According to the latest scientific data, the ancestors of modern fish - jawless animals resembling their appearance - lived in early Cambrian, about 530 million years ago. It is thought that such creatures, found in 1999 in Yunnan, may be the progenitors of all vertebrates.
At the moment, cartilaginous fish, bone fish (lobe-fin and ray-finned) make up more than half of all vertebrates living on the planet. In total, there are about 31 thousand species of the most diverse forms, sizes, living in salt and fresh water bodies. The study of ancient creatures is a separate science - ichthyology. Let us dwell in more detail on the classes, their features and differences.
Cartilaginous fish
The main feature of all members of the class is that their skeleton consists of cartilage, which over time as a result of the deposition of minerals can become quite hard. For this reason, they were previously considered prehistoric animals. However, many of them are characterized by live births, sometimes even with the formation of a bile placenta - this is how cartilaginous fish differ radically from bone fish.
In addition, they have several more anatomical structural features. Firstly, the absence of a swimming bladder. Therefore, they have to move in order to sink to the bottom of the reservoir. Secondly, cartilaginous fish lack gill covers, and the gills open outward with characteristic gaps. Thirdly, they are all covered with placoid scales, which are similar to the teeth of vertebrates. It consists of dentin and a layer of enamel covering it. Such scales do not recover with loss, however, with the growth of fish, its amount increases.
Cartilage fish life support systems
The characteristics of cartilaginous and bone fish will not be complete if we do not mention the basic systems: circulatory, digestive and reproductive, in which differences are observed. Cartilage has red blood (due to the presence of hemoglobin and red blood cells), which is produced by the spleen. The circulatory system itself in structure resembles that of cyclostomes. Along the spine stretch in the form of two dark red stripes of the kidney. The intestines of cartilaginous fish consists of three departments, these are: large and small intestines, rectum. Well developed liver, pancreas. But the main thing is that the classes of Cartilaginous fish and Bone fish differ in the structure of the reproductive system. The first is characterized by internal fertilization with the formation of an egg, which can be laid in the external environment or remain in the lower part of the oviduct. In the second case, an embryo begins to develop in the maternal body.
Cartilage fish classification
All of the existing members of the class Cartilaginous fish are divided into three superorders.
- Stingrays - cartilaginous fish (bone fish - hereinafter referred to as the text) with a characteristic "flattened" body shape and large pectoral fins fused with the head. Many of them have reduced scales. One of the largest representatives is the manta ray.
- Sharks. Fish possessing such distinctive features: a torpedo-shaped body, elongated, heterocercal large caudal fin and a large number of teeth on the jaws. Some of the oldest representatives on Earth, whose ancestors appeared 420-450 million years ago. Sizes range from 17 cm to 20 m (whale shark - pictured above), most predators. Each species has a certain life expectancy, but for the most part they differ in longevity - 20-30 years.
- Whole-headed - represented by only one detachment - Chimeras, including about 50 mainly deep-sea fish species. This explains their little knowledge.
Bone fish: general description
For a long time, up to the 21st century, cartilaginous fish, bone fish were considered as two classes. However, in the scientific community, a different point of view is becoming more widespread. For example, the Canadian zoologist in his work identifies lobedate and ray-finned fish in separate classes, and bone, respectively, in the superclass. These are the most diverse inhabitants of all types of reservoirs. Their mouth is formed by grasping jaws and teeth located on them, the gills are located on the branchial arches, and the nostrils are paired.
Differences from Cartilaginous Fish
The most important difference between bone fish and cartilaginous is clear from the name - the skeleton. He is made of bones. In the internal cavity are located circulatory system, excretion, reproduction and digestion. Scales are also characteristic, of one of three types: cycloid, ctenoid or ganoid.
The next difference is the presence of a swimming bladder located under the spine and filled with gases that secrete blood vessels. With an increase in its volume, the fish easily floats to the surface, with a decrease, it is sent to the depth.
Differences have not only external signs of cartilage and bone fish, but also reproductive organs, as already mentioned. Most of the representatives of the second group are characterized by external fertilization, which occurs in the aquatic environment. This process is called spawning, it occurs at certain times and is accompanied by characteristic behavior.
Rayfin fish
This is the numerically predominant class in the modern variety of fish; they number more than 20 thousand species, which is about 95%. They inhabit all corners of the planet, from the Arctic seas to the hot equator, sizes range from 8 mm to 11 meters, and the weight of individual individuals reaches more than two tons. The name, as you might guess, is associated with the structure of paired fins, in which there is no basal axis. The class, in turn, is divided into two groups: New feathers (the most prosperous species) and Bone-cartilaginous fish. The structure of the latter has distinctive features. They have a swimming bladder, but at the same time, their skeleton consists mainly of cartilage. The chord has only cartilaginous arches and is not dissected; vertebral bodies as such are absent. A distinctive feature is the rostrum and lower mouth. Many of them are commercial, in particular sturgeon (the photo below shows the catch of the beluga).
Lobed fish
A small class of fish based on the skeleton of which is an elastic chord. They combine progressive and archaic features, all representatives belong to two modern superorders - Kistepery and Lung-breathing. Both groups unite ancient fish. Breathless live in freshwater bodies of Australia, South America and Africa. They possess not only gills, but also lungs. This allows them to do without water for some time and feel free in oxygen-depleted ponds. In total, 6 species are known: four African protopters (photo below), Australian horned tooth and South American flake.
Squad
It is considered almost extinct. Only one genus has survived to our days - Latimeria (pictured below), which has two species. Moreover, both of them were discovered relatively recently, the first specimen was caught in the Indian Ocean in 1938. It is believed that bristled fish are inhabitants of fresh water bodies in which oxygen was lacking. In this regard, they developed musculature at the base of the fins and a double way of breathing (lungs and gills). This subsequently allowed some to move back to the seas, and freshwater eventually became extinct. There is an assumption that it is the brush-headed fish that gave rise to the Amphibian class.
Thus, cartilaginous fish, bone fish have a number of individual characteristics. The main ones are observed in the structure of the skeleton (cartilage or bone), the presence or absence of a swimming bladder, type of scales, reproductive system and method of reproduction.