Osteictes: The Bone Fish

Bone fish are the largest group (corresponding to 9 out of 10 species) and diverse of current fish.

These animals inhabit all types of water, fresh, brackish, salty, hot or cold (although most are limited to temperatures between 9 and 11ºC). This is the latest class from a phylogenetic point of view, as well as the most evolved class. The taxonomy within this class has often been altered due to the discovery of new species as well as new relationships between known ones.

Typically bony fish are no larger than 1 m in length but there are small (some gobies are only 10 mm long) and gigantic (3.70 m swordfish, 3.80 m and 590 kg weight sturgeon) or fish 900 kg weight).

They have adapted to live in sometimes difficult conditions such as high altitude lakes, polar zones, hydrothermal vents, high salinity or oxygen poor ponds, etc.

Many fish make periodic migrations from place to place or from deep water to the surface for both spawning and feeding.

Its main features include a body, taller than broad and oval in silhouette, which makes it easy to move through the water.

The head extends from the tip of the muzzle to the opening of the operculum, the trunk from there to the anus, behind which is the tail. The body has a strong segmental musculature - myomers -, separated by delicate connective septa.

The skeleton is made up of true bones, although some species may have cartilage bones (sturgeon, for example) with numerous distinct vertebrae, although notochord persistence in intervertebral spaces is frequent.

The skeleton has 3 main parts: spine, skull and fin rays. From the spine, the ribs and pectoral girdle start (there is no pelvic girdle, and these fins are connected by tendons, without connection to the spine). Numerous other small bones support the rays of the fins.

The skull is articulated with the well-developed jaws and jaws and supports the gill arches. The articulation of the skull with the spine is so strong that fish cannot turn their heads. The tail is usually homocerca.

The skin covers the entire body and contains numerous mucous glands, whose secretion facilitates gliding through water and protects against infections, and is covered in the trunk and tail. Scales can come in many forms, but they are always of dermal origin. Some species have no scales or they may be coated with enamel.

At Scales they are thin, rounded and set in longitudinal and diagonal rows, interwoven like the tiles of a roof. The free ends of the scales are covered by a thin layer of skin that protects against parasites and disease. In some species, this layer of skin helps maintain moisture when the animal is emerged.

Each scale is fixed in a dermal pouch and grows over the life of the animal, which usually gives rise to growth rings (larger in summer and very small in winter). These rings are most noticeable in temperate fish. Because the pattern of distribution, shape, structure and number of scales is almost constant in each species, this is an important systematic feature of this class.

The fins are supported by bony or sometimes cartilage rays. Odd fins include two dorsal and one anal, as well as symmetrical tail fin. The shape of the caudal fin conditions the shape of the animal's movement: rounded fins increase maneuverability but generally speed is slow, while forked or sickle-shaped fins allow for high speeds. The dorsal fin has skeletal support and varies greatly in shape according to the animal's habits. The even fins are the pectoral fins just behind the operculum and the pelvic fins. Each fin has its own set of muscles, which allows for independent movement, increasing maneuverability.

Unlike cartilage fish, and due to the presence of a swim bladder, bony fish do not need their fins to keep floating, only using them to maneuver in water.

Nervous system

It includes a distinct brain and developed sense organs, namely:

  • Eyes - Large, lateral and without eyelids, probably only able to focus precisely on nearby objects, but easily perceive distant movements, including above the water surface. The retina contains cones and rods, which allows color vision in most cases;

  • Ears - With three semicircular canals arranged perpendicular to each other (functioning as a balancing organ, thus, as in all upper vertebrates), they allow for accurate hearing, because the sound propagates quite well in water. Many fish communicate with each other by making sounds, either by rubbing body parts with each other or with their swim bladder;

  • Nostrils - located in the dorsal part of the muzzle, communicate with a cavity covered with cells sensitive to molecules dissolved in water;

  • Later linel - Located longitudinally along the flank of the animal, it is composed of a row of small pores, in communication with a channel below the scales, where are mechanoreceptors. The effectiveness of this system to detect movements and vibrations caused by it in water allows the formation of shoals, fundamental as a defense strategy of these animals.

Digestive system

It has a large mouth in terminal position, surrounded by distinct jaws and jaws, where tapered and thin teeth are implanted. There are other teeth, located in the first gill arches, useful for gripping and grinding food. In the mouth there is also a small tongue, attached to the floor of the cavity and that helps in breathing movements.

It has a heart with two cavities (atrium and ventricle) through which only venous blood circulates. Blood is pale and scarce compared to a terrestrial vertebrate.

Continues after advertising

Respiratory system

It typically has comb-shaped gills, supported by bony or cartilage gill arches and located within a common chamber on either side of the pharynx. This chamber is covered by an operculum, thin and with free margins below and behind. Gill arches have expansions that protect the gill filaments from hard particles and prevent the passage of food through the gill slits. In gills there is a countercurrent mechanism between water and blood that irrigates them, increasing the efficiency of gas exchange.

There is usually swimming bladdera large, thin-walled, irrigated sac derived from the anterior intestine that occupies the dorsal zone of the body cavity. This cavity is filled with gases (O2, N2, CO2), acting as a hydrostatic organ, adjusting the body weight of the fish according to the depth. The adjustment is made by secretion or absorption of gases into the blood.

The capacity of the swimming bladder is higher in freshwater fish because it is less dense than saltwater and cannot support the fish as easily. The swimming bladder can help with breathing (lung fish) or as a resonating box for sense organs or sound production.

Excretory system

It is formed by mesonephric kidneys.

Reproductive system

The sexes are separated, each individual presenting generally even gonads. The vast majority are oviparous with external fertilization, although there are species with internal fertilization and hermaphrodites.

Some species undergo sex changes, with males becoming larger in size and females becoming dominant in schools as they become males. The eggs are small and without embryonic attachments but with a very variable amount of calf. Deep-sea species produce huge amounts of eggs, since most do not survive, which become part of plankton, while coastal species place them between debris and leaves or at the bottom. Some species take care of eggs and / or juveniles, guarding their nests and keeping them oxygenated with water spurts. Others incubate their eggs in their mouths or allow young people to gather there when threatened.

Several species migrate large distances (either from saltwater to freshwater, such as some salmon species, or the reverse, such as eels) to spawn.

Bone fish are the only ones that form shoals, sometimes with tens of thousands of individuals. In schools the fish move synchronously, as if they were one. Each fish runs parallel to its neighbor at a distance of about one body length and maintains its position due to the action of sight, hearing and lateral line. The silver color of most schooling fish is critical as it helps to detect each other's movements (a slight change in direction produces a big difference in reflected light).

In a shoal the fish are safer because there are more senses aware of a predatory potential and it becomes more difficult to choose prey among so many moving bodies. Group life also helps to find food and sexual partners.