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Elasmobranchii (/ɪˌlæzməˈbræŋkiaɪ/[1]) is a subclass of Chondrichthyes or cartilaginous fish, including modern sharks (division Selachii), and batomorphs (division Batomorphi, including rays, skates, and sawfish). Members of this subclass are characterised by having five to seven pairs of gill slits opening individually to the exterior, rigid dorsal fins and small placoid scales on the skin. The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper. The details of this jaw anatomy vary between species, and help distinguish the different elasmobranch clades. The pelvic fins in males are modified to create claspers for the transfer of sperm. There is no swim bladder; instead, these fish maintain buoyancy with large livers rich in oil.
The definition of the clade is unclear with respect to fossil chondrichthyans. Some authors consider it as equivalent to Neoselachii (the crown group clade including modern sharks, rays, and all other descendants of their last common ancestor). Other authors use the name Elasmobranchii for a broader branch-based group of all chondrichthyans more closely related to modern sharks and rays than to Holocephali (the clade containing chimaeras and their extinct relatives).[2] Important extinct groups of elasmobranchs sensu lato include the hybodonts (Order Hybodontiformes), xenacanths (order Xenacanthformes) and Ctenacanthiformes. These are also often referred to as "sharks" in reference to their similar anatomy and ecology to modern sharks.
The name Elasmobranchii comes from the Ancient Greek words elasmo- ("plate") and bránchia ("gill"), referring to the broad, flattened gills which are characteristic of these fishes.
From a practical point of view the life-history pattern of elasmobranchs makes this group of animals extremely susceptible to over fishing. It is no coincidence that the commercially exploited marine turtles and baleen whales, which have life-history patterns similar to the sharks, are also in trouble.[7]
Members of the Elasmobranchii subclass have no swim bladders, five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins, and small placoid scales. The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper.
Extant elasmobranchs exhibit several archetypal jaw suspensions: amphistyly, orbitostyly, hyostyly, and euhyostyly. In amphistyly, the palatoquadrate has a postorbital articulation with the chondrocranium from which ligaments primarily suspend it anteriorly. The hyoid articulates with the mandibular arch posteriorly, but it appears to provide little support to the upper and lower jaws. In orbitostyly, the orbital process hinges with the orbital wall and the hyoid provides the majority of suspensory support.
In contrast, hyostyly involves an ethmoid articulation between the upper jaw and the cranium, while the hyoid most likely provides vastly more jaw support compared to the anterior ligaments. Finally, in euhyostyly, also known as true hyostyly, the mandibular cartilages lack a ligamentous connection to the cranium. Instead, the hyomandibular cartilages provide the only means of jaw support, while the ceratohyal and basihyal elements articulate with the lower jaw, but are disconnected from the rest of the hyoid.[8][9][10] The eyes have a tapetum lucidum. The inner margin of each pelvic fin in the male fish is grooved to constitute a clasper for the transmission of sperm. These fish are widely distributed in tropical and temperate waters.[11]
Many fish maintain buoyancy with swim bladders. However elasmobranchs lack swim bladders, and maintain buoyancy instead with large livers that are full of oil.[12] This stored oil may also function as a nutrient when food is scarce.[7][13]
The oldest unambiguous total group elasmobranch, Phoebodus, has its earliest records in the Middle Devonian (late Givetian), around 383 million years ago.[14] Several important groups of total group elasmobranchs, including Ctenacanthiformes and Hybodontiformes, had already emerged by the latest Devonian (Famennian).[15] During the Carboniferous, some ctenacanths would grow to sizes rivalling the modern great white shark with bodies in the region of 7 metres (23 ft) in length.[16] During the Carboniferous and Permian, the xenacanths were abundant in both freshwater and marine environments, and would continue to exist into the Triassic with reduced diversity.[17] The hybodonts had achieved a high diversity by the Permian,[18] and would end up becoming the dominant group of elasmobranchs during the Triassic and Early Jurassic. Hybodonts were extensively present in both marine and freshwater environments.[19] While Neoselachii/Elasmobranchi sensu stricto (the group of modern sharks and rays) had already appeared by the Triassic, they only had low diversity during this period and only began to extensively diversify from the Early Jurassic onwards, when modern orders of sharks and rays appeared.[20] This co-incided with the decline of the hybodonts, which had become minor components of marine environments by the Late Jurassic but would remain common in freshwater environments into the Cretaceous.[21] The youngest remains of hybodonts date to the very end of the Cretaceous.[22]
Elasmobranchii was first coined in 1838 by Charles Lucien Bonaparte. Bonaparte's original definition of Elasmobranchii was effectively identical to modern Chondrichthyes, and was based around gill architecture shared by all 3 living major cartilaginous fish groups. During the 20th century it became standard to exclude chimaeras from Elasmobranchii; along with including many fossil chondrichthyans within the group. The definition of Elasmobranchii has since been subject to much confusion with regard to fossil chondrichthyans. Maisey (2012) suggested that Elasmobranchii should exclusively be used for the last common ancestor of modern sharks and rays, a grouping which had previously been named Neoselachii by Compagno (1977).[2] Other recent authors have used Elasmobranchii in a broad sense to include all chondrichthyans more closely related to modern sharks and rays than to chimaeras.[14]
The total group of Elasmobranchii includes the cohort Euselachii Hay, 1902, which groups the Hybodontiformes and a number of other extinct chondrichthyans with Elasmobrachii sensu stricto/Neoselachii, to the exclusion of more primitive total group elasmobranchs, which is supported by a number of shared morphological characters of the skeleton.[23][24][25][26]
The 5th edition of Fishes of the World sets out the following classification of the Elasmobranchs:[29]
Infraclass Elasmobranchii
Division Selachii (sharks)
Superorder Galeomorphi
†Order Synechodontiformes
Order Heterodontiformes
Order Orectolobiformes
Suborder Parascyllioidei
Suborder Orectoloboidei
Order Lamniformes
Order Carcharhiniformes
Superorder Squalomorphi
Series Hexanchida
Order Hexanchiformes
Series Squalida
Order Squaliformes
Series Squatinida
†Order Protospinaciformes
Order Echinorhiniformes
Order Squatiniformes
Order Pristiophoriformes
Division Batomorphi (rays)
Order Torpediniformes
Order Rajiformes
Order Pristiformes
Order Myliobatiformes
Suborder Platyrhinoidei
Suborder Myliobatoidei
Recent molecular studies suggest the Batoidea are not derived selachians as previously thought. Instead, skates and rays are a monophyletic superorder within Elasmobranchii that shares a common ancestor with the selachians.[30][31]
^Bigelow, Henry B.; Schroeder, William C. (1948). Fishes of the Western North Atlantic. Sears Foundation for Marine Research, Yale University. pp. 64–65. ASINB000J0D9X6.
