Structure and topographic distribution of oral denticles in elasmobranch fishes

Morphological study of the oral denticles of the porbeagle shark Lamna nasus

Oral denticles of sharks are composed by a crown, dentine covered by a layer of enameloid and pulp cavity, the same structure of the dermal denticles found across the body surface of most elasmobranchs. In addition, oral papillae and taste buds are distributed among denticles within the oropharyngeal cavity, playing a fundamental role for tasting as part of the chemosensory system of fishes. Scanning electron microscopy (SEM) has been employed as an important tool for the study of dermal denticles and other structures, as well as histology and more recently computed tomography (CT) scan analysis. Herein, the authors used two methods for the study of the morphology of the oropharyngeal cavity of Lamna nasus (Lamniformes), an oceanic and pelagic shark: SEM and CT scan. The general morphology of oral denticles studied herein is related to abrasion strength as they are diamond-shaped, lack lateral cusps and have less pronounced ridges. In addition, smooth ridges and broad rounded denticles could be related to prevent abrasion during food consumption and manipulation. Oral papillae had a round shape and were observed only under SEM. The densities of papillae were estimated in 100 per cm² , whereas denticles were 1760 and 1230 cm² over the dorsal and ventral regions, respectively. The high numbers of denticles are inversely proportional to papillae density; denticles seem to restrict papillae distribution. Regarding the differences between methodologies, under SEM, only the crown was visualized, as well the papillae, allowing the estimation of size and density of both structures. Nonetheless, under CT scan, the whole components of denticles were clearly visualized: different views of the crown, peduncle, basal plate, and pulp cavity. On the contrary, oral papillae were not visualized under CT due to the tissue preparation. Furthermore, both methods are complementary and were important to extract as much information as possible from denticles and papillae.

The conundrum of pharyngeal teeth origin: the role of germ layers, pouches, and gill slits

There are several competing hypotheses on tooth origins, with discussions eventually settling in favour of an 'outside-in' scenario, in which internal odontodes (teeth) derived from external odontodes (skin denticles) in jawless vertebrates. The evolution of oral teeth from skin denticles can be intuitively understood from their location at the mouth entrance. However, the basal condition for jawed vertebrates is arguably to possess teeth distributed throughout the oropharynx (i.e. oral and pharyngeal teeth). As skin denticle development requires the presence of ectoderm-derived epithelium and of mesenchyme, it remains to be answered how odontode-forming skin epithelium, or its competence, were 'transferred' deep into the endoderm-covered oropharynx. The 'modified outside-in' hypothesis for tooth origins proposed that this transfer was accomplished through displacement of odontogenic epithelium, that is ectoderm, not only through the mouth, but also via any opening (e.g. gill slits) that connects the ectoderm to the epithelial lining of the pharynx (endoderm). This review explores from an evolutionary and from a developmental perspective whether ectoderm plays a role in (pharyngeal) tooth and denticle formation. Historic and recent studies on tooth development show that the odontogenic epithelium (enamel organ) of oral or pharyngeal teeth can be of ectodermal, endodermal, or of mixed ecto-endodermal origin. Comprehensive data are, however, only available for a few taxa. Interestingly, in these taxa, the enamel organ always develops from the basal layer of a stratified epithelium that is at least bilayered. In zebrafish, a miniaturised teleost that only retains pharyngeal teeth, an epithelial surface layer with ectoderm-like characters is required to initiate the formation of an enamel organ from the basal, endodermal epithelium. In urodele amphibians, the bilayered epithelium is endodermal, but the surface layer acquires ectodermal characters, here termed 'epidermalised endoderm'. Furthermore, ectoderm-endoderm contacts at pouch-cleft boundaries (i.e. the prospective gill slits) are important for pharyngeal tooth initiation, even if the influx of ectoderm via these routes is limited. A balance between sonic hedgehog and retinoic acid signalling could operate to assign tooth-initiating competence to the endoderm at the level of any particular pouch. In summary, three characters are identified as being required for pharyngeal tooth formation: (i) pouch-cleft contact, (ii) a stratified epithelium, of which (iii) the apical layer adopts ectodermal features. These characters delimit the area in which teeth can form, yet cannot alone explain the distribution of teeth over the different pharyngeal arches. The review concludes with a hypothetical evolutionary scenario regarding the persisting influence of ectoderm on pharyngeal tooth formation. Studies on basal osteichthyans with less-specialised types of early embryonic development will provide a crucial test for the potential role of ectoderm in pharyngeal tooth formation and for the 'modified outside-in' hypothesis of tooth origins.

