Encyclopedia of Animal Cognition and Behavior

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| Editors: Jennifer Vonk, Todd Shackelford

Actinopterygii (Bony Fish)

  • Pedro F. AmorimEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-319-47829-6_839-1
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Synonyms

Introduction

Actinopterygii is the most diverse group of Chordata, with about 470 families and more than 33,500 extant species. The most ancient fossils known of the group are scales from the Late Silurian. The fossil record of the group is more diverse, and the fossils are more complete in deposits of the Devonian and Carboniferous (Choo 2015; Nelson et al. 2016).

Along this 400 million years, the ray-finned fishes evolved reaching almost every aquatic environment in both salt and freshwater, from deep abyssal areas to the highest river basins. Throughout the evolution of these fishes, almost every kind of aquatic habitat has been reached, so that they are found in areas with large differences in temperature, elevation, pressure, salinity, pH, turbidity, luminosity, and availability of oxygen. This diversity of environments reflects the adaptations of the sensory organs and behavior found in the ray-finned fishes species (Helfman et al. 2009). In this entry, we will see more about the structures and senses of the Actinopterygians and how they are adapted to the habitats occupied by these species.

Anatomy

Skeleton

The anterior portion of the skeleton on every Actinopterygii is the cranium, where the brain and most sensorial organs are found. This complex structure is divided in Neurocranium and Branchiocranium. The neurocranium is divided in two portions, each one derived from a different embryonic tissue, Chondrocranium and Dermatocranium. The chondrocranium is formed from endochondral bones, meaning that the brain case is originally cartilaginous, which is later replaced by bones during the ontogenetic development. The dermatocranium develops from dermal bones, which are derived from groups of scales that change into bones. The histology of both kinds of bones is very similar; however, the embryonic development is completely different. The neurocranium is usually divided in four areas, each one associated with a different sense organ: (1) ethmoid region (related to the olfactory nasal capsule); (2) orbital region (around the eyes); (3) optic region (encompassing the ear chamber); and (4) basicranial region (connecting the brain to the spinal cord).

The branchiocranium, also known as visceral cranium, is composed of the bones present in jaws, branchial, opercular, pharyngeal, and ventral orbital regions of the cranium. This portion is derived from dermal bones. It is in the branchiocranium where the teeth are usually found among the species of Actinopterygii. The teeth are found not only in the jaws, but they also may occur in branchial and pharyngeal bones. There is a great diversity of teeth forms among ray-finned fishes: (1) caniniform (tooth is slender and conical); (2) cardiform (thin teeth ordered in a series of rows); (3) molariform (robust teeth used to smash food); and (4) villiform (short and pointed teeth). Other patterns found in Actinopterygii include: (1) teeth fused in a plate; (2) triangular arrowhead shaped teeth; and (3) species with no teeth in the jaws but only in pharyngeal bones.

In the vertebral column, each vertebra develops from cartilaginous structures named arcualia. The formation of vertebrae may occur in two different ways: (1) monospondylus, when there is only a single vertebra for each arcualia, and (2) diplospondylous, when two vertebrae are formed from each arcualia. Among Actinopterygii, the diplospondylous condition is only found in the monotypic order Amiiformes. Two kinds of vertebrae are recognized in each species of Actinopterygii: (1) precaudal vertebrae, present from the anterior portion of the column to the end of the body cavity, and (2) caudal vertebrae, in the posterior portion of the body. The caudal vertebrae feature a closed hemal canal and a completed hemal spine ventrally. The dorsal portion of both caudal and precaudal vertebrae is similar, presenting a neural spine supported by the neural arches.

Ribs and intramuscular bones are found next to precaudal vertebrae. Most of Actinopterygii have the pleural ribs distributed between the third and the last precaudal vertebrae, and these structures are positioned around the visceral cavity protecting the organs. Other intramuscular bones found in Actinopterygii are epineurals, epicentrals, and epipleurals.

