In this article we will discuss about the habit, habitat and life history of Obelia.
Habit and Habitat of Obelia:
Obelia is a typical representative of colonial hydrozoa (Fig. 12.14). It is exclusively marine. It exists in two principal forms, the polyp and the medusa. The polyp or hydroid form represents the asexual phase of its life-history and the medusoid form represents the sexual phase. There is an alternation of these two phases or generations in its lifecycle. Thus it shows a typical instance of Metagenesis.
I. Hydroid or Polyp phase:
Hydroid stage of Obelia is a colonial form. It is a branched filament-like structure and remains attached with the substratum. The colony is polymorphic, i.e., a number of individuals or zooids are present which are morphologically as well as functionally different from each other.
A. Hydrorhiza and Hydrocaulus:
Obelia colony is constituted of two portions—the horizontal portion is called the Hydrorhiza and the vertical portion bearing the zooids is named as the Hydrocaulus. Hydrorhiza is a branched structure and gives only mechanical anchorage to the whole colony.
From the hydrorhiza arises the vertical hydrocaulus having short lateral alternate branches bearing zooid at the terminal ends. Two types of zooids are seen—Gastrozooids having nutritive function and Blastostyles for reproduction.
In addition to the fully-formed gastrozooids and blastostyles, the lateral branches bear short club-like structures which are either primordial or developing gastrozooids or blastostyles. Both the hydrorhiza and the hydrocaulus are hollow tubes called the coenosarc, covered externally with perisarc. Coenosarc is made up of two cellular layers, the outer one is designated as ectoderm and the inner layer is called endoderm.
In between these two cellular layers there is a thin non-cellular mesoglea or mesolamella. Coenosarc contains a tubular cavity known as coelenteron which is continuous throughout the colony and is filled with a fluid. Histological picture of the cellular layers resembles more or less to that of Hydra.
Obelia possesses one kind of nematocyst, called the basitrichous isorhizas. The capsule is oval, the thread tube is open at the tip with spiners at the base only. The tentacles of Obelia are solid and contain a core of endoderm cells. The perisarc is a cuticle-like transparent non-cellular layer secreted by the ectoderm of coenosarc.
Perisarc is separated from the coenosarc and at places it shows attachment with the coenosarc and thus showing some constrictions which are known as annuli of perisarc or periderm.
B. Gastrozooid or Trophozooid or Nutritive Zooid:
Most of the zooids present in the hydroid stage of Obelia are the gastrozooids (Fig. 12.15A). They are specially designated to perform nutritive function and feed the whole colony. Each gastrozooid has a short tube-like body having at its distal end a conical projection, called hypostome or manubrium. The mouth is situated at the terminal end of the manubrium.
Surrounding the manubrium there is a circlet of about twenty four solid tentacles. This zooid is enveloped by a base-like investment, called hydrotheca.
Hydrotheca is formed by the modification of the perisarc and is perfectly transparent. It is provided with a circular shelf at its proximal end upon which the whole zooid rests. The circular shelf has a central aperture through which the tubular body of the zooid remains continuous with the rest of the colony.
C. Blastostyle or Gonozooid or Reproductive Zooid:
These particular types of the zooids are few in number in comparison to the number of gastrozooids. Each blastostyle has a long cylindrical body without mouth and tentacles (Fig. 12.15B). The coelenteron is greatly reduced. It is enclosed by a transparent covering, called gonotheca, a modified form of perisarc.
The lateral wall of the body gives of small lateral buds, called the medusa-buds. They exhibit great structural variations depending upon the development sequences. When the medusa-buds become mature the gonotheca ruptures and thus allows the medusa-buds to escape.
II. Medusoid phase:
Medusa develops as hollow offshoot of the blastostyle. When fully-formed, it assumes the appearance of an umbrella with a convex surface by which the medusa was attached with the blastostyle (Fig. 12.15C). This convex side is called the exumbrella and the concave side of the umbrella is known as the sub-umbrella.
From the centre of the subumbrellar surface emerges a hanging tube, called manubrium, bearing a square mouth at its terminal end (Fig. 12.15D). The edge of the umbrella gives rise to a very short circular shelf, called velum. At the junction of the exumbrellar side and velum there is a circlet of tentacles. The number of tentacles is sixteen in a newly-formed stage but the number may increase with age.
