Nuptial tubercles (also called nuptial efflorescences, breeding tubercles or pearl organs) are

dermal The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided in ...

structures present in 15

families Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideall ...

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...

belonging to 4 orders of

Actinopterygii Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species. The ray-finned fishes are so called because their fins are webs of skin supported by bony or h ...

–

Salmoniformes Salmonidae is a family of ray-finned fish that constitutes the only currently extant family in the order Salmoniformes . It includes salmon (both Atlantic and Pacific species), trout (both ocean-going and landlocked), chars, freshwater whitefis ...

Gonorhynchiformes The Gonorynchiformes are an order (biology), order of Actinopterygii, ray-finned fish that includes the important food source, the milkfish (''Chanos chanos'', family Chanidae), and a number of lesser-known types, both ocean, marine and freshwat ...

Cypriniformes Cypriniformes is an order of ray-finned fish, including the carps, minnows, loaches, and relatives. Cypriniformes is an Order within the Superorder Ostariophysi consisting of "Carp-like" Ostariophysins. This order contains 11-12 families, ...

and

Perciformes Perciformes (), also called the Percomorpha or Acanthopteri, is an order or superorder of ray-finned fish. If considered a single order, they are the most numerous order of vertebrates, containing about 41% of all bony fish. Perciformes means ...

– used in the courtship and reproduction process. They consist of partially or fully keratinised

cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

that form a perceptible protrusion in certain regions, used to stimulate females during courtship. Their development is stimulated by

hormones A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and beh ...

secreted by the

pituitary In vertebrate anatomy, the pituitary gland, or hypophysis, is an endocrine gland, about the size of a chickpea and weighing, on average, in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain. The hypop ...

and

adrenal The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys. Each gland has an outer cortex which ...

glands, and is induced shortly before the

breeding season Seasonal breeders are animal species that successfully mate only during certain times of the year. These times of year allow for the optimization of survival of young due to factors such as ambient temperature, food and water availability, and cha ...

and discarded after. In some species, at least, there is a correlation between the level of

androgens An androgen (from Greek ''andr-'', the stem of the word meaning "man") is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This inc ...

present in the

endocrine system The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neu ...

and the volume and complexity of tubercle growth.

Use and description

Nuptial tubercles are skin nodules made of

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...

, the same material as hair, hooves, and fingernails. They normally form on the heads of male fish, often covering the whole of the top part of the snout, but may also occur on fins, or anywhere else on the scaled

integumentary system The integumentary system is the set of organs forming the outermost layer of an animal's body. It comprises the skin and its appendages, which act as a physical barrier between the external environment and the internal environment that it serves ...

. The actual purpose of nuptial tubercles is not definitely known. There are many theories about their function, which include: use to stimulate females during courtship, in aggressive rituals performed by males during mate selection, or to maintain contact between individuals during reproduction. There is definite proof that they grow mainly on areas with greater mechanical stress, which means that those species that head-butt females to stimulate them to release eggs will have more on the head, whereas those that dig nests in gravel substrates will develop them more on their fins. Tubercles occasionally develop on female fish also, but this is rare, and they are often barely visible to the naked eye. Nuptial tubercles have been described as an

honest signal Within evolutionary biology, signalling theory is a body of theoretical work examining communication between individuals, both within species and across species. The central question is when organisms with conflicting interests, such as in sex ...

of the general health of the male bearer. Indeed, research has proven a correlation between tubercle density and parasite resistance, though it is not always a positive correlation. In species that use

lek mating A lek is an aggregation of male animals gathered to engage in competitive displays and courtship rituals, known as lekking, to entice visiting females which are surveying prospective partners with which to mate. A lek can also indicate an avail ...

, the females choose the males with the roughest skin, but were not confused by males that had

papillomatosis Papillomatosis of skin is skin surface elevation caused by hyperplasia and enlargement of contiguous dermal papillae.Kumar, Vinay; Fausto, Nelso; Abbas, Abul (2004) ''Robbins & Cotran Pathologic Basis of Disease'' (7th ed.). Saunders. Page 1230. . ...

(a skin disease resulting in rough skin). The presence of nuptial tubercles has been used to distinguish speciation. The genus ''

Leucos ''Leucos'' is a genus of fishes in the family Cyprinidae, from Southern Europe. They are carp close to the genus '' Rutilus'', and were only recently taxonomically distinguished from that genus.Bianco, P.G., Ketmaier, V. (2014)A revision of the ...

'' was described from similar ''

Rutilus ''Rutilus'' is a genus of fish in the family Cyprinidae found in Eurasia. This genus is a widely distributed lineage of cyprinids and ranges from West Europe to East Siberia. Species In FishBase (2022), nine species are included in the genus: * ...

