Dental sexual dimorphism and morphology of Urotrygon microphthalmum

Secondary sexual dimorphism and ontogenetic shifts in habitat use by the lesser guitarfish Zapteryx brevirostris

Sexual dimorphisms are generated by divergent processes, such as natural or sexual selection and niche convergence. Males and females of the lesser guitarfish, Zapteryx brevirostris, present morphological differences in their discs, and the relationships with the species biology and ecology were unrecognized. Analysing the morphometry of 201 specimens and the influence of bottom features on the frequencies of 188 specimens among life stages and sexes, we found strong evidence that gonadal maturation leads to dimorphisms on discs, validating a concavity on male pectoral fins as a secondary sexual dimorphism and rejecting the hypothesis that such dimorphisms were related to ecological pressures. The principal component analysis (PCA) and permutational MANOVA (PERMANOVA) analyses revealed that males and females shared similar body aspects until they reached maturity, mainly due to lower variations in WD, WR, LD, DPRO, and LSC at younger life stages. The relationships of these variables with LT corroborate the former results, showing a changing point around LT > 30 cm where females started to attain larger measurements than males. Moreover, we revealed ontogenetic shifts, with adults from both sexes exploring different habitats than juveniles and subadults. Differences in frequencies of each life stage were best explained by organic matter (OM) with the adults exploring bottom habitats of higher concentrations of OM than juveniles and subadults, strengthening the assumption that body differences between sexes are not related to ecological pressures. These results bring not only new insights about the possible advantages that those morphometric differences provide to males while mating but also information about the abiotic influences on species distribution, which, along with knowledge of local oceanographic dynamics and benthic community patterns, would inform actions for species conservation.

The First Southern Hemisphere Occurrence of the Extinct Cretaceous Sclerorhynchoid Sawfish Ptychotrygon (Chondrichthyes, Batoidea), With a Review of Ptychotrygon Taxonomy

A new extinct sclerorhynchoid sawfish, Ptychotrygon ameghinorum sp. nov., is presented here based on abundant isolated teeth and some dermal denticles, which were recovered from the Mata Amarilla Formation, belonging to the lower Upper Cretaceous of the Santa Cruz Province in the Austral Basin of Patagonia, Argentina. This new species is the first Ptychotrygon occurrence in the southern hemisphere, which so far only has been reported from northern hemisphere deposits (Europe, North Africa, and North America). The presence of P. ameghinorum sp. nov. in these southern high-latitude deposits of Patagonia, Argentina, extends the geographic range of Ptychotrygon considerably southwards. This distribution pattern in the "middle" Cretaceous seems to correlate with the South Atlantic opening at the end of the Albian. The presence of lateral cephalic dermal denticles and the simultaneous absence of rostral denticles in the abundant fossil material support the view that Ptychotrygon did not develop such rostral structures. A reinvestigation of all known species assigned to Ptychotrygon reveals that P. ellae is a junior synonym of P. boothi, P. benningensis belongs to Texatrygon, P. rugosum belongs to Asflapristis, and P. clementsi represents an unidentifiable species (Ptychotrygon? sp.). The stratigraphic distribution demonstrates that Ptychotrygon might have originated in the Albian in south-western Europe and subsequently dispersed to obtain its widest distribution during the Cenomanian. In the Coniacian, a steep diversity decline is recognizable with a subsequent distribution shift from Europe to North America.

Cranial morphology of the orectolobiform shark, Chiloscyllium punctatum Müller & Henle, 1838

Elasmobranchs, comprising sharks, skates, and rays, have a long evolutionary history extending back into the Palaeozoic. They are characterized by various unique traits including a predominantly cartilaginous skeleton, superficial prismatic phosphatic layer, and permanent tooth replacement. Moreover, they exhibit a more or less marked sexual dimorphism. Especially the morphology of the chondrocranium and the elements of the whole cranial region of extant and extinct chondrichthyans can provide valuable information about corresponding functions, e.g. the feeding apparatus might reflect the diet of the animals. However, studies on sexual dimorphisms are lacking in orectolobiform sharks, therefore, little is known about possible sexual dimorphic characters in the cranial region in this group. For this reason, we present in this study a comprehensive morphological description of the cranial region of the brownbanded bamboo shark Chiloscyllium punctatum Müller & Henle, 1838, with a special focus on its sexual dimorphic characters. Our results reveal clear morphological differences in both sexes of the examined C. punctatum specimens, particularly in the chondrocranium and the mandibular arch. The female specimen shows a comparatively more robust and compact morphology of the chondrocranium. This pattern is also evident in the mandibular arch, especially in the palatoquadrate. The present study is the first to describe the morphology of an orectolobiform shark species in detail using both manual dissection and micro-CT data. The resulting data furthermore provide a starting point for pending studies and are intended to be a first step in a series of comparative studies on the morphology of the cranial region of orectolobiform sharks, including the determination of possible sexual dimorphic characteristics.

