Shark Anatomy: Skeleton, Skin, and Sensory Organs

Sharks belong to the class Chondrichthyes, a group defined by a skeleton made of cartilage instead of bone. This article details the cartilaginous skeleton, the dermal denticle skin covering, and the sensory organs that distinguish sharks from bony fish, with attention to the structural and functional evidence from recent research. The content serves students, researchers, life-science professionals, and informed general readers who need a precise anatomical reference grounded in peer-reviewed findings.

The Cartilaginous Skeleton

Sharks do not have bones. Their entire internal framework consists of cartilage, a flexible connective tissue that is lighter than bone and well suited to an aquatic existence. The cartilaginous skeleton includes the cranium, the jaws, the vertebral column, and the fin supports. Unlike the bony skeletons of teleost fish and terrestrial vertebrates, shark cartilage lacks the marrow cavities and the cellular bone remodeling systems found in mammals.

Vertebral Centra Structure

The vertebral column of sharks is composed of individual vertebrae, each with a central body called the centrum. Research published in Acta biomaterialia in 2024 examined the vertebral centra of lamniform and carcharhiniform sharks using synchrotron microComputed Tomography, scanning electron microscopy, and light microscopy. The study focused on the blue shark and the shortfin mako and found that the main centrum structures, the corpus calcareum and intermedialia, consist of fine, closely spaced, mineralized trabeculae. Mean trabecular thickness ranged from 4.5 to 11.2 micrometers, and mean spacing ranged from 4.5 to 15.6 micrometers across the species examined. The trabeculae form an uninterrupted, interconnected network, and the unmineralized volumes are similarly interconnected. Growth band pairs in the centra consist of locally higher and lower mineral volume fractions, which is how age records are preserved in the skeleton. See the NCBI Literature Resources portal and the specific study on micrometer-scale structure in shark vertebral centra for the full methodology and data.

Mineralization Without Bone

The vertebral cartilage of sharks contains bioapatite, a calcium phosphate mineral related to that found in bone. However, the arrangement differs fundamentally. A 2024 study in Acta biomaterialia on blacktip and shortfin mako sharks used polarized light microscopy, atomic force microscopy, confocal Raman microspectroscopy, and nanoindentation to characterize the material. The highly calcified corpus calcarea show a ridged morphology, while the less mineralized intermedialia contain a chain-like network. Raman spectromicroscopy demonstrated a relative increase of glucosaminoglycans with respect to collagen in mineral-rich zones, indicating that these molecules modulate bioapatite mineralization. Local Young's moduli from mineralized regions exceeded bulk values by a factor of 10, showing that the mechanical properties vary dramatically within a single vertebra. See the PubMed record for the chemistry-structure-function relationships in shark vertebrae.

Tessellated Cartilage

A defining feature of shark and ray skeletons is tessellated cartilage, where only the outer surface of the cartilage mineralizes, forming a layer of small tiles called tesserae. A review in the Journal of fish biology describes this architecture as curious and complex, with features that would be considered anomalous or even pathological in human skeletons. The review synthesizes older anatomical literature with recent high-resolution structural and materials characterization work to frame the form-function relationships in this tissue. See the Journal of fish biology review on tessellated cartilage for the full synthesis.

Chondrocytes, the cells within cartilage, behave differently in elasmobranchs than in other vertebrates. A 2020 study in Bone used high-resolution synchrotron microCT scans of stingray tesserae to characterize chondrocyte lacunae, the spaces where these cells reside. The study found that cell density is comparable in unmineralized and mineralized tissue, and cells maintain similar volume even after incorporation into tesserae. This supports the hypothesis that elasmobranch chondrocytes do not proliferate, hypertrophy, or undergo apoptosis during mineralization, unlike the chondrocytes of other vertebrates. Tessera lacunae show zonal variation in shape, being flatter further from the unmineralized cartilage matrix and more spherical closer to it. The lacunae organize into parallel layers and orient toward neighboring tesserae. See the Bone study on co-aligned chondrocytes for the detailed morphometric analysis.

