Obelia (Phylum Cnidaria)
HABIT AND HABITAT
Obelia is commonly referred to as the sea fur. It is a marine, sedentary, colonial, delicate and filamentous organism. It is found up so a depth of 80 meters. Its small (2 to 3 cm in height) branched colonies are attached to the rocks, stones, molluscan shells, wooden piles, and seaweeds in shallow sea water. Obelia looks like whitish or light brown fur on the substratum. It occurs as asexual and sexual forms. The asexual form is a prominent branched hydroid colony found attached to rocks, stones, shells of animals, wooden piling. The sexual form is an inconspicuous bell or an umbrella-like free-swimming stage, called medusa. The sexual form is a bell-like medusa which possesses gonads and disperses sperms and ova in water. Obelia is a marine colonial trimorphic hydrozoan having polyp, medusa and blastostyle stages in its life history. It exhibits phenomena of polymorphism, alternation of generations and metagenesis. To complete its life cycle, Obelia’s asexual and sexual generations alternate. The life cycle of Obelia regularly alternates between hydroid and medusoid phases. Obelia does not have any respiratory organs and the gas exchange takes place by diffusion through the general body surface. Oxygen diffuses directly from the surrounding water into the epidermal cells and carbon dioxide is diffused out. Obelia does not have special excretory or osmoregulatory organs. It excretes nitrogenous waste in the form of ammonia that diffuses through the body wall. Excess water is thrown out of the gastrovascular cavity through the mouth. Thus, mouth being the single opening functions as a contractile vacuole also.
Feeding: The animal is carnivorous as it feeds upon insects, larvae, worms, crustaceans etc. the food is captured & paralysed by stinging cells. Digestion is both intracellular & intercellular. Reproduction: By both methods, asexual reproduction occurs by external budding and sexual reproduction by medusae which are unisexual.
Fertilization: is external as it occurs in sea water. Development: is indirect as life history includes a larva Planula.
Metagenesis: Life history of Obelia shows alternation of Generation. Asexual polypoid phase alternates with the free swimming sexual medusoid phase. This phenomenon is called Metagenesis.
STRUCTURE
Zooids and polymorphism. The colony of Obelia is polymorphic showing the phenomenon of polymorphism. When two or more members of a species or zooids of a colony are structurally modified for different functions, it is called polymorphism. A young colony of Obelia is termed dimorphic, since it contains only two types of zooids, namely gastrozooids (i.e., polyps or hydranths) and gonozoids (i.e., blastostyles). Mature blastostyles produce sexual zooids, the medusae by asexual budding. So, a mature Obelia colony is termed trimorphic, since it contains three types of zooids: gastrozooids, gonozooids and medusae.
The Obelia is a trimorphic colony, that is, having three kinds of zooids which are as follows:
1. Polyps or hydranths (nutritive zooids).
2. Gonangia or blastostyles (budding zooids).
3. Medusae, (sexual zooids).
Obelia exists in following two distinctly different forms during its life history: 1. Polypoid, hydroid or hydranth phase; 2. Medusoid phase.
Hydroid Colony A hydroid colony of Obelia contains different types of individuals, called zooids. Each colony of Obelia consists of a horizontal thread-like branched root, called hydrorhiza which is attached to the substratum and helps in the attachment of the colony. The hydrorhiza gives off several vertical stems, 2-3 cm long called hydrocauli Colony of Obelia geniculata. Both hydrorhiza and hydrocauli are hollow tubes. Each hydrocaulus gives off short lateral branches in an alternate manner which terminates into a polyp. Each hydrocaulus grows in a scorpoid cymose manner as each branch stops growing, ends into a zooids & gives rise a new branch. Each lateral branch bears a zooid. These zooids are nutritive in function and help in feeding. These are called gastrozooids.
The axils of proximal branches bear cylindrical Club- shaped zooids, bearing the medusae buds called reproductive zooids. These are termed as gonozooids, blastozooids or blastostyles. Medusae buds. Umbrella-like reproductive zooids bearing gonads, enclosed in a gonotheca.
Microscopic Structure of Obelia Colony
Coenosarc: Whole colony of Obelia; hydrorhiza, hydrocaulus and zooids contain living tissue, called coenosarc. Coenosarc encloses a cavity or canal, called enteron or gastrovascular cavity. Through this canal digested food is distributed throughout the hydroid colony. The coenosarc is diploblastic comprising of two layers. The wall of gastrovascular cavity is formed of outer epidermis (ectodermal in origin) and an inner gastrodermis (endodermal in origin). A gelatinous and non-cellular mesoglea lies in between that two layers of the diploblastic animal. A narrow canal, called coenosarcal canal runs through whole colony of Obelia which is continuous with the gastrovascular cavity of the zooids. The continuity of the canal system helps to transport the digested food throughout the colony. The Obelia Gastrozooid is a feeding polyp. Its purpose is to nourish the entire colony. The combination of the gastrozooid and hydrotheca constitutes hydranth.