^Bone, Q.; Roberts, B. L. (2009). "The density of elasmobranchs". Journal of the Marine Biological Association of the United Kingdom. 49 (4): 913. doi:10.1017/S0025315400038017. S2CID85871565.
^Schultze, H.-P., Bullecks, J., Soar, L. K., & Hagadorn, J. (2021). Devonian fish from Colorado's Dyer Formation and the appearance of Carboniferous faunas in the Famennian. In A. Pradel, J. S. S. Denton, & P. Janvier (Eds.), Ancient Fishes and their Living Relatives: a Tribute to John G. Maisey (pp. 247–256.). Verlag Dr. Friedrich Pfeil.
^Rees, J. A. N., and Underwood, C. J., 2008, Hybodont sharks of the English Bathonian and Callovian (Middle Jurassic): Palaeontology, v. 51, no. 1, p. 117–147.
^Ebert, David A.; Fowler, Sarah; Dando, Marc (2021). Sharks of the world: a complete guide. Princeton: Princeton University Press. ISBN978-0-691-20599-1.
Elasmobranchii is a subclass within the class Chondrichthyes, comprising sharks, skates, and rays, characterized by endoskeletons of cartilage, five to seven pairs of gill slits exposed directly to the exterior without an operculum, and dermal denticles known as placoid scales covering the skin.[1][2][3]
These fishes exhibit internal fertilization via claspers in males and lack swim bladders, relying on hydrodynamic lift from fins and oil-filled livers for buoyancy.[1][4]
Elasmobranchs occupy a wide array of aquatic environments, from deep oceans to freshwater rivers, playing key roles as apex predators and influencing trophic dynamics through their predatory behaviors and life histories.[5]
Their evolutionary lineage traces back over 400 million years, with a fossil record primarily of teeth and spines revealing diversification through periods of mass extinction and recovery, underscoring their resilience amid environmental upheavals.[6][7][8]
Morphology and Anatomy
General Body Plan
Elasmobranchii encompass sharks, rays, and skates, exhibiting diverse body plans adapted to pelagic, benthic, and demersal habitats. Sharks typically possess a fusiform, spindle-shaped body with a pointed snout, aiding in efficient cruising through water, whereas batoids (rays and skates) feature dorso-ventrally flattened bodies with enlarged pectoral fins that extend anteriorly, facilitating undulatory locomotion along the substrate.[1][9]A defining external feature across elasmobranchs is the presence of five to seven pairs of gill slits opening individually to the exterior, without an opercular cover, allowing direct water expulsion. The skin is adorned with placoid scales—small, dentine-based structures resembling tiny teeth—that provide abrasion resistance, hydrodynamic benefits, and sensory functions. Paired pectoral and pelvic fins, along with unpaired dorsal and caudal fins, support propulsion and stability; in batoids, the pectoral fins dominate for flapping motion, while shark caudal fins are often heterocercal, with an enlarged dorsal lobe enhancing thrust.[1][9][10]
Skeletal and Dermal Structures
The endoskeleton of elasmobranchs consists primarily of unmineralized hyaline-like cartilage that forms both embryonic and adult structures without undergoing ossification into true bone, distinguishing it from the bony endoskeletons of osteichthyan fishes.[11] This cartilage core is enveloped by a thin, mineralized surface layer known as tessellated cartilage, where polygonal tesserae—small blocks typically under 500 μm in diameter—tile the exterior, providing enhanced rigidity in high-stress areas such as the jaws, vertebral column, and fin supports.[12][11] Tesserae mineralization involves calcium phosphate deposits, including apatite crystals, arranged in patterns with Liesegang rings and radial spokes at intertesseral joints, developing from isolated globular islets in early embryos (around 6 cm disc width in some species) and expanding via appositional growth without the cellular remodeling seen in bone.[11] Chondrocytes reside in lacunae within the uncalcified cartilage, connected by canalicular networks, but the structure lacks osteocytes and repair mechanisms, relying instead on the fibrous perichondrium for integration.[11][12]In vertebral centra, supplementary calcification forms areolar cartilage, characterized by concentric mineralized annuli surrounding the notochordal remnant, which contribute to axial support but remain less extensively studied than tessellation.[12] Prismatic calcified cartilage may appear in specific elements like the spine, where mechanical demands necessitate denser mineralization, though overall mineral content varies (e.g., approaching levels in compact bone in some shark vertebrae).[11]Dermal structures in elasmobranchs are dominated by placoid scales, or dermal denticles, which embed in the skin as an exoskeletal armor and are developmentally homologous to gnathostome teeth, featuring similar odontogenic tissues.[13] Each denticle comprises a rectangular basal plate anchored in the dermis and a projecting, often posteriorly oriented spine that imparts a rasping texture to the skin.[13] Internally, a vascular pulp cavity supplies nutrients, surrounded by a middle layer of dentine and an outer capping of vitrodentine—a hard, enameloid material resistant to abrasion.[13] Unlike cycloid or ctenoid scales in bony fishes, placoid scales do not expand with somatic growth; juveniles add new denticles interstitially, resulting in denser coverage over time, with size, shape, and spination varying by species (e.g., more pronounced thorns in certain sharks) and body region to optimize protection or hydrodynamics.[13]
Sensory Organs
Elasmobranchs possess a diverse array of sensory organs finely tuned to detect chemical, mechanical, electrical, and visual stimuli in aquatic environments, enabling prey detection, navigation, and predator avoidance. These include the olfactory system for chemical cues, vision adapted for low-light conditions, electroreception via the ampullae of Lorenzini, mechanoreception through the lateral line, and audition via the inner ear.[14][15]The olfactory system features paired nares leading to olfactory sacs with rosettes containing 31 to 300 lamellae, which increase surface area by 70 to 495% through secondary folding, enhancing detection of amino acids at concentrations from 10^{-7} to 10^{-9} M; the scalloped hammerhead shark detects alanine at 10^{-11} M.[15] Olfactory bulbs can comprise over 30% of brain mass in deep-water species, reflecting specialization for odor localization via bilateral timing differences and somatotopic mapping.[15] This system drives behaviors such as prey tracking in nurse sharks, which rely on olfactory cues for feeding, and detection of sex pheromones or predator odors.[15][14]Vision occurs through duplex retinas with rods and cones, supported by a tapetum lucidum—a reflective layer of mirrored crystals behind the retina that amplifies light in dim conditions, allowing sharks to see approximately 10 times better than humans in low light.