Discipline Course I. Semester - II. Paper : Obelia. Lesson: Morphology and Physiology of Obelia. Lesson Developer: Sarita Kumar

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1 Discipline Course I Semester - II Paper : Obelia Lesson: Morphology and Physiology of Obelia Lesson Developer: Sarita Kumar College: Acharya Narendra Dev College University of Delhi

2 Table of Contents Introduction Habit and Habitat Morphology Hydrorhiza Hydrocaulus Living Tissue of Obelia - Coenosarc Epidermis Gastrodermis Protective Covering - Perisarc Morphology of a Gastrozooid Morphology of a Gonozooid Morphology of a Medusa Locomotion in Obelia Nutrition in Obelia Respiration in Obelia Excretion and Osmoregulation in Obelia Sense Organ - Statocyst Reproduction in Obelia Asexual Reproduction Sexual Reproduction Metagenesis Polymorphism Summary Exercise/Practice Glossary References/Bibliography/Further Reading 1

3 Introduction Obelia is a sedentary colonial marine cnidarian which grows upright in a branching tree-like form and has several specialized feeding and reproductive polyps. It is commonly called sea-fur and exists in both asexual, sessile, polypoid stage and sexual, free-swimming medusoid phase. Value Addition: Interesting to Know!! Heading Text: Origin of word Obelia Body Text: The word Obelia is probably derived from the Greek word obeliās, which means a loaf baked on a spit; obel (ós) - a spit + -ias noun suffix. Source: The common species of Obelia are: a) Obelia geniculata (Knotted thread hydroid) b) Obelia longissima (Sessile hydroid) c) Obelia dichotoma (Sea thread hydroid) d) Obelia bidentata (Double toothed hydroid) Value Addition: Fact File! Heading Text: Different species of Obelia Body Text: Obelia longissima is a long, flexible hydroid colony with a prominent main stem and branches. It is usually up to 20 cm in length but may reach 35 cm in British waters. Side branches of uniform length but shorter distally give the colony a tapering outline. This species may be confused with other Obelia species. Obelia dichotoma may also be elongate but lacks the regular shape and extreme length of Obelia longissima. Its colony may be distinguished from Obelia longissima growing in 2

4 rock pools in spring by its long tubular, nearly straight and darkening internodes Obelia bidentata has multiple branched stems even when young. Still no reliable key is available to distinguish between the medusae of Obelia species. Fig. 1: Obelia longissima Source: Habit and Habitat Obelia is cosmopolitan in distribution, only exception being the high-arctic and Antarctic seas. They grow in shallow water, in intertidal rock pools and are usually found up to meters of depth from the water's surface. The medusa stage of Obelia species is commonly found in coastal and offshore plankton around the world. The colonies of Obelia are often found as a delicate fur-like growth on the rocks, stones, mollusc shells, sea weeds, wooden pilings and wharves. Obelia geniculata normally grows on kelp fronds, especially on Laminaria hyperborea in conditions of moderate wave exposure. 3

5 (A) (B) Fig. 2: Obelia sp. growing on (A) Kelp stipes of Laminaria hyperborea; (B) Rocky bottom Source: Smale, D.A., Burrows, M. T., Moore, P., O'Connor, N. and Hawkins, S. J. (2013) Threats and knowledge gaps for ecosystem services provided by kelp forests: a northeast Atlantic perspective. Ecology and Evolution, 3: CC 4

6 Fig. 3: Point Map of Obelia geniculata Source: CC Morphology Obelia is a very small marine hydroid. It looks like a small branching tree exhibiting whitish or brown colour. The height of Obelia varies from 2 cm or more. Fig. 4: Colony of Obelia geniculata Source: CC The body of Obelia consists of two kinds of filaments, horizontal hydrorhiza and vertical hydrocaulus. a) Hydrorhiza (Root of a hydroid) Hydrorhiza is the basal part of the colony consisting of tubular processes called stolons. 5

7 It encrust over the surface of substratum and helps in the attachment of the colony. b) Hydrocaulus (Stem of a hydroid) A few small vertical filaments, 2-3 cm long, arise from the hydrorhizas. These are called hydrocauli (Sing., us). Each hydrocaulus branches alternately, each of which terminates into a polyp. The polyps collectively are termed as zooids. These zooids are nutritive in function and help in feeding. These are called gastrozooids. The axils of proximal branches bear cylindrical reproductive zooids. These are termed as gonozooids, blastozooids or blastostyles. 6