The body wall of the colony is composed of two layers, the outer epidermis and inner gastrodermis and a thin, transparent, non-cellular gelatinous layer called mesogloea sandwiched in between.
EPIDERMIS
The epidermis is thin and made up of typical cells of Cnidaria. These include epithelio-muscular cells, mucus-secreting cells, interstitial cells, nerve cells and nematoblasts. A nerve-net composed of branched nerve cells is present on each side of mesogloea. The nematocysts consist of an oval capsule, a long thread bearing spines and open at the tip which are especially abundant on tentacles forming batteries.
GASTRODERMIS
It is made up of endothelial-muscular cells, nutritive cells (nutritive-muscle cells and narrower gland cells that secrete digestive enzymes and help in digestion, which is both extracellular and intracellular), gland cells, and nerve cells and forms the lining of the gastrovascular cavity.
Perisarc: The coenosarc of entire colony is covered by a tough, yellow, or brown transparent non-living protein called perisarc. It is secreted by epidermis over the coenosarc and forms the exoskeleton of the colony. Perisarc covers hydrorhiza, hydrocauli, their branches and zooids. Around each polyp, perisarc expands to form a clear widely open wine glass-shaped structure, called hydrotheca. Inside the hydrotheca is present a circular, centrally perforated disc-like basal plate, called shelf, to support the polyp or hydranth. The hydranth can contract and withdraw into the hydrotheca.
The perisarc around the blastostyle forms a transparent cylindrical capsule called gonotheca which is open at its free end by gonopore. The blastostyle and gonotheca are called a gonangium. At the base of each branch and each zooid, perisarc has annular (ring-like) constrictions called perisarcal annuli which permit some swaying of the branches and zooids caused due to water currents. It makes the vertical part of the colony firm and rigid. Perisarc is secreted by the epidermis and is separated from the coenosarc by a thin fluid-filled space. Each branch consists of a granular coenosarc made of two cell layers enclosing the coelenteron and surrounded by a thin transparent horny perisarc.
Gastrozooids
They are also called polyps (Gr., polypus- many-footed), hydranths (Gr., hydra or hydor = water + anthos = flower) or trophozooids. All polyps are feeding structures. Polyps are situated singly at apices of each main stem (hydrocaulus) and all branches. Each polyp is cylindrical vase-like structure attached to the axis of the hydrocaulus by its proximal end and remains free at its distal end. The free distal end of the polyp is produced into a conical elevation called manubrium or hypostome which is about one-third of the total length of hydranth. The apex of the hypostome bears a circular terminal aperture, called oral aperture or mouth. A circlet of numerous (about 24) slender and solid tentacles occurs around the base of the hypostome. Rising from the base of hypostome is a circlet of up to 30 filiform tentacles which are much longer than hypostome. Mouth is capable of great dilation and contraction. Below the hypostome is the stomach or gastric region of polyp which forms about two-third of the length of hydranth.
Mouth opens into a spacious gastrovascular cavity or enteron of the polypoid body, i.e., cavity of hypostomal and gastric part of polyp. At the base of the polyp, the cavity is continuous with coenosarcal cavity of concerned branch (hydrocaulus). The polyp is covered by cup-shape perisarcal hydrotheca. The annuli of the perisarc present around the stalk of polyp allow the swaying movements due to the force of water current.
Histology of polyp: The wall of hydranth is composed of an outer ectodermal epidermis and an inner endodermal layer of gastrodermis. There is an acellular, thin, transparent, and gelatinous mesoglea in between the two layers. The tentacles of polyp are solid with no enteron. Each tentacle consists of an outer layer of epidermal cells and a single row of thick walled cubical vacuolated gastrodermal cells.
Epidermis. Epidermis of polyp is a continuous layer of epithelio-muscular cells which are long, conical, and columnar in shape. The inner ends of these cells are produced into muscular processes which run longitudinally. Epidermis of hypostome and tentacles also consists of sensory and epithelio-glandular cells and batteries of nematocyst containing cnidocytes. Epidermis of polyp contains very few interstitial cells. Branched nerve cells occur in deeper zones of epidermis. The cnidoblasts are found in the basal part of the hydranth and in the coenosarc. They form nematocyst and migrate actively to reach their final positions. Obelia has only one kind of nematocyst called basitrichous isorhizas in which the capsule is oval, the butt is absent, the thread is open at the tip and has spines at its base.