The final portion of the Actinopterygians body is the caudal fin, which is a complex structure composed of vertebral centrum, vertebral accessories, and also fin rays. Over the different Actinopterygii lineages, the caudal fin has been modified, adapting to each environment reached by the ray-finned fishes. The classification of caudal fins is based on dorsal-ventral symmetry and the internal morphology of the fin. In Actinopterygii, there are two kinds of fins found: heterocercal and homocercal. Species with heterocercal pattern present the column extending to the superior portion of the tail, in the dorsal lobe. This type of fin is observed in the plesiomorphic orders Acipenseriformes (sturgeons and paddlefishes), Lepisosteiformes (gars) and Polypteriformes (bichirs and reedfish). The second pattern found in Actinopterygii is the homocercal, in which the caudal fin is symmetric externally and internally. This internal symmetry is possible due to a series of bones named hypurals, positioned after the last vertebra. The external symmetry comes from the disposition of the caudal-fin rays connecting with the hypurals. Only one species presents the caudal fin externally symmetric but not internally, Amia calva, the Bowfin (Amiiformes). This intermediate pattern is named hemihomocercal.

Three kinds of unpaired fins may be found in Actinopterygians: dorsal fin, anal fin, and adipose fin. These fins are always aligned with the sagittal plane of the body, with the dorsal and adipose fins positioned dorsally and anal fin ventrally. The dorsal and anal fins are externally supported by lepidotrichia, small bones derived from scales. Internally these fins are held by radials, which are named interneural bones in the dorsal fin and interhemal bones in the anal fin.

Plesiomorphic lineages of Actinopterygii have only one dorsal fin with soft rays. In Teleostei, an anterior dorsal fin with spines is found; in species with two dorsal fins, the first is always composed of spines and the second by soft rays. The primary function of the dorsal fin was related to making the swim more stable, but this function has been modified along the evolution of different lineages. Some of the most abrupt changes are found in remoras (Echeneidae), in which the dorsal fin is modified into a suction disk allowing these fishes to attach themselves to larger animals, and in anglerfishes (Lophiiformes), the dorsal fin spine is modified in a structure named illicium and is used as a fishing rod to capture prays.

Unlike the dorsal fin, few lineages show diversification in the anal fin. In some families of the order Cyprinodontiformes, males have the anal fin modified as a reproductive structure, the gonopodium. A similar modification may be found in the genus Zenarchopterus (Beloniformes), where the anal fin is modified in an andropodium. In some lineages, as Gymnotiformes and Notopteridae (Osteoglossiformes), the anal fin is used as the main locomotory structure.

The adipose fin is an extra fin found in some lineages of Teleostei, positioned posterior to the dorsal fin. Despite the name, this fin is not usually composed of adipose tissue. Lepidotrichia and bones of support are generally absent in this fin. The adipose fin has evolved independently in different lineages, yet, the function of this fin is still unknown.

Ray-finned fishes present two pairs of paired fins, the pectoral and pelvic fins. The pectoral fin is positioned in the anterior portion of the flank. The pectoral-fin rays are supported by radials attached to the pectoral girdle. In tetrapods this girdle is connected with the vertebral column. However, in Actinopterygii, the pectoral girdle is connected to the posterior portion of the neurocranium. The pelvic fin is positioned in the ventral portion of the body, anterior to the anal fin. Plesiomorphic groups present the pelvic fin in an abdominal position; however, along the evolution of the ray-finned fishes, this fin became anteriorly positioned in the thorax or even assuming a jugular position. Pelvic fins in jugular position may be found even anteriorly to the pectoral fins. The pelvic girdle of tetrapods is connected to the vertebral column. In ray-finned fishes, the pelvic girdle usually has no connection with other structures, except for the species with jugular pelvic fins, that are generally connected with the pectoral girdle.

