Age and growth of three endemic threatened guitarfishes Pseudobatos horkelii, P. percellens and Zapteryx brevirostris in the western South Atlantic Ocean.

The age and growth of three endemic threatened guitarfish species were analysed using vertebrae of Pseudobatos horkelii, P. percellens and Zapteryx brevirostris. Edge and marginal-increment analyses were used to evaluate the periodicity of the formation of the band-pairs, suggesting deposition of one band-pair per year, from late winter to late spring. The von Bertalanffy growth model was used to describe the growth of these species with the following parameters, for pooled sexes: P. horkelii L∞ = 126.93, k = 0.19 and t0 = -1.51; P. percellens L∞ = 109.31, k = 0.16 and t0 = -1.78; Z. brevirostris L∞ = 60.37, k = 0.24 and t0 = -1.42. Our results are essential to understanding the resilience and vulnerability of these species to harvest, which can contribute to management and conservation actions of these species.

A re-evaluation of the size of the white shark (Carcharodon carcharias) population off California, USA.

White sharks are highly migratory and segregate by sex, age and size. Unlike marine mammals, they neither surface to breathe nor frequent haul-out sites, hindering generation of abundance data required to estimate population size. A recent tag-recapture study used photographic identifications of white sharks at two aggregation sites to estimate abundance in "central California" at 219 mature and sub-adult individuals. They concluded this represented approximately one-half of the total abundance of mature and sub-adult sharks in the entire eastern North Pacific Ocean (ENP). This low estimate generated great concern within the conservation community, prompting petitions for governmental endangered species designations. We critically examine that study and find violations of model assumptions that, when considered in total, lead to population underestimates. We also use a Bayesian mixture model to demonstrate that the inclusion of transient sharks, characteristic of white shark aggregation sites, would substantially increase abundance estimates for the adults and sub-adults in the surveyed sub-population. Using a dataset obtained from the same sampling locations and widely accepted demographic methodology, our analysis indicates a minimum all-life stages population size of >2000 individuals in the California subpopulation is required to account for the number and size range of individual sharks observed at the two sampled sites. Even accounting for methodological and conceptual biases, an extrapolation of these data to estimate the white shark population size throughout the ENP is inappropriate. The true ENP white shark population size is likely several-fold greater as both our study and the original published estimate exclude non-aggregating sharks and those that independently aggregate at other important ENP sites. Accurately estimating the central California and ENP white shark population size requires methodologies that account for biases introduced by sampling a limited number of sites and that account for all life history stages across the species' range of habitats.

Comparative demography of skates: life-history correlates of productivity and implications for management.

Age-structured demographic models were constructed based on empirical estimates of longevity and maturity for five deepwater Bering Sea skates to investigate how observed differences in life history parameters affect population growth rates. Monte Carlo simulations were used to incorporate parameter uncertainty. Estimated population growth rates ranged from 1.045 to 1.129 yr(-1) and were lower than those reported for other Alaskan skates and most chondrichthyans. Population growth rates of these and other high-latitude skates increased with relative reproductive lifespan, but displayed no significant relationship with body size or depth distribution, suggesting that assemblage shifts may be difficult to predict for data-poor taxa. Elasticity analyses indicated that juvenile and adult survival had greater per-unit effects on population growth rates than did egg-case survival or fecundity. Population growth rate was affected more by uncertainty in age at maturity than maximum age. The results of this study indicate that if skates are deemed to be a management concern, gear modifications or depth-specific effort controls may be effective.

Egg cases of the genus Apristurus (Chondrichthyes: Scyliorhinidae): phylogenetic and ecological implications.

The genus Apristurus has been divided formerly into three species subgroups based on morphological and molecular phylogenies. Previous studies have agreed on the separation into distinct species groups, the largest two of which have been described as the brunneus and spongiceps groups. An examination of the egg cases produced by these oviparous catsharks corroborates the distinction in species groups: egg cases of the brunneus group have long fibrous and curly tendrils at either end, whereas egg cases of the spongiceps group lack tendrils. The egg cases of A. kampae, A. manis, A. spongiceps and Apristurus sp. D are described for the first time, and the egg cases of A. brunneus and A. riveri are re-described to include comparable morphological characteristics. The ecological implications of egg case morphology on oviposition and ventilation are also discussed.

Calcitonin-like immunoreactivity in serum and tissues of the bonnethead shark, Sphyrna tiburo.

Calcitonin is a 32-amino acid peptide hormone that is best known for its actions in maintaining skeletal integrity and calcium homeostasis in mammals. Calcitonin also appears to function in regulating certain aspects of animal reproduction, but the nature of this role remains unclear, particularly in nonmammalian vertebrates. The present study investigated the relationship between calcitonin and reproduction in the bonnethead shark (Sphyrna tiburo), a well-studied member of the oldest living vertebrate group (i.e. elasmobranchs) known to possess a calcitonin-producing organ. Serum calcitonin concentrations were measured in 28 reproductively mature female S. tiburo using a heterologous enzyme-linked immunosorbent assay (ELISA) system. Sites of calcitonin immunoreactivity were detected in tissues of mature female and embryonic S. tiburo using immunocytochemistry. Significant increases in serum calcitonin concentrations of mature female S. tiburo occurred during early stages of gestation, a period characterized by yolk-dependency of developing embryos. Immunoreactive calcitonin was detected in the duodenum and pancreas of embryonic S. tiburo sampled during the same period. The results from this study suggest that calcitonin obtained from endogenous and/or maternal sources may function in regulating yolk digestion in embryonic S. tiburo. Therefore, the association between calcitonin and reproduction in elasmobranchs may reflect an important role for this hormone in embryonic development.