Gastrodermis. Gastrodermis of polyp is a continuous inner layer of long, granular endothelio-muscular cells, their muscle processes point outwards and are circular. These gastrodermal cells have flagella towards their inner ends; flagella produce a current in the fluid of enteron. Pseudopodial gastrodermal cells can form pseudopodia for engulfing food. Gastrodermis of polyp also contains club-shaped glandular cells. Glandular cells of hypostome secrete mucus, while those of gastric region secrete the proteolytic digestive enzymes. Interstitial cells, sensory cells and nerve cells have same distribution as in epiderms. The nerve cells form an intra-gastrodermal nerve net upon the mesogloea. The two nerve nets: epidermal and gastrodermal are inter-connected.
Function of Polyp: The polypoid colony of Obelia is sessile and attached to the substratum. It does not move from place to place. The polyp are nutritive zooids of the colony. Most of their diet consists of small crustaceans, tadpoles, worms, insect larvae, etc. They capture, ingest and digest food. Obelia is carnivorous and feeds upon aquatic crustaceans, nematodes, and other worms. Tentacles help in catching and conveying the prey to the mouth. They do this job by the help of nematocysts which penetrate, hold, and kill the prey. The highly developed longitudinal muscle tails of the tentacles holding the prey contract and succeed in carrying the prey to the mouth where it is ingested and passed into enteron. Digestion of food is mainly extracellular. It occurs in the enteron by the help of enzymes secreted by glandular gastrodermal cells. Pseudopodial gastrodermal cells ingest partly digested food (food particles) by phagocytosis. Intracellular digestion within the food vacuole completes the digestive process. Through cell-to-cell diffusion, aided by the beating flagella of gastrodermal cells, the digested components of food are transported throughout the body; the gastrovascular cavity serves both digestion and transportation of food.
After digestion has been completed in the gastrodermis, soluble food diffuses to the epidermis through the mesogloea. Any unwanted/undigested food remains are egested via the oral aperture. Beating of flagella of gastrodermal cells and rhythmic contraction (= peristaltic movement) of hydranth circulates the digested food (along with food particles) throughout the coenosarc of the colony. The polyps can also undergo contraction and extension because of the presence of longitudinal and circular muscles in their body wall.
Respiration In Obelia
Obelia lacks respiratory organs; hence, gas exchange occurs by diffusion across the entire body surface. Oxygen diffuses straight into the epidermal cells from the surrounding water, while carbon dioxide diffuses out. As there is a continual flow of water into the gastrovascular cavity of a polyp or medusa, diffusion of gases can also occur during water circulation. Here, water and gastrodermal cells exchange gases, from which oxygen diffuses to each Obelia cell.
Excretion And Osmoregulation in Obelia
Obelia lacks specialised excretory and osmoregulatory organs. It eliminates nitrogenous waste as ammonia, which diffuses through the body wall. Excess water is expelled through the mouth from the gastrovascular cavity. As the only aperture, the mouth also functions as a contractile vacuole.
Gonozooid
When Obelia colony has reached its full development, it produces special non-feeding club-shaped bodies called blastostyles, or blastozooids, or gonozooids. These zooids are called gonangia. These are less numerous than hydranths and occur in the axils of older hydranths. Gonangia occur in the axils of older hydranths, towards the proximal end of hydrocauli. Each gonangium comprises of medusae-bearing blastostyle and gonotheca. A blastostyle is a club-shaped cylindrical and hollow extension of the coenosarc. It has no mouth or tentacles and is surrounded by a portion of perisarc, called the gonotheca. The gonotheca opens at its distal end by a gonopore. Blastostyle bears many saucer-shaped, hollow buds, called medusae or gonophores. The blastostyles are the reproductive zooids as they reproduce asexually to give rise to numerous lateral buds called medusa buds or gonophores. These buds develop into third type of zooids of the colony called medusae. Fully formed medusae detach from blastostyle to escape into surrounding water through an aperture, the gonopore, formed by the rupture of gonotheca at its distal end. Gonotheca together with blastostyle and gonophores medusae is referred to as the gonangium.
Histology of blastostyle: A blastostyle has two layered body wall containing outer epidermis and inner gastrodermis, both layers being separated by mesogloea. Its cavity, the gastrovascular cavity, is continuous with the coenosarcal canal of the hydrocaulus and hydrorhiza. Blastostyle is rudimentary and does not open to exterior.