8 Fig. 5: Outline structure of Obelia showing alternate branches with zooids Source: Living Tissue of Obelia - Coenosarc Whole colony of Obelia; hydrorhiza, hydrocaulus and zooids; contain living tissue, called coenenchyme or coenosarc. The coenosarc is diploblastic comprising of two layers; outer epidermis and inner, gastrodermis. A middle non-cellular layer of mesoglea is present in between epidermis and gastrodermis. 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. Epidermis The epidermis is thin and made up of typical cells of Cnidaria (Refer to Chapter on General Characteristics of Phylum Cnidaria). These include; epithelio-muscular cells, mucus-secreting cells, interstitial cells, nerve cells and nematoblasts. The nematocysts are basitrichous isorhizas. These consist of an oval capsule, a long thread bearing spines and open at the tip. Gastrodermis It forms the lining of gastrovascular cavity and consists of endothelio-muscular cells, nutritive cells, gland cells and nerve cells. Protective Covering Perisarc 7

9 Entire colony of Obelia is surrounded by a protective covering, called perisarc. It is noncellular, tough, transparent, yellowish-brown and cuticular in nature and is called perisarc or periderm. 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. However, the coenosarc and perisarc are in contact making the colony more rigid. At some points, the perisarc is arranged in flexible rings called annuli. These allow the swaying movements due to the force of water currents. The perisarc of hydranth is termed as hydrotheca and that of gonozooid is called gonotheca. 8

10 Fig. 6: Part of the stem and branches of Obelia colony showing annuli Source: Morphology of a Gastrozooid Gastrozooid of Obelia is a feeding polyp. Its function is to feed the whole colony. Gastrozooid is a tubular and diplobastic zooid with a central gastrovascular cavity continuous with the coenosarcal canal. The polyp is attached to the hydrocaulus by a hollow stalk while its distal end is produced into a conical elevation called manubrium or hypostome. The apical portion of the manubrium bears a terminal mouth encircled by numerous long, solid tentacles, often 24, loaded with nematoblasts. The perisarc of gastrozooid, called hydrotheca, is transparent and cup-shaped invaginated as a platform or shelf at the base of the gastrozooids for polyp to rest. The gastrozooid and hydrotheca collectively form hydranth. In case of any emergent situation, the polyp can withdraw itself into the hydrotheca and the tentacles fold over the manubrium covering the mouth. The presence of shelf prevents the polyp to retract into the hydrocaulus. The annuli of the perisarc present around the stalk of polyp allow the swaying movements due to the force of water current. 9

11 Fig. 7: Structure of a gastrozooid and gonozooid Source: Morphology of a Gonozooid The gonozooids, also called blastozooids or blastostyles are cylindrical rod-like reproductive bodies present in the axils of hydrocaulus and stalk of gastrozooids. Gonozooids are less in number than gastrozooids as these are present only in the proximal part of the colony. 10

12 It has a reduced gastrovascular cavity and is devoid of mouth and tentacles. It, thus, can not feed and receives food digested by the gastrozooids and transported through the gastrovascular cavity. Like other parts of the colony, gonozooids are also enclosed in a perisarc, called gonotheca. It is constricted distally and constricted by annuli proximally. The apical part of the gonotheca has an opening called gonopore. Gonozooid produces numerous small medusae or gonophores by the asexual process of budding. Mature medusae detach from the gonozooids and escape into the surrounding water through the gonopore. The gonozooids, gonophores and gonotheca collectively form gonangium. Morphology of a Medusa Medusa of Obelia is radially symmetrical, umbrella-like zooid which measures approximately 6-7 mm in diameter. The outer surface of medusa is convex and known as ex-umbrellar surface, while the inner concave surface is called sub-umbrellar surface. A short manubrium containing a quadrangular mouth at its distal end hangs from the centre of the sub-umbrellar surface. The medusa is craspedote type as its edge is produced inwards into an insignificant rudimentary velum. The margins of the medusa bear initially 16 short, contractile tentacles; which gradually increase in number. The mouth open into a short gullet which leads to a wide expanded stomach from which arise four narrow, radial canals which mark the four principal per-radii. The radial canals extend till the margin of the umbrella and open into a circular canal running parallel to the margin. 11