Scymnodon plunketi (Waite, 1910): a junior synonym of Scymnodon macracanthus (Regan, 1906) (Somniosidae: Elasmobranchii)

The taxonomic status and validity of Scymnodon macracanthus (Regan, 1906) and Scymnodon plunketi (Waite, 1910) are revised in light of new material from the Southern Pacific and Indian Oceans. Despite being historically accepted as distinct taxa, recent studies suggested the possibility that these species could represent a single taxon. Morphometrics, meristics and morphology of dermal denticles show that S. plunketi is indeed a junior synonym of S. macracanthus. Previous distinctive characters proved to be the result of intraspecific variation. S. macracanthus is therefore redescribed including an updated comparative diagnosis for the genus Scymnodon in the family Somniosidae.

An ultrastructural focus on the buccal cavity of the grey gurnard Eutrigla gurnardus (Linnaeus, 1758): Adaptative dietary implications

The current investigation represented the first anatomical description of the buccal cavity of the grey gurnard Eutrigla gurnardus. For the achievement of this study, 10 heads of mature fish were used for SEM examinations. The morphological characteristic appearance of the buccal cavity has shown several adaptations in relation concerning its feeding habits. The median fissure of the upper lip divided the upper lip into two halves, this fissure appeared to be formed from many micro-tubercles with numerous taste buds which aid in holding mucus secretion to the cell surface, help in mucus spread, and provide mechanical protection to the epithelium. Velum had two regions: median and peripheral region. The epithelium covering of the median region of the upper velum had dots-like epithelial protrusion that carrying numerous taste buds, meanwhile, the epithelium covering of the lower velum not carried taste buds but instead carried numerous small scales-like structures. The palate is divided into two semicircular regions by small region of microridges into the anterior and posterior region. The slightly bifid tongue had a median tubercle with numerous small taste buds on its dorsal surface. Concluded, these data were necessary to understand the adaptation of fish to its habitat and in developing a new and better method for fishing of this type of fish.

Age-related ultrastructural characterizations of the teeth of the white grouper (Epinephelus aeneus) in the different three age-stages

This study represents the first attempt to describe ultrastructural features of teeth in different three ages of white grouper (Epinephelus aeneus) grossly and by the aid of the electron microscope. The current study depends on 18 oropharyngeal cavity of E. aeneus from each age-stages. There are common and special characterization between each age. Five teeth bands in the roof (incisive, canine, upper molar, palatine, and vomer), while two bands in the floor (incisive, molar). The apical teeth part resembles the arrowhead that bordered by groove distally. In 5 cm age, small upper incisive teeth had two appearance (straight and curved), and vomer teeth arranged in triangular in only one row (while, in 12 cm fish arranged in two rows and in 15 cm fish arranged in more than two rows), the palatine teeth began rostrally as one row then two rows and terminated by one row, while in other two ages began by two, then three and ended by two rows. There is no canine teeth in lower jaw in all age. In 12 cm fish, the rostral row of lower incisive teeth usually contain small straight teeth, while the posterior row mainly contain large with some medium upper incisive teeth. In 18 cm fish, upper incisive teeth located within two incisive fossa that separated from each other by longitudinal part of T-shaped upper incisive ridge and small incisive teeth had two appearance (straight and curved), and each canined teeth group contain four teeth (while in other two ages each group contain only two canine teeth).

The megamouth shark, Megachasma pelagios, is not a luminous species

Despite its five meters length, the megamouth shark (Megachasma pelagios Taylor, Compagno & Struhsaker, 1983) is one of the rarest big sharks known in the world (117 specimens observed and documented so far). This filter-feeding shark has been assumed to be a luminous species, using its species-specific white band to produce bioluminescence as a lure trap. Another hypothesis was the use of the white band reflectivity to attract prey or for social recognition purposes. However, no histological study has ever been performed to confirm these assumptions so far. Two hypotheses about the megamouth shark's luminescence arose: firstly, the light emission may be intrinsically or extrinsically produced by specific light organs (photophores) located either on the upper jaw white band or inside the mouth; secondly, the luminous appearance might be a consequence of the reflection of prey luminescence on the white band during feeding events. Aims of the study were to test these hypotheses by highlighting the potential presence of specific photophores responsible for bioluminescence and to reveal and analyze the presence of specialized light-reflective structures in and around the mouth of the shark. By using different histological approaches (histological sections, fluorescent in situ hybridization, scanning electron microscopy) and spectrophotometry, this study allows to unravel these hypotheses and strongly supports that the megamouth shark does not emit bioluminescence, but might rather reflect the light produced by bioluminescent planktonic preys, thanks to the denticles of the white band.

