Skate Fish: Anatomy, Reproduction, and Ray Differences

Skates are flat-bodied, bottom-dwelling cartilaginous fish closely related to sharks and rays, belonging to the order Rajiformes. With roughly 150 described species spread across oceans from tropical shallows to abyssal depths beyond 2,800 meters, they are among the most diverse groups of cartilaginous fish on the planet. Their diamond-shaped bodies, wing-like pectoral fins, and slender tails give them a distinctive silhouette, but what makes skates genuinely fascinating sits beneath the surface: electric organs in their tails, teeth designed for crushing rather than cutting, and a reproductive strategy that involves encasing embryos in leathery capsules that wash up on beaches worldwide.

Skates Versus Rays

People routinely confuse skates with rays, and it is easy to see why. Both are flattened, both have gill slits on their undersides, and both glide across the seafloor. But the two groups diverged tens of millions of years ago and differ in several practical ways. Rays generally have whip-like tails, often armed with one or more venomous barbs. Skates lack a sting altogether. Their tails tend to be thicker, shorter, and sometimes lined with small thorns or spines instead. The reproductive difference is even starker: most rays give live birth, while skates are exclusively egg-layers, depositing tough rectangular egg cases that beachcombers know as “mermaid’s purses.”

Body shape offers another clue. Many ray species have rounded or circular disc outlines, while skates lean toward a more angular, kite-like shape with a pointed snout. None of these distinctions are absolute across every species, but taken together they let you tell the two groups apart in most encounters.

How Skates Swim

Skates move through the water by sending undulatory waves along their broad pectoral fins, a locomotion style sometimes compared to underwater flight. Research on the longnose skate showed that even small increases in the wave frequency produced significant gains in swimming speed, and the animals favored a narrow frequency and wavenumber range that appeared to be tuned to the body’s own structural properties.¹ That same study compared the live animal’s movements with the passive motion of a deceased specimen driven by a mechanical actuator. The dead skate’s body produced similar wave patterns at similar peak frequencies, suggesting that the skate’s musculoskeletal architecture alone supports wave propagation without active muscle input. In other words, much of what makes a skate’s swimming look so fluid is baked into its anatomy.

The skeleton underpinning those fins is unusual even among cartilaginous fish. Because skates lack true bone, their pectoral fin skeleton is reinforced by a mosaic of mineralized tiles called tesserae. A structural analysis of the longnose skate’s fin skeleton found that this tessera arrangement distributes shear, tensile, and compressive stress in a way that supports both lift and thrust forces during swimming while still allowing the fin to flex.² It is a lightweight structural solution that achieves something rigid bone cannot: a fin that is simultaneously strong enough to push water and flexible enough to ripple.

The Genetics Behind Wing-Like Fins

Those oversized pectoral fins are an evolutionary novelty, and researchers have started tracing the genetic changes that made them possible. A chromosome-scale genome of the little skate revealed that its genome retains many ancient features of jawed vertebrates, including numerous small microchromosomes that other lineages have lost. More relevant to fin shape, the genome analysis identified skate-specific chromosomal rearrangements that alter how certain developmental genes are regulated. When researchers experimentally blocked the signaling pathway affected by those rearrangements, the anterior portion of the fin shrank, confirming that this pathway is a major contributor to the expanded, wing-like fin shape.³

A separate line of work showed that the front and back halves of the skate’s pectoral fin are actually built by different genetic programs. The posterior portion develops much like any other fish fin, through a standard signaling ridge. But the anterior portion uses an alternative genetic module to create a ridge-like structure that drives the dramatic forward expansion of the fin.⁴ This dual-program architecture is part of why skate fins look and function so differently from a typical fish fin. It also means that the skate’s body plan is a patchwork of ancient gene-expression patterns and newly derived ones, which makes these animals unexpectedly useful for understanding how vertebrate limbs and fins evolved in general.

