Coelacanth

Coelacanths are the well-known and oldest living lineage of the taxon Sarcopterygii that are lobe-finned fish and tetrapods, they are more closely related to tetrapods and lungfish. These tetrapods include birds, amphibians, reptiles, and mammals. These fishes are found along the coastline of the regions of Indonesia and the Indian Ocean. In the Western part of the Indian Ocean, the coelacanth is considered a critically endangered species.

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Coelacanths belong to the subclass Actinistia, this sub-class consists of lobed-finned fish that are closely related to the lungfish and certain extinct Devonian fish. Latimeria chalumnae is the scientific name of coelacanths. Since their morphology remained static over hundreds of millions of years these are sometimes referred to as living fossils. Also, there are many characteristics that are found in common with class Osteichthyes like their bony skeleton and diphycercal tail, their reliance on fat for buoyancy, and ovoviviparity. The features that are usually found in the class Chondrichthyes other features are derived specializations such as a vestigial lung and intracranial joint.

Coelacanth Classification

Kingdom: Animalia

Phylum: Chordata

Class: Sarcopterygii - It is also considered as Crossopterygii, it includes bony fishes. The members of this class are known as lobe-finned fishes.

Order: Coelacanthiformes - The order of lobe-finned fishes is collectively known as the coelacanth, it consists of five families among which four of these are extinct. 

Family: Latimeriidae - It is a monospecific family consists of large bulky fishes that are characterized by a peculiar stalk that supports the second dorsal, pelvic, and pectoral fins.

Genus: Latimeria - It is a rare genus of fish that contains only living species of coelacanths.

Species: Chalumnae

The coelacanths are members of a now-rare order of fish that includes two extant species in the genus Latimeria:

1. The West Indian Ocean coelacanth with the scientific name Latimeria chalumnae is primarily found near the Comoro Islands off the east coast of Africa.

2. The Indonesian coelacanth with the scientific name Latimeria menadoensis, is found in the waters of Sulawesi, Indonesia.

The name coelacanth originates from the Permian genus Coelacanthus, it was the first scientifically named coelacanth. Many scientists believe that these animals have unique characteristics that represent an early step in the evolution of the fishes to the terrestrial ecosystem as four-legged animals like amphibians.

Coelacanth Habitat

Coelacanth fishes can live in temperate waters in special zones called the "twilight zone”. These zones are found off steep rocky slopes of volcanic islands generally at the depth of 152-243 m. During the daytime, the specimens of Comoran are known to cluster together in the caves in submarine lava deposits, from which they venture at night to feed.

Physical Description

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Coelacanths can grow up to the length of two meters. Their bodies are covered with blue scales, these turn to brown color after their death. These consist of a white speckling that is found unique to each individual. These consist of seven fins that have fleshy lobes and they can move their two paired sets of fins such as pectoral and pelvic in a diagonally synchronous manner. This feature is found in four-limbed terrestrial animals. Coelacanths can possess an intracranial joint and an associated basicranial muscle like all other vertebrates. The main purpose of this structure is still under dispute among various experts, where some of the experts argue that it helps with suction feeding, while others suggest that it helps to increase the bite force. In coelacanths, a notochord is a hollow tube that is completely filled with fluid.

A fatty organ is found in coelacanths that serve the same purpose as that of the swim bladder. It was discovered recently that, inside this fatty organ consists of a vestigial lung and it is surrounded by some small hard plates. These plates are involved in the regulation of lung volume in an ancestral species. Females are larger than males, and the thickness ratio and metabolic rates are higher than males. The adult female’s average weight is about 82.1 kg, and the average length is about 170 cm, whereas the average mass of males is 37.2 kg, and length is about 125 cm.

Development

Coelacanths are ovoviviparous animals, the females can bear their offsprings internally for about thirteen months to three years. The gestation period of these animals is found to be 13 months that was determined originally by the scientists Ozouf and Hureau. Froese and Palomares argued that the growth function discovered by Karl Ludwig von Bertalanffy can be applied to coelacanth embryos, and found that their gestation period can be about three years. Along with this, the supporting evidence is found that is based on scale rings. Eggs are measured to be about one to nine cm in diameter and they can weigh up to 100 to 350g. Few juvenile specimens have been caught following birth, whereas none of them have been witnessed in the presence of adults.