Scales

Scales are found covering the body of almost every species of Actinopterygii. In this clade there are two basic kinds of scales: (1) the ganoids and (2) the cycloids and ctenoids (Roberts 1993). Ganoid scales are found is Polypteriformes, Acipenseriformes, and Lepisosteiformes. This scale generally presents spots of connection and articulation. Sturgeons and Paddlefishes (Acipenseriformes) show the most modified ganoid scales, which are featured as large plates letting most of the body naked, covered only by the skin. Cycloids and ctenoids scales are found in Teleostei and have two main layers. The first one is composed of salts (mostly calcium carbonate and hydroxyapatite) distributed in an organic matrix. The second layer is more internal and mainly composed of collagen fibers. These scales are aligned overlapping the next scale, giving much more flexibility to the individual than ganoid scales. Cycloids and ctenoids scales differ from each other due to the absence of ctenii (spinules on posterior margin) in cycloid scales. There are three kinds of ctenoid scales: (1) crenates, with spines only in the margin of the scale; (2) spinoids, when the ctenii reach the main body of the scale; and (3) ctenoids, with the spinules found as ossifications separated from the scale. During the ontogenetic process, the development of the scales begins on the caudal peduncle in the lateral line. After this, the scales advance in dorsal and ventral rows parallel to the lateral line, propagating to the anterior portion of the body. When the progress of the scales is finished, their number remains unchanged during the life of the individual.

Cardiovascular System

This system is responsible for the distribution of nutrients, excretory metabolites, and respiratory gases, making the cardiovascular system also important for osmoregulation, excretion, and respiration. In Actinopterygii, the basic pattern of the blood flow starts in the heart, proceeds to the gill arches, and one part goes to the head and another part to the body, and then back to the heart. The pericardial cavity shelters the heart, and this cavity is positioned posteriorly and ventrally to the gill arches. The heart of Actinopterygians is divided into four sequential chambers: sinus venosus, atrium, ventricle, and bulbus arteriosus (Farrell and Jones 1992). The last chamber is a nonmuscular bulbus, which stabilizes the pressure of the blood coming from the ventricle. When compared with tetrapods, the heart of ray-finned fishes shows a very small weight relative to the body weight. Lethargic species present a proportion of less than 0.1% between heart weight and body weight. More active species have heavier hearts, but never reaching more than 0.25% of the body weight.

Respiratory System

Aquatic breathing demands more energy than aerial breathing. While the oxygen composes about 20% of the atmosphere, in water environments this gas may be diluted in less than 1%. Thus, the respiratory system of the Actinopterygians needs to extract as much oxygen as possible from the water (Graham and Lee 2004). The organs responsible for gas exchange are the gills. They have an osteologic structure supporting the secondary lamellae (thin membranes well-vascularized with a large surface).

Oxygen is absorbed and carbon dioxide is released by diffusion, and to optimize this process the blood in the secondary lamellae flows in countercurrent with the water passing through the gills. This opposite flow is necessary for the correct function of the gills. To ensure this countercurrent flow, two strategies are used by the Actinopterygii species. The process most commonly used is to alternate the size of the oral and opercular chambers, increasing and decreasing the volumes. The water flows from the mouth to operculum keeping the correct direction and the respiratory cycle. The second process is named ram ventilation, where the individuals simply swim with the mouth slightly opened, keeping the water flow through the gills. Ram ventilation is used by pelagic high-speed species and it saves energy, since only the swim muscles are used to breathe, in opposition to the previous pattern of ventilation.

In some species the oxygen may also be acquired from the atmosphere. Gills usually need to be in an aquatic environment to keep the correct shape, since when exposed to an aerial habitat the gills collapse and stick together, making gas exchange impossible. Therefore, species with this ability to breathe atmospheric air also present morphological adaptations in some structures to enable them to utilize the atmospheric air. Those modifications are divided into three categories: (1) species with air-breathing organs derived from the alimentary tract, as gas bladder, stomach or intestines; (2) modifications in the head, such as gills modified to work properly in atmospheric environments; and (3) a well vascularized skin allowing the cutaneous gas exchange (Graham 1997).

Swim Bladder

The bladder is filled with oxygen, nitrogen, and carbon dioxide, being also known as gas bladder. The function of this organ has changed through the Actinopterygii evolution, as probably the original function of the gas bladder was similar to the one of a lung. In extant species this bladder is used mainly for hydrostatic and buoyancy control, although in some species the swim bladder is also used for breathing and in sound perception or production. In some deep-sea or benthic species, this bladder is lost. The most internal layer of the swim bladder, the bladder epithelium, has a different origin than the other layers, as this epithelium has an endodermic origin, while the other tissues are derived from the mesoderm.