Functions of gonozooid: The main function of gonozooids is to take part in reproduction, hence are called reproductive zooids. They produce gametes (sperm and eggs) and release them into the surrounding environment. When fertilization occurs, new individuals are formed, contributing to the growth and expansion of the colony. Each blastostyle produces numerous lateral buds (by asexual budding) which develop into medusae. Each saucer-shaped medusa is attached by middle of its convex surface to the blastostyle, when fully formed it breaks free and escapes out in water through the gonopore. The free-swimming medusae eventually develop gonads and represents the sexual phase of the life cycle of Obelia.
Medusa Production: In some hydrozoan species, gonozooids give rise to medusae, which are free-swimming, jellyfish-like individuals. Medusae are a part of the life cycle of some colonial organisms, and they may be responsible for dispersing the colony's offspring to new locations.
Gonophores: Gonozooids may also produce specialized structures called gonophores, which are asexual reproductive structures. These gonophores can develop into new polyps or medusae, depending on the species, and they help the colony reproduce without relying solely on sexual reproduction.
Medusae
Medusae are the third type of zooids of Obelia colony. These are modified zooids. Medusae are produced as hollow buds from the coenosarc of blastostyle by asexual budding during spring and summer seasons. They are solitary and pelagic, i.e., they swim freely on the surface of water. Mature medusae bear gonads and reproduce sexually. They are unisexual (dioecious), i.e., male gonads (testes) and female gonads (ovaries) are borne by separate individuals.
Structure of medusa: A medusa is radially symmetrical. It is tiny and circular having umbrella-like shape. The convex outer surface is known as the exumbrella, and the concave inner surface is the subumbrella. Hanging down from the centre of the subumbrellar surface is a short, hollow manubrium at the end of which is a four-lobed oral aperture or mouth. Mouth opens into a part of gastrovascular cavity, called stomach or gastric cavity situated inside the manubrium. From the stomach arise four radial canals which are delicate flagellated tubes, they pass outwards to the edge of umbrella and join a flagellated circular canal running near the margin of umbrella. The free edge of umbrella becomes folded inward towards manubrium to form a rudimentary velum. The medusa with a velum is called craspedote and those without the velum are called acraspedote.
Tentacles. From the edge of the umbrella arise numerous small, solid, and highly contractile tentacles. There are 16 tentacles in young medusa, but this number increases in the adult stage. According to their position, 16 tentacles are classified as follows.
1. Per-radial tentacles: These are four in number, and they are situated against the four radial canals.
2. Inter-radial tentacles: These are four in numbers and are located on radii, bisecting the angle between two per-radial tentacles.
3. Ad-radial tentacles: These are eight in number and found in between the per-radial and inter-radial tentacles.
4. Sub-radial tentacles: These tent- acles are found in between the ad-radial and per-or inter-radial tentacles.
Histology of medusa: Like the hydranth, medusa is diploblastic having outer ectodermal epidermis and inner endodermal gastrodermis. A gelatinous mesoglea exists between the two epithelia; it becomes thickened towards exumbrella forming the main bulk of the body.
1. Epidermis: The epidermis covers manubrium, subumbrella surface, the margins of the umbrella and entire exumbrella surface of medusa. It consists of epitheliomuscular cells which are produced into muscle processes.
Muscle processes run longitudinally in the manubrium and tentacles. They form striated circular muscles and some radial muscles of subumbrella and bring about locomotory movements. The epidermis of exumbrella is devoid of musculature. Velum is the double fold of epidermis. It contains no endoderm but has thick mesogloea in the middle. The structure of tentacles is essentially the same as in the polyp, but in addition they each possess a swelling at their base where interstitial cells have accumulated. The interstitial cells are used to replace lost or damaged tentacular nematoblasts.
2. Gastrodermis: The endodermis or gastrodermis lines the gastrovascular cavity of manubrium, stomach, radial canals, and circular canals. The epithelial cells of endodermis have no muscle processes, many of them are flagellated cells, and some are glandular cells secreting digestive enzymes. Interstitial cells are mainly accumulated at the swollen bases of tentacles and give rise to cnidoblasts which are particularly abundant on margin of umbrella, on tentacles and around mouth.
Sensory cells are most abundant around mouth and tentacles. Inside the bell, between radial canal and between ex umbrella and sub umbrella layers of epidermis, there is a thin sheet of gastrodermis, called gastrodermal lamella.
It is formed by the fusion of upper and lower layers of gastrodermis. Velum is composed of double layer of epidermal cells enclosing a middle narrow layer of mesogloea, there being no gastrodermis. This type of velum is called true velum. Tentacles are solid, each containing a core of vacuolated gastrodermal cells covered by epidermis.