Nanoscale Architecture and Deformation

A 2025 study in ACS nano examined the ultrastructure and deformation mechanism of mineralized shark vertebrae from the blacktip shark using synchrotron X-ray nanotomography and deep-learning image segmentation. The corpus calcareum consists of stacked, interconnected, curved mineralized planes permeated by a network of organic occlusions. The mineral network in the intermedialia resembles trabecular bone, with thicker struts oriented opposite to the predominant biological strain. Collagenous fiber elements wind around lacunar spaces in the intermedialia, and the researchers hypothesize that the swirling arrangement and elasticity of the fibers distribute stress. With little permanent deformation detected in mineralized structures, the soft organic matrix likely absorbs energy through deformation, irreversible damage, and viscoelastic behavior. In the corpus calcareum, cracks typically terminate toward thick struts along the mineral planes, resembling the microscale crack deflection and arrest mechanism found in nacre and bone. Transmission electron microscopy revealed preferentially oriented, needlelike bioapatite crystallites and d-band patterns of collagen. See the ACS nano study on nanoscale shark vertebral cartilage for the complete findings.

Jaw Mechanics

The jaws of cartilaginous fishes vary in structure and mechanical properties according to diet and phylogenetic position. A 2022 study in The Journal of experimental biology investigated the jaws of the spotted ratfish, a holocephalan and the sister group to elasmobranchs. The compressive stiffness of the jaws ranged from 13.51 to 21.48 MPa, similar to silicone rubber. In holocephalans, the upper jaw is fused to the cranium, and this fusion reduces deformation during feeding. The lower jaw resists bending primarily in the posterior half, which occludes with the wider and flatter region of the upper jaw, providing an ideal location for crushing or cracking prey. The study concluded that while low compressive stiffness is a material limit of jaw cartilage, spotted ratfish evolved structural solutions, including the fused upper jaw and shape variation along the lower jaw, to meet the demands of a durophagous diet. See the Journal of experimental biology study on spotted ratfish jaws for the biomechanical analysis.

Hematopoiesis Without Bone Marrow

The cartilaginous skeleton of sharks has no bone marrow. A 2020 review in Fish & shellfish immunology explains that cartilaginous fish have Leydig's and epigonal organs that provide hematopoiesis analogous to mammalian bone marrow. The review also notes that the thymus and spleen of sharks resemble those of mammals in appearance and function, and that gut-associated lymphoid tissue is conserved across all vertebrate phylogeny. See the Fish & shellfish immunology review on cartilaginous fish lymphoid tissue for the comparative analysis.

Hormonal Regulation of Skeletal Tissues

The parathyroid hormone gene family, which regulates calcium and bone metabolism in vertebrates, has ancient origins in cartilaginous fish. A 2010 study in the Journal of bone and mineral research identified three members of the parathyroid hormone gene family in the elephant shark: Pth1, Pth2, and Pthrp. Phylogenetic analysis suggested that elephant shark Pth2 is an ancient gene whose orthologue is lost in bony vertebrates. The three genes showed distinct patterns of expression, with Pth2 expressed only in the brain and spleen. The elephant shark Pth1 and Pthrp peptides stimulated cAMP accumulation in mammalian cells, but Pth2 did not show such PTH-like biologic activity. The presence of these genes in the elephant shark indicates that they played fundamental roles before their recruitment to bone development in bony jawed vertebrates. See the Journal of bone and mineral research study on the parathyroid hormone gene family for the phylogenetic and functional analysis.

Shark Skin and Dermal Denticles

Shark skin is a biological composite of dermal denticles embedded in a multilayered network of collagen fibers. The denticles are toothlike structures that cover the body surface, each with a enamel-like cap and a dentine base. They reduce drag during swimming, provide protection, and give shark skin its characteristic abrasive texture.

Mechanical Properties Across Species

A 2025 study in Acta biomaterialia tested shark skin in uniaxial tension to failure and calculated tensile strain and mechanical properties from 20 species. The orientation of collagen fibers results in mechanical anisotropy: shark skin is more extensible when stressed longitudinally (anteroposterior) and stiffer when stressed perpendicularly (dorsoventral). Across ecomorphotypes, skin from deeper-water, non-migratory species was stronger and tougher than skin from small-bodied, non-migratory species, and medium-bodied, migratory species had stiffer skin than large-bodied, migratory species. Skin from mature sharks was stronger, stiffer, tougher, and more extensible than skin from pups. These results indicate that ontogeny and ecomorphotype impact skin mechanics among sharks. See the Acta biomaterialia study on shark skin mechanics for the comparative data.