At the bases of alternate tentacles there lies a sense organ in the form of lithocyst or marginal sense organ. Each lithocyst has a very small spherical sac-like body that encloses a central calcareous mass and sensory cell. These sense organs regulate and coordinate the movement of the organism.
The mouth leads into the coelenteron lodged inside the manubrium. From the base of the coelenteron emerges four equidistant radial canals, which ultimately open into a ring canal situated at the margin of the body of the umbrella. Through these canals food matters are conveyed to the different parts of the body.
The histological structure of the medusa is same as that in the hydroid stage. Mesoglea is thick and often contains wandering cells and fibres. The only peculiarity is the velum which lacks the endoderm. The two sides are composed of ectoderm with a median layer of mesoglea.
In the margins of both the exumbrella and subumbrella, the ectoderm contains nerve cells. In exumbrella, the nerve cells crowd together to form an outer ring and in subumbrella it constitutes an inner margin.
The whole organisation of the body of the medusa exhibits distinct radial symmetry. The medusae are unisexual (dioecious). Gonads are ectodermal in origin and remain in close association with the radial canals towards the subumbrellar surface. They are round in appearance and are four in number. Both the male and female goands are similar externally.
Life-History of Obelia:
The male gametes or spermatozoa, after maturation, are liberated into water, and one spermatozoon fertilizes a female gamete or ovum and thus results into the formation of a zygote. The single-celled zygote divides repeatedly and the daughter cells reorganise to form stages like a hollow blastula (coeloblastula) and then the solid gastrula or stereo-gastrula in due course.
The blastocoel of the blastula is filled with the cells which are budded off from the wall of the blastula. Thus a double layered stereo-gastrula is formed. Finally, a larval form— the planula larva, is produced.
Planula larva:
The planula larva has an elongated and ovoid appearance. The outer layer of the body is composed of ciliated ectodermal cells and the inner layer is the endoderm. Mouth is absent.
It is free-swimming and contains a cavity which is the primordial coelenteron. After a very brief free-swimming existence, the planula larva settles down and fixes itself to the substratum by one pole and transforms into the next stage—the hydrula stage.
Hydrula stage:
The fixed end of this form is designated as the aboral end and the free or oral end develops a manubrium and a circle of tentacles. Then a mouth is formed at the centre of the manubrium. The hydrula, thus formed, gives out lateral buds and transforms into an Obelia colony and thus exhibits a typical instance of metagenesis.
Metagenesis in Obelia:
Amongst Cnidarians, Obelia shows excellent phenomenon of metagenesis. The term was first introduced by Haeckel in 1866. Metagenesis is a phenomenon where diploid (2n) sexual and diploid (2n) asexual generations alternate each other cyclically to complete the life cycle of a sexually reproducing individual. Between the two generations the sexual reproduction is postponed than the asexual reproduction.
In most cnidarians where metagenesis is seen, the medusae (2n) or sexual individuals do not develop directly from the eggs but develop from the polyp (2n) by budding. The polyp form in Obelia develops asexually and it is diploid (2n) and does not contain sex organs. It represents the asexual generation.
The Obelia colony represents the fixed, sexless and colonial hydroid or asexual stage (2n), which produces medusa-buds by budding (Fig. 12.16). These medusa-buds subsequently transform into full-fledged medusae. The medusae represent the solitary, free-swimming sexual stage (2n) and possess male or female gonads. The male and the female gametes are produced in the respective gonads.
Mature male and female gametes unite and result into the formation of a zygote, which in turn passes through the usual developmental stages. The developmental process in Obelia is indirect and intervened by the presence of larval forms, which gives rise to the Obelia colony.
So in the life-cycle of Obelia there is a distinct alternation of two diploid generations (Fig. 12.17). Such a phenomenon is called metagenesis. Hartman (1939) considered that the metagenesis is a kind of secondary alternation of generations.
The life cycle of Obelia does not represent the alternation of generations because alternation of generation is a phenomenon where diploid (2n) asexual generation alternates with the haploid (n) sexual generation cyclically to complete the life cycle and the phenomenon of metagenesis represents in the life cycle of Obelia.