Class Actinopterygii
ray-finned fishes

Actinopterygians, or ‘ray-finned fishes,’ are the largest and most successful group of fishes and make up half of all living vertebrates. While actinopterygians appeared in the fossil record during the Devonian period, between 400-350 million years ago (Ma), it was not until the Carboniferous period (360 Ma) that they had become dominant in freshwaters and started to invade the seas. At present, approximately 42 orders, 431 families, and nearly 24,000 species are recognized within this class but there are bound to be taxonomic revisions as research progresses. Teleosts comprise approximately 23,000 of the 24,000 species within the actinopterygians, and 96 percent of all living fish species (see Systematic/Taxonomic History). The latter estimates, however, will probably never be accurate because actinopterygian species are becoming extinct faster than they can be discovered in some areas, such as the Amazon and Congo Basins. Unfortunately, habitat destruction, pollution and international trade, among other human impacts, have contributed to the endangerment of many actinopterygians (see Conservation Status). ()

Clearly, given the enormous diversity of this class, entire books could be (and are) written for each of the categories below, so this account does not attempt an exhaustive summary of the diversity of habitats, body forms, behaviors, reproductive habits, etc. of actinopterygians. Instead, each section introduces important ichthyological concepts and terminology, as well as numerous examples from a diverse range of ray-finned fish families. A section of particular interest is Systematic/Taxonomic History because salient features of the evolutionary history of actinopterygians are discussed. The phylogenetic trends within early actinopterygians provide a basis for understanding why this group has been so successful, as more derived forms (i.e. neopterygians and teleosts), which make up nearly all existing ray-finned fishes, have repeated and extended early trends. Many of the sections, such as Physical Description, Reproduction, Behavior and Ecosystem Roles merely scratch the surface, but there are numerous links to family-level ray-finned fish accounts. (‘Fishes’ is used interchangeably with ‘ray-finned fishes’ and 'actinopterygians' from this point forward). ()

Ray-finned fishes inhabit a variety of extreme environments. These include high altitude lakes and streams, desert springs (e.g. pupfishes), subterranean caves (e.g. cavefishes), ephemeral pools, polar seas, and the depths of the ocean (e.g. deepsea anglerfishes). Across these habitats water temperatures may range from -1.8˚C to nearly 40˚C, pH levels from 4 to 10+, dissolved oxygen levels from zero to saturation, salinities from 0 to 90 parts per million and depths ranging from 0 to 7,000 m (Davenport and Sayer 1993 in Moyle and Cech 2004:1)! Some fish even spend considerable time outside of water: mudskippers prey on the invertebrates of mudflat habitats, while airbreathing catfishes and gouramies live in stagnant, low oxygen ponds (among other habitats) or migrate over land to colonize new areas. Another extreme example of habitat adaptation is found in hillstream loaches , which live in the steep, torrential watercourses of Asiatic hillstreams. Hillstream loaches have flattened bodies and utilize suckers, permanently clinging to rock faces so they are not swept downstream. Lanternfishes , hatchetfishes , dragonfishes , deep-sea codfishes , halosaurs and spiny eels all have lights (flashing or constant), created by luminescent bacteria or special glandular cells, to find prey, communicate with other individuals, or for defense in the blackness of their deepsea habitats (see Communication, Food Habits, and Predation). ()

Disparate localities may have similar geographic conditions, yet fish species composition varies widely across similar regions. In other words, patterns of fish distribution are not simply related to how well a fish is adapted to a particular type of environment, which is why invasive species can be so devastating (see Conservation). The study of zoogeography attempts to answer questions about how and why fish (and other animal) faunas differ across geographic regions. Zoogeography integrates a variety of disciplines within ichthyology (ecology, physiology, systematics , paleontology, geology and biogeography) to explain patterns of fish distribution. While ichthyologists certainly have incomplete knowledge in many of these areas, advances in plate tectonics and phylogenetic systematics have allowed them to define various zoogeographic (or biogeographic) regions (also subregions) and types. ()

Fresh water covers only a tiny fraction of the earth’s surface (.0093 percent), yet it is home to approximately 41 percent of all fish species. Most of these are concentrated in the tropics (1,500 different species in the Amazon Basin alone), and Southeast Asia probably has the most diverse assemblage of freshwater species. In marine areas, species concentrations are highest around coral reefs, where butterflyfishes and angelfishes , wrasses , parrotfishes and triggerfishes are common. In the arctic seas five notothenoid families dominate: thornfishes , plunderfishes, Antarctic dragonfishes , and notothens. ()