Denticle Layer and Imaging Challenges

The high electron density of dermal denticles creates challenges for imaging internal structures. A 2025 study in Scientific Reports presented an AI-based solution for digitally removing the dermal denticle layer from CT scans of shark craniofacial skeletons. The researchers developed a geometric AI-driven software called SKINPEELER that selectively removes high-intensity voxels corresponding to the denticle layer while preserving underlying anatomical structures. The approach demonstrated high accuracy of 86.16 to 98.52 percent relative to manual measurements, with high reproducibility and repeatability. See the Scientific Reports study on digital denticle removal for the methodology and validation.

Sensory Organs

Sharks possess a suite of sensory organs that allow them to detect prey, navigate, and interact with their environment. These include the electrosensory ampullae of Lorenzini, the lateral line system, vision, olfaction, and hearing.

Ampullae of Lorenzini

The ampullae of Lorenzini are specialized electrosensory organs found in the heads of sharks and other elasmobranchs. They consist of jelly-filled canals that open to the skin surface and connect to sensory chambers. These organs detect weak electric fields produced by living organisms and are also sensitive to temperature and salinity changes.

The jelly within the ampullae has unusual properties. A 2007 study in Neuroscience Letters titled Semiconductor gel in shark sense organs investigated the electrical properties of this material, though the abstract was not available for detailed summary.

The peripheral morphology of the electrosensory system shapes behavior. A 2001 review in Environmental Biology of Fishes titled The Neuroecology of the Elasmobranch Electrosensory World examined how the distribution and morphology of electrosensory organs influence foraging and behavioral ecology, though the abstract was not available for detailed summary.

The ampullae of Lorenzini are vulnerable to environmental contamination. A 2022 study in Marine Pollution Bulletin assessed metal and metalloid contamination in the ampullae of Lorenzini of the Brazilian sharpnose shark. The study found no differences between sexes regarding jelly metal concentrations at either sampling site. Statistically significant correlations were noted between total lengths and condition factors and several metals at both sites, demonstrating the potential for sensory capacity disruption and possible effects on foraging success. Maternal metal transfer to ampullae jelly was confirmed. See the Marine Pollution Bulletin study on ampullae of Lorenzini contamination for the ecotoxicological data.

Gill Structure and Respiratory Function

The gills of sharks are housed in antero-posteriorly compressed gill pouches. A 2026 study in Tissue and Cell examined the gill structure of the tiger shark. The first pair of gills are hemibranchs, and the remaining pairs are holobranchs. The respiratory surfaces consist of cartilage-supported gill arches bearing numerous filaments with secondary lamellae lined by thin epithelium comprising pavement cells, chloride cells, and sparse goblet cells. The well-developed interbranchial septum is a distinguishing feature of elasmobranch gills. Histochemical staining demonstrated the presence of polysaccharides and neutral mucins in the basement membranes and goblet cells, weak alcian blue staining for acid mucin, abundant collagen fibers in supporting structures, and elastic fibers in the pharyngeal cavity, gill arches, and blood vessel walls. See the Tissue and Cell study on tiger shark gills for the histological and ultrastructural characterization.

At a Glance

Practical Assessment of Shark Anatomy

For researchers, fisheries biologists, and veterinary professionals who handle sharks, a systematic approach to anatomical assessment supports accurate data collection and minimizes specimen damage.

Step 1: External Examination

Begin with the skin surface. Note the density and arrangement of dermal denticles across body regions. The denticle layer can obscure internal structures in CT imaging, so account for this when planning imaging protocols. The SKINPEELER approach demonstrates that digital removal of the denticle layer improves measurement accuracy for craniofacial structures.

Step 2: Skeletal Assessment

Examine the vertebral column for centrum morphology. The corpus calcareum and intermedialia have distinct microarchitectures that vary between lamniform and carcharhiniform sharks. Trabecular thickness and spacing can be measured with microCT or histology. Growth band pairs in the centra record age and can be counted for demographic studies. See the vertebral centra study for reference values.

Step 3: Sensory Organ Sampling

If collecting ampullae of Lorenzini for contamination studies, note that metal concentrations in the jelly can correlate with total length and condition factor. Maternal transfer of metals to ampullae jelly has been confirmed, so account for reproductive status in sampling designs. See the contamination study for the ecotoxicological context.

