Ceratioidei, the pelagic anglerfishes or deep-sea anglerfishes, is a suborder of marine ray-finned fishes, one of five suborders in the order Lophiiformes, the anglerfishes. These fishes are found in tropical and temperate seas throughout the world, living above the bottom of the deep sea, in the pelagic zone. The esca, the anglerfishes' lure and a defining feature of all anglerfish groups, is bioluminescent in the deep-sea anglerfishes, attracting prey in the vast darkness of the bathypelagic zone which they inhabit.
The deep-sea anglerfishes have extreme sexual dimorphism; the males are many times smaller than the females. To reproduce, a male seeks out a female, using his sharp teeth-like denticles to clamp onto the female. The details of this sexual parasitism varies between the species; in some, the male permanently becomes part of the female's body, their tissues fusing with each other. This is the only known natural example of a process called parabiosis occurring in adult vertebrates.
Etymology Ceratioidei takes its name from the genus Ceratias, the type genus of the family Ceratiidae and of the suborder. Ceratias means "horn bearer", an allusion to the esca sticking up from the snout, like a horn.
Description
All pelagic anglerfish displays extreme sexual dimorphism, the loss of the pelvic fins found in other anglers, relocation of the pectoral fins, and a general reduction in density through the loss of bony parts, a general decrease in ossification and muscle mass, and the infusion of lipids throughout the body. Elongated dorsal and anal fin rays along with characters of the illicium and esca also support this group as natural. Ceratioids are highly variable in body plan, with the species ranging from elongated to globe-like in body shape.
Ceratioids are relatively small, with the warty anglerfish Ceratias holboelli being the largest, measuring up to in TL. Like other anglerfish this group is sexually dimorphic, with the deep-sea anglerfishes being the most extreme examples of such; male C. holboelli can reach up to long (SL), while females are commonly around TL, weighing an order of magnitude more than her mate. Male Photocorynus spiniceps were measured to be at maturity, and were at one time claimed to be the smallest vertebrate known. However, due to not being free-living (being parasitic males) and the females being , they are now often excluded from the records.
The larvae have well developed eyes, though they stop developing in adult females, where they are thought to be vestigial and only used for light-detection.Munk, O. 1964. The eyes of some ceratioid fishes. Dana Rept., p. 61-62Munk, O. 1966. Ocular anatomy of some deep-sea teleosts. Dana Rept., 70, 62 pp. The eyes of males are variable in development depending on whether the species relies on them to locate females: they may be very well-developed, or they may be highly reduced, as those species rely on olfaction (their sense of smell) instead. The lateral line system is relatively simple, and its pores are often elevated on papillae.
Light organs The illicial apparatus or luring system is a modified first dorsal fin, and is composed of several parts: the pterygiophore (a bone which anchors the structure into the body), the illicium (the free-moving "rod"), and the esca (tip). In most species, the esca contains bioluminescent bacteria, allowing them to produce light. An intricate system of musculature controls this apparatus, allowing fine control over its movement; sliding, vibrating, and twisting motions have been observed in living ceratioids. Most female ceratioids can also retract their illicial apparatus into a groove on their cranium's surface. Additional light organs are present in some families: dorsal caruncles (wart-like protrusions) are present in Ceratiidae, while a hyoid barbel is known in Centrophryne and Linophryne. More light organs may be found throughout the skin and on the fins.
Males do not have an illicial apparatus. The equivalent bones that would form this apparatus instead form a specialized structure used to attach to the females. This structure is mainly composed of the denticular bones, formed through the fusion of modified dermal spinules on the front of the head. These spinules resemble hooked "teeth" and allow the males to attach with the females. In some families, the pterygiophore is integrated into this system.
Evolution
It is presumed that the ancestor of ceratioids resemble the modern Chaunacoids (deep-sea toads) or Ogcocephaloids (batfish), which live in benthic and/or littoral habitats. These ancestors would eventually retain the pelagic habits of larva Lophiiformes into adolescence.
The divergence of Chaunacidae and Ceratioidei is thought to have occurred after the common ancestor of this group adapted to depths of . A 2024 study found that while the ceratioids likely diverged from the Chaunacidae during the Paleocene, the diversification into their various extant families only occurred throughout the Eocene, following the Paleocene-Eocene Thermal Maximum. This likely also coincides with their colonization of deep sea habitats. Prior to these radiations, ancestors of ceratioids evolved extreme sexual size dimorphism and independently lost adaptive immune genes such as aicda, which allowed male anglerfishes to fuse with females, ultimately leading to the evolution of their sexual parasitism.
Elongated genera and species are thought to have arisen from the globose forms, in various, independent adaptive radiation events.
Fossils Owing to the extreme environments they inhabit, fossil remains of deep-sea anglerfishes are very rare in the geologic record. Only a few formations worldwide preserve them, which tend to have been deposited in tectonically active regions where deep-sea sediments could be uplifted to the surface. These include the Puente Formation of California, USA, and the Kurasi Formation of Sakhalin Island, Russia. These formations date to the mid-late Miocene, and specimens recovered from them are assigned to extant genera.
