks display small amplitude accommodative excursions. Thus, refractive state measurements on restrained sharks in general may not reflect the natural, resting state of the shark eye, but rather, an induced hyperopia and lack of accommodative function. Such an artifact may be present in other vertebrate species, underscoring the need to obtain measurements of refractive state in unrestrained animals.

Serum steroid hormones and the reproductive cycle of the female bonnethead shark, Sphyrna tiburo.

The bonnethead shark Sphyrna tiburo reproduces by placental viviparity with one of the shortest gestation periods (4.5-5 months) known in sharks. In southwest Florida, mating in this species occurs in November, sperm is stored until ovulation/fertilization the following March-April, and parturition occurs in August. Serum concentrations of four steroid hormones (17 beta-estradiol, progesterone, testosterone, and dihydrotestosterone) were determined by radioimmunoassay over a complete reproductive cycle in mature females from a wild population. Serum 17 beta-estradiol and testosterone levels are high during mating and preovulatory stages. Preovulatory concentrations of testosterone are greater in female S. tiburo than in any other female elasmobranch previously studied. Progesterone levels are significantly elevated during preovulatory, ovulatory, and postovulatory stages, while serum dihydrotestosterone levels increase significantly during the preovulatory stage. Our study is the first to demonstrate a sustained rise in progesterone during gestation in a placental shark and suggests a regulatory role for this hormone during the period prior to implantation of the embryos in the uterus.

The presence of a porphyropsin-based visual pigment in the juvenile lemon shark (Negaprion brevirostris).

The visual pigment from the juvenile lemon shark has been extracted and is a homogeneous vitamin A2-based porphyropsin with maximum absorption at 522 nm. This is the first report of a porphyropsin visual pigment extracted from the retina of an elasmobranch. In contrast, the visual pigment from the adult lemon shark yields a homogeneous vitamin A1-based rhodopsin with maximum absorption at 501 nm. We conclude that the porphyropsin of the juvenile lemon shark changes over to a rhodopsin as the animal matures.

Very large neurons of the inner retina of humans and other mammals.

Large ganglion cells, called parasol cells, are known to occur in the Golgi-stained, human retina. This report describes a population of much larger cells that is not stained by Golgi technique. These cells may be located in the human ganglion cell layer using Nomarski differential interference contrast optics and unstained, flatmounted tissue. These cells are regularly distributed in young and old adults in a Gaussian fashion along the radii that extend from the perimacula toward the far periphery. The author did not find the cells in the central retina. The most frequent (J-type) cells have soma diameters between 26 and 40 microns. Rare (S-type) cells measure up to 55 microns in diameter. Many cells have processes that appear to be axons or dendrites. These cell types may be especially sensitive to damage early in diseases of the inner retina.