These types of fishes arose at the late Silurian but they really peaked at the Devonian stage. There are three types which are:
- Placoderms – extinct
- Chondricthyes – Sharks and the rays, Chimeras. The first fossils were found in the late Silurian stage and more were found during Devonian to Mesozoic. The decline occured at the Paleozoic stage.
- Osteichthyes – Bony fishes
So now lets focus on 2 and 3!
CLASS CHONDRICHTHYES
The fishes in this class are mainly marine with a cartilaginous skeleton (secondarily derived and evolved from fish with bone and then it lost it). They also have very powerful jaws.
Subclass Elasmobranchii
The sharks and the ray are in this subclass with around 937 species. They are mostly marine but they have a diversity of sizes, habitats and ecological niches.
BODY PLAN
So we already discussed they have a cartilaginous skeleton, but they also have specialized fins! There are paired pectoral and pelvic fins with skeletal support to provide better muscle attachment. They also have a heterocercal tail which is when the vertebrae extend into the upper lobe of the tail, and this makes it longer.
There are 5 gill slits which are modified to have water passage, and the skin is covered with dermal placoid scales. The scales cover the skin which feels like sand paper, and help reduce turbulence.
There is a spiracle located in the 1st gill slit which was reduced due to jaw development.
The caudal fin is asymmetrical and with the vertebral column extending this far it allows thrust and lift.
There is also neutral buoyancy which is when sharks have very large livers filled with fatty hydrocarbon (squalene) lighter than water. The liver acts like a sack of buoyant oil which helps compensates for their heavy body.
Food?
They are predators with the ability to detect prey from afar due to olfaction and vibration (can detect chemicals!). The hammer head shark is an example of a shark making use of this olfaction by having nostrils far apart to have more detection.
There is an upper and lower jaw with functional teeth in the front and developing teeth in the back. Shedding is important in order to keep the sharp teeth for their diet.
There is a pharynx and gill slits, which lead to a J stomach. This is to help increase the surface area to allow breakdown, and the intestines also have increased surface area with a spiral valve and it food then enters the rectum and exits via the anus.
Nervous system?
The brain is well developed with the 3 structures (forebrain, midbrain and hindbrain) and yes it does look similar to a human vagina. They have specialized senses now including sight, smell, vibrations and even electrical fields.
There are olfactory lobes which are essentially paired nostrils to help detect smell.
They detect vibrations via neuromast cells near the lateral line, and these are sensory cells with are filled with hair. They are able to distort the hair cells and am able to hear low frequency sounds.
Sight is good but only in the dim light and they can see by a refletive layer behind the retina known as the tapetum lucidum.
The Ampullae of Lorenzini is an interesting structure concentrated near the nose and dispersed on the head which helps with detecting electrical fields and temperature gradients. This features helps them detect the potential difference between the tissues and the external environment. It acts sort of like an electroconductive gel.
How do they detect prey?
They use their sight and if they recognize their prey they immediately attack. If not they will circle their prey and attack without opening its jaws.They bump into the prey with their nostrum and there is much debate on why they do that.
Breathing?
This is done via the gills with the gill slits and they swim to actually ventilate.
Circulation?
There is a heart and the blood pumps through the ventral aorta then the gills then through the dorsal aorta and into the tissues. It doesn’t return to the heart.
Sexy time?
They have internal fertilization and they are able to lay large yolk filled eggs (oviparous). The yolk nourishes the young for 6 months to 2 years. The pelvic fins is how females are inseminated. The eggs contain tendrils which are used to help anchor the eggs.
Some species are able to retain the eggs in the uterus and the young is nourished by the yolk sac (ovoviparous) but it is independant of the mother (no placenta).
There are even some species which are viviparous where there is a placenta and they give birth to live young!
Subclass Holocephali
This subclass includes the Chimeras with around 33 species. They have large flat plates as teeth and the jaws are actually fused to the cranium. They are a rare species.
OSTEICHTHYES – BONY FISHES
This is where 96% of fishes come from and this the lineage that gave rise to the early tetrapods. So how did they have such success?
- Bony operculum – increased effeciency respiration
- swim bladder – provides buoyancy
- further refined jaw structure and jaw muscles allowing greater flexibility in feding
- mobility which is well developed with overlapping musculature.
The operculum is used for gill ventilation and this structure is rigid, well muscularized. This is important as it allows the fish to breathe with continuously moving.