[14] Eye size varies from 0.3 cm in small dogfish to 6.2 cm in thresher sharks, with rod-dominated sensitivity peaking around 500 nm and potential dichromatic or trichromatic capabilities in some batoids and sharks via multiple cone pigments.[15] Temporal resolution reaches 20 to 45 Hz, up to 54 Hz in sandbar sharks, aiding pursuit of fast-moving prey.[15]Electroreception is mediated by the ampullae of Lorenzini, dermal pores connected by jelly-filled canals to electrosensory cells that detect electric fields as weak as 5 nV/cm, primarily in the 0.1 to 15 Hz range for bioelectric signals from prey muscle activity.[16] These organs, numbering in the hundreds around the head, facilitate prey localization even when buried or hidden, predator avoidance, and geomagnetic navigation via field polarity and intensity.[14] Sensitivity varies by species, with neural gain up to 24 spikes/s per μV/cm in some rays.[14]The lateral line system comprises fluid-filled canals beneath the skin, housing neuromasts that sense water velocity on the order of μm/s and acceleration up to mm/s², sensitive to low-frequency vibrations (≤200 Hz) from nearby movements.[14] This mechanoreceptive array detects hydrodynamic disturbances up to 1-2 body lengths away, supporting rheotaxis for odor plume tracking, school coordination, and prey localization via eddies.[14]Audition and balance are handled by the inner ear, featuring three semicircular canals and end organs including the unique macula neglecta, which enhances low-frequency detection (best sensitivity 20-300 Hz).[15]Inner ear size scales hypoallometrically with body mass and is larger in piscivorous and reef-associated species, correlating with enhanced auditory sensitivity for prey sounds and habitat-specific soundscapes; semicircular canals are smaller in pelagic forms, reflecting locomotor demands.[17]
Physiology
Osmoregulation and Metabolism
Elasmobranchs maintain osmotic balance primarily through a ureosmotic strategy, retaining high concentrations of urea (typically 350–600 mmol L⁻¹) and trimethylamine oxide (TMAO, around 70–120 mmol L⁻¹) in their plasma, resulting in total osmolarities of approximately 1000–1100 mOsm L⁻¹, which is iso- or slightly hyperosmotic to seawater (about 1000 mOsm L⁻¹).[18][19] This approach minimizes passive water loss across permeable gills and skin while countering the inward diffusion of salts, differing from the hypoosmotic strategy of marine teleosts that rely on active ionextrusion.[20]Urea serves as the primary osmolyte, synthesized in the liver via the ornithine-urea cycle and actively reabsorbed in the kidneys through specialized urea transporters (e.g., UT family proteins), preventing excessive urinary loss despite glomerular filtration rates of 15–20% of plasma flow.[21][22]Excess sodium and chloride ions gained from seawater are excreted via the rectal gland, a specialized organ that secretes a fluid isoosmotic to plasma but with high NaCl concentrations (up to 500 mmol L⁻¹ each), driven by Na⁺/K⁺-ATPase pumps and coupled chloride channels.[19] The gills contribute minimally to ion extrusion compared to teleosts but handle some Na⁺ and Cl⁻ uptake regulation, while the kidney produces dilute urine (200–400 mOsm L⁻¹) relative to plasma, aiding water conservation.[20] In euryhaline species, such as bull sharks (Carcharhinus leucas), reduced salinity triggers downregulation of urea synthesis and retention, lowering plasma urea to 100–300 mmol L⁻¹ to avoid toxicity in hypoosmotic environments, supplemented by increased active ion uptake at gills and kidneys.[23] TMAO counteracts urea's perturbing effects on proteins, maintaining cellular function at these elevated levels.[18]Elasmobranch metabolism is generally ectothermic and lower than in comparably sized teleosts, with routine metabolic rates scaling at 1.5–3.0 mg O₂ kg⁻¹ h⁻¹ for many species at 20°C, reflecting adaptations to intermittent feeding and energy conservation in cartilaginous skeletons that reduce structural mass.[24] Preferred fuels include lipid-derived ketones (e.g., β-hydroxybutyrate) over glucose, enabling sustained activity during fasting via efficient hepatic ketogenesis, though amino acids contribute during protein catabolism.[25] Select taxa, such as lamniform sharks (e.g., great white, Carcharodon carcharias), exhibit regional endothermy via vascular counter-current heat exchangers, elevating red muscle temperatures by 10–20°C above ambient to support burst swimming, which increases metabolic scope but raises overall energy demands by 2–3 times compared to ectothermic congeners.[26] In rays, benthic habits correlate with even lower rates (0.5–2.0 mg O₂ kg⁻¹ h⁻¹), with diel rhythms showing peaks at dusk in species like epaulette sharks (Hemiscyllium ocellatum), independent of temperature fluctuations.[27] Osmoregulatory costs, including urea synthesis (about 10–15% of total energy budget), integrate with metabolism, as high protein turnover for urea production demands dietary nitrogen intake exceeding that of teleosts.[20]
Locomotion and Respiration
Elasmobranchs demonstrate specialized locomotion adapted to aquatic environments, with sharks relying on lateral undulations of the body and tail that propagate from an anguilliform to thunniform mode depending on speed and species. The heterocercal caudal fin, featuring a larger dorsal lobe, generates thrust by producing a posteroventral water jet angled 40–45° below horizontal, simultaneously providing lift to counteract negative buoyancy.[28] Swimming speeds typically range from 0.5 to 2.0 body lengths per second, with body angle decreasing toward horizontal at higher velocities for streamlined efficiency.[28] Pectoral fins in sharks contribute minimally to steady propulsion but adjust dynamically for vertical maneuvers, such as increasing chord angle to +14° for ascent.[28]In batoids (rays and skates), propulsion shifts to undulatory or oscillatory motions of enlarged pectoral fins, which form a disc-like structure enabling hovering, precise turns, and benthic punting via pelvic fins in some species.[28] Chimaeras employ pectoral fin flapping combined with body undulations. Dermal denticles covering the skin reduce drag by aligning flow and minimizing turbulence, particularly evident in fast-swimming forms.[28] These mechanisms optimize energy use, with undulation suiting slow, maneuverable swimming and oscillation favoring sustained cruising.[28]Respiration in elasmobranchs occurs through 5 to 7 pairs of external gill slits per side, facilitating water flow over vascularized gill lamellae for oxygen extraction via counter-current exchange.[29] Ventilation modes include ram ventilation, where forward motion passively streams water through the open mouth and over the gills, predominant in active pelagic species, and buccal pumping, involving pharyngeal contractions to actively draw and expel water, common in demersal forms.[30] Most elasmobranchs are facultative, switching modes to rest without suffocation, but obligate ram ventilators—such as great white sharks (Carcharodon carcharias), shortfin makos (Isurus oxyrinchus), and whale sharks (Rhincodon typus)—lack sufficient buccal musculature and must swim continuously to maintain gill perfusion, achieving high respiratory efficiency at speeds above 0.5 body lengths per second.[31][32]Batoids supplement primary ventilation with spiracles—vestigial first gill slits posterior to the eyes—that intake oxygenated water from above the head, crucial for buried or substrate-resting postures where the mouth ingests sediment.[33]Gill function yields efficient oxygen uptake, often exceeding arterial partial pressure relative to expired water, supporting active metabolisms despite urea-based osmoregulation.[34]