Ray-finned fishes inhabit a variety of extreme environments. These include high altitude lakes and streams, desert springs (e.g. pupfishes), subterranean caves (e.g. cavefishes), ephemeral pools, polar seas, and the depths of the ocean (e.g. deepsea anglerfishes). Across these habitats water temperatures may range from -1.8˚C to nearly 40˚C, pH levels from 4 to 10+, dissolved oxygen levels from zero to saturation, salinities from 0 to 90 parts per million and depths ranging from 0 to 7,000 m (Davenport and Sayer 1993 in Moyle and Cech 2004:1)! Some fish even spend considerable time outside of water: mudskippers prey on the invertebrates of mudflat habitats, while airbreathing catfishes and gouramies live in stagnant, low oxygen ponds (among other habitats) or migrate over land to colonize new areas. Another extreme example of habitat adaptation is found in hillstream loaches , which live in the steep, torrential watercourses of Asiatic hillstreams. Hillstream loaches have flattened bodies and utilize suckers, permanently clinging to rock faces so they are not swept downstream. Lanternfishes , hatchetfishes , dragonfishes , deep-sea codfishes , halosaurs and spiny eels all have lights (flashing or constant), created by luminescent bacteria or special glandular cells, to find prey, communicate with other individuals, or for defense in the blackness of their deepsea habitats (see Communication, Food Habits, and Predation). ()

Researchers have long divided freshwater and saltwater habitats. However, habitat boundaries are often crossed by migratory species, some of which are diadromous – meaning they migrate between fresh water and the sea. Depending on the type of migration, they can be anadromous (migrate up rivers to spawn), with a pattern of freshwater-ocean-freshwater (typical of salmon and lampreys), or catadromous (migrate from freshwater to the sea to spawn), which is characteristic of freshwater eels . In the latter family juveniles, carried to river mouths by ocean currents, migrate upstream and live for up to 10 years before returning to spawning grounds in the ocean and dying shortly after (see Behavior as well). ()

Fresh water covers only a tiny fraction of the earth’s surface (.0093 percent), yet it is home to approximately 41 percent of all fish species. Most of these are concentrated in the tropics (1,500 different species in the Amazon Basin alone), and Southeast Asia probably has the most diverse assemblage of freshwater species. In marine areas, species concentrations are highest around coral reefs, where butterflyfishes and angelfishes , wrasses , parrotfishes and triggerfishes are common. In the arctic seas five notothenoid families dominate: thornfishes , plunderfishes, Antarctic dragonfishes , and notothens. ()

Even though there is not a strong set of derived characters (characteristics not present in ancestral species) for actinopterygians, they are believed to be monophyletic. The known ancestral features include lepidotrichia (“like scales, [they] are of dermal origin and probably are derived from scales. They form the soft rays of the fins, which are segmented and dumbbell-shaped in cross-section.” Moyle and Cech, 2004:231), heavy ganoid scales , structurally distinctive pelvic and pectoral girdles, fins attached to the body by the fin rays (rather than with a fleshy lobe), branchiostegal rays (a series of long, curved bones supporting the gill membrane), and no internal nares (or choana – separate internal openings to the lungs). ()

Two subclasses are recognized, the neopterygians (new-finned) and chondrosteans , the latter being non-monophyletic. Within the Chondrostei, only sturgeons , bichirs and paddlefishes survive today, and many are threatened (see Conservation). The rest of the actinopterygians, which includes the vast majority of species, are in the subclass Neopterygii: “In their great numbers and degree of anatomical diversity, the modern ray-finned fishes may be considered the most successful of all vertebrates” (Caroll, 1986:136 in Helfman et. al. 1997:162). Further, modern teleosts represent the culmination of continuous ‘improvements’ on the basic fish design, within the Neopterygii. Investigators have traced these changes in the basic fish design with the aid of the fossil record, and have identified important trends in actinopterygian evolution. Refinements in the structure of scales, branchiostegal rays, swimbladder, jaws, tail and fins have all contributed to the diverse radiation (increase in number of species) of actinopterygian fishes. The trend among actinopterygians has been toward lighter, more flexible bones and scales, an internal muscular-tendonous system, neutral buoyancy via the swimbladder, greater maneuverability, mobility and speed via changes in the tail and fins, and improvements in mouth structure (as described in detail below). ()