CLASS ACTINOPTERYGII
This is the class where the fishes have bony rays and it is a large group with 27,000 species.
Subclass Cladistia
Birchirs are in this subclass which have 16 species that live in the streams in Africa. They have lungs and ganoid scales.
Subclass Chondrostei
This includes paddlefishes and sturgeons, and there are 27 species. They have a heterocercal tail and they also have ganoid scales.The tail is to provide some lift as the scales are heavy.
Ganoid scales are rigid and heavy and hard to bend, and they are covered in ganoine which is silvery enamel.
Subclass Neoptergii
This are the teleosts which include 96% of all living fishes and 50% of vertebrates.
The primitive examples of this subclass was Bowfins and Gars which arose at Permian stage and radiated in the Mesozoic stage. Both of them had to gulp air in order to fill the swim bladder.
Gars are heavily armored and lived in temperate waters. They were medium to large predators.
Bowfins are small and had thin scales. They tend to prey on smaller organisms.
The teleosts have cycloid or cytenoid scales, and these are modified to allow greater mobility. The cytenoid scales have overlapping of scales to provide additional protection.
There is the homocercal tail which allows better movements and it allows better focussed muscle contractions on the tail. They also have W shaped musculature, and there are fins to help control movement.
The swim bladder is still present to provide neutral buoyancy.
Feeding is also more interesting as the have a pre-oral cavity which helps expand quickly for sucking motion. This allows the fish to suck in prey and increase the speed of attack.
Some fishes are able to modify their dorsal fins such as the lion fish which made the dorsal fin have venom instead. The angler fish is also an example with the dorsal fins modified for bioluminescence light.
CLASS SARCOPTERYGII
The lobe-finned fishes which are ancestors to all the tetrapods, there are 6 species of lung fishes and 2 extant species of coelacanth. They have lobes which allowed for further development for the tetrapods. They are less abundant now, but was flourishing in the Devonian stage.
They use their swim bladder for gas exchange as well as gills and they also have powerful jaws and heavy enameled scales.
They have fleshy fins and a diphycercal tail (spinal column extending horizontally to the end of the tail).
CLADE OSTEICHTHYES
There is no single origin of this bit slowly there are changes. The body plan includes:
- Endochondral bone – replacing cartilage during development (skeletal support important)
- Notochord which is reduced
- Jaws derived from 1st and 2nd gill arches
- Paired pelvic and pectoral fins supported by bony rays
- Overlapping scales whether cycloid or ctenoid
Filter feeding has slowly changed to respiration. It started when the Aganathans had gills, and then the 1st gill arch moved forward and form an upper and lower jaw. This then led to the 2nd gill arch moving forward to brace jaws and become Gnathostomes.
The swim bladder is a gas filled organ that contributes to buoyancy, and it develops as an outpocket of the gut and it can be filled based on the need to rise or fall.
When it goes up it is expanding, and when it goes down it is compressed.
Gas volume can also be changed from two things:
- Pnematic duct: which is connects the swim bladder to the esophagus. The fish can gulp air to increase the volume or expel to decrease.
- Gas exchange with blood: The gas enters the swim bladder via the gas gland and the gas then diffuses out of the swim bladder at the ovale.
How to fill with gas?
The gas gland secretes lactic acid which then increases the acidity causing the hemoglobin to release oxygen. The rete mirabile acts as an counter current multiplier which helps keeps things in balance. The high amounts of oxygen allows the diffusion of the swim bladder.
Breathing?
There is a bony operculum which helps draw water over the gills, and there is countercurrent gas exchange which occurs.When the mouth is open the operculum expands and pulls water across the gills, and when the mouth is closed the water is pushed across the gills.
There is vascularised lamellae which helps with the counter current flow between blood and oxygen. The idea is to transfer oxygen from the water to the blood. The idea of flowing in the opposite direction is to prevent equilibrium from occurring as the oxygen if flowing in the same way could reduce intake.
Circulation?
There is a single circuit and are some features such as:
- It must pass through 2 sets of capillaries (lungs and tissue)
- Low blood pressure to body tissues, as the pressure is reduced when it enters the capillaries. The heart has to work harder.
Reproduction?
They are diecious with external fertilization where the eggs develop externally. There is plenty of exceptions as it is such as wide group.
As interesting as fishes are, we are now going to slowly move onto land in the next blog with the amphibians!
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