Evolutionary History
Origins and Early Diversification
The origins of Elasmobranchii, the subclass encompassing modern sharks, skates, and rays, trace to the Devonian Period, with the earliest evidence of shark-like chondrichthyans appearing approximately 410 million years ago in the Early Devonian. Fossils such as teeth from Doliodus problematicus exhibit primitive features including multiple cusps and vascularization patterns indicative of early cartilaginous fish dentition, marking the onset of chondrichthyan evolution before the full diversification of elasmobranch-specific traits.[35] These forms likely arose from stem-group chondrichthyans amid the adaptive radiation of aquatic vertebrates following the Silurian, driven by ecological opportunities in marine predation niches.[36]By the Middle Devonian (late Givetian stage, around 383 million years ago), unambiguous total-group elasmobranchs emerged, represented by Phoebodus, whose dental morphology and skeletal elements from Late Devonian (Famennian) deposits in Morocco reveal hybodontiform affinities with cladodont teeth suited for grasping prey.[36] This genus underscores the transition from basal chondrichthyans to more derived elasmobranchs, featuring enhanced jaw mechanics and body streamlining for active swimming. Early diversification accelerated in the Late Devonian and extended into the Carboniferous, where fossil assemblages from regions like the Canning Basin in Western Australia document at least 18 shark taxa, including ctenacanthiforms with robust, multi-cuspidate teeth adapted to durophagous diets.[37] These developments coincided with the proliferation of reefal and nearshore habitats, facilitating niche partitioning among predatory forms.[8]Carboniferous elasmobranchs, such as Ctenacanthus and cladoselachians, exemplify early morphological experimentation, with elongated bodies, heterocercal tails, and calcified cartilage skeletons preserving well in anoxic sediments.[38] Diversity peaked with over 20 genera in some assemblages, reflecting adaptations to both marine and freshwater incursions during the period's climatic fluctuations. However, Paleozoic elasmobranchs remained stem-like compared to Mesozoic neoselachians, lacking the specialized fins and placoid scales of modern lineages, and many lineages experienced bottlenecks at the Permo-Triassic extinction.[39] This early phase laid foundational traits like internal fertilization and viviparity precursors, setting the stage for subsequent radiations.[6]
Fossil Record and Extinctions
The fossil record of Elasmobranchii, comprising sharks, rays, and skates, is dominated by disarticulated elements such as teeth, fin spines, and calcified vertebral centra, as their cartilaginous endoskeletons rarely preserve intact. Earliest elasmobranch remains appear in the Devonian Period, around 400 million years ago, with primitive taxa like ctenacanthiforms documented from Late Devonian deposits.[40] Diversification accelerated in the Carboniferous and Permian, encompassing diverse morphologies including hybodonts, before a Mesozoic radiation of neoselachians around 250 million years ago.[41] All modern orders and most families have fossil representatives, with sharks exhibiting deeper temporal ranges than rays; for instance, extant genera trace back to the Early Jurassic (~190 million years ago), and some species to the Late Cretaceous (~66 million years ago).[41]Elasmobranchs demonstrated resilience across major mass extinctions, surviving the end-Permian event (~252 million years ago) through habitat contraction into deep-sea refugia, as evidenced by the persistence of stem chondrichthyan lineages into the Cretaceous.[42] The Cretaceous-Paleogene extinction (66 million years ago) inflicted heavier tolls on neoselachians, extinguishing 17% of families (7 of 41) and 56% of genera (60 of 107), with disproportionate impacts on batoids (rays and skates) and open-marine apex predators like anacoracid sharks, while benthopelagic and deep-water forms fared better.[43] Post-extinction recovery involved the emergence of modern families such as Carcharhinidae by the Danian stage of the Paleocene, restoring diversity by the early Eocene.[43] Overall, elasmobranchs endured at least four mass extinctions with selective survivorship favoring adaptable, ecologically flexible lineages.[40]
Modern Evolutionary Patterns
The Cretaceous-Paleogene (K-Pg) mass extinction event, occurring approximately 66 million years ago, resulted in the loss of over 60% of elasmobranch diversity, with net diversification rates turning strongly negative during this interval. Recovery followed in the Paleocene and Eocene, marked by increased speciation in surviving lineages, though overall rates remained subdued compared to ray-finned fishes (teleosts), which experienced more pronounced post-extinction radiations due to higher speciation efficiencies. Elasmobranch persistence is evidenced by the survival of basal groups like Hexanchiformes, contributing to a modern total of over 1,200 species across sharks, skates, and rays.[44][45]Cenozoic diversification showed episodic bursts, particularly in the Oligocene and Miocene, linked to biogeographic drivers such as continental drift, tectonic uplift, and eustatic sea-level changes that fragmented habitats and opened new niches. For instance, Carcharhiniformes (requiem sharks) and Orectolobidae (wobblegills) exhibited rapid cladogenesis during these periods, coinciding with the proliferation of coastal and reef environments. Batoids (rays and skates) displayed relatively higher diversification within Elasmobranchii, with molecular phylogenies indicating Cenozoic radiations tied to benthic adaptations and viviparity, contrasting slower turnover in shark-dominated Selachii. Key innovations, such as bioluminescence in deep-sea Etmopteridae, further accelerated speciation in isolated oceanic realms by facilitating prey detection and mate recognition.[44][44]Trophic ecology has profoundly shaped modern patterns, with tooth morphology and dietary specialization driving partitioning rather than direct competition. Time-calibrated phylogenies reveal stable dental disparity in Carcharhiniformes since the Mesozoic, peaking with generalist diets amid Eocene coral reef expansions, while Lamniformes experienced Late Cretaceous highs followed by post-K-Pg declines and Holocene recoveries, correlating with piscivory and environmental shifts like sea-level regression. Gigantism evolved convergently multiple times (e.g., in filter-feeding rhincodontids and lamnids), tied to resource abundance in open oceans, but increased extinction vulnerability in apex predators during Miocene-Pliocene cooling. Biotic interactions, including prey availability and competitor exclusion, underscore low net rates, with viviparous lineages diversifying at roughly twice the pace of oviparous ones due to enhanced maternal investment and juvenile survival.[46][44][47]