Scales: Heavy and complex scales composed of three layers (ganoid) served as a relatively inflexible suit of armor for ancestral species but the general trend in actinopterygians has been to reduce weight and complexity and increase flexibility. The result is the elasmoid scale (ctenoid and cycloid), found in teleosts , which is thin, light, flexible and composed of only two layers (an external fibrous layer and an internal bony layer). In fact, many teleosts and all the surviving chondrosteans (sturgeons and paddlefishes) have taken the final step and shed their scales entirely. ()

Branchiostegal rays: Branchiostegal rays developed from the bones at the base of the branchial cavity, and increased the efficiency with which water was pumped across the gills (“two pump” respiratory system) . The interopercular bones, which were formed by the modification of branchiostegal rays at the bottom of each gill cover (operculum), further improved the pumping efficiency by enlarging the opercular cavity, thus increasing the volume of water that could flow through the gills. Branchiostegal rays have also allowed actinopterygians to utilize suction feeding methods, rather than simply grabbing – a function equally significant as improved respiration. ()

Swimbladder: The swimbladder probably functioned as a lung in the earliest actinopterygians, but in more derived ray-finned fishes it is used primarily as a hydrostatic organ (to maintain buoyancy). With neutral buoyancy achieved by controlling the amount of gas in the swimbladder, the pectoral fins no longer needed to function as hydroplanes, so they evolved to aid in greater maneuverability. Finally, in some actinopterygians, such as gouramies (and others), the swimbladder is utilized for non-respiratory functions such as hearing and sound production. ()

Jaws: Jaws developed in conjunction with the branchiostegal rays and show a trend towards more flexibility. (See an illustration of jaw anatomy). Two major bones of the upper jaw, the maxilla, and the premaxilla, were previously firmly attached to the skull and had teeth. However, in recently derived actinopterygians, there are fewer attachments and teeth are rarely present. This has allowed for the upper jaw to extend, making it protrusible (dramatically illustrated by some wrasses), and permitted a variety of feeding specializations to develop, such as plankton straining (usually zooplankton). One result of increased flexibility of the upper jaw has been that processing could not easily occur at the rim of the mouth any longer. Therefore, many fish with protrusible jaws have a second set of jaws in the throat, termed pharyngeal jaws , that process food and free the outer jaw to continue feeding. ()

Tail: Changes in the lobes of the tail, from heterocercal (upper lobe longer than lower lobe) to homocercal (upper and lower lobes same size) in recently derived actinopterygians, are also related to the achievement of neutral buoyancy. Heterocercal tails (still found in sharks) provide lift, which is unnecessary for neutrally buoyant fish. Homocercal tails provide uniform thrust and allow for precise movements (each ray can be controlled individually), which are quite important for fast-swimming, pelagic fish and small, maneuverable fish, respectively. (See an illustration of locomotion in fish). ()

Fins: With the loss of heavy, armoring scales, actinopterygians developed spines, which are used as anti-predator devices when individuals are unable to use speed to escape (see Predation). The positioning of the pelvic and pectoral fins also changed along with the achievement of neutral buoyancy. Instead of having pelvic fins located well behind the pectoral fins, in more derived actinopterygians the pelvic fins are located just below or even slightly in front of the pectoral fins; here they aid in increased maneuverability instead of being used simply as stabilizers, as in earlier actinopterygians. (See an illustration of fin function). ()

The truly spectacular array of body forms within this class can only be appreciated by familiarizing oneself with some of the more than 25,000 species of actinopterygians – the largest and most diverse of all vertebrate classes – that exist today. Consider the fact that actinopterygians may fly, walk, or remain immobile (in addition to 'swimming'), exist in virtually all types of habitats except constantly dry land (though some can walk over land), feed on nearly every type of organic matter, utilize several types of sensory systems (including chemoreception, electroreception, magnetic reception and a “distance-touch” sensation – see Communication), and some even produce their own light or electricity. In addition, color diversity in ray-finned fishes is “essentially unlimited, ranging from uniformly dark black or red in many deepsea forms, to silvery in pelagic and water-column fishes, to countershaded in nearshore fishes of most littoral [near-shore] communities, to the strikingly contrasted colors of tropical freshwater and marine fishes” (Helfman et al. 1997:367). Of course, extravagant coloration is not helpful for fish at risk of being eaten, yet bright coloration is environment-specific (see Helfman et al. 1997:367) and bright colors at one depth are cryptic at others due to light attenuation (see Communication). Further, color change is common in brightly colored (as well as many other) fishes and occurs under a variety of circumstances. Pigments are responsible for a many types of color change, but there are also structural colors, resulting from light reflecting off of crystalline molecules housed in special chromatophores (cells located mainly in the outer layer of skin). The silvery sheen displayed by many pelagic fishes is an example of structural color. Numerous actinopterygians are also sexually dimorphic (males and females look different), and body form changes drastically during development, so there is significant diversity within, as well as among, species. ()