Two kinds of swim bladder are found in adults of Actinopterygii: physostomous and physoclistous. In physostomous species, the acquisition and release of gas occurs by a connection between the swim bladder and the alimentary tract, called the pneumatic duct. The physostomous pattern is plesiomorphic among the ray-finned fishes, and species with adults presenting the physoclistous pattern, juveniles present a physostomous swim bladder.

In physoclistous species there is no duct connecting the bladder with other organs, and the addition of gas is performed by the gas gland. This gland transfers gas dissolved on the blood, by diffusion, to the air bladder. The gas is released by a modified area of the swim bladder, called the oval. This structure transfers the air to the blood, and the excess gas is carried to the gills and released in the environment.

Nervous System

In Actinopterygii, the nervous system is divided into cerebrospinal, encompassing the central nervous system and peripheral nervous system, and the autonomic systems, with sympathetic and parasympathetic ganglia.

The central nervous system contains the brain and spinal cord. Anatomically the brain is divided into five portions: telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon. Each portion has a different function, processing information of different parts of the body. The telencephalon is responsible for the perception of the olfactory system; this part of the brain is homologous to the brain of tetrapods. The diencephalon function is to connect the information arriving and departing related to the endocrine system and homeostasis. In the dorsal portion of the diencephalon there is a well-vascularized structure named pineal body. The function of this structure is associated with color change, seasonal cycles, and circadian clock. Despite being a structure of the brain, the pineal body has light-sensitive cells, similar to the retinal cones. The mesencephalon is connected with the optic nerve, and all the information received by the eyes is processed by this portion of the brain. The next portion of the brain is the metencephalon, which comprises the cerebellum, responsible for the equilibrium of the swim, eye muscles, and muscular tonus. The last portion of the brain is the myelencephalon, controlling osmoregulation, breathing, and transmitting the information of each sensory systems, with the exception of the olfactory and visual systems. The myelencephalon also connects the brain to the spinal cord.

Sensory Systems

An important step to understand the sensory systems of Actinopterygians is to be aware that the perception of stimuli in aquatic environments differs from that in aerial environments. Taste and smell, that are very different from each other for land organisms, are very similar in water. Some wavelengths of light begin to fade according to the depth, and the sound travels faster and further than through the air. The perception of the environment may also change along the ontogeny of some species, when adults and juveniles are found in different niches (Poling and Fuiman 1998; Braun et al. 2002).

Mechanoreception

In Actinopterygii, this sense perceives the vibration of the water. These stimuli are received by sensory hair cells, featuring cilia (Schellart and Wubbels 1997). Modifications on the vibration of the water change the frequency of nerve impulses emitted by the sensory cell to the brain. Two main mechanosensory systems are found in ray-finned fishes: the lateral line (receiving stimuli of turbulence around the fish, helping in hunting, positioning in school, and avoiding obstacles) and the inner ear (helping in hearing and equilibrium).

Among the Actinopterygii the lateral line system is a plesiomorphic feature, found even in Silurian fossil jawless fishes. The sensitive hair cells of this system are found clustering together with a gelatinous dome covering the cells, in a structure named neuromast. The function of the gelatinous cupula is to protect the hair cells from background noise, thus only stronger vibrations will be detected by the neuromasts. In the lateral line system, the neuromasts may be found in two possible conformations: (1) free neuromasts over the skin, or (2) in channels under the skin and scales (in the trunk) or dermal bones (in the head).

In the early ontogenetic stages of ray-finned fishes, all neuromasts are superficial and mostly disposed on the head. Along the development the distribution of the neuromasts extends to the trunk and the channels are formed encompassing them, under the skin. Those channels have pores, allowing the water to enter and interact with the neuromasts. The protection provided by the channels helps to avoid the permanent background noise of the water around the fish, making those neuromasts very sensitive to modifications on the water flow or high frequency vibrations (20–100 Hz). This kind of neuromasts is usually well-developed in species inhabiting turbulent waters habitats or in fast swimmers. Superficial neuromasts are more sensible to movements in environments of slow currents and low frequencies (below 20 Hz), making them very useful to detect prays and predators. Free neuromasts are more abundant in species inhabiting slow current environments and sedentary species.