Systematics and Taxonomy
Higher Classification
Elasmobranchii is recognized as a subclass within the class Chondrichthyes, which encompasses all cartilaginous fishes distinguished by their endoskeleton composed primarily of cartilage rather than bone, multiple unpaired gill slits, and placoid scales in most species.[48][49] The class Chondrichthyes diverged from bony fishes (Osteichthyes) approximately 420–450 million years ago during the Silurian-Devonian periods, forming one of the two major lineages of jawed vertebrates (Gnathostomata).[50] This division is supported by molecular phylogenies, including mitochondrial genome analyses, which confirm Elasmobranchii's monophyly alongside the sister subclass Holocephali (chimaeras).[49]Higher in the hierarchy, Chondrichthyes belongs to the subphylum Vertebrata (craniates with a vertebral column), phylum Chordata (characterized by a notochord, dorsal nerve cord, pharyngeal slits, and post-anal tail at some life stage), and kingdom Animalia.[48] This placement reflects shared deuterostome traits with other vertebrates, including internal fertilization and advanced sensory systems, though Chondrichthyes uniquely retain urea-based osmoregulation and spiral valve intestines. Taxonomic ranks above subclass remain stable in contemporary classifications, with no major revisions altering Elasmobranchii's position since the integration of molecular data in the early 2000s.[51]Phylogenetic studies, such as those using mitogenomic sequences from over 80 species, reinforce Elasmobranchii as a well-supported clade within Chondrichthyes, excluding Holocephali based on differences in jaw suspension, cloacal structure, and reproductive anatomy.[49] While some early classifications elevated Elasmobranchii to class rank, current consensus subordinates it to subclass to accommodate the basal split with Holocephali, avoiding paraphyly.[52] This hierarchy aligns with fossil evidence from Devonian deposits, where primitive elasmobranch-like forms predate holocephalan diversification.[53]
Major Orders and Families
The subclass Elasmobranchii comprises approximately 1,192 species across 14 orders and 60 families, representing the sharks, rays, skates, and allied forms.[54] These taxa are united by features such as multiple rows of replaceable teeth, placoid scales, and a spiral valve intestine, with modern diversity concentrated in the superorder Euselachii under the subclass. Extant elasmobranchs divide into two primary clades: Selachimorpha (sharks, ~500 species in ~34 families) and Batoidea (batoids, including rays and skates, ~689 species in ~42 families), reflecting adaptations to predatory and benthic lifestyles.[55] Phylogenetic analyses confirm this bipartition, with Selachimorpha branching earlier and exhibiting greater morphological disparity in body form compared to the dorsoventrally flattened batoids.[54]Within Selachimorpha, eight principal orders encompass the shark diversity, each characterized by distinct dentition, fin morphology, and habitat preferences:
Hexanchiformes: Primitive six- or seven-gilled sharks, including families Hexanchidae (e.g., bluntnose sixgill shark) and Chlamydoselachidae (frilled shark), totaling ~6 species; these retain archaic traits like multiple gill slits.[56]
Squaliformes: Dogfish and gulper sharks, with families such as Squalidae and Centrophoridae (~100 species); known for luminescent organs and deep-sea adaptations.[56]
Squatiniformes: Angelsharks (family Squatinidae, ~20 species), ambush predators with pectoral fins fused to the head, resembling flattened rays.[56]
Pristiophoriformes: Sawsharks (family Pristiophoridae, ~7 species), distinguished by elongate rostral teeth used for prey manipulation.[56]
Heterodontiformes: Bullhead sharks (family Heterodontidae, ~9 species), with molariform posterior teeth for crushing mollusks.[56]
Orectolobiformes: Carpet sharks, including families Orectolobidae (wobbegongs) and Hemiscylliidae (~45 species); feature barbels and nocturnal habits in tropical reefs.[56]
Lamniformes: Mackerel sharks, with families Lamnidae (e.g., great white shark) and Alopiidae (thresher sharks, ~60 species); noted for regional endothermy and high-speed cruising.[56]
Carcharhiniformes: Ground sharks, the most speciose order (~280 species in ~8 families like Carcharhinidae and Scyliorhinidae); includes requiem and catsharks, dominant in coastal and pelagic zones.[56][55]
The Batoidea exhibit greater uniformity in body plan, with enlarged pectoral fins for undulatory or oscillatory locomotion, and include four to six orders in contemporary schemes, aggregating ~20 families:
Rhinopristiformes: Guitarfishes, wedgefishes, and sawfishes (families Rhinobatidae, Pristidae; ~60 species); transitional forms between sharks and rays, with sawfishes possessing rostral serrations for hunting.[56]
Torpediniformes: Electric rays (families Torpedinidae, Narcinidae; ~60 species), equipped with paired electric organs derived from kidney tissue for prey stunning and defense.[56]
Rajiformes: Skates (families Rajidae, Arhynchobatidae; ~200 species), oviparous benthic forms with thorned dorsal surfaces and tail fins for propulsion.[56]
Myliobatiformes: Stingrays, eagle rays, and manta rays (families Dasyatidae, Myliobatidae; ~300 species); venomous tail spines and diamond-shaped discs, with some species performing benthic "aquatic flight" via pectoral undulations.[56]
These classifications draw from morphological and molecular data, with ongoing revisions; for instance, the erection of Rhinopristiformes in 2016 separated guitarfishes from traditional Rajiformes based on cladistic analyses. Family-level diversity peaks in Carcharhiniformes and Myliobatiformes, reflecting ecological opportunism in nearshore environments.[54]
Phylogenetic Insights and Recent Updates