13 The radius bisecting two per-radii is called inter-radius (four in number) and that bisecting per-radius and adjacent inter-radius is termed as ad-radius (eight numbers). The tentacles present at the end of these radii are named accordingly, such as per-radial tentacles; inter-radial tentacles and so on. Whole system of canals is lined by inner layer of gastrodermis and both the ex-umbrellar and sub-umbrellar surfaces are covered by epidermis. Fig. 8: Oral view of a medusa Source: Nervous system consists of two diffused nerve nets which are concentrated around the margins of the umbrella and form two circular nerve rings. Eight receptor organs, called statocysts, are present at the bases of ad-radial tentacles. These are the organs of balance, muscular co-ordination and equilibrium. Medusa possesses four gonads on the sub-umbrellar surface. These are per-radial in position and each of these is present in the middle of each radial canal. These are dioecious, male and female medusae being separate individuals. 12

14 Fig. 9: Lateral view of a medusa Source: Locomotion in Obelia Movement in Polyps The polypoid colony of Obelia is sessile and attached to the substratum. It does not move from place to place. However, polyps exhibit certain movements under the force of water currents due to the presence of annuli in the perisarc. The polyps can also undergo 13

15 contraction and extension because of the presence of longitudinal and circular muscles in their body wall. Locomotion in Medusa a) Hydro propulsion: Medusae are free swimming forms. They generally swim in the water by jet propulsion method. 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. b) Passive drifting: Medusae also drift and float passively in sea water under the force of strong water currents and wind. Thick mesoglea of medusae provides them buoyancy and helps in floating. Nutrition in Obelia Nutrition in Polyps The gastrozooids are the nutritive zooids of the Obelia colony. They are primarily carnivorous and feed upon small crustaceans, tadpoles, worms, insect larvae, etc. The gastrozooids capture the food with the help of nematocysts present on the tentacles. The food is pushed into the gastrovascular cavity through the mouth where the proteolytic enzymes secreted by the gastrodermal gland cells partially digested the food. The semidigested food is engulfed by the food vacuoles of the nutritive cells for complete digestion. Thus, digestion is both extracellular and intracellular. The digested products of the food are distributed throughout the body by cell-to-cell diffusion helped by beating of flagella of 14

16 gastrodermal cells; the gastrovascular cavity thus serving for both digestion and transportation of food. The undigested food material is egested through the mouth of the gastrozooids. Nutrition in Medusa The process of feeding in medusa is similar to that in polyps. Medusa is strictly carnivorous and captures food with the help of tentacles beset with nematocysts. As in polyps, the food is digested both extracellularly and intracellularly but exclusively in stomach. The digested food is distributed to whole body through the network of radial and circular canals present in medusa. Value Addition: Video Heading Text: Feeding by Obelia polyps Body Text: These are interesting videos which describe the process of feeding by the nutritive polyps, gastrozooids of Obelia colony. The second video by Laura Gonzalez also shows the digestion of ingested food in the gastrovascular cavity of the polyp and the transportation of digested food in entire body of Obelia. Source: Respiration in Obelia 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. 15

17 The diffusion of gases can also take place during circulation of water in the gastrovascular cavity of polyp or medusa as there is a continuous influx of water. Here, exchange of gases takes place between water and the gastrodermal cells from where oxygen diffuses to each cells of Obelia. Excretion and Osmoregulation in Obelia 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. Sense Organs Statocyst Polyps of Obelia are sessile zooids and they do not require any sense organs. However, medusae are free-swimming zooids and while swimming, their body may tilt and lose balance. Thus, they possess balancing organs, statocysts with the help of which they can regain their position. Structure A statocyst is a fluid-filled sac lined by sensory epithelial cells. The basal part of the cells is connected to the nerve cells while the inner ends bear sensory processes. The cavity of statocyst contains a round particle of calcium carbonate, called statolith or otolith. The particle is movable and is secreted by a large cell, lithocyte. 16

18 Fig. 10: Structure of a Statocyst Source: Function The statocysts help in balance and equilibrium of medusa. 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. The nerve impulse causes the rapid contraction of the muscle tails of the stimulated side regaining the original position of medusa. Reproduction in Obelia The life cycle of Obelia includes both polyp and medusa stages. Polyp is an asexual form and reproduces by asexual means while medusa is a sexual zooid and reproduces sexually. a) Asexual Reproduction The polyps reproduce asexually by the process of budding. The hydrocaulus gives rise to a number of gastrozooids and as the colony matures, blastostyles bud from the axils of proximal gastrozooids and hydrocaulus. 17

19 Each blastostyle produces a large number of medusa buds in spring and summer. These buds gradually develop and mature. When fully formed, they detach from the blastostyles and escape into the water through the gonopore. 18

