Cartilaginous Fish: Habitat, Types And Characteristics
Belonging to the class Chondrichthyes , cartilaginous fish are animals that have cartilage skeletons. This class includes sharks, rays and chimeras. These taxa have an ancient evolutionary history, since they appeared more than 450 million years ago.
The Chondrichthyes are mostly predators. Due to this, they have an important ecological role, by controlling the populations of their prey. Its meat provides a large amount of protein, fat and vitamin A, which is why they are used for human consumption. If you want to know more about these species, keep reading. You will surely be amazed.
Types and characteristics of cartilaginous fish
Cartilaginous fish, also called chondrichthyans , are characterized by having a cartilage skeleton. In addition, their skin is covered by placoid scales, which closely resemble a vertebrate tooth. That is why these organisms have a rough texture to the touch.
Its mouth contains numerous partially calcified teeth. These are not fused to the jaw, so they also have several spare parts. These fish carry 2 nostrils, a blowhole, and 5 to 7 gill openings. Their eyes do not have eyelids like those of the human, but they have a light and transparent membrane, called nictitante , which fulfills the same function.
Another main characteristic is the presence of pelvic fins in the lower part of their body, which serve as reproductive organs for these fish. These fins, also called claspers , are modified to be able to deposit the gametes (sperm) inside the female. That is why only the males present them, but it is a characteristic of the whole group.
According to the UNAM Institute of Biology , the group of chondrichthyans contains about 900 species. This taxon is divided into 2 subclasses: the elasmobranchs (sharks and rays) and the holocephalus (chimeras). We tell you its peculiarities.
Holocephalos (chimeras)
These fish are common inhabitants of deep waters at the bottom of the oceans. In addition, they have a jaw attached to the skull and their teeth are divided into 3 pairs of dental plates, which grow slowly and continuously without changing. The first fossil representative of this group is Callorhinchus , from Germany, dating from the Middle Jurassic.
Elasmobranchs (sharks and rays)
Members of this subclass have torpedo- shaped (fusiform) bodies, flattened on the sides. Although they have eyes, their sight is not very good, so they rely on their smell to detect their prey. The olfactory bulbs, the areas of the brain that are responsible for processing information from smell, are highly developed.
Elasmobranchs also have a sense of electrosensitivity, with which they can detect variations in low-frequency electrical stimuli. They do this by means of Lorenzini’s ampoules, which serve them for orientation by electric fields and to detect bioelectric fields from their prey.
The jaws of this group are mobile, since they are suspended by a cartilage that joins them to the skull. This allows the lower jaw to project outward, allowing it to catch its prey.
How do cartilaginous fish swim?
Cartilaginous fish need constant swimming for the water to pass through the gill slits and for them to breathe. For this reason, they have developed different swimming mechanics: wave propulsion and appendage propulsion.
The first mechanism refers to the use of its tail, with oscillating movements, which allow the fish to move forward. It is used by most sharks, because their pectoral fins only allow them to maintain stability, but they do not have flexibility.
Meanwhile, the second mechanism involves the use of pectoral fins or lateral appendages. This locomotive adaptation is used by stingrays and rays, which raise and lower their appendages to be able to propel themselves in the water, something similar to “flying”, but under water.
All species have one or both swimming mechanisms. This will depend on the body structure of each of the cartilaginous fish, since the shape of the body affects the locomotion and buoyancy of the body.
These fish are machines optimized for swimming, since their modifications give them a great hydrodynamic capacity. Having a (lightweight) cartilage skeleton improves buoyancy and the use of scales reduces water turbulence. In addition, these species do not have a swim bladder, so their liver fulfills this function.
Reproduction of cartilaginous fish
Chondrichthyans have an internal fertilization process, but they are capable of presenting all 3 types of reproduction: viviparous, oviparous and ovoviviparous. We tell you about each of these strategies below.
Ovoviviparous
This mode of reproduction is at an intermediate point of oviparity and viviparity, because the development of the embryo takes place within an egg that is retained by the mother. When the egg hatches, the mother gives birth to fully developed young.
Viviparous
It is the type of reproduction in which the mother provides nutrients to her young through the placenta. With this, the female maintains the health and growth of her son. Some examples of viviparous species are the hammerhead shark ( Sphyrnidae sp. ) And the blue shark ( Prionace glauca ).
Oviparous
Once mating is over, females deposit hard capsules on the ground, or they tie them to rocks or algae. The embryos use the yolk of the egg – yolk sac – to feed and develop until hatching.
Habitat of cartilaginous fish
These fish can have both marine and freshwater habitats. They are usually found in tropical coral reefs, deep ocean trenches, in rivers or in areas where rivers converge with the sea (estuaries).
Although they present ancestral and conserved characteristics, the chondrichthyans have had to adapt to their environment. That is why the evolutionary process has enhanced certain characteristics, such as their smell and mechanical perception, to ensure the capture of their prey.
Evolution is a process that promotes the conservation and improvement of the characteristics that most benefit the species. For this reason, although a bone skeleton is more resistant, in some cases the cartilaginous skeleton is preferred for its flexibility and lightness. Sharks, rays, and chimeras are living examples of this.