Among the largest actinopterygians are the pirarucu (also known as giant arapaima , up to 2.5m in length) in freshwater and the black marlin (up to 900kg) in saltwater; the longest is the oarfish , Lampris guttatus, which averages between 5 and 8m in length; and the smallest, a variety of diminutive gobies in saltwater and minnows , catfishes and characins in freshwater. At various points in this account, there is further discussion of physical characteristics as they relate to particular topics (i.e. Systematic/Taxonomic History, Communication, Food Habits and Predation), but for a technical description of actinopterygians, see below. (View an illustration of external fish parts or a fish skeleton). ()

Actinopterygians may have ganoid, cycloid, or ctenoid scales, or no scales at all in many groups. With the exception of Polypteriformes, the pectoral radials are attached to the scapulo-coracoid, a region of the pectoral girdle skeleton. (The pectoral radials are one of a series of endochondral - growing or developing within cartilage - bones in the pectoral and pelvic girdle on which the fin rays insert). Most have an interopercle and branchiostegal rays and the nostrils are positioned relatively high on the head. Finally, the spiracle (respiratory opening between the eye and the first gill slit – connects with the gill cavity) and gular plate (behind the chin and between the sides of the lower jaw) are usually absent, and internal nostrils are absent. ()

In general, five major developmental periods are recognized in fish: embryonic, larval, juvenile, adult, and senescent. Fish development is known for its confounding terminology, so there are many gray areas within these major categories, and, as with many other animals, many species tend to defy classification into discrete categories. For instance, species in several teleostean families bear live young (viviparous) – Poeciliidae, Scorpaenidae, and Embiotocidae (to name a few), and the young in some families (Salmonidae) seem to emerge as juveniles after hatching (externally) from the egg. ()

There are two important developmental characteristics that separate fish from most vertebrates: indeterminate growth (growing throughout life) and a larval stage. The fact that most fish (although there are always exceptions) are always growing means they constantly change in terms of anatomy, ecological requirements, and reproduction (i.e. larger size means larger clutches, more mates, better defense, etc. in most species). Increased age is also associated with better survivability, As physiological tolerances and sensitivity improve, familiarity with the local environment accrues, and behavior continues to develop. The larval stage is usually associated with a period of dispersal from the parental habitat. Also, the disappearance of the yolk sac (the beginning of the larval stage according to most researchers) marks a critical period in which most individuals die from starvation or predation. ()

Recently, researchers of coral reef fishes (mostly of the order Perciformes) have made significant advances concerning the life history of larvae. Nearly all bony coral reef fishes produce pelagic young (meaning they live in the water column for a period of time before settling on reefs), and the length of the stage is highly variable, from only a week in some damselfishes to greater than 64 weeks in some porcupine fishes . Initially, researchers made relatively simplistic assumptions about the pelagic phase, "portray[ing] larvae as little more than passive tracers of water movement that 'go with the flow,' doing nothing much until they bump into a reef by chance and settle at once" (Lies and McCormick 2002:171). Actually, the larvae of most coral reef fishes are endowed with good swimming abilities, good sensory systems, and sophisticated behavior that is quite flexible. And, while mortality rates are quite high at this stage (as with many other actinopterygian larvae), many larvae are able to detect predators at a considerable distance, and they are often transparent (usually larvae) or cryptically colored (many juveniles). ()

