1. One of the smallest adult bony fishes is the dwarf pygmy goby (Pandaka pygmaea), a freshwater fish of the Philippines that reaches a length of only 8 mm (0.3 in.) at maturity (Bond, 1979).
2. One of the largest bony fishes is the common mola (Mola mola), which lives throughout warm and temperate seas worldwide. A large mola can reach 3.7 m (1 3 ft.) and 1,500 kg (3,307 lb.) (Miller, 1972).
Many sturgeons (family Acipenseridae) grow large. The largest is the beluga sturgeon (Huso huso), which inhabits the Caspian, Black, and Adriatic Seas. It reaches 5 m (16.4 ft.), and very large specimens have been weighed and documented at 1,220 kg (2,689 lb.). The largest American sturgeon is Acipenser transmontanus, which can reach at least 4.6 m (15 ft.). The record weight is 816 kg (1,799 lb.), but few specimens that have been taken in the 1900s have exceeded 136 kg (300 lb.), probably a result of increased fishing pressures (Wheeler, 1975).
1 . Most bony fishes have a fusiform (rounded and tapering at both ends) body shape. This body shape reduces drag and requires a minimum amount of energy to swim.
2. Bony fish forms deviate from the fusiform body shape in three directions: compression, depression, and elongation.
a. A laterally compressed body shape is common in bony fishes that live in dense cover or within coral reefs. The compressed body is laterally flattened. Butterflyfishes (family Chaetodontidae) are good examples of bony fishes with a laterally compressed body shape.
Butterflyfishes (family Chaetodontidae) show a laterally compressed body shape.
b. A depressed body shape is common in bottom-dwelling bony fishes. Goosefishes (family Lophiidae) and batfishes (family Ogcocephalidae) are good examples of bony fishes with a depressed body shape.
c. Elongated body shapes are common in open ocean fishes and eels. The elongated body is horizontally lengthened. The morays (family Muraenidae) are good examples of fishes with an elongated body shape.
Most bony fishes have a fusiform body shape. Others may have a compressed, depressed, or elongated body shape.
1. Most fish species have pigmentation.
a. Pigment is mostly contained in cells called chromatophores. Most fishes can contract and expand their chromatophores to change colors.
b. Reflective cells called iridocytes can change color rapidly.
c. Species such as cave-dwelling fishes (family Amblyopsidae) lack pigmentation.
Some species of bony fishes, such as this ghost glass caff ish (Kryptopterus bicirrhus), lack pigmentation.
2. Since different wavelengths of light are absorbed at different depths, fishes may appear a different color underwater than they do at the surface.
3. Coloration may serve as camouflage.
a. Most species of fishes are countershaded. Countershading is a type of camouflage in which the dorsal (top) side is darker than the ventral (bottom) side. The dorsal side of a countershaded fish blends in with the dark ocean depths or ocean bottom when viewed from above. The ventral side blends in with the lighter surface of the sea when viewed from below. Under ideal circumstances, neither the fish's predators nor the fish's prey can differentiate between the countershaded animal and the environment.
b. The coloration of some fishes helps them blend in with their environment. The northern pike (Esox lucius) has coloration that provides camouflage in weedy areas, making it a more successful predator.
c. Some fishes show disruptive coloration, a camouflage in which the color pattern of the animal contradicts the animal's body shape.
4. In some species of bony fishes, coloration serves as advertisement to other fishes.
a. Some fishes rely on coloration for species recognition and sexual distinction. The stoplight parrotfish (Sparisoma viride) female and male are completely different colors, although they are similar in shape and size. Some species of fishes become brighter in color during breeding season to attract potential mates.
b. In some species, coloration may trigger territorial behavior. After establishing a territory, the male stickleback's (family Gasterosteidae) belly turns red. He then actively defends his territory only from other fish with red bellies, notably other male sticklebacks.
c. A garibaldi's (Hypsypops rubicundus) bright orange color warns other fishes that the garibaldi will defend its territory.
