Introduction:
Phylum Coelenterata is also called as phylum Cnidaria due to the presence of cnidoblast cells. It includes about 9000 species.
General characteristics of phylum Cnidaria
Habit and habitat : These are aquatic, mostly marine and few are also found in freshwater (e.g- Hydra).
Sessile or free-swimming.
Level of Organisation : Tissue level of Organisation.
Symmetry : Radially symmetrical.
Germ layers : These are diploblastic, which means their body is derived from two embryonic germ layers i.e., ectoderm and endoderm. Undifferentiated mesoglea is present between them.
Body wall : Body wall is made up of layers-
Epidermis- Outer ,ectodermal layer having Interstitial cells which are totipotent in nature and form nerve cells. Gastrodermis- Endodermal layer, present around gastrovascular cavity.
Body plan : These have blind-sac body plans.
Coelom : These are acoelomates. Central cavity called gastrovascular cavity present.
Digestion : Incomplete digestive systems present i.e., they have a central gastro-vascular cavity which acts as both mouth and anus. Digestion is both extracellular and intracellular. Nutrition is holozoic.
These are carnivorous in nature.
Respiration : Respiration takes place by the general body surface through diffusion.
Excretion : Excretion takes place by the general body surface through diffusion. These are ammonotelic i.e., ammonia is the nitrogenous waste.
Skeleton : Usually absent; soft bodied animals. But in corals body is covered by horny or calcareous exoskeleton.
Nervous system : A primitive type of nervous system is present in them which consist of a network of non-polar nerve cells or neurons and their processes. Statocyst is seen for the first time, which acts as a sense organ for balance.
Reproduction : Both sexual (formation of gametes) and asexual reproduction (budding) is found in them. Asexual buds and sexual parts/gonads develop from the interstitial cells.
Power of regeneration is present.
Development is indirect in them which means larval stages are formed. For eg,
Larvae of Obelia - planula (free living)
Larva of Aurelia - Ephyra
Special characteristics
i. Polymorphism
These show polymorphism i.e., exhibit two body forms which are polyp and medusa.
a. Polyp
A cylindrical and sessile form which may be colonial and solitary.
Their mouth directed upwards. Example - Hydra, Adamsia, etc.
b. Medusa
An umbrella shaped and free swimming. They are always solitary.
Their mouth directed downwards. Example - Aurelia or jellyfish.
If both the forms i.e polyp and medusa are found in a species then it shows metagenesis.
ii. Metagenesis
The process of alternation of generations, in which polyp produce medusae by asexual reproduction (budding) and medusae produce polyps by sexual reproduction. Example - Obelia
Both polyp and medusae phases are diploid.
iii. Cnidocytes
Also known as cnidoblast.
These consist of a hypnotoxin filled stinging capsule or nematocyst.
These are present on the body and the tentacles.
Function- i.anchorage (attachment), ii.defense, iii. capturing the prey.
Classification of phylum Cnidaria
Cnidarians are divided into 3 classes, on the basis of development of zooids or dominance of medusoid or polypoid phase in the life cycle.
1. Hydrozoa
Either only polyps are present or polyps and medusae both are present.
Examples: Hydra, Obelia (sea-fur) and Physalia(Portugese man of war).
2. Scyphozoa
Only medusae forms are present in them.
Class of Jelly fishes.
Examples: Aurelia(Jelly fish).
3. Anthozoa
Only polyp forms are found in them.
Medusa form is absent in them.
Class of corals and sea anemones
Examples : Gorgonia (Sea fan), Adamsia(Sea anemone), Alcyonium (dead man's finger), Pennatula ( Sea pen), Corallium (red coral), , Meandrina (Brain coral), Madrepora (stag-horn coral), Tubipora (organ pipe coral).
Australian barrier reef is formed by members of this class, i.e., the corals.
Examples
1. Hydra
They are found in freshwater.
Some species of Hydra are found in symbiotic association with algae called Zoochlorella and it gives green colour to Hydra.
2. Physalia
They are commonly known as Portuguese Man of war.
3. Aurelia
They are commonly known as jellyfish.
They are carnivorous.
4. Adamsia
They are commonly known as “The Sea anemone”.
They show commensalism with Eupagurus prideuxi (hermit crab).