Feeding habits of Urotrygon microphthalmum (Myliobatiformes: Urotrygonidae) caught off northeastern Brazil

Abstract The feeding habits of Urotrygon microphthalmum, a Critically Endangered (CR) species, were investigated through stomach contents analysis from specimens caught on bottom double rigged otter trawls in Pernambuco state, Brazil, between March of 2010 and March of 2012. A total of 338 stomachs were analyzed, and 31 food items were identified in the diet of U. microphthalmum. The species ingests mainly shrimps. The diets between males and females were not different, and an ontogenetic diet shift was not observed. The estimated species’ trophic level is 3.5, classifying it as a secondary order consumer.

The intraspecific diversity of tooth morphology in the large-spotted catshark Scyliorhinus stellaris: insights into the ontogenetic cues driving sexual dimorphism

Teeth in sharks are shed and replaced throughout their lifetime. Morphological dental changes through ontogeny have been identified in several species and have been correlated with shifts in diet and the acquisition of sexual maturity. However, these changes were rarely quantified in detail along multiple ontogenetic stages, which makes it difficult to infer the developmental processes responsible for the observed plasticity. In this work, we use micro-computed tomography and 3D geometric morphometrics to describe and analyze the tooth size and shape diversity across three ontogenetic stages (hatchling, juvenile, and sexually mature) in the large-spotted catshark Scyliorhinus stellaris (Linnaeus, 1758). We first describe the intra-individual variation of tooth form for each sex at each ontogenetic stage. We provide a tooth morphospace for palatoquadrate and Meckelian teeth and identify dental features, such as relative size and number of cusps, involved in the range of variation of the observed morphologies. We then use these shape data to draw developmental trajectories between ontogenetic stages and for each tooth position within the jaw to characterize ontogenetic patterns of sexual dimorphism. We highlight the emergence of gynandric heterodonty between the juvenile and mature ontogenetic stages, with mature females having tooth morphologies more similar to juveniles' than mature males that display regression in the number of accessory cusps. From these data, we speculate on the developmental processes that could account for such developmental plasticity in S. stellaris.

Intraspecific dental variations in the deep-sea shark Etmopterus spinax and their significance in the fossil record

An important character on several taxonomic levels for shark identification is the tooth morphology. Sharks show a variety of highly specialized dentitions reflecting adaptations to their feeding habits. Intraspecific variation of tooth morphology such as sexual or ontogenetic dimorphism is poorly known in many species, even though tooth morphology plays a decisive role in the characterization of the fossil record of sharks, which comprises mostly fossil teeth. Here we analyzed the dentition of 40 jaws of the Velvet Belly Lantern Shark Etmopterus spinax and identified ontogenetic and sexual dimorphic characters such as total number of teeth, number of upper teeth, cusplet numbers in upper jaw teeth and width of lower jaw teeth. Dimorphic characters may reduce intraspecific competition for food, as E. spinax segregates by sex and size and may allow for identifying the male sex. The lower jaw tooth height, a sexually non-dimorphic character, was used to re-calculate the total length of specimens, which represents the first such approach for a squaliform shark. Results derived from the extant E. spinax are subsequently applied to fossil Etmopterus sp. teeth (Miocene) to gain individual information such as sex or size, but also characterize the extinct population from the excavation site by a size distribution profile in comparison to data from extant populations. This approach indicates the presence of multiple ontogenetic stages in the extinct population.

Biological notes on the reef stingray, Urobatis concentricus, an endemic species of Mexico

This study found that the reef stingray Urobatis concentricus presents a matrotrophic aplacental viviparous reproduction with yolk sac and trophonemata formation. Growth of ovarian follicles occurs asynchronously and continuously with fecundity of three embryos per female. A maximum size of 58·4 cm total length and 37·6 cm disc width was recorded, corresponding to a mature female of 3 or 4 years of age.