Batomorphi

• 1. Introduction

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  Batomorphi is the scientific name for a group of cartilaginous fishes that most people simply call rays. With well over 600 species spread across 26 families, rays represent the most species-rich group among all cartilaginous fishes. That puts them ahead of their famous cousins, the sharks, in sheer diversity. Yet despite their abundance, they remain far less studied and far less protected. A 2021 study published in Nature found that oceanic sharks and rays had declined globally by 71 percent over the preceding 50 years. Three-quarters of those species are now threatened with extinction. How did a lineage that survived from the Devonian period reach this point? And what makes rays so distinct, so varied, and so ecologically important that their loss could threaten the health of entire ocean ecosystems?

• 2. A Body Built Differently

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  The most immediately recognizable feature of a batomorph is its flattened body, which looks almost like a living kite pressed flat from above. This shape is described scientifically as dorsoventrally flattened, meaning the compression runs from back to belly rather than side to side. The pectoral fins, which in other fish are modest paddle-like structures, have in most rays expanded into enormous wing-like appendages that fuse directly to the head. These are what give manta rays their soaring, wing-flapping style of swimming. The anal fin, by contrast, is absent entirely in batomorphs.

  Breathing works differently in rays than in most fish. Bottom-dwelling batomorphs draw water in through openings called spiracles, located on top of the head, then push it out through gill slits on the underside. This arrangement is the reverse of a shark, whose gill slits open on the sides of the head. The Hexatrygonidae, a family of sixgill stingrays, are unusual even among batomorphs in having six ventral gill slits rather than the five that most other batomorphs possess.

  The skeleton beneath all of this is made not of bone but of cartilage, tough and elastic, a trait shared with all elasmobranchs including sharks. The jaw structure is equally specialized: batomorphs can protrude their upper jaw, technically called the palatoquadrate cartilage, away from the skull to seize prey. The jaw suspension type is euhyostylic, relying entirely on the hyomandibular cartilages for support. Most species have developed dense, rounded teeth built for crushing the hard shells of snails, clams, oysters, and crustaceans. Manta rays are the notable exception, feeding on plankton by filtering water through dense gill rakers.

• 3. Where Rays Live

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  Most batomorphs are demersal, meaning they live close to or on the sea floor. They are found across a wide range of geographical regions, from shallow coastal waters to depths of at least 3,000 meters. Their distribution is broadly cosmopolitan, with a preference for tropical and subtropical marine environments, though temperate and cold-water species also exist. The guitarfishes occupy something of a middle ground, with some demersal behavior and some time spent in the water column.

  Open-ocean life is rare among batomorphs; only a few species, including manta rays, consistently inhabit the open sea. Freshwater is rarer still, with only a small number of species making rivers and lakes their home. Some batomorphs tolerate brackish conditions, inhabiting bays and estuaries where salt and fresh water mix. This breadth of habitat preference, from tropical reef flats to cold deep-sea floors to Amazonian rivers, gives the group a global footprint that few other fish families can match.

• 4. How Rays Reproduce

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  All batomorphs fertilize internally, a trait shared across elasmobranchs. Internal fertilization offers specific biological advantages: it conserves sperm, shields eggs from being eaten by predators, and keeps the energy invested in reproduction within the animal rather than releasing it into the surrounding water. The fate of the fertilized egg varies by species. All skates and some rays are oviparous, meaning they lay eggs. Those eggs are enclosed in tough, leathery cases widely known as mermaid's purses, which wash up empty on beaches in areas where skates are common. Other rays are ovoviviparous, giving birth to young that develop inside a womb but without the involvement of a placenta.

  One threat to batomorph reproduction that fisheries managers have largely overlooked is capture-induced parturition, the premature birth or abortion triggered when a pregnant animal is caught. A documented phenomenon in sharks and rays alike, it has been shown to occur in at least 12 percent of live-bearing species, covering 88 species identified to date. The event happens frequently when animals are fished, yet it is rarely factored into the calculations used to manage fish populations.

• 5. Ancient Origins

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  The oldest confirmed ray is Antiquaobatis, a fossil found in deposits from the Pliensbachian stage in Germany. The Pliensbachian falls within the Early Jurassic, placing the earliest identifiable ray ancestor deep in the age of dinosaurs. The lineage that contains batomorphs stretches back even further: fossil denticles, the tooth-like scales embedded in cartilaginous fish skin, have been found in rocks dating at least to the Ordovician period. The oldest unambiguous complete fossils of cartilaginous fish come from the middle Devonian. A subset of cartilaginous fishes called Neoselachii, which includes the modern sharks and rays, had emerged by the Triassic, with the best-understood neoselachian fossils coming from the Jurassic period. The clade represented by those ancient ancestors is still alive today as sharks, sawfish, rays, and skates.

• 6. Sorting Out the Family Tree

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  Molecular evidence has firmly refuted the once-entertained idea that skates and rays are simply derived forms of sharks; they represent a distinct evolutionary path. For much of the 20th century, batomorphs were organized into four major lineages: skates, stingrays, electric rays, and the group called Rhinopristiformes, which includes sawfishes and various guitarfishes. This framework was codified in Nelson's 2006 edition of Fishes of the World.

  A comprehensive morphological analysis published in 2004 offered one picture of how these lineages relate to one another. A 2011 study then substantially revised that picture using nuclear and mitochondrial DNA drawn from 37 taxa, representing almost all recognized families. That sample was far larger and more diverse than any previous study. The 2011 work confirmed the internal unity of skates, stingrays, and electric rays. It also placed panrays as the closest relatives of stingrays, consistent with older morphological work. But it found that Rhinopristiformes, previously treated as a single coherent group, actually breaks into two separate clusters: one containing only the family Trygonorrhinidae, and another containing the families Glaucostegidae, Pristidae, Rhinidae, and Rhinobatidae. The 2011 study also found a contested arrangement at the base of the batomorph family tree, identifying a polytomy among skates, electric rays, and thornbacks, with weak support for skates as the most ancestral lineage.

  The long-puzzling Mesozoic group Sclerorhynchoidea complicated the picture for years. Their shark-like snouts led researchers to associate them with sawfishes, but they are now classified as true skates within Rajiformes. A 2022 morphological study by Villalobos-Segura and colleagues produced updated cladograms that further refined understanding of how batomorph families relate to one another.

• 7. The Conservation Crisis

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  The 2021 Nature study placed a number on what ocean scientists had long feared: a 71 percent decline in oceanic sharks and rays over 50 years, driven above all by overfishing. That scale of loss has pushed three-quarters of these species to the point where extinction is a genuine threat. The consequences the study identified are not confined to the animals themselves. The researchers warned that such declines jeopardize the health of entire ocean ecosystems and threaten food security for some of the world's poorest countries.

  The Mediterranean Sea stands out as the worst-affected region, described as the most impacted by unregulated fishing. A recent survey conducted by the Mediterranean Science Commission found that only 38 species of rays and skates still persist there. The failure to account for capture-induced parturition in fisheries management, despite evidence it affects at least 88 live-bearing species, means that the true reproductive toll of fishing pressure remains systematically undercounted.