It is important to note that the young of reef fishes develop quite differently from most temperate fishes that have been studied. While the eggs of most temperate fishes hatch from 3 to 20 days after laying, the eggs of most coral reef species hatch within only a day. Also, at any given size, the larvae of reef fishes are more developed than most temperate, non-perciform fish: they have "more complete fins, develop scales at smaller size, [have] seemingly better sensory apparatus at any size, and are morphologically equipped for effective feeding within a few days of hatching" (173). Finally, the settling habitat for reef fishes (coral reefs) tends to be relatively fragmented and, therefore, much more difficult to locate, unlike the habitat of temperate fishes, which tends to have large expanses suitable for settling. This brief glimpse into the pelagic stage of reef fishes reveals the diversity and complexity of development in actinopterygians. ()

Not surprisingly, the lifespan of ray-finned fishes varies widely. In general, smaller fish have shorter lives and vice versa. For instance, many smaller species live for only a year or less, such as North American minnows in the genus Pimephales, a few galaxiids from Tasmania and New Zealand, Sundaland noodlefishes , a silverside , a stickleback , and a few gobies . However, researchers of coral reef fishes are beginning to find that this correlation does not hold for some families. While many people, especially in the business of fisheries, assumed short lifespans for many fish, researchers are starting to find that many live much longer than previously expected. For example, common species, such as the European perch (aka river perch) and largemouth bass can live 25 and 15 to 24 years respectively. Even more impressive, some sturgeons (which are severely threatened) can live between 80 to 150 years. Several species of rockfish (deepwater rockfish , silvergray rockfish and rougheye rockfish) live from 90 to 140 years! These long lifespans have quickly become a serious issue for some fisheries because populations can be decimated if individuals that naturally accumulate in older age classes are removed (see Conservation). ()

Many ray-finned fishes exhibit migratory behavior; daily migrations are usually related to feeding or predator avoidance while longer migrations are usually for reproduction purposes. Some fishes stay within saltwater (oceanodromous) or fresh water (potamodromous) their entire lives, while others migrate between the salt and fresh water as part of their life cycle (e.g. to reproduce) or to feed (diadromous). Diadromous species can be broken down into three types: those in which growth occurs primarily in saltwater but move into freshwater to spawn (termed anadromous) – e.g. salmon; those in which growth occurs primarily in freshwater but move into saltwater to spawn (termed catadromous) – e.g. anguillid eels; and those that migrate between salt and fresh water for purposes other than spawning, such as feeding (termed amphidromous) – e.g. various gobies , sleepers and galaxiids. While many ray-finned fishes migrate well outside their home range – in many cases hundreds of kilometers, against current and even up waterfalls – they have remarkable abilities to find their way back. For instance, salmon can remember the odor of the rivers they originated from, as well as the odor of other rivers they have passed during migration. In addition, salmon (among other actinopterygians) use currents, salinity and temperature gradients, and topographic cues (buoys or islands) for orientation. Tidepool sculpins separated from their home pool by 100 m can also find their way back using olfactory and visual cues. While younger fish rely on visual or olfactory cues, some older fish, even if removed from their original locale for multiple years, only require visual cues, utilizing a cognitive map to navigate. ()

When ray-finned fishes group together, either for spawning migration, feeding or protection, they sometimes form shoals. While in some cases fishes simply form aggregates (no social interaction but a mutual attraction to resources), shoaling represents a continuum of fascinating social behaviors. Schooling, in which individuals form a synchronized, polarized group, is actually an extreme form of shoaling and represents one of many types of shoal formation. The formation changes shape depending on whether the group is resting, foraging, traveling, spawning or avoiding predators. Approximately 25 percent of fishes shoal throughout life (e.g. herrings , anchovies , minnows , silversides) and about half form shoals at some point during their lifetime. ()

Another common characteristic of ray-finned fishes is aggressive behavior, which results from competition for valuable resources, such as feeding, refuge and mating territories, mates, eggs, and young. One form of aggressive behavior is dominance hierarchies, which are found in many groups (e.g. catfishes , minnows , cods , ricefishes , topminnows , cichlids , wrasses , blennies , and boxfishes). The hierarchy is determined through a variety of factors, including size, sex, age, previous residency, and previous experience. In most actinopterygian species males dominate females, subordinate individuals are relegated to suboptimal sites in terms of cover availability, current velocity and prey densities, and dominant individuals have favorable habitats, higher feeding rates and tend to remain dominant. Another aggressive behavior is territoriality, which is found in numerous ray-finned fishes and spread across a wide variety of groups, such as freshwater eels , cyprinids , knifefishes , salmonidsfrogfishes , rockfishes , sculpins , sunfishes and black basses , butterflyfishes , cichlids , damselfishes , barracuda , blennies , gobies , surgeonfishes and labyrinthfishes. Territorial interactions primarily occur along territorial boundaries and usually involve displays, vocalizations, chasing, and biting as a last resort. As with dominance hierarchies, prior experience, previous residency, and individual size are all important in determining the outcome of an altercation. (Behavioral characteristics relating directly to Reproduction, Food Habits, defense (Predation), or Ecosystem Roles can be found in their respective sections). ()

