Chordata is the phylum of the animal kingdom that includes a large number of animal species, including humans. Phylum Chordata animals include all animals that possess a notochord at least at one point in their lifetime.
The term chordate is used to refer to any animal belonging to the phylum Chordata. We can define chordate as follows: The chordates are the class of animals that possess four anatomical features namely:

 
notochord
dorsal nerve cord
post-anal tail
pharyngeal slits
 
at least during some part of their development into maturity.


 Characteristics of Chordates :
Animals belonging to this phylum share four key features: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Let us read more about each one below.






Figure 1: Characteristics of Chordates: four key features: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail


Phylum Chordata has three subphyla: (1) Urochordata, (2) Cephalochordata, and (3) Vertebrata, as depicted in Figure 6. There are very few species that belong in Cephalochordata and Urochordata.

Most chordates belong in the subphylum Vertebrata (vertebrates), e.g. fishes, animals, birds, and reptiles that form backbones made up of cartilage and bone and a brain enclosed in a hard skull. Approximately, there are fifty thousand species in this subphylum. They are ubiquitous. They are found in various habitats, e.g. marine water, freshwater, and terrestrial.

Chordata has a very peculiar feature; they possess bodies bilaterally symmetric making them unique to other animal phyla. The chordates are a member of the deuterostomes. One of the characteristics of this group is the anus gets developed in the early embryonic stages before the mouth.

Vertebrates are further divided into many classes, such as fishes, birds, reptiles, amphibians, and mammals. Reptiles are vertebrates with scaly, water-resistant skins. They lay shelled eggs and breathe air. They are cold-blooded, which means that they are unable to withstand the cold weather and widely depend on the temperature of its surrounding to maintain normal body temperature. They usually have four limbs. Turtles, chameleons, crocodiles, and snakes are some of the most common examples of reptiles.

Another famous class of chordates is the amphibians. Their name amphibian refers to the capacity to live on both land and aquatic habitats. They are born in water, possess tails and gills, which with the passage of time as they grow into adults, develop lungs and legs, enabling them to live on land. The common feature between reptiles and amphibians is that both of them are cold-blooded animals. Frogs, newts, blindworms, salamanders, caecilians, and toads are common species found in this class.

In class Chondrichthyes (cartilaginous fish), their skeleton consists of cartilage. They have an upward curving and asymmetrical tail. On the body of Chondrichthyes, there are five to seven various gills and the cycle of reproduction is achieved via modified fins that pass the sperms to the females. Examples of class Chondrichthyes include rays, skates, sharks, and chimeras. The oldest known class of chordates, specifically, vertebrates, is the class Agnatha (jawless fish). Hagfish and lampreys are the two primary groups of Agnatha. Hagfishes are specialized scavengers whereas the lampreys are the parasites that attach to other fishes in order to get nourishment via suctions.

Class Mammalia consists of warm-blooded animals that produce milk in their bodies that they use to feed their babies. They maintain the same body temperature throughout their bodies and they have the ability to sustain body temperature in any environmental conditions. They have a larger brain than the other vertebrates. Bears, camels, bats, dolphins, monkeys, and cheetahs are some of the most common animals that belong to the class of Mammalia. 

We will take a sperm whale as an example about chordatas.


sperm Whale:


The sperm whale or cachalota (Physeter macrocephalus) is the largest of the toothed whales and the largest toothed predator. It is the only living member of the genus Physeter and one of three extant species in the sperm whale family, along with the pygmy sperm whale and dwarf sperm whale of the genus Kogia.

The sperm whale is a pelagic mammal with a worldwide range, and will migrate seasonally for feeding and breeding. Females and young males live together in groups, while mature males (bulls) live solitary lives outside of the mating season. The females cooperate to protect and nurse their young. Females give birth every four to twenty years, and care for the calves for more than a decade. A mature sperm whale has few natural predators, although calves and weakened adults are sometimes killed by pods of killer whales (orcas).

Mature males average 16 metres (52 ft) in length but some may reach 20.7 metres (68 ft), with the head representing up to one-third of the animal's length. Plunging to 2,250 metres (7,382 ft), it is the third deepest diving mammal, exceeded only by the southern elephant seal and Cuvier's beaked whale.The sperm whale uses echolocation and vocalization as loud as 230 decibels (re 1 �Pa m) underwater.It has the largest brain on Earth, more than five times heavier than a human's. Sperm whales can live 70 years or more.

Spermaceti (sperm oil), from which the whale derives its name, was a prime target of the whaling industry, and was sought after for use in oil lamps, lubricants, and candles. Ambergris, a solid waxy waste product sometimes present in its digestive system, is still highly valued as a fixative in perfumes, among other uses. Beachcombers look out for ambergris as flotsam. Sperm whaling was a major industry in the 19th century, depicted in the novel Moby-Dick. The species is protected by the International Whaling Commission moratorium, and is listed as vulnerable by the International Union for Conservation of Nature.