Molecular phylogenetic analyses, particularly those utilizing complete mitochondrial genomes from 82 elasmobranch species, have resolved key relationships within Elasmobranchii, confirming the monophyly of Batoidea (rays and skates) and their sister-group position to Pristiophoriformes (sawsharks), rendering traditional shark groupings paraphyletic.[57] Squatiniformes (angelsharks) emerge as the basal lineage, followed by the clade comprising Pristiophoriformes + Squalea, where Squalea encompasses Squalomorphii (e.g., dogfish sharks) and Galeomorphii (e.g., requiem and carpet sharks) as reciprocally monophyletic sister groups to Batoidea.[58] These findings, derived from concatenated nucleotide sequences of 13 protein-coding genes and ribosomal RNAs, underscore convergent morphological traits such as body flattening in batoids, which had previously obscured relationships in morphology-based trees.[59]Recent genome-scale studies have extended these insights into chromosomal evolution, revealing that elasmobranch karyotypes typically feature high chromosome counts (often 80–200), attributable to repeated fission events rather than fusions seen in teleost fishes.[60] For instance, chromosome-level assemblies from species like the whale shark (Rhincodon typus) and small-spotted catshark (Scyliorhinus canicula) highlight conserved synteny blocks amid variability, suggesting adaptive chromosomal rearrangements linked to ecological diversification.[60] Such data challenge earlier assumptions of chromosomal stasis in chondrichthyans and provide a framework for integrating fossil calibrations into divergence time estimates, placing crown-group Elasmobranchii origins in the Permian-Triassic boundary.[60]Diversification analyses from 2023–2025 indicate heterogeneous speciation and extinction rates across elasmobranch clades, with batoids exhibiting elevated net diversification during the Cretaceous, driven by ecological opportunities in benthic habitats, while squalomorph sharks show slower rates in deep-sea niches.[6] These patterns, modeled using time-calibrated phylogenies from nuclear and mitochondrial loci, resolve prior conflicts between fossil and molecular clocks by accounting for incomplete sampling and trait-dependent shifts.[61]Taxonomic updates informed by phylogenomics include molecular re-evaluations of batoid interrelationships, affirming Myliobatiformes (e.g., eagle rays) as derived within Batoidea but noting homoplasy in pectoral fin morphology.[62] In 2025, a holomorphic stem batomorph fossil from the Late Jurassic integrated into Bayesian phylogenetic frameworks extended the lineage's ghost range, supporting a mid-Mesozoic radiation predating modern skate-ray splits.[63] Regional molecular-assisted revisions, such as those in southeastern Arabia, have synonymized cryptic species in carcharhinid sharks using COI barcoding, refining alpha taxonomy amid phylogenetic stability.[64]
Reproduction and Life History
Reproductive Modes
Elasmobranchii universally employ internal fertilization, with males using paired claspers—modified pelvic fins equipped with grooves and spines—to insert sperm directly into the female's oviduct during copulation.[65][66] This mechanism ensures high fertilization efficiency in aquatic environments, contrasting with external fertilization in most bony fishes, and supports diverse reproductive strategies adapted to varying ecological pressures such as predation risk and habitat stability.[67]Reproductive modes in elasmobranchs are classified primarily by embryonic development and nutrition: oviparity, ovoviviparity (including yolk-sac viviparity), and viviparity (encompassing aplacental and placental forms). Oviparity, the ancestral mode, involves females depositing fertilized eggs encased in a tough, leathery egg case that affords mechanical protection and gas exchange via fibrous strands anchoring to substrates.[68][65] Development relies solely on yolk reserves (lecithotrophy), with hatching times ranging from weeks to over a year depending on species and temperature; examples include skates (Rajidae), which produce "mermaid's purses," and oviparous sharks like catsharks (Scyliorhinidae) and horn sharks (Heterodontidae).[69] Approximately 40% of elasmobranch species are oviparous, predominantly in smaller, benthic taxa where egg cases mitigate high predation.[70]Ovoviviparity features retention of eggs within the uterus until embryos hatch internally, nourished exclusively by yolk-sac lecithotrophy without maternal input beyond shelter.[69] This mode, common in about 10-20% of sharks, reduces exposure to external threats but demands extended gestation (up to 24 months in some species like the Greenland shark).[71] It prevails in mid-sized pelagic and reefsharks, such as requiemsharks (Carcharhinidae), where unhatched siblings may compete for limited uterine space.Viviparity, the derived and most prevalent mode (roughly 50% of species), yields live young after maternal provisioning via matrotrophy, supplementing yolk with nutrients like histotroph (uterine "milk" secretions) or, in placental forms, direct yolk-sac placental transfer of uterine fluids and proteins.[68][65] Aplacental viviparity involves oophagy (embryos consuming unfertilized eggs) or intrauterine cannibalism in species like sand tiger sharks (Carcharias taurus), where the strongest embryo devours siblings.[71] Placental viviparity, seen in hammerhead and tiger sharks, features a transient yolk-sac placenta enabling efficient nutrient uptake, correlating with larger body sizes and higher fecundity (litters of 10-100 pups).[69] These modes enhance offspring survival in open-water habitats but impose energetic costs on females, often leading to biennial or triennial reproductive cycles.[72]Rare variants include facultative parthenogenesis in captive bamboo sharks (Chiloscyllium punctatum), where unfertilized eggs develop into female offspring via automixis, observed as early as 2001 but not confirmed in wild populations.[73] Such asexuality may serve as a reproductive assurance mechanism under low mate availability, though it risks inbreeding depression. Overall, mode distribution reflects phylogenetic patterns, with viviparity evolving multiple times from oviparous ancestors, driven by selection for increased maternal guarding in predator-rich environments.[68]
Growth, Maturity, and Longevity