Taxonomic history Ceratioidei was first proposed as a grouping in 1912 by the English ichthyologist Charles Tate Regan as the division Ceratiformes within the suborder Lophoidea of the order Pediculati, which included the Batrachoididae. The Batrachoididae are no longer considered to be closely related to the anglerfishes, which are now included in the order Lophiiformes; within that clade the Ceratioidei are in the same clade as the Chaunacoidei with the Antennarioidei and the Ogcocephaloidei as the sisters of that clade. The 5th edition of Fishes of the World treats this grouping as a suborder within the Lophiiformes.
The family , with its type genus Aceratias (Brauer, 1902), was established to accommodate certain fish which resembled the Ceratiidae and Gigantactinidae, but lacked a luring apparatus. Through closer examination, it was eventually determined that aceratiid fish were actually the males of several distantly related anglerfish species, and so the family is no longer used.
Families
Monophyly (whether a group is "natural") is supported in this group through shared anatomical features. Identification and classification of this suborder is largely based on characters specific to the females, such as the escal morphology, though some osteological characteristics and meristics (traits that can be counted) are shared between the sexes. Male anglerfish can be identified to the genus level using characteristics of the denticular "teeth" and nostril morphology, but species-level identification has not been possible, even when examining parasitic males.
Ceratioidei contains the following families:
Ceratiidae Gill, 1861 (Warty seadevils) Himantolophidae Gill, 1861 (Footballfishes) Melanocetidae Gill, 1878 (Black seadevils) Oneirodidae Gill, 1878 (Dreamers) Caulophrynidae Goode & Bean, 1896 (Fanfins) Gigantactinidae Boulenger, 1904 (Whipnose anglers) Thaumatichthyidae Smith & Radcliffe, 1912 (Wolftrap anglers) Linophrynidae Regan, 1925 (Leftvents) Neoceratiidae Regan, 1926 (Spiny seadevils) Diceratiidae Regan & Trewavas, 1932 (Double anglers) Centrophrynidae Bertelsen, 1951 (Prickly seadevils)
Phylogeny The cladogram below is based on a phylogenetic tree in Pietsch & Orr (2007);
The following cladogram is based on a 2025 analysis of Total Evidence consisting of ultraconserved elements (UCEs), mitochondrial DNA sequence data, and morphological characters. This study reranked a number of clades, and considers suborder Ceratioidei as the infraorder Ceratioideo:
Biology
Behaviour
Deep-sea anglerfish have a low rate of metabolism,Cowles, D.L., and J.J. Childress. 1995. Aerobic metabolism of the anglerfish Melanocetus johnsoni, a deep-pelagic marine sit-and-wait predator. Deep-Sea Res., 42(9):1631–1638. and often drift without actively swimming; In situ observation of female Oneirodes and whipnose anglerfish (from ROVs) recorded that they often passively float in place or in a current, but they were sometimes observed to attempt to flee from the ROV, beating its pectoral fins in-phase while undulating its tail fin. Notably, the whipnose angler (of the genus Gigantactis) maintained an upside-down position. Cryptopsaras couesii has been observed to maintain a "heads-up" position, facing upwards. While most species are thought to inhabit midwater, specimens of diceratiids and Thaumatichthys have been collected near the seabed, with gut content containing benthic prey such as polychaetes, gastropods, sea urchins, and sea cucumbers.Uwate, K.R. 1979. Revision of the anglerfish family Diceratiidae, with descriptions of two new species. Copeia, 1979(1): 129–144.
Larvae are found in much shallower depths than the adults, generally occurring "at depths of less than and rarely below ". Metamorphosis causes a rapid shift in depth, though post-metamorphic juvenile females still occur at somewhat shallower depths than adults. Adults of most species primarily inhabit a range of between in depth, though Thaumatichthys is known from the abyssal zone, up to in depth.
Bioluminescence Light production is likely controlled through esca's blood supply, which provides oxygen and secretions that are used by the bacteria to produce light. Depending on the species, the color of light produced by the esca may vary from pink or purple, to white, yellow, orange, yellowish green, blue, and bluish green, though the peak light frequency is thought to be within the blue-green spectrum as that color is transmitted the furthest in water, and is the frequency that the majority of deep-sea animals are most sensitive to.Herring, P.J. 1983. The spectral characteristics of luminous marine organisms. Proc. Roy. Soc. London, B, 220(1219): 183–217. Cryptopsaras couesii possesses light organs on its skin, thought to allow counterillumination through precise adjustment of these lights. An observed specimen was able to closely match the overhead lighting in the lab; in nature, this ability allows them to camouflage with the little sunlight available.Young, R.E., and C.F.E. Roper. 1977. Intensity regulation of bioluminescence during countershading in living midwater animals. U.S. Fishery Bulletin, 75(2):239–252.
Trophic ecology