Hair cells are also responsible for equilibrium and balance of ray-finned fishes. In the pars superior of the inner ear, there are three semicircular canals, that contain endolymph, and the terminal ampulla, connected with the semicircular canals, containing sensory hair cells. This portion of the inner ear receives the stimuli related to equilibrium and balance. The movement of the fish is reflected in the endolymph and changes in the posture, orientation, or speed are perceived by the hair cells. These cells send this information to the brain, which analyzes and corrects the balance and equilibrium of the individual.

The cavities of inner ear named saccule and lagena (in pars inferior) and utricle (in pars superior) are the portions responsible by hearing. Each cavity holds an otolith (structures of the inner ear made from calcium carbonate), which is surrounded by the otolithic membrane. This membrane connects the otolith with the tissue of the cavity. The tissue of each cavity is composed of sensorial hair cells that send to the brain the information from mechanic stimuli related to the sound. Otoliths react to the vibration due to their density. Most structures of the ray-finned fishes body have the same density as water, not interacting with its vibration. However, the vibration of the water may interact with spaces filled with gas inside the body of the fish, such as the gas bladder. Some species have adaptations that connect the gas bladder to the inner ear, improving their hearing. Better hearing species include cods, in which the anterior portion of the bladder is positioned very closely to the inner ear; sardines, in which the gas bladder is in contact with the inner ear; African mormyrids, which present independent gas bladders close to the inner ears, and the Othophysi species. In the species of Othophysi, small bones connect the gas bladder to the inner ear. This adaptation is derived from the modified anterior vertebrae, and this structure is named Weberian ossicles. Thus, the vibration of the gas bladder is directly transmitted to the inner ear, making the hearing of Othophysi the most complex and accurate among the Actinopterygii.

Chemoreception

In aerial environments the difference among olfaction, the sense of molecules dissolved in air, and gustation, the perception of chemicals dissolved in water, is very clear. However, for species inhabiting aquatic environments, those limits are not so clear, and chemoreceptors may be found in different structures than just mouth and nostrils. Several kinds of molecules dilute in water and the capacity of noticing these molecules allow the ray-finned fishes to locate shelter, to communicate with other individuals, to find food, and to evade predators. In Actinopterygii, the chemoreceptors related to the perception of the environment are usually associated with the olfaction, whereas the receptors related to the identification of the food are generally linked to the gustation (Hara 1986).

The chemoreceptors of the ray-finned fishes are mainly concentrated in the paired olfactory chambers. These structures are invaginations usually on the anterior portion of the head. Inside the chambers there is the olfactory epithelium usually organized in rosettes. The stimuli received by this tissue are transferred to the olfactory lobes of the brain. Inside the chamber there are cells with cilia generating the movement of the water through the sensitive cells. This water flux allows the animal to continuously perceive the molecules around. Among the ray-finned fishes the olfaction is mainly used to find partners, receiving stimuli of sex steroids, of pheromones, and of alarm substances. Thus, this sense is extremely important to avoid predators and to reproduction. Damages in those receptors may affect the behavior of the individual, disturbing the social interactions of the specimen.

The sense of taste in Actinopterygians is mainly used to recognize what may be eaten. Sensory cells responsible for the gustation may be found isolated or grouped in taste buds. Isolated cells may be very abundant, with up to 4000 per mm2. The taste buds contain a smaller number of cells, usually between 30 and 100. Those chemoreceptors are found inside the mouth but also in structures around the mouth, as lips and barbels, or even in trunk and fins.