Elasmobranchs generally display slow somatic growth rates as part of their K-selected life history strategy, characterized by low intrinsic population growth potential (r') and extended developmental periods, with growth coefficients (k) in the von Bertalanffy model often ranging from 0.05 to 0.3 yr⁻¹ across species, lower than typical teleost values.[74][75] This pattern stems from physiological constraints including slow digestion, intermittent feeding, and cartilaginous skeletal deposition, though growth is indeterminate and continues post-maturity at decelerating rates.[76] Empirical studies using vertebral band counts, tag-recapture, and multi-model approaches confirm variability, with smaller coastal species like the Australian sharpnose shark (Rhizoprionodon taylori) exhibiting faster initial growth (k ≈ 3.69 yr⁻¹ for males) compared to larger pelagic forms.[77][78]Sexual maturity in elasmobranchs is typically delayed, with age at 50% maturity (A₅₀) correlating positively with body size and often reached at approximately 50% of maximum lifespan, reflecting trade-offs in energy allocation between growth and reproduction.[79] Males generally attain maturity earlier and at smaller sizes than females; for example, in the crocodile shark (Pseudocarcharias kamoharai), males mature at 4.55 years versus 5.91 years for females.[80] Larger species delay maturity further, as seen in great white sharks (Carcharodon carcharias), where females reach A₅₀ around 33 years and males around 26 years, validated via bomb radiocarbon analysis of vertebral cores.[81] Across taxa, maturity ogives are size-dependent, with batoids often maturing later relative to sharks due to discoidal body plans affecting locomotion and energy budgets.[82]Longevity estimates for elasmobranchs, derived from age-validation techniques like tag-recapture, oxytetracycline marking, and radiometric dating, reveal extended lifespans often exceeding prior assessments, with many species surviving 20–70+ years and natural mortality (M) scaling inversely with maximum age.[83][75] Females typically outlive males, as in shortfin mako sharks (Isurus oxyrinchus), where maximum ages approach 30–40 years amid rapid early growth tapering to slower rates.[84] Reassessments using multi-decadal data indicate underestimation in historical studies, with white sharks reaching 73 years in males and potentially over 100 in some rays, underscoring low annual mortality (e.g., M ≈ 0.1–0.2 yr⁻¹) that buffers populations against perturbations but heightens vulnerability to chronic exploitation.[81][82]
Ecology and Distribution
Habitats and Geographic Range
Elasmobranchs occupy diverse marine habitats globally, distributed across all major ocean basins from polar regions to the equator, including coastal shelves, pelagic zones, coral reefs, mangroves, and deep-sea environments.[85][86] Their latitudinal range spans Arctic and Antarctic waters to tropical seas, with species adapted to temperatures from near-freezing to over 30°C.[87] Depth utilization varies extensively, from intertidal and shallow coastal areas to abyssal depths exceeding 3,000 meters for certain deep-sea species like the bluntnose sixgill shark (Hexanchus griseus) and various skates.[88][89]While predominantly marine, approximately 5% of elasmobranch species, or around 60 taxa, inhabit freshwater or euryhaline environments beyond tidal influence, primarily in rivers and lakes of South America (e.g., potamotrygonid stingrays), Africa, Southeast Asia, and Australia.[90][91] Obligate freshwater elasmobranchs, such as the Ganges shark (Glyphis gangeticus) and various river stingrays, are confined to inland systems, whereas euryhalinespecies like the bull shark (Carcharhinus leucas) migrate between marine and freshwater habitats.00745-9) These freshwater occurrences represent a small fraction of the subclass's overall diversity, with most species showing strong marine fidelity shaped by physiological tolerances to salinity, oxygen levels, and temperature.[86] Batoidea (rays and skates) predominantly favor benthic substrates on continental shelves and slopes, whereas many Selachii (sharks) exhibit pelagic or migratory behaviors across open ocean expanses.[92]
Trophic Roles and Interactions
Elasmobranchs occupy diverse trophic positions in marine food webs, ranging from low-level filter feeders like the basking shark (Cetorhinus maximus) to apex predators such as the great white shark (Carcharhinus carcharias), with most species functioning as mid-level or mesopredators.[93] Their diets typically include teleost fishes, cephalopods, crustaceans, and occasionally other elasmobranchs, reflecting opportunistic or generalist feeding strategies that contribute to trophic structuring and energy transfer across levels.[94] In tropical marine networks, certain elasmobranch species participate in multiple roles simultaneously, acting as both predators and prey across up to four trophic levels, thereby enhancing food web connectivity and stability.[95]Sharks often exert predation pressure that influences prey behavior, distribution, and population dynamics, with large-bodied species mediating habitat partitioning and preventing overexploitation of lower trophic resources through top-down control.[96] Rays and skates, predominantly benthic feeders, primarily consume crustaceans, polychaetes, and small fishes, occupying lower trophic positions than many sharks and facilitating nutrient recycling in demersal ecosystems.[97] Trophic interactions among elasmobranchs include intraguild predation, where larger individuals prey on smaller conspecifics or heterospecifics, and egg predation, which can significantly impact reproductive success in oviparous species.[98]As prey, elasmobranchs are consumed by larger sharks, marine mammals like orcas, and humans, with juveniles and egg cases particularly vulnerable, underscoring their role in supporting higher trophic levels.[95] Declines in elasmobranch populations due to overfishing can lead to mesopredator release and cascading effects, such as altered prey abundances and reduced ecosystem resilience, though the strength of these top-down impacts varies by species and habitat.[99] In subtropical food webs, elasmobranchs demonstrate topological importance, with some species identified as keystones that, if removed, could destabilize network structure.[100]
Conservation and Human Impacts
Population Status and Threats
Approximately one-third of assessed elasmobranch species—sharks, rays, and skates—are classified as threatened with extinction (Vulnerable, Endangered, or Critically Endangered) on the IUCN Red List, with overfishing identified as the primary driver affecting all such species.[101][102] Global population abundances of sharks and rays have declined by more than 50% since 1970, with oceanic species experiencing a 71% reduction due to intensified fishing pressure.[103][104] These declines are exacerbated by elasmobranchs' slow growth rates, late maturity, and low reproductive output, which limit population recovery even under reduced exploitation.[105]Overfishing, encompassing targeted fisheries for fins, meat, and gill plates as well as incidental bycatch in trawl and gillnet operations, accounts for the sole or primary threat to nearly two-thirds of threatened species.[102]Habitat degradation from coastal development, dredging, and pollution further compounds risks, particularly for benthic rays and skates dependent on shallow nurseries.[106]Persecution driven by human-shark conflicts and the illegal trade in body parts also contributes, though less universally than fishing pressures.[102]Regional variations highlight acute vulnerabilities: wedgefishes and giant guitarfishes face near-total depletions in some Indo-Pacific fisheries, while Mediterranean elasmobranch assemblages show accelerating declines from small-scale and industrial catches.[101] Approximately 10% of species remain Data Deficient, underscoring gaps in monitoring that may conceal additional at-risk populations.[107] Despite these trends, localized recoveries in protected areas demonstrate potential for rebound where fishing mortality is curtailed.[105]