Step 4: Tissue Preservation

Cartilage and skin samples require appropriate fixation for histological and mechanical analysis. For mechanical testing, fresh or properly frozen skin maintains its anisotropic properties. The collagen fiber orientation produces different mechanical responses along the anteroposterior versus dorsoventral axes, so record the orientation of each sample. See the skin mechanics study for testing protocols.

Records and Measurements

Accurate anatomical records depend on standardized measurements. The following table summarizes key parameters for documenting shark anatomy.

Common Failure Patterns in Anatomical Studies

Several recurring problems affect shark anatomical research and specimen handling.

Denticle Artifact in Imaging

The high electron density of dermal denticles creates beam-hardening artifacts in CT scans that obscure internal structures. Traditional specimen preparation often damages cranial landmarks. The AI-based denticle removal approach addresses this by digitally removing the denticle layer, achieving 86.16 to 98.52 percent accuracy relative to manual measurements.

Cartilage Deformation During Handling

Shark cartilage is flexible and can deform during dissection or fixation. The compressive stiffness of jaw cartilage is similar to silicone rubber, so mechanical testing must account for this material property. See the spotted ratfish jaw study for reference values.

Misidentification of Mineralized Tissues

The mineralization patterns in shark cartilage differ from bone. The corpus calcareum is a hypermineralized double cone, while the intermedialia are blocks of mineralized cartilage interspersed by unmineralized arches. Researchers unfamiliar with these structures may misidentify them as pathological calcification. See the nanoscale study for the structural distinctions.

Welfare and Safety Context

Handling sharks for anatomical study requires attention to both human safety and animal welfare. Live sharks should be handled with wet gloves or towels to protect the skin and the handler. The dermal denticles can cause abrasions, so appropriate protective equipment is necessary. For euthanized specimens, follow institutional animal care protocols and applicable regulations.

The cartilaginous skeleton has no bone marrow, so hematopoiesis occurs in the Leydig's and epigonal organs. Researchers should be aware of these organs when dissecting the body cavity. See the lymphoid tissue review for the anatomical context.

Limitations of Current Knowledge

Several aspects of shark anatomy remain incompletely characterized. The functions of chondrocytes within mineralized tesserae are unknown, as noted in the co-aligned chondrocytes study. The electrical properties of the ampullae of Lorenzini jelly require further investigation, as indicated by the semiconductor gel study. The relationship between skin morphology and mechanical function appears to be less direct than expected, as noted in the skin mechanics study.

The parathyroid hormone gene family in cartilaginous fish includes an ancient gene, Pth2, that is lost in bony vertebrates. The functional significance of this gene in sharks remains unclear. See the elephant shark study for the phylogenetic context.

Professional Escalation Criteria

When anatomical findings fall outside expected parameters, consult a specialist. Escalate to a veterinary pathologist or comparative anatomist when you observe:

• Mineralization patterns that do not match the described trabecular architecture of the corpus calcareum or intermedialia
• Skin mechanical properties that deviate substantially from the ecomorphotype ranges described in the comparative study
• Ampullae of Lorenzini jelly with unusual color, consistency, or contamination levels that may indicate environmental exposure
• Skeletal deformities that may indicate developmental abnormalities or nutritional deficiencies

For imaging questions, consult a radiologist experienced with cartilaginous fish. The denticle layer creates unique artifacts that require specialized processing, as demonstrated by the SKINPEELER software.

Frequently Asked Questions

Do sharks have bones?

No. Sharks have a skeleton made entirely of cartilage. The vertebral centra contain mineralized tissue with bioapatite, but the structure is trabecular cartilage instead of bone. The vertebral centra study describes the fine, closely spaced mineralized trabeculae that form the centrum structure.

What is shark skin made of?

Shark skin is a biological composite of dermal denticles embedded in a multilayered network of collagen fibers. The denticles are toothlike structures with an enamel-like cap over a dentine base. The collagen fiber orientation creates mechanical anisotropy, with skin more extensible longitudinally and stiffer perpendicularly. See the skin mechanics study for the comparative analysis.

How do shark vertebrae differ from mammal vertebrae?