Certain Organ Systems of Medusa
Nervous system: In the medusa the epidermal nerve cells and fibres form two circular nerve rings, one external and one internal to circular canal. Primarily the inner nerve ring controls the subumbrellar muscle fibres, whilst the outer nerve ring receives impulses from the statocyst. However, they are also interconnected with each other, the sensory cells and with processes innervating the tentacles.
Sense organs (statocysts): Medusa contains eight receptor organs, called statocysts. Statocysts are located inside the bases of eight ad radial tentacles. Each contains a fluid-filled sac lined with ectodermal cells. Each sac possesses movable particle of calcium carbonate, called statolith or otolith. Alongside the statolith is a series of sensory protoplasmic processes. The whole apparatus of the statocyst is an organ of equilibrium and helps to bring about coordination in the movement of medusa.
Reproductive organs: Each medusa of Obelia contains only four gonads which may be either testes or ovaries depending on the sex of the medusa. Halfway along each radial canal and protruding from the subumbrellar surface are the gonads. They consist of an outer layer of ectodermal cells and a core of endodermal cells derived from the radial canal itself. Germ cells originate in the ectoderm of manubrium and migrate to gonads. It is here that meiosis occurs. Each medusa produces either flagellate sperms or spherical ova. Liberation of the gametes occurs when the ripe gonads burst. After the release of gametes, the medusa dies.
Functions of medusa:
Movement: Medusa floats passively in water. It is simply drifted here and there by water current with manubrium hanging downward and tentacles swaying freely. It also swims actively by muscular contraction started by impulses which originates in the nerve at the umbrellar margin. By rhythmic contraction and expansion of bell the water in sub-umbrellar cavity is propelled behind (hydro propulsion), and medusa moves forward in a series of jerks. Closure of bell is affected by contraction of epidermal muscle-tails that are best developed on sub-umbrellar surface, specially along the margin of bell, where they form a muscle ring, but remain undeveloped on exumbrellar surface. Opening of bell is brought about mainly by the elastic mesogloea regaining its original shape and to some extent due to contraction of muscle tails in the middle of upper surface. During swimming, body may be tilted thrown aside, or more commonly turned inside out so that the sub-umbrellar surface becomes outer convex, with the manubrium springing up from its apex. The contraction and expansion of bell muscles alternatively closes and opens the bell which forces water out of the sub-umbrellar cavity downwards and propels the body in upward direction. The contraction of the epidermal muscle tails of the sub- umbrellar surface helps in the closure of the bell cavity while the opening of the bell is brought about by elastic mesoglea and contraction of the muscle tails in the middle of upper surface. This kind of jet propulsion method is called hydro propulsion.
Nutrition. Medusa is strictly carnivorous. Food includes minute worms, nematodes, insects, etc. These are captured by crustaceans, nematocyst-bearing tentacles and ingested by the highly contractile mouth. Prey is digested exclusively in stomach. Digestion is both extracellular and intracellular like that of hydranth. Digested food is distributed to entire medusa through the system of radial and circular canals.
Respiration and excretion. Respiration and excretion are carried on individually by each cell by diffusion, as in hydranth, and special organs are absent.
Statocysts are the organs of equilibrium and muscular coordination. Their presence in medusae is associated with their active free-swimming habit. During swimming if body becomes tilted, the statolith falls over the tilted side against the processes of sensory cells which become stimulated. In this way, nerve impulse is created and transmitted to the nerve ring. As a result, the muscle tails of the stimulated side contract more rapidly and medusa is brought back to its normal horizontal position. While swimming, if the medusa tilts, the movable particle of statolith rolls over the tilted side and presses against the sensory processes. The stimulated cells transmit the nerve impulse to the nerve ring which is connected to the muscle tails.
Sexual reproductive Medusae are the sexual zooids, or gonozooids, possessing gonads. They are dioecious, i.e., testes and ovaries are borne by separate individuals Each medusa bears only four gonads situated on sub-umbrellar surface, one in the middle of each radial canal. Gonads are formed as ventral diverticula of radial canals. They mature after the medusae escape from gonangia. Each gonad (testis or ovary) is an ovoid, knob-like body, having an outer covering of epidermis. continuous with that of sub-umbrella, and an inner lining of gastrodermis, continuous with that of radial canal. The space between these two layers is filled with a mass of sex cells. which become differentiated into sperm or ova. as the case may be. Sex cells originate in the epidermis of manubrium, while medusa is still attached to the blastostyle. They soon pass into gullet through gastrodermis and finally make their way into gonads. Here they undergo maturation divisions and become gametes. These gonads are not true gonads but only aggregations of developing gametes. Outer wall (epidermis) of mature gonads ruptures to release the gametes in water.