Microstructural morphology of dermal and oral denticles of the sharpnose sevengill shark Heptranchias perlo (Elasmobranchii: Hexanchidae), a deep-water species

The dermal denticles are among the unique morphological adaptations of sharks, which have been acquired throughout their long evolutionary process of more than 400 million years. Species-specific morphological characteristics of these structures has been applied specially as tools for functional and taxonomic (family-level) studies. Nevertheless, few studies have explored the diversity of denticle structure in different around the body and oral cavity. In the present study, we described the morphological differences observed in skin and oral cavity of sharpnose sevengill shark Heptranchias perlo, using scanning electron microscopy. Our findings demonstrate substantial variation in morphological structure of the denticles of the body and oral cavity. Overall, the dermal denticles observed across body surface were overlapped, tricuspid, with the central cuspid being more pronounced, pointed, and triangular in shape compared with lateral ones. Unlike, the denticles on the tip of the nose had a smooth crown, with rounded edges, being compact, and overlapped. The oral denticles were found in the ventral and dorsal region of the oral cavity. They also were tricuspid, but with differences in arrangement and ridges. These results suggest a strict functional relationship with the morphological characteristics observed. Such morphological diversity body-region-dependent highlights the need for comparative studies that include oral denticles, since this structure has an important functional role in sharks and can be found in fossil and recent records.

Taxonomy through the lens of neutral helium microscopy

The field of taxonomy is critically important for the identification, conservation, and ecology of biological species. Modern taxonomists increasingly need to employ advanced imaging techniques to classify organisms according to their observed morphological features. Moreover, the generation of three-dimensional datasets is of growing interest; moving beyond qualitative analysis to true quantitative classification. Unfortunately, biological samples are highly vulnerable to degradation under the energetic probes often used to generate these datasets. Neutral atom beam microscopes avoid such damage due to the gentle nature of their low energy probe, but to date have not been capable of producing three-dimensional data. Here we demonstrate a means to recover the height information for samples imaged in the scanning helium microscope (SHeM) via the process of stereophotogrammetry. The extended capabilities, namely sparse three-dimensional reconstructions of features, were showcased via taxonomic studies of both flora (Arabidopsis thaliana) and fauna (Heterodontus portusjacksoni). In concert with the delicate nature of neutral helium atom beam microscopy, the stereophotogrammetry technique provides the means to derive comprehensive taxonomical data without the risk of sample degradation due to the imaging process.

A preliminary investigation into the morphology of oral papillae and denticles of blue sharks (Prionace glauca) with inferences about its functional significance across life stages

Sensory organs in elasmobranchs (sharks, skates, rays) detect and respond to a different set of biotic and/or abiotic stimuli, through sight, smell, taste, hearing, mechanoreception and electroreception. Although gustation is crucial for survival and essential for growth, mobility, and maintenance of neural activity and the proper functioning of the immune system, comparatively little is known about this sensory system in elasmobranchs. Here we present a preliminary investigation into the structural and dimensional characteristics of the oral papillae and denticles found in the oropharyngeal cavity of the blue shark (Prionace glauca) during embryonic development through adulthood. Samples were obtained from the dorsal and ventral surface of the oropharyngeal cavity collected from embryos at different development stages as well as from adults. Our results suggest that development of papillae occurs early in ontogeny, before the formation of the oral denticles. The diameter of oral papillae gradually increases during development, starting from 25 μm in stage I embryos, to 110 μm in stage IV embryos and 272-300 μm in adults. Embryos exhibit papillae at early developmental stages, suggesting that these structures may be important during early in life. The highest density of papillae was observed in the maxillary and mandibular valve regions, possibly related to the ability to identify, capture and process prey. The oral denticles were observed only in the final embryonic stage as well as in adults. Accordingly, we suggest that oral denticles likely aid in ram ventilation (through reducing the hydrodynamic drag), to protect papillae from injury during prey consumption and assist in the retention and consumption of prey (through adhesion), since these processes are only necessary after birth.