Shark vertebrae consist of mineralized cartilage instead of bone. The centrum has a corpus calcareum, a hypermineralized double cone, and intermedialia, blocks of mineralized cartilage interspersed by unmineralized arches. The trabeculae form an interconnected network at the micrometer scale. See the nanoscale study for the ultrastructural details.

What are tesserae in shark skeletons?

Tesserae are the small tiles of mineralized cartilage that form the outer surface layer of shark and ray skeletons. Unlike the cartilage of other vertebrates, elasmobranch chondrocytes survive mineralization and remain alive in lacunae within the tesserae. See the co-aligned chondrocytes study for the cellular analysis.

How do the ampullae of Lorenzini work?

The ampullae of Lorenzini are electrosensory organs consisting of jelly-filled canals that open to the skin surface and connect to sensory chambers. They detect weak electric fields produced by living organisms. The jelly within the canals has unusual electrical properties that are still under investigation. See the semiconductor gel study for the material characterization.

Why is shark cartilage important for understanding vertebrate evolution?

Cartilaginous fish are the oldest living group of jawed vertebrates. Their skeletal tissues represent an intermediate stage in the evolution of bone. The parathyroid hormone gene family, which regulates bone development in bony vertebrates, has ancient members in sharks. See the elephant shark study for the genetic evidence.

How does shark skin mechanics vary among species?

Shark skin mechanics vary by ecomorphotype and ontogenetic stage. Deeper-water, non-migratory species have stronger and tougher skin than small-bodied, non-migratory species. Medium-bodied, migratory species have stiffer skin than large-bodied, migratory species. Mature sharks have stronger, stiffer, tougher, and more extensible skin than pups. See the comparative skin study for the species-level data.

What challenges do dermal denticles create for imaging?

The high electron density of dermal denticles obstructs clear visualization of internal structures in CT scans. Traditional specimen preparation often damages cranial landmarks. AI-based software can digitally remove the denticle layer while preserving underlying anatomy, achieving high accuracy relative to manual measurements. See the denticle removal study for the validation data.

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References and Further Reading

• NCBI Literature Resources. National Center for Biotechnology Information.
• PubMed. National Library of Medicine.
• Micrometer-scale structure in shark vertebral centra.. Acta biomaterialia, 2024.
• Revealing chemistry-structure-function relationships in shark vertebrae across length scales.. Acta biomaterialia, 2024.
• Biomechanics of the jaws of spotted ratfish.. The Journal of experimental biology, 2022.
• The multiscale architecture of tessellated cartilage and its relation to function.. Journal of fish biology, 2021.
• Comparative study of cartilaginous fish divulges insights into the early evolution of primary, secondary and mucosal lymphoid tissue architecture.. Fish & shellfish immunology, 2020.
• A Nanoscale View of the Structure and Deformation Mechanism of Mineralized Shark Vertebral Cartilage.. ACS nano, 2025.
• Parathyroid hormone gene family in a cartilaginous fish, the elephant shark (Callorhinchus milii).. Journal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research, 2010.
• Co-aligned chondrocytes: Zonal morphological variation and structured arrangement of cell lacunae in tessellated cartilage.. Bone, 2020.
• Histochemical and ultrastructural characterization of the gills in tiger shark, Galeocerdo cuvier (Péron &amp, Lesueur, 1822): Structural strategies for functional adaptation.. 2026.
• Ecomorphology and ontogeny modulate the mechanical properties of shark skin.. 2025.
• Digital removal of dermal denticle layer using geometric AI from 3D CT scans of shark craniofacial structures enhances anatomical precision.. 2025.
• Semiconductor gel in shark sense organs?. Neuroscience Letters, 2007.
• The Neuroecology of the Elasmobranch Electrosensory World: Why Peripheral Morphology Shapes Behavior. Environmental Biology of Fishes, 2001.
• First report on metal and metalloid contamination of Ampullae of Lorenzini in sharks: A case study employing the Brazilian sharpnose shark Rhizoprionodon lalandii from Southeastern Brazil as an ecotoxicological model.. Marine Pollution Bulletin, 2022.
• The skeleton | Cartilaginous Fish Skeletal Tissues. Encyclopedia of Fish Physiology, 2011.
• Evolution of the parathyroid hormone family and skeletal formation pathways. General and Comparative Endocrinology, 2011.

This article is educational and does not replace institutional policy, professional advice, or applicable safety and regulatory requirements.