Ray-finned fishes perceive the external environment in five major ways – vision, mechanoreception, chemoreception, electroreception and magnetic reception, and to humans several of these sensory systems are entirely alien. Many types of perception are also used by ray-finned fishes to communicate with individuals of the same (conspecifics) or other species (heterospecifics). ()

Vision is the most important means of communication and foraging for many ray-finned fishes. The eyes of fish are very similar to terrestrial vertebrates so they are able to recognize a broad range of wavelengths. A species’ ability to perceive various wavelengths corresponds to the depth at which it lives since different wavelengths attenuate (become weaker) with depth. In addition to the normal spectrum perceived by most vertebrates, several shallow-water species are able to see ultraviolet light; others, such as anchovies , cyprinids , salmonids and cichlids , can even detect polarized light! Many fishes also have specially modified eyes adapted for sight in light-poor environments and even outside of water (e.g. mudskippers). For example, several families of deepsea fishes (deepsea hatchetfishes , pearleyes , giganturids , barreleyes) have elongate (long and narrow), upward-pointing, tubular eyes that enhance light gathering and binocular vision, providing better depth perception. Also, several deepwater, midwater and a few shallow species actually have internally generated lights around the eyes to find and attract prey and communicate with other species (see below). Light is usually produced in two ways: by special glandular cells embedded in the skin or by harnessing cultures of symbiotic luminous bacteria in special organs. ()

One way fishes communicate visually is simply through their static color pattern and body form. For instance, juveniles progress through a range of color and shape patterns as they mature, and sexes are often colored differently (sexual dimorphism). In addition, some fishes are quite good at identifying other species; the Beau Gregory damselfish is apparently able to distinguish 50 different reef fish species that occur within its territory. A second way fishes communicate visually is through dynamic display, which involves color change and rapid, often highly stereotyped movements of the body, fins, operculae, and mouth. Such displays are often associated with changes in behavioral state, such as aggressive interactions, breeding interactions, pursuit and defense. A third form of visual communication is light production, found among numerous fishes in deepsea habitats. Midwater species, such as lanternfishes , hatchetfishes and dragonfishes have rows of lights along the underside of the body, probably for mating and identification as well as foraging. Even some shallow-water species, such as pineconefishes , cardinalfishes and flashlight fish (family Anomalopidae) of the Red Sea utilize internal light sources to form nighttime feeding shoals or for other behavioral interactions. ()

Mechanoreception includes equilibrium and balance, hearing, tactile sensation, and a ‘distance-touch-sense’ provided by the lateral line (Wheeler, Alwyne 1985:viii). Detecting sound in water can be difficult because waves pass through objects of similar density. Therefore, ray-finned fishes have otoliths, which have greater density than the rest of the fish, in the inner ear attached to sensory hair cells. Since gas bubbles increase sensitivity to sound, many ray-finned fish (e.g. herrings , elephantfishes and squirrelfishes) have modified gas bladders and swimbladders adjacent to the inner ear. Most ray-finned fishes have keen hearing ability and sound production is common but not universal. In groups that do utilize sound for communication, the most common purpose is territorial defense (e.g. damselfishes and European croakers) or prey defense (e.g. herrings , characins , catfishes , cods , squirrelfishes and porcupinefishes). Sound production is also used in mating (for attraction, arousal, approach or coordination) and communication between shoal mates. Stridulation, which involves rubbing together hard surfaces such as teeth (e.g. filefishes) or fins (e.g. sea catfishes), or the vibration of muscles (e.g. drums), is the most common way sound is produced. Often the latter structures have a muscular connection to the swimbladder to amplify sound. Accordingly, the swimbladder itself is the source of the most complex forms of sound production in many groups (e.g. toadfishes ,