'' species in Europe as they do not develop them.Bianco, P.G., Ketmaier, V. (2014)
A revision of the ''Rutilus'' complex from Mediterranean Europe with description of a new genus, ''Sarmarutilus'', and a new species, ''Rutilus stoumboudae'' (Teleostei: Cyprinidae).
''Zootaxa'', 3841 (3): 379–402. Similarly, the description of ''

Messinobarbus ''Barbus carottae'' is a species of ray-finned fish in the genus ''Barbus'' from Lake Yliki in Greece Greece,, or , romanized: ', officially the Hellenic Republic, is a country in Southeast Europe. It is situated on the southern tip of t ...

'' (now a synonym of ''

Luciobarbus ''Luciobarbus'' is a genus of ray-finned fishes in the family Cyprinidae. Its members are found in fresh and brackish waters of southern Europe, northern Africa, the wider Near East, the Aral and Caspian Seas, and rivers associated with these. Se ...

'') in 1994 was an attempt to distinguish those species with them from ''

Barbus ''Barbus'' is a genus of ray-finned fish in the family Cyprinidae. The type species of ''Barbus'' is the common barbel, first described as ''Cyprinus barbus'' and now named ''Barbus barbus''. ''Barbus'' is the namesake genus of the subfamily B ...

'' that do not have them.

Similar structures in other fishes

Visually similar structures to nuptial tubercles are produced by male

loricariid The Loricariidae is the largest family of catfish (order Siluriformes), with 92 genera and just over 680 species. Loricariids originate from freshwater habitats of Costa Rica, Panama, and tropical and subtropical South America. These fish are not ...

catfishes during the mating season, in similar places to nuptial tubercles (snout and fins). These are actually

hypertrophied Hypertrophy is the increase in the volume of an organ or tissue due to the enlargement of its component cells. It is distinguished from hyperplasia, in which the cells remain approximately the same size but increase in number.Updated by Linda J. ...

odontode Odontodes, or dermal teeth, are hard structures found on the external surfaces of animals or near internal openings. They consist of a soft pulp surrounded by dentine and covered by a mineralized substance such as enamel, a structure similar to th ...

s, which are therefore dermal teeth, rather than keratin, and are covered by flesh containing taste buds. It has been proposed that these ornaments may assist breeding success by mimicking fry, tempting females to lay their eggs in the nest cave of a successful male, already guarding his own offspring. Instead of tubercles,

cichlid Cichlids are fish from the family Cichlidae in the order Cichliformes. Cichlids were traditionally classed in a suborder, the Labroidei, along with the wrasses ( Labridae), in the order Perciformes, but molecular studies have contradicted this ...

males produce nuchal humps (koks), Dual function and associated costs of a highly exaggerated trait in a cichlid fish Sina J. Rometsch, Julián Torres-Dowdall, Gonzalo Machado-Schiaffino, Nidal Karagic, Axel Meyer First published: 23 November 2021 https://doi.org/10.1002/ece3.8383 where additional fat is laid under the skin over the skull forming a hump on the head.

List of species

The following species are known to produce nuptial tubercles. *

Common bream The common bream, freshwater bream, bream, bronze bream, carp bream or sweaty bream (''Abramis brama''), is a European species of freshwater fish in the family Cyprinidae. It is now considered to be the only species in the genus ''Abramis''. ...

, ''Abramis brama'' *Stone-roller, ''

Campostoma anomalum The central stoneroller (''Campostoma anomalum'') is a fish in the family Cyprinidae endemic to North America. Biology Stonerollers have a rounded snout overhanging a crescent-shaped mouth, a hard ridge of cartilage on the lower lip, and irr ...

'' *

Goldfish The goldfish (''Carassius auratus'') is a freshwater fish in the family Cyprinidae of order Cypriniformes. It is commonly kept as a pet in indoor aquariums, and is one of the most popular aquarium fish. Goldfish released into the wild have ...

, ''Carassius auratus'' *

Zebrafish The zebrafish (''Danio rerio'') is a freshwater fish belonging to the minnow family ( Cyprinidae) of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (and thus often ...

, ''Danio rerio'' *'' Knodus nuptialis''Menezes, N.A., and M.M.F. Marinho (2019). A New Species of ''Knodus'' Eigenmann (Characiformes: Characidae: Stevardiinae) with Comments on Nuptial Tubercles and Gill Gland in Characiform Fishes. PLoS ONE 14(7): e0217915. *

Fathead minnow Fathead minnow (''Pimephales promelas''), also known as fathead or tuffy, is a species of temperate freshwater fish belonging to the genus ''Pimephales'' of the cyprinid family. The natural geographic range extends throughout much of North Ameri ...

, ''Pimephales promelas'' * Common roach, ''Rutilus rutilus''

Use and description

Similar structures in other fishes

List of species

See also

References

Further reading