Commensalism is a relationship between two organisms in which one gets benefited while the other neither gets benefits nor harmed.
It gets transported to other places by hermit crabs that lives inside the shell.
Physalia
Physalia, a genus of the order Siphonophorae, found on the surface of the Indian, Pacific and Atlantic oceans, are colonies of polyps and medusoids. They may appear like a single multicellular organism but are made up of minute colonies called zooids. Two species of the genus are known: Physalia physalis and Physalia utriculus. Let us now study the genus in detail.
Systematic Position of Physalia
Kingdom : Animalia
Phylum. : Cnidaria
Class : Hydrozoa
Order. : Siphonophorae
Family. : Physaliidae
Genus. : Physalia
Characteristics of Physalia
There are two species found in the genus Physalia: Physalia physalis (Portuguese man o’ war) and Physalia utriculus (Pacific man o’ war or blue bottle). The Portuguese man o’ war thrives in the Indian and Atlantic oceans, whereas the blue bottle thrives in the Pacific ocean. Both species are considered to be the same, below is a detailed description of Physalia physalis.
It is a marine hydrozoan that belongs to the Neuston community. Neuston is a community of floating ocean organisms that live at the interface between air and water.
It is named Portuguese man o’ war because of its great resemblance to the sailing warship.
The pneumatophore of the species lives on the surface while the remainder of the body is submerged under water.
Physalia physalis is a conspicuous, accessible and robust siphonophore as compared to other members of the order that are fragile and difficult to collect.
It consists of numerous nematocysts that sting painfully, killing fishes and even humans.
It is found commonly on the coasts of Indonesia, Japan and Eastern Australia, stinging thousands of beachgoers.
Anatomy
The species is composed of smaller units called zooids that hang in clusters from a large gas-filled structure called pneumatophores.
Almost seven kinds of different zooids are described in the species, namely nectophores, vestigial nectophores and gonophores in the medusoid body and tentacle-bearing zooids, gonopalpons, gonozooids and free gastrozooids in the polyploid body.
The pneumatophore, also called bladder, is a conspicuous, translucent and tinged (mauve, purple, pink or blue) part that rises as high as 15cm above the water.
It is a floatation device that allows the colony to move in the direction of the wind.
The pneumatophores are filled with carbon dioxide that is actively produced by the animal. This gas diffuses into the surrounding area in the event of an attack, thus allowing the colony to submerge in water for protection.
The colony of Physalia hunts and feeds with the help of gastrozooids and tentacle-bearing zooids.
The tentacle-bearing zooids, also known as dactylozooids or tentacular palpons, are equipped with tentacles of about 10 m lengths that can extend up to 30 metres.
The tentacles have small, thread-like, coiled structures that are called nematocysts.
The nematocysts have venom that is injected at the time of attack by squids and fishes. It paralyses and kills the organisms.
For preying, the cnidarian contracts its tentacles to catch the prey upwards and bring it near the gastrozooids (digestive zooids). The gastrozooids surround the prey and secrete enzymes for digestion.
Gonophores, the reproductive zooids, are situated in branches called gonodendra.
The gonophores produce sperm and eggs, the gonozooids are accessory gastrozooids, and the nectophores allow the detached gonodendra to swim.
Ecology of Physalia physalis
The species is a carnivore that preys on fishes (both young and adult), zooplanktons, shrimps and other small crustaceans.
The species has many predators, such as loggerhead turtle, blue sea slug, violet snail and ocean sunfish.
The blanket octopus is a species that is immune to the toxic venom of Physalia physalis.
The man-of-war fish (Nomeus gronovii) lives and feeds on the tentacles and gonads of the Portuguese man o’ war.
Yellow jack is another species of marine fish that gains shelter from the stinging tentacles of Physalia, and in return, the species attracts fish to eat.
Life Cycle
They are dioecious in nature, which means that each colony is either male or female. The gonophores, borne on branch-like structures called gonodendron, produce either egg or sperm. The eggs and sperm are released into the water, and mating takes place primarily in the autumn season.
A fertilised egg develops into a larva and starts budding zooids. The growth usually takes place underwater. Pneumatophores are grown first, followed by gastrozooids and tentacle-bearing zooids. The pneumatophore eventually becomes buoyant enough for the colony to float on the surface of the water.