Fisheries Exploitation and Management
Elasmobranchs are harvested globally in both directed fisheries targeting species such as dogfish, requiem sharks, and stingrays, and as bycatch in tuna and billfish longline fisheries, with reported annual landings to the FAO exceeding 500,000 metric tons in the early 2010s, reflecting an upward trend driven by expanded fishing into previously unexploited areas and shifts toward lower-value species.[108] Primary products include fins for soup (primarily from larger sharks), meat for human consumption, and gill plates from mobulid rays for traditional medicine, though official statistics underrepresent true exploitation levels, with less than 25% of shark catches identified to genus or species and significant omissions from artisanal and unreported fisheries.[109] Approximately 50% of global elasmobranch catches occur as bycatch, often discarded at sea, exacerbating mortality due to the group's K-selected life history traits including slow growth, late maturity, and low fecundity.[110]Shark finning—the removal of fins for high-value trade while discarding the carcass—has historically amplified waste and overexploitation, prompting regulatory responses such as the EU's 2003 finning ban requiring a fin-to-carcass weight ratio no greater than 5% and similar "fins-attached" policies in the US since 1993, which facilitate enforcement and stock assessments by mandating whole-animal landings.[111][112] These measures address inefficiencies where fins constitute only 2-5% of body weight but up to 90% of economic value in some markets, though illegal, unreported, and unregulated (IUU) fishing persists, particularly in developing regions with limited monitoring.Management frameworks include the FAO's International Plan of Action for Sharks (IPOA-Sharks), adopted in 1999, which urges voluntary national shark plans encompassing assessments, research, and precautionary quotas tailored to productivity; implementation varies, with stronger adherence in high-capacity nations like the US, where species-specific quotas (e.g., for blacktip and bonnethead sharks) are set based on annual stock assessments.[113][114] Regionally, RFMOs such as the International Commission for the Conservation of Atlantic Tunas (ICCAT) enforce binding measures including retention bans on overfished species like bigeye thresher sharks and finning prohibitions since 2004, while CITES Appendix II listings for over 60 elasmobranch species since 2013 regulate international trade through non-detriment findings and export quotas to curb unsustainable harvests.[115][116]Despite these advances, enforcement gaps, data deficiencies, and capacity constraints in small-scale fisheries hinder recovery, with studies indicating that well-enforced spatiotemporal closures and gear restrictions could rebuild depleted stocks, but global declines persist absent comprehensive monitoring and incentives for compliance.[117][118] Trade-driven regulations have improved reporting in some Appendix II species, yet multispecies exploitation and transboundary stocks challenge unilateral efforts, underscoring the need for integrated, science-based approaches prioritizing empirical catch data over generalized assumptions of resilience.[119]
Debates on Resilience and Policy
Debates persist regarding the intrinsic resilience of elasmobranch populations to exploitation, with life history traits such as slow growth rates, late maturity (often 10-20 years for large species), and low fecundity (typically 1-20 offspring per reproductive cycle) cited as factors rendering many species K-selected and thus prone to overfishing collapse.[106] However, empirical studies from marine protected areas (MPAs) demonstrate rapid recovery potential in certain contexts; for instance, shark abundance in Australia's Great Barrier Reef MPA increased significantly within 5-10 years of no-take enforcement, suggesting that well-managed protections can reverse declines for some coastal species.[120] Critics of uniform vulnerability assessments argue that these overlook species-specific variabilities and historical biases in data collection, which often emphasize declines while underreporting rebounds, as seen in gradual recoveries of white shark populations off California following 1990s protections, where sightings rose over two decades.[121][122]Policy responses reflect these tensions, with advocates for stringent measures like global finning bans and shark sanctuaries pointing to overfishing as the primary driver of declines in over one-third of assessed species, per IUCN data from 2021.[106] In contrast, fisheries scientists contend that blanket prohibitions may hinder sustainable management, noting that few elasmobranch fisheries have collapsed irreversibly when quotas align with productivity estimates, and that bycatch reductions through gear modifications offer more targeted efficacy than broad bans.[123] International forums, such as CITES meetings, have highlighted U.S. advocacy for evidence-based quotas over outright trade restrictions, amid critiques that non-governmental organizations sometimes amplify extinction risks without sufficient stock assessment data, potentially skewing public policy toward emotion-driven rather than data-driven outcomes.[124][125]Media portrayals exacerbate policy divides, with analyses showing frequent misrepresentation of shark declines—such as inflating global fintrade impacts or ignoring regional recoveries—which fuels support for overly restrictive policies disconnected from local ecological realities.[126] Effective resilience-building policies, proponents argue, require integrating spatial data and fishery-independent surveys to tailor protections, as evidenced by positive trajectories for large coastal species under U.S. rebuilding plans since the 1990s, though challenges remain for highly migratory rays and deep-sea sharks where enforcement lags.[127][128] These debates underscore the need for causal assessments prioritizing verifiable population metrics over precautionary narratives, to balance conservation with viable human uses.
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