Vision

In aquatic environments the intensity of light and the wavelength available differ according to the depth and to what chemical substances are dissolved in the water, as salts and organic matter. These differences among the habitats lead to distinct adaptations related to ray-finned fishes vision organs. Wavelengths closer to the red end (lower frequencies) are the firsts to disappear with the increasing depth, while those closer to the violet end (higher frequencies) reach deeper into water. Despite that, ultraviolet light does not reach very deep in the water (Guthrie and Muntz 1993).

The morphology of the ray-finned fishes’ eyes is very similar to the other vertebrates. The most external layer is a thin transparent cornea, followed by the pupil. Unlike tetrapods, in which the pupil diameter is variable according to the light available, in Actinopterygii the pupil size is fixed. The next structure found in the eye is a spherical lens, contrasting with the eyes of tetrapods, which have a convex and less dense lens. The center of the eye is a cavity filled with liquid, the posterior portion of which is composed of the layers of the retina, where the light stimuli are received by the photosensory cells and sent to the brain. In Actinopterygians, the focus of the image inside the eye is obtained by moving the lens to anterior or posterior portion of the eye, adjusting according to the distance between the fish and the object (Hawryshyn 1998).

Two kinds of photosensory cells are found in the retina of Actinopterygii species: cones and rods. Rods are the most sensitive to the light, allowing to see even with low light. However, these cells generate images with low resolution. The rods are more numerous in species occupying niches with low light, as deep sea, nocturnal, or crepuscular species. Cones have a lower sensitivity to light, and therefore more light is necessary to stimulate it, but these cells can generate images with higher resolution than rods. These cells are sensitive to different wavelengths, depending on which kind of opsin, the photoreceptive glycoproteins, is present in each cell. Cones with porphyropsins perceive longer wavelengths, or colors between red and yellow. These cells are usually found in species living near the water surface. Photosensory cells with rhodopsin are sensitive to wavelengths of the colors between green and blue; thus, these cells are more common in species living in habitats somewhat deeper. In species inhabiting deep sea areas, the most usual opsin found in the cones is the chrysopsin, and these cells detect colors from blue to violet, which reach deeper in the water. In some species inhabiting areas near the surface or shallow habitats, an opsin sensible to ultraviolet light also may be present.

As the light intensity may vary according to each habitat, the Actinopterygii species feature three types of cones, with a few species featuring more than three or only two types. The arrangement of the photosensory cells may vary according to the habitat of the species and also along the ontogenetic development. In some species in which juveniles and adults occupy different niches, not only the distribution of the cells may be modified, but even the opsin expressed by each cell may be modified. Despite the variety of light wavelengths that can be detected by Actinopterygians, reaching even ultraviolet light, some species are also able to see polarized light.

In most species of ray-finned fishes there is a U-shaped choroid gland around the optic nerve. The function of this gland is to provide oxygen to the retina because of its high need for the gas. The last part of the eye is the sclera, a layer to protect those delicate internal structures. In Actinopterygii, the sclera is usually found as sclerotic bones, while in Myxiniformes and Petromyzontiformes the sclera is fibrous, and in Chondrichthyes this structure is presented as cartilaginous plates.

Between the retina and the protective sclera there is a well-vascularized layer named choroid. In a few species of Actinopterygii the choroid may present a tapetum lucidum. This is a structure made by guanine crystals that reflect back to the eye the light that passed through the retina and was not absorbed by it. This adaptation increases the visual perception in conditions of low light.

Electroreception

In Actinopterygii, few groups have the capacity to feel electric fields. This sense is perceived by electroreceptor organs that only have evolved in some lineages. The cells that compose this organ are derived from mechanoreceptor hair cells during the ontogeny (Whitehead and Collin 2004). In ray-finned fishes, there are two kinds of electroreceptor organs: ampullary receptors and tuberous receptors.

Ampullary receptors are found in invaginations of the skins. Externally there is a pore, connecting the receptors to the environment. This pore leads to a canal containing a conductive gel, and to the receptor. This kind of organ detects stimuli of low frequency electric fields, between less than 0.1 and 25 Hz, perceiving electrical signals coming from other animals. In this organ the perception of electricity is due to differences in electric potential between the apical and basal membranes.