Electroreception and the Ampullae of Lorenzini

Like sharks, skates can sense weak electric fields in the water around them, a talent most vertebrates on land lost hundreds of millions of years ago. The organs responsible are the ampullae of Lorenzini, gel-filled pores clustered on the underside of the head and around the snout.⁵ Every time a buried clam contracts its muscles or a worm twitches in the sediment, it generates a tiny voltage change, and a skate cruising just above the bottom can detect it. Structural work on the skate’s ampullae has shown that the organ is specifically tuned to pick up the weak electric field changes produced by prey muscle contractions.⁶

This sensory system matters because skates spend much of their lives resting on or gliding just above the seafloor, where visibility is often poor and prey is hidden beneath sand or mud. Electroreception gives them a way to hunt effectively in the dark, literally sensing the bioelectric signatures of organisms they cannot see.

Electric Organs in the Tail

Here is where skates diverge sharply from most of their relatives. In addition to passively sensing electric fields, skates actively produce their own electrical discharges. Paired electric organs sit within the tail, running as two longitudinal columns of specialized cells called electrocytes on either side of the vertebral column.⁷ These electrocytes develop from skeletal muscle fibers and come in cup-shaped or disc-shaped varieties depending on the species. The discharges themselves are weak, asynchronous, and long-lasting compared to those of other electric fish.⁸

What skates use these signals for is still being studied, but the leading hypotheses center on communication rather than predation or defense. Unlike electric rays, which can deliver a powerful shock to stun prey, skates produce signals far too weak for that purpose. The electrical pulses may help skates find and recognize each other, especially during mating season, or serve as a form of social signaling on the murky seafloor where visual cues are unreliable. The fact that skates possess both the ability to sense external electric fields and the ability to generate their own gives them a two-way electrical channel that few other fish share.

Reproduction and Mermaid’s Purses

Every skate species lays eggs, and those eggs come packaged in remarkably tough cases. The “mermaid’s purse” is produced by a specialized organ in the female called the oviduct gland, which synthesizes a highly cross-linked biomaterial to form a protective leathery shell around the developing embryos.⁹ The cases are rectangular, with horn-like extensions at each corner that anchor them to substrate or seaweed. Once deposited, the embryos develop entirely outside the mother’s body, drawing nutrients from a yolk sac.

Development times vary enormously. Shallow-water species in temperate waters may hatch in a few months under the right conditions. Studies on the mottled skate found that a single egg capsule can contain one to seven fertilized eggs, with hatching rates reaching 100% at optimal water temperature around 18°C, yielding four to five hatchlings per capsule.¹⁰ Deep-sea species face a radically different timetable, as discussed below.

Deep-Sea Skates and Hydrothermal Nurseries

Some of the most extreme adaptations among skates belong to species living at bathyal and abyssal depths. At those pressures and near-freezing temperatures, biochemistry needs help. Deep-sea skates accumulate high levels of trimethylamine oxide, or TMAO, in their muscle tissue. Measurements from skates collected off Oregon found TMAO concentrations around 215 mmol per kilogram at bathyal depths and 244 mmol per kilogram on the abyssal plain, while urea levels, which are typically high in shallow-water relatives, dropped with increasing depth.¹¹ Laboratory experiments have confirmed that TMAO at these concentrations protects enzymes against the destabilizing effects of hydrostatic pressure, keeping proteins functional in conditions that would otherwise distort their shape.¹²

Deep-sea skates also face an extraordinary reproductive challenge. Cold water slows embryo development to a crawl. One estimate for the Pacific white skate at ambient deep-sea temperatures of about 2.76°C places the likely incubation period at over 1,500 days, more than four years spent inside an egg case on the ocean floor. Researchers exploring the Galapagos Rift discovered a nursery of egg cases belonging to a deep-sea skate species clustered around an active hydrothermal vent, where water temperatures were elevated above the ambient deep-sea chill. The hypothesis is that these skates are deliberately using volcanic heat to speed up embryonic development, a behavior that would make them one of the few animals known to exploit geothermal energy for reproduction.¹³

What Skates Eat

Most skates are generalist bottom-feeders, hoovering up whatever invertebrates the seafloor offers. A detailed dietary study of a southern ocean species found that polychaete worms dominated the menu, followed by amphipods, isopods, and decapods. What the animals ate shifted as they grew: smaller, immature individuals favored amphipods, while larger adults switched toward larger crustaceans like isopods and decapods. Depth and region also influenced the diet, meaning a skate in one area might eat a very different mix of prey from one of the same species in another.¹⁴