20 Fig. 11: Development of medusa buds in Obelia Source: b) Sexual Reproduction The sexual reproduction in Obelia takes place in the medusa stage; the male and female medusa being separate. The medusae produce ova and sperms and release them into the water where fertilization takes place. Sperms may also enter the female medusa along with the water current and fertilization may take place inside the body of female medusa. Development of Fertilized Egg The fertilized egg undergoes complete and equal cleavage resulting in the formation of solid ball of cells, called morula. It develops a central cavity, blastocoel surrounded by loosely arranged blastomeres. This hollow blastula is termed as coeloblastula. Gradually, the new cells cut off from the blastomeres and start migrating in the blastocoel from one end of the coeloblastula. Slowly, entire blastocoel is filled with the cells and hollow blastula converts into solid gastrula, called stereogastrula by delamination. The outer surface of the embryo becomes ciliated forming a ciliated larva, planula larva. It is double layered ovoid larva, consisting of outer ciliated ectoderm and inner solid mass of endodermal cells. It actively swims in the water and helps in the dispersal of species. After a short period of time, the larva settles down and attaches itself to the substratum by one of its ends. The attached end forms a basal disc while a mouth surrounded by tentacles is formed at the distal end. This sessile stage is termed as hydrula stage as it resembles a hydra. Gradually, hydrula undergoes asexual reproduction repeatedly and converts into an adult Obelia colony. 19

21 Fig. 12: Diagrammatic view of the life cycle of Obelia Source: Metagenesis The life cycle of Obelia represents a remarkable example of alternation of generation where the asexual and sessile phase of Obelia reproduces asexually by budding and gives rise to sexual and free-swimming medusa. The medusa reproduces sexually and forms new polyps. Thus, a diploid asexual hydroid phase alternates with another diploid sexual medusoid phase. This phenomenon of alternation between two diploid phases is termed as metagenesis. 20

22 Fig. 13: Detailed view of the life cycle of Obelia Source: Polymorphism Thus, the life cycle of Obelia includes three distinct types of zooids; 21

23 a) Nutritive polyps hydranths b) Asexual reproductive polyps blastostyles c) Sexual reproductive polyps medusa This phenomenon, where Obelia is represented by structurally and functionally different individuals, is called polymorphism. Initially the colony of Obelia is represented by only two forms, gastrozooids and blastozooids and is called dimorphic. Later, when gonophores develop on the blastozooids by the process of budding, the colony is considered trimorphic represented by three kinds of zooids. Fig. 14: Mature colony of Obelia Source: Value Addition: Video Heading Text: Structure and Life cycle of Obelia 22

24 Body Text: These videos describe the detailed structure and life cycle of Obelia colony. The videos interestingly explain the phenomenon of polymorphism, and structure of gastrozooids, blastostyles and medusa. These further illustrate the phenomenon of metagenesis describing the development of polyp and medusa stages. Source:

25 Summary Obelia (sea-fur) is a sedentary colonial marine hydroid and exists in both asexual, sessile, polypoid stage and sexual, free-swimming medusoid phase. The colonies of Obelia are often found attached on the rocks, stones, mollusc shells, sea weeds, wooden pilings, wharves and on kelp fronds, especially on Laminaria hyperborea. The body consists of two kinds of filaments, horizontal root-like hydrorhiza and vertical stem hydrocaulus. Each hydrocaulus branches alternately, each of which terminates into a nutritive polyp, gastrozooid. The axils of proximal branches bear cylindrical reproductive zooids, called gonozooids, blastozooids or blastostyles. Whole colony of Obelia contains diploblastic living tissue, coenosarc through which runs a narrow coenosarcal canal continuous with the gastrovascular cavity of the zooids. Whole colony is covered by a non-cellular, tough, and cuticular perisarc or periderm which is arranged in flexible annuli at some points for movement. The gastrozooid is covered by hydrotheca and is attached to the hydrocaulus by a hollow stalk. Its distal end is produced into a conical elevation, manubrium with a terminal mouth encircled by numerous long, solid tentacles. The gastrozooid and hydrotheca collectively form hydranth. 24