Life cycle:

Sperm whales can live 70 years or more.They are a prime example of a species that has been K-selected, meaning their reproductive strategy is associated with stable environmental conditions and comprises a low birth rate, significant parental aid to offspring, slow maturation, and high longevity.

How they choose mates has not been definitively determined. Bulls will fight with each other over females, and males will mate with multiple females, making them polygynous, but they do not dominate the group as in a harem.Bulls do not provide paternal care to their offspring but rather play a fatherly role to younger bulls to show dominance.

Females become fertile at around 9 years of age.The oldest pregnant female ever recorded was 41 years old.Gestation requires 14 to 16 months, producing a single calf.Sexually mature females give birth once every 4 to 20 years (pregnancy rates were higher during the whaling era).[181] Birth is a social event, as the mother and calf need others to protect them from predators. The other adults may jostle and bite the newborn in its first hours.

Lactation proceeds for 19 to 42 months, but calves, rarely, may suckle up to 13 years.Like that of other whales, the sperm whale's milk has a higher fat content than that of terrestrial mammals: about 36%, compared to 4% in cow milk. This gives it a consistency similar to cottage cheese,[185] which prevents it from dissolving in the water before the calf can drink it.[186] It has an energy content of roughly 3,840 kcal/kg,compared to just 640 kcal/kg in cow milk.Calves may be allowed to suckle from females other than their mothers.

Males become sexually mature at 18 years. Upon reaching sexual maturity, males move to higher latitudes, where the water is colder and feeding is more productive. Females remain at lower latitudes.Males reach their full size at about age 50.

Social behaviour:

Like elephants, females and their young live in matriarchal groups called pods, while bulls live apart. Bulls sometimes form loose bachelor groups with other males of similar age and size. As they grow older, they typically live solitary lives, only returning to the pod to socialize or to breed .Bulls have beached themselves together, suggesting a degree of cooperation which is not yet fully understood.The whales rarely, if ever, leave their group.

A social unit is a group of sperm whales who live and travel together over a period of years. Individuals rarely, if ever, join or leave a social unit. There is a huge variance in the size of social units. They are most commonly between six and nine individuals in size but can have more than twenty. Unlike orcas, sperm whales within a social unit show no significant tendency to associate with their genetic relatives. Females and calves spend about three-quarters of their time foraging and a quarter of their time socializing. Socializing usually takes place in the afternoon.

When sperm whales socialize, they emit complex patterns of clicks called codas. They will spend much of the time rubbing against each other. Tracking of diving whales suggests that groups engage in herding of prey, similar to bait balls created by other species, though the research needs to be confirmed by tracking the prey.

Relations with other species:
The most common natural predator of sperm whales is the orca (killer whale), but pilot whales and false killer whales sometimes harass them.Orcas prey on target groups of females with young, usually making an effort to extract and kill a calf. The females will protect their calves or an injured adult by encircling them. They may face inwards with their tails out (the 'marguerite formation',named after the flower). The heavy and powerful tail of an adult whale is potentially capable of delivering lethal blows.Alternatively, they may face outwards (the 'heads-out formation'). Other than sperm whales, southern right whales had been observed to perform similar formations.However, formations in non-dangerous situations have been recorded as well. Early whalers exploited this behaviour, attracting a whole unit by injuring one of its members.Such a tactic is described in Moby-Dick:

"Say you strike a Forty-barrel-bull-poor devil! all his comrades quit him. But strike a member of the harem school, and her companions swim around her with every token of concern, sometimes lingering so near her and so long, as themselves to fall a prey."

If the killer whale pod is large, its members may sometimes be able to kill adult female sperm whales and can at least injure an entire pod of sperm whales.Bulls have no predators, and are believed to be too large, powerful and aggressive to be threatened by killer whales.Solitary bulls are known to interfere and come to the aid of vulnerable groups nearby. However, the bull sperm whale, when accompanying pods of female sperm whales and their calves as such, may be reportedly unable to effectively dissuade killer whales from their attacks on the group, although the killer whales may end the attack sooner when a bull is present. However, male sperm whales have been observed to attack and intimidate killer whale pods in competitive feeding instances. An incident was filmed from a long-line trawler: a killer whale pod was systematically taking fish caught on the trawler's long lines (as the lines were being pulled into the ship) when a male sperm whale appeared to repeatedly charge the killer whale pod in an attempt to drive them away; it was speculated by the film crew that the sperm whale was attempting to access the same fish. The killer whales employed a tail outward and tail-slapping defensive position against the bull sperm whale similar to that used by female sperm whales against attacking killer whales. However, at some potential feeding sites, the killer whales may prevail over sperm whales even when outnumbered by the sperm whales. Some authors consider the killer whales "usually" behaviorally dominant over sperm whales but express that the two species are "fairly evenly matched", with the killer whales' greater aggression, more considerable biting force for their size and predatory prowess more than compensating for their smaller size.

Sperm whales not known for forging bonds with other species, but it was observed that a bottlenose dolphin with a spinal deformity had been accepted into a pod of sperm whales. They are known to swim alongside other cetaceans such as humpback, fin, minke, pilot,and killer whales on occasion.