Their teeth match this lifestyle. Unlike the sharp, pointed teeth of most sharks, skate teeth are typically flattened into dense crushing pads designed to crack the shells and exoskeletons of bottom-dwelling invertebrates. Research on the thornback skate and little skate confirmed that their dental development follows a pattern conserved across all cartilaginous fish, but with modifications to the signaling centers that shape each tooth, resulting in the flat morphology suited to their diet.¹⁵ Skates also continuously regenerate their teeth throughout life, replacing worn dental plates with fresh ones on a rolling basis.

Conservation and Vulnerability

Skates face conservation pressures that are easy to underestimate. Because they are cartilaginous fish with relatively slow reproductive rates and long generation times, they cannot bounce back from population declines as quickly as many bony fish. A modeling study examining extinction risk across sharks, skates, rays, and chimaeras found that deep-water species are especially vulnerable: the average level of fishing mortality needed to drive a deep-water species to extinction was only about 38% of the level estimated for continental shelf species.¹⁶ That study also made a point worth emphasizing: body size was a weak predictor of extinction risk. Many small skate species, particularly deep-water ones, are just as vulnerable as larger, more charismatic species. Conservation priority lists that focus only on big animals miss a large chunk of the species at greatest risk.

Several skate species have already experienced dramatic population crashes. The common skate in European waters, once abundant enough to be considered a nuisance bycatch, is now critically endangered across much of its former range. The barndoor skate in the Northwest Atlantic showed similar declines before fishing restrictions were implemented. Recovery, when it happens, is slow because most skate species produce relatively few eggs per year and take years to reach sexual maturity.

Mercury and Contaminant Accumulation

For people who eat skate wings, which are served in cuisines from Korean to French, contaminant levels are worth knowing about. Like other long-lived predatory fish, skates accumulate mercury in their muscle tissue over time. A study of cartilaginous fish in southern New England found that total mercury in both dogfish and skates increased with body size and age, a clear bioaccumulation pattern.¹⁷ Measurements from two offshore European skate species showed mercury concentrations in muscle ranging from 0.02 to 1.8 mg per kilogram depending on the species and the individual’s size, with larger animals carrying higher loads.¹⁸

These numbers place large skates in roughly the same mercury range as many other predatory marine fish. If you eat skate regularly, choosing smaller individuals and limiting meal frequency is the same precaution that applies to any mercury-accumulating species. The advice is not unique to skates, but because skates are not as commonly discussed in consumer seafood guidelines, some people do not realize the concern applies to them at all.

The Fossil Record and Evolutionary Puzzle

Despite being a large and speciose group today, skates have an oddly thin fossil record. Their cartilaginous skeletons rarely survive long enough to fossilize, which means paleontologists mostly have isolated teeth and dermal denticles to work with. The first unambiguous skeletal fossil of a skate comes from Miocene deposits in Austria, described as the genus Ostarriraja.¹⁹ Older fossils from the Late Cretaceous of Lebanon have been tentatively aligned with skates, but without the kind of complete preservation that removes all doubt. This gap makes it difficult to nail down exactly when modern skate body plans first appeared, though molecular clock estimates from genomic data suggest the group diversified well before the Miocene.

Parasites and the Skate Microbiome

Like any wild fish, skates host a community of parasites, and the composition of that community varies between species in ways that may reflect each host’s ecology. A survey of seven sympatric skate species in the English Channel and Celtic Sea found that parasite communities differed between species, with some species carrying higher parasite loads than others.²⁰ These differences likely stem from variations in habitat use, diet, and body size, since all of those factors influence which parasites a host encounters and how many it accumulates. For researchers, parasite community data can serve as an indirect window into a host’s ecology, revealing where and what an animal has been eating even when direct observation is impractical.

The practical relevance for fisheries science is that parasite loads can be used as biological tags, helping identify distinct populations of the same species when genetic differentiation is too subtle to detect with standard methods. For skate species that are poorly studied, and many are, parasitology sometimes fills gaps that other tools cannot.