Squamation and ecology of thelodonts

Thelodonts are an enigmatic group of Paleozoic jawless vertebrates that have been well studied from taxonomical, biostratigraphic and paleogeographic points of view, although our knowledge of their ecology and mode of life is still scant. Their bodies were covered by micrometric scales whose morphology, histology and the developmental process are extremely similar to those of extant sharks. Based on these similarities and on the well-recognized relationship between squamation and ecology in sharks, here we explore the ecological diversity and lifestyles of thelodonts. For this we use classic morphometrics and discriminant analysis to characterize the squamation patterns of a significant number of extant shark species whose ecology is well known. Multivariate analyses have defined a characteristic squamation pattern for each ecological group, thus establishing a comparative framework for inferring lifestyles in thelodonts. We then use this information to study the squamation of the currently described 147 species of thelodonts, known from both articulated and disarticulated remains. Discriminant analysis has allowed recognizing squamation patterns comparable to those of sharks and links them to specific ecological groups. Our results suggest a remarkable ecological diversity in thelodonts. A large number of them were probably demersal species inhabiting hard substrates, within caves and crevices in rocky environments or reefs, taking advantage of the flexibility provided by their micromeric squamations. Contrary to classical interpretations, only few thelodonts were placed among demersal species inhabiting sandy and muddy substrates. Schooling species with defensive scales against ectoparasites could be also abundant suggesting that social interactions and pressure of ectoparasites were present in vertebrates as early the Silurian. The presence of species showing scales suggestive of low to moderate speed and a lifestyle presumably associated with open water environments indicates adaptation of thelodonts to deep water habitats. Scale morphology suggests that some other thelodonts were strong-swimming pelagic species, most of them radiating during the Early Devonian in association with the Nekton Revolution.

Morphology and distribution of taste papillae and oral denticles in the developing oropharyngeal cavity of the bamboo shark, Chiloscyllium punctatum

Gustation in sharks is not well understood, especially within species that ingest food items using suction. This study examines the morphological and immunohistochemical characterisation of taste papillae and oral denticles in the oropharynx of the brown-banded bamboo shark Chiloscyllium punctatum and compares their distribution during development. Taste papillae of C. punctatum are located throughout the oropharyngeal region and are most concentrated on the oral valves (2125-3483 per cm² in embryos; 89-111 per cm² in mature adults) close to the tooth territories. Papillae appearance is comparable at all stages of development, with the exception of the embryos (unhatched specimens), where no microvilli are present. Oral valve papillae are comparable in structure to Type I taste buds of teleost fishes, whereas those of the rest of the oropharyngeal region are comparable to Type II. Both types of papillae show immunofluorescence for a number of markers of taste buds, including β-Catenin and Sox2. Taste papillae densities are highest in embryos with 420-941 per cm² compared to 8-29 per cm² in mature adults. The total number of papillae remains around 1900 for all stages of development. However, the papillae increase in diameter from 72±1 μm (mean±s.e.m.) in embryos to 310±7 μm in mature individuals. Microvilli protrude in multiple patches at the apical tip of the papilla covering ∼0.5% of the papillar surface area. We further document the relationship between taste papillae and the closely associated oral denticles within the shark orophayngeal cavity. Oral denticles first break through the epithelium in the antero-central region of the dorsal oral cavity, shortly after the emergence of teeth, around time of hatching. Denticles are located throughout the oropharyngeal epithelium of both immature and mature stages, with the highest concentrations in the antero-dorsal oral cavity and the central regions of the pharynx. These denticle-rich areas of the mouth and pharynx are therefore thought to protect the epithelium, and importantly the taste papillae, from abrasion since they correlate with regions where potential food items are processed or masticated for consumption. Taste papillae and denticles are more dense in anterior oropharyngeal regions in close association with the oral jaws and teeth, and in the juvenile or hatchling shark taste units are functional, and innervated, allowing the shark to seek out food in utero, at birth or on emergence from the egg case.

Summary: Characterisation of taste buds in the developing bamboo shark, Chiloscyllium punctatum, reveals that taste papillae are functional, innervated units, allowing the shark to seek out food in utero, at birth or following hatching.

Morphological study of the asymmetrical buccal cavity of the flatfish common solea (Solea solea) and its relation to the type of feeding

OBJECTIVE

To investigate the surface architecture of the asymmetrical buccal cavity of Solea solea which are considered one of the most important predators in benthic communities.

METHODS

Adult Solea solea were obtained from Mediterranean Sea near Damietta. The heads were removed and processed for scanning electron microscopy. Its buccal cavity is asymmetrical and divided into roof and floor and the tongue for histological studies.

RESULTS

The buccal cavity roof is formed from upper jaw, velum and the palate. The upper jaw has several wing like processes with teeth arranged in several rows which may help in cutting and pushing the food to the entrance of the digestive canal while the floor is formed from the lower jaw and the tongue. The tongue is divided into apex, body and root. There is a gradual decrease of goblet cells in the tongue from anterior to posterior. These goblet cells function in protection of the epithelium.

CONCLUSIONS

Teeth in the floor of the buccal cavity and taste buds can be considered adaptive changes of the oral cavity related to the feeding habits and was a source to identify new and better methods of nutrition in aquaculture of Solea solea.