Reproduction

Internal fertilization is found for coelacanths and males possess a cloaca in modified form for this purpose due to the absence of a copulatory organ. About the life history of these species, only a little information is available. At earlier stages of development, the number of eggs that are found can be about 60 or more than that which suggests a decline in the size of the brood during the period of gestation. The females may reproduce at intervals of two or more years due to the presence of a long gestation period. Reproductive maturity in females can be achieved after more than 20 years.

Lifespan or Longevity

The lifespan of the coelacanth fishes is not known for certain. Based on the repeated observation of the adult individuals over that period of time these may live up to 22 years. Froese and Palomares determined that the expected lifespan of these coelacanths could live longer than 100 years using an alternate method, and pointed out that there are other deep-water fish species with similar lifespans.

Behaviour

Coelacanths have the capability to show a tendency to aggregate in their daytime cave habitats. However, the observation of certain social interactions among coelacanths in their aggregations has not been found in any kind. None of the patterns have been found discerned in the case of cave selection, the coelacanths require beyond temperature and depth requirements. Coelacanths have been observed staying individually in different caves from night to night, but also will sporadically revisit the same cave over many years.

At night, they drift hunt individually. From the few observations that have been made so far related to their feeding, it appears that coelacanths do not have prey. But they will simply catch any prey of suitable size that can pass within 20 cm in the front of their mouths. Because of their specialized fin anatomy, coelacanths possess remarkable agility in the water. They are capable to swim in any direction and have been witnessed to swim inverted, or in a vertical, face-down posture.

Communication and Perception

Coelacanth fishes have a rostral organ in their snouts that are believed to have an electroreceptive function. They also possess a color vision that is strongly adapted to a deepwater environment. Most of the visible light is found at that depth has a wavelength of about 480 nm, and the visual pigments of these animals are more sensitive to the wavelengths of range 478 nm and 485 nm, which is a blue shift of roughly 20 nm relative to the corresponding orthologous pigments.

Food Habits

Coelacanths, when comes to feeding habits they are more opportunistic. Some of their known prey species are fishes that include: Beryx splendens, Coranthus polyacanthus, Lucigadus ori, and Brotula multivariate. Their intracranial joint and basicranial muscle that is associated can likely play an important but unresolved role in feeding.

Predation

Humans are the only predators of coelacanths. These animals are considered as unfit for eating and they are usually caught by accident while the fishermen are angling for oilfish (Ruvettus pretiosus). The color patterns of scales of coelacanths resemble the walls of the caves found in the Comoros where these fishes spend most of the daytime hours and may play a role in crypsis.

Coelacanth Fossil Records

The Coelacanth specimen was caught in the year 1938, it is still considered to be the zoological find of the century. This 'living fossil' comes from a lineage of fishes that was thought to have been extinct since the time of the dinosaurs.

The oldest known coelacanth fossils are over 410 million years old. Coelacanths were thought to have become extinct around 66 million years ago in the Late Cretaceous, but they were rediscovered in the year 1938 off the coast of South Africa.

Since scientists thought that these were the sole remaining members of a taxon, the coelacanth was long considered as a "living fossil". Otherwise only from fossils, there are no close relations that are alive, and these are evolved roughly into their current form approximately 400 million years ago. However, several more recent studies have shown that the body shape of coelacanths is much more diverse than previously thought.

How could these Coelacanths disappear for about 80 million years and then turn up to be alive and well in the twentieth century? The answer to this question seems to be the Coelacanths that are found from the fossil records are lived in environments that can favor fossilization. Modern Coelacanths, which are found both in the regions of Comoros and Sulawesi were found in environments that do not favor fossil formation. They can inhabit caves and overhangs that are nearer to vertical marine reefs, at about 200 m depth, off newly formed volcanic islands.

The discovery of the Coelacanth by science in the year 1938 caused so much excitement because at that time these Coelacanths were thought to be the ancestors of the tetrapods that are land-living animals, including humans. It is now believed that the closest living relative of tetrapods are lungfishes.