26 The gonozooids, enclosed in a gonotheca, are cylindrical rod-like reproductive bodies with a reduced gastrovascular cavity and devoid of mouth and tentacles. The gonozooids, gonophores and gonotheca collectively form gonangium. Medusa of Obelia is radially symmetrical, umbrella-like sexual zooid with an outer convex ex-umbrellar surface, an inner concave sub-umbrellar surface, a short manubrium containing a quadrangular mouth and four gonads. Obelia thus exhibits polymorphism being represented by structurally and functionally different individuals; nutritive polyps hydranths; asexual reproductive polyps blastostyles and sexual reproductive polyps medusa. The polypoid colony of Obelia is sessile and attached to the substratum while medusae generally swim in the water by jet propulsion method or passive drifting. Gastrozooids capture the food with the help of nematocysts present on the tentacles and feed the whole colony. Digestion is both extracellular and intracellular. Circulatory and respiratory system are absent. Nervous system is primitive and consists of single diffuse nerve net in polyps and double nerve nets in the form of two circular nerve rings in medusa. Meduase possess balancing organs, statocysts with the help of which they maintain their balance and equilibrium while swimming. Reproduction is both by asexual and sexual means. Gonozooids form medusa buds which detach and escape into the water where they release gametes for external fertilization. The zygote undergoes radial and determinate cleavage and develops into adult polypoid colony through a ciliated planula larva. 25

27 The life cycle of Obelia exhibits phenomenon of metagenesis; alternation between two diploid phases; where the asexual and sessile phase of Obelia asexually forms sexual and free-swimming medusa by budding which in turn reproduces sexually and forms new polyps. 26

28 Exercise/Practice Q1. Differentiate between (a) Hydranth and Hydrotheca (b) Gonangium and Gonophore (c) Hydrorhiza and Hydrocaulus (d) Coenosarc and Perisarc (e) Nutritive polyp and Reproductive polyp (f) Planula and Hydrula Q2. Name the following (a) Ciliated larva of Obelia (b) Solid gastrula of Obelia (c) Flexible part of perisarc of polyps (d) Inner surface of Obelia medusa (e) Organs of balance and equilibrium in Obelia (f) Mode of locomotion in medusa (g) Movable particle in the cavity of statocyst (h) Phenomenon of alternation of generation in Obelia Q3. Answer the following questions in short. (a) Name the different kinds of zooids present in Obelia and write their functions. (b) Explain the significance of perisarc present in the Obelia colony. (c) What is metagenesis? Explain with the help of a flow diagram. (d) Describe the structure and function of statocyst present in the Obelia medusa. Q4. Answer the following questions in detail. (a) Explain the polymorphism in Obelia colony and write the significance. (b) Differentiate between three morphological forms of Obelia based on their structure and function. (c) Describe the life cycle of Obelia in detail. Support your answer with the help of diagrams. 27

29 (d) Obelia is sessile and cosmopolitan in distribution. Justify the statement and discuss how does dispersal of Obelia take place. (e) Explain the phenomenon of metagenesis in Obelia and add a note on its significance. 28

30 Glossary Coenosarc: Diploblastic living tissue of Obelia colony comprising of an outer epidermis and inner gastrodermis with a middle non-cellular layer of mesoglea Coeloblastula: Hollow blastula with a central cavity, blastocoel surrounded by loosely arranged blastomeres Ex-umbrellar surface: Outer convex surface of medusa Gastrozooids: Cup-shaped nutritive polyps of Obelia colony Gonangium: Collective term for gonozooids, gonophores and gonotheca Gonotheca: Perisarc of a gonozooid Gonozooids: Rod-like reproductive polyps of Obelia colony, also called blastozooids or blastostyles Hydranth: Collective term for gastrozooid and hydrotheca Hydrocaulus: Small vertical filaments of the Obelia colony which branch alternately, each of which terminates into a polyp. Hydropropulsion: Propulsion of the medusa in upward direction by forcing water out of the body through alternate contraction and expansion of muscles Hydrorhiza: Basal part of the Obelia colony consisting of tubular stolons which help in its attachment Hydrotheca: Perisarc of a hydranth 29

31 Manubrium: A central tubular projection in the body of medusa which bears a quandrangular mouth Medusa: Umbrella-like, free-swimming form of Obelia which reproduce sexually to form polypoid colony Perisarc: Non-cellular, tough, transparent, yellowish-brown and cuticular protective covering of Obelia colony Polyp: Tubular, sessile and asexual morphological form of Obelia Statocysts: Fluid-filled organs of balance and equilibrium in medusa of Obelia Statolith: Round and movable particle of calcium carbonate secreted by lithocyte and present in the cavity of statocyst Sub-umbrellar surface: Inner concave surface of medusa 30

32 References/ Bibliography/ Further Reading Suggested Readings Barnes, R.D. (1982). Invertebrate Zoology, V Edition. Holt Saunders International Edition. Barnes, R.S.K., Calow, P., Olive, P.J.W., Golding, D.W. and Spicer, J.I. (2002). The Invertebrates: A New Synthesis, III Edition, Blackwell Science Barrington, E.J.W. (1979). Invertebrate Structure and Functions. II Edition, E.L.B.S. and Nelson Useful Web Links