5. Coloration may change.
a. In some species, coloration changes with age or sex change. Some species show completely different coloration and markings as they grow from juveniles to adults.
b. Some color change may be rapid and temporary. Alarmed fish, for instance, often change color. Some bottom-dwelling fishes change color almost instantly to match the substrate.
a. Some marine species of bony fishes produce light through a chemical reaction process called bioluminescence. Bioluminescence may be used to attract mates, to deter or confuse predators, or to attract prey (Lagler, 1962).
b. Light production may occur through photophores, luminescent cells on the fish, or by light-producing bacteria that live in or on the fish in a symbiotic relationship.
1. All fishes have fins. Various bony fish families show various degrees of fin fusion and reduction.
2. Fins help stabilize or propel the fish in the water.
3. Except in the lungfishes and the coelacanth, fins lack bones. In Actinopterygians, fins are supported by structures called rays.
a. Some bony fishes have soft, flexible fin rays.
b. Other bony fishes have spiny, rigid fin rays at the leading edges of the dorsal, anal, and pelvic fins.
c. Both soft and spiny fin rays are modified scales.
d. The spiny fin rays of some species are associated with venom glands. Fishes in the family Scorpaenidae include the stonefish (Synanceja spp.), the lionfish (Pterois spp.), and the scorpionfish (Scorpaena spp.)-some of the most venomous fishes in the world. Glands in the dorsal, anal, and pelvic spines produce a venom that is intensely painful and is occasionally fatal to humans. There are several records of human deaths due to stonefish stings.
4. Fishes have two kinds of fins: paired fins (pectoral and pelvic) and median fins (dorsal, caudal, and anal). a. The paired pectoral fins are usually responsible for turning. In some fishes, pectoral fins are adapted for other functions.
(1) Some bony fishes, such as the hawkfishes (Cirrhitichthys spp.), use their pectoral fins to help them stay at the bottom and on reef areas. Mudskippers (family Periophthalmidae) support themselves on land with their pectoral fins.
(2) The pectoral fins of flying fishes (family Exocoetidae) are extremely long, an adaptation that allows flying fish to remain airborne for as long as 20 seconds and glide a distance of 150 meters (492 ft.) or more (Bond, 1979).
(3) Some bottom-dwelling fishes such as threadfins (family Polynemidae) have taste buds and touch receptors on their pectoral fins to locate food.
(4) For some fishes, such as wrasses (family Labridae), pectoral fins are the main source of power for swimming.
b. Paired pelvic fins add stability and are used for slowing some bony fishes. In the clingfishes (family Gobiesocidae), the pelvic fins have modified into a sucking appendage, which help the fishes hold on to stationary objects on the ocean bottom.
c. The dorsal fin may be a single fin or separated into several fins. In most bony fishes, the dorsal fin is used for sudden direction changes and acts as a "keel" to keep the fish stable in the water. In some fishes, the dorsal fin is adapted for other functions.
(1) In the anglerfishes (order Lophiiformes), the dorsal fin is modified into a lure that attracts prey.
(2) The dorsal fin of remoras (family Echeneidae) is modified into a sucking disc. Remoras cling to large fishes and mammals with this dorsal disc and are carried along as hitchhikers.
A remora's dorsal fin is modified into a sucking disc.
(3) An African knifefish (Gymnarchus niloticus) undulates its dorsal fin to move forward or backward.
d. The caudal fin, or tail, is responsible for propulsion in most bony fishes. Caudal fins come in many shapes: rounded, truncate, forked, and lunate.
(1) Fishes with truncate or rounded caudal fins are usually strong, but slow, swimmers.
(2) Many continuously swimming fishes have forked caudal fins.
(3) Fishes with lunate caudal fins tend to be the fastest fishes and maintain a rapid speed for long durations.
e. The anal fin adds stability. In some fishes, the anal fin is adapted for other functions.
(1) The black ghost knifefish (Apteronotus albifrons) undulates its anal fin as a means of propulsion.
(2) In some bony fishes, the anal fin plays a role in reproduction. The anal fin may fan sperm over eggs, or may concentrate sperm into a particular area.