Tuberous receptors are placed over the skin, covered by a layer of epithelial cells. These receptors are sensitive to electric fields of higher frequency, between 50 and 2000 Hz, and are used for electrolocation, detecting electric pulses of the individual itself. Thus, only species with electric organ discharge have tuberous receptors. These receptors may be divided into two main kinds according with which stimuli are detected: the frequency of the pulses or their amplitude. With those receptors the individual is able to detect any kind of object passing through the coverage area of the electric field. Electric organs are composed of modified muscle cells named electrocytes (Von der Emde 1998). These cells usually have a disk-like shape and are arranged in rows. The electric pulse is generated by an ion flux over the membrane of the electrocytes. Each cell generates a small electric current, but because they are aligned in rows and the electric discharge is simultaneous, the effect is additive, amplifying the discharge, as little batteries in a row.

Reproduction

Several kinds of reproductive patterns are found among Actinopterygii species, the differences range from the number of mating partners along the life of the individual, to the existence or not of parental care. For most species of the group the sex of the individual is defined in early life stages and kept along the lifetime (Bone and Moore 2008). Nevertheless, in some species both gametes are produced, and this species are divided into two categories: simultaneous hermaphrodites and sequential hermaphrodites. Species considered simultaneous hermaphrodites are very rare, and the individuals are able to produce fertilized eggs, or sperm and eggs in one spawning. Sequential hermaphrodites change the gender along the lifetime, and all the reproductive structures correspondent to the present sex become functional. Species in which individuals are born as females and become males throughout development are named protogynous, and species where the first stage is male and the second female are named protoandrous (Emlen and Oring 1977).

Most of the ray-finned fishes’ species have more than one opportunity to reproduce in their lifetime, and these species are named iteroparous. However, in a few species only one single spawning event occurs in life. These species are known as semelparous, and they are usually diadromous or perform a great migration. Diadromous species are divided into two kinds: (1) anadromous (the individuals are born in freshwater, migrate and grow in the sea, and then migrate back to freshwater to reproduce) and (2) catadromous (the individuals are born in the sea, migrating to freshwater to grow, and then migrate to the sea to reproduce). A well-known example of semelparous species is the salmon, living most part of the life in the sea but going up rivers to spawn.

The number of reproductive partners also varies in the different species of the group, and may be divided into three main patterns. (1) Promiscuous species, when the partner is chosen with little or no degree of selection. In those species multiple individuals of both sexes spawn together at the same time or in a short time. (2) Monogamous species, in which the individuals remain breeding exclusively with the same partner. In this case, the couple not necessarily stays together after the mating season. (3) Polygamous species, in which individuals of only one sex have many partners. When the female presents several partners it is named polyandry, and when the male has different partners it is called polygyny, which is more common. Among the polygynous strategies the formation of harems may also be found. In this case a group of females stay with one male and only reproduces with him, who protects the whole group. Another strategy comprised by the polygamous pattern is the formation of leks, in which many males concentrate in a specific area only to display to females.

Most of the ray-finned fishes’ species leaves the eggs after spawning, not presenting any kind of parental care. However, this behavior is found in several lineages of Actinopterygii, in different levels and time periods. The first stage of parental care includes to build a nest. In some species the eggs are buried in the nest after the spawning and the parents leave the eggs. Other species stay with the nest until the birth of the offspring, protecting the eggs from predators, cleaning the nest, removing sediments, unviable eggs, and keeping a water flux to maintain the eggs oxygenized. In other species the eggs are eventually carried in the mouth to provide a better protection to the offspring. In some species the development of the eggs is internal, and this kind of bearing is divided in ovoviviparous (when the eggs are kept inside the mother and embryos feed on the yolk present in the egg) or viviparous (when there is a specialized tissue responsible for feeding the embryos and performing gas exchange). In a few species, after the birth, the adults also may produce mucus to feed the juveniles or help them to obtain food.

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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Institute of BiologyFederal University of Rio de Janeiro (UFRJ)Rio de JaneiroBrazil

Section editors and affiliations

  • Mystera M. Samuelson
    • 1
  1. 1.The Institute for Marine Mammal StudiesGulfportUSA