ChordatesBy Samuel Davis

Introduction:-Vertebrates are in the phylum Chordate(containing the spinal cord)3 Characteristics of Vertebrates within life histories:• Vertebrates first evolved in the ocean.• The invertebrate ancestors of vertebrates had gill slits, but these were used primarily for filter feeding; these organisms took up most of the oxygen they needed through the skin.• Vertebrates are found from the tropics to the polar regions, from the deep sea to high mountains, and even the air.

Sexual Reproduction

Reproduction in beings of the phylum Chordata is sexual, with the exception of urochordates that can also reproduce asexually. In some classes (cyclostomes, osteichthyes fishes and amphibians) there is larval stage.

Sexual Reproduction cont.With rare exceptions, fishes, amphibians, reptiles and monotreme mammals are oviparous, egg-laying (embryos develop within eggs and outside the mother’s body) and marsupial and placental mammals are viviparous (embryos develop inside the mother’s body, feeding from her).

Evolution

Timeline

All chordate evolved from a basic eukaryote with a spine.

Different classes of Marine ChordateJawless fishCartilaginous fishBony fish

Jawless FishLamprey HagfishLampreys (sometimes also called lamprey eels) are an order of jawless fish-like vertebrates, whose adults are characterized by a toothed, funnel-like sucking mouth.

Hagfish,  (also known as Hyperotreti), are

eel-shaped slime-producing marine

animals (occasionally called slime eels).

They are the only known living

vertebrates that have a skull but not a

vertebral column.

Chondricthyes

Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, a heart with its chambers in series, and skeletons made of cartilage rather than bone. The class is divided into two subclasses: Elasmobranchii (sharks, rays and skates) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class).

Chondricthyes

The skeleton is cartilaginous. Their tough skin is covered with dermal teeth (again with Holocephali as an exception as the teeth are lost in adults, only kept on the clasping organ seen on the front of the male's head), also called placid scales or dermal denticles, making it feel like sandpaper.All Chondrichthyes breathe through 5-7 gills, depending on the species.

Chondricthyes Reproduction

Fertilization is internal. Development is usually live birth (ovoviviparous species) but can be through eggs (oviparous). Some rare species are viviparous. There is no parental care after birth; however, some Chondrichthyes do guard their eggs.

Chondricthyes

Skates and RaysBatoids are flat-bodied, and, like sharks, are cartilaginous , meaning they have a boneless skeleton made of a tough, elastic substanceMost species live on the sea floor, in a variety of geographical regions — many in coastal waters, few live in deep waters to at least 3,000 meters (9,800 ft)Batoids reproduce in a number of ways. As is characteristic of elasmobranches, batoids undergo internal fertilization. Internal fertilization is advantageous to batoids as it conserves sperm, does not expose eggs to consumption by predators, and ensures that all the energy involved in reproduction is retained and not lost to the environment.

Sharks

Shark teeth are embedded in the gums rather than directly affixed to the jaw, and are constantly replaced throughout life. Multiple rows of replacement teeth grow in a groove on the inside of the jaw and steadily move forward in comparison to a conveyor belt; some sharks lose 30,000 or more teeth in their lifetime. The rate of tooth replacement varies from once every 8 to 10 days to several months. Shark skeletons are very different from those of bony fish and  vertebrates. Cartilage is flexible and durable, yet is about half the normal density of bone. This reduces the skeleton’s weight, saving energy. Because sharks do not have rib cages, they can easily be crushed under their own weight on land.

Like other fish, sharks extract oxygen from seawater as it passes over their gills. Unlike other fish, shark gill slits are not covered, but lie in a row behind the head. A modified slit called a spiracle lies just behind the eye, which assists the shark with taking in water during respiration and plays a major role in bottom–dwelling sharks.Digestion can take a long time. Unwanted items never get past the stomach, and instead the shark either vomits or turns its stomachs inside out and ejects unwanted items from its mouth.

Senses

Smell: Sharks have the ability to determine the direction of a given scent based on the timing of scent detection in each nostril. This is similar to the method mammals use to determine direction of sound.They are more attracted to the chemicals found in the intestines of many species, and as a result often linger near or in sewage outfalls. Some species, such as nurse sharks, have external barbells that greatly increase their ability to sense prey.Sight: Shark eyes are similar to the eyes of other vertebrates, including similar lenses, corneas and retinas, their eyesight is well adapted to the marine environment. Tthis means that sharks can contract and dilate their pupils, like humans, something no other fish can do. This tissue is behind the retina and reflects light back to it, thereby increasing visibility in the dark waters.Hearing: Although it is hard to test sharks' hearing, they may have a sharp sense of hearing and can possibly hear prey many miles away. A small opening on each side of their heads (not the spiracle) leads directly into the inner ear through a thin channel

FEEDING Most sharks are carnivorous. Basking sharks, whale sharks,

and mega mouth sharks have independently evolved different strategies for filter feeding plankton:

basking sharks practice ram feeding, whale sharks use suction to take in

plankton and small fishes mega mouth sharks make suction

feeding more efficient by using the  tissue inside of their mouths to attract prey in the deep ocean

Feeding Cont

Other highly specialized feeders include cookie cutter sharks, which feed on flesh sliced out of other larger fish and marine mammals.Cookie cutter teeth are enormous compared to it’s size.Although they have never been observed feeding, they are believed to latch onto their prey and use their thick lips to make a seal, twisting their bodies to rip off flesh.Many sharks, including the white tip reef shark are cooperative feeders and hunt in packs to herd and capture elusive prey. These social sharks are often migratory, traveling huge distances around ocean basins in large schools. These migrations may be partly necessary to find new food sources.

The End