5. Some species of bony fishes have reduced or absent fins. For example, morays (family Muraenidae) lack pectoral fins and pelvic fins, and several species lack an anal fin.
a. Eye size and position vary depending on the particular habitat and behavior of the species. In general, deep-water fishes have large eyes, allowing them to absorb as much light as possible in their dark environments. Shallow-water fishes generally have smaller eyes.
b. The pupils of some species of bony fishes, such as eels, contract and dilate depending on how much light is needed. In most species of bony fishes, however, pupils can't contract or dilate.
c. Flounders and other bony fishes in the order Pleuronectiformes have one eye that migrates across the top of their skull during development to lie adjacent to the eye on the other side.
d. The four-eyed fishes (family Anablepidae) have hourglass-shaped pupils that lie at the waterline as the fish swims at the surface. Four-eyed fishes can focus on images above and below the water simultaneously (Lagier, 1962). The half of the pupil above the waterline focuses on images above the water, while the half of the pupil below the waterline focuses on images under water.
a. Most bony fishes have a single pair of gill openings. In most bony fishes, the gill openings are covered by a flexible plate called an operculum that protects the gills. Oxygenated water enters through the fish's mouth and is forced over the gills. It exits behind the operculum.
b. Some species of bony fishes such as eels (family Anguillidae) have a pair of gill holes or pores that aren't covered by an operculum.
The nostrils of most bony fishes have no internal connections with the oral cavity. In some bony fishes such as eels, the nostrils' incurrent and excurrent openings are widely separated.
a. Mouth shape and size are good indications of a bony fish's feeding habits. Most bony fishes have mouths at the front end of the head.
(1) Some bottom-feeding species have mouths on the underside of the snout, angled toward the bottom.
(2) Some surface-feeding species have mouths that angle upwards.
(3) Butterflyfishes (family Chaetodontidae) have thin snouts and small mouths that are useful in reaching food located in crevices and cracks.
b. Some species of bony fishes, like the goatfishes (family Mullidae), have fleshy barbers that fringe the mouth. These barbers are used to detect food.
Bony fishes have separate incurrent and excurrent nostril openings. Note the prominent nostrils of this Queensland grouper (Epinephelus lanceolatus).
1 . Most species of bony fishes are covered with and protected by a called scales. Some bony fishes may have scales only on portions of their body, and some species have no scales.
2. Scales lay head to tail in all bony fishes. This helps reduce drag.
3. There are four different kinds of bony fish scales: cosmoid, ganoid, cycloid, and ctenoid.
a. True cosmoid scales are found only on extinct Crossopterygians. The inner layer of a cosmoid scale is compact bone. On top of this bone layer lies a spongy layer and then a layer of dentinelike material called cosmine. The upper surface is enamel. The coelacanth has modified cosmoid scales, which are thinner than true cosmoid scales.
b. Ganoid scales are found on bichirs and reedfishes (family Polypteridae) and gars (family Lepisosteidae). Ganoid scales are similar to cosmoid scales, but a layer of ganoin lies over the cosmine layer and under the enamel. Ganoid scales are diamond-shaped, shiny, and hard.
c. Most bony fishes have cycloid or ctenoid scales. Both cycloid and ctenoid scal consist of an outer layer of calcium and an inner layer of connective tissue. Cycloid and ctenoid scales overlap from head to tail.
(1) As they grow, cycloid and ctenoid scales add concentric layers.
(2) Cycloid scales are circular and smooth. They are most common on fish with soft fin rays.
(3) Ctenoid scales have a characteristic toothed edge. They are most common on fishes with spiny fin rays.
1 . Protective spines are common in slow-swimming bony fishes and bony fishes that need to protect themselves without moving.
2. Body spines are modified scales.
a. Most surgeonfishes (family Acanthuridae) have mobile, razor-sharp precaudal fin spines that they use to protect themselves.
b. The triggerfishes (family Balistidae) have three dorsal spines that lock together These spines may allow a triggerfish to securely lodge itself between rocks and keep predators from swallowing it.
c. Some pufferfishes (family Tetraodontidae) have spines that cover the entire body. The spines lay flat until the pufferfish inflates its body.
1. As do other classes of fishes, bony fishes secrete a layer of mucus that covers the entire body.
2. Mucus helps protect a fish from infection.
3. In some bony fishes, mucus may serve additional functions.
a. Some species of parrotfishes (family Scaridae) envelop their bodies in mucus bubbles at night while they rest. This may offer a form of protection for the parrotfish. Nocturnal predators that rely on their sense of smell to locate prey may not be able to detect the parrotfish.
b. Young discus (Symphysodon discus) feed on mucus on the skin of the parents.
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