ZLY 303: Biology of Free-living Non-Arthropods

1. PHYLUM CNIDARIA ZLY 303: Biology of Free-living Non-Arthropods

2. Classification • Domain: Eucaryota • Kingdom: Animalia • Subkingdom: Eumetazoa • Phylum: Cnidaria Cnidarians were classified into the following Subphyla viz: • Anthozoa (Sessile) e.g. Sea anemones, Corals, Sea pens • Ceriantharia (tube anemones), • Hexacorallia, • Octocorallia,

3. Classification • Medusozoa • Scyphozoa (Swimming) e.g. true jellyfish • Cubozoa e.g. box jellyfish, Sea wasps • Hydozoa (Sessile) e.g. Hydra and colonial swimmers e.g. Portuguese Man o’ War (A diverse group that include all freshwater cnidarians as well as many marine forms). Recently, the following group has been recognized as a class rather than a sub-group of Scyphozoa; they include: • Staurozoa (Stalked Jellies) • Myxozoa(Parasites) • Polypodiozoa (Parasites) There is still debate whether they are Cnidarians or Bilaterians

4. Phylogeny of Cnidaria Four main classes (even though most sources consider 3) stem group - possess basic structural organization of other more advanced metazoa • have radiated into many habitats – yet body plan has been retained.

5. Classes For study purpose, the following classes shall be considered:- • HYDROZOA e.g. Hydra, Obelia • SCYPHOZOA e.g. Aurelia aurita(Jelly fish) • CLASS ANTHOZOA e.g. Sea anemones, Corals • CLASS CUBOZOA e.g. Tripedaliacystophora, Sea wasp

6. General Characteristics • Possess the simplest degree of structural and functional organization • Exclusively marine, few sp. in freshwater • They are acoelomates (lack coelom) • Radially symmetrical • There is diversity of body types found among the members of this phylum. • Has 2 distinct layers i.e. Ectoderm and Endoderm and a thin layer of non-living jelly-like mesoglea

7. General Characteristics • Tentacles around the mouth • Tissues level of organization • No choanocytes (flagellated collar cells of the sponges) • Alternation of generation with 2 morphologically distinct stages: • Polyp (sessile, remain attached, asexually reproducing form) • Medusa(motile, sexually reproducing form)

8. General Characteristics • No excretory or respiratory organs • Nerve net with symmetrical & asymmetrical synapses • Gastrovascular cavity used for both digestion and excretion. • Mesoglea (jelly-like non-living substance between the two tissue layers (epidermis & gastrodermis) for support). • Hydrostatic Skeleton - chitinous, calcerous, or protein.

9. General Characteristics • Radiata have only two embryonic tissue layers. • Specifically, no mesoderm (the tissue that gives rise to structures, including muscles, in triploblastic organisms). • Lack sophisticated movement seen in triploblastic organisms. • Mouth: both ingestion & egestion- no anus • Asexual reproduction by budding in polyps • Sexually by gametes in all medusa & some polyps

10. Body Cavity Diversity

11. Why Cnidarians are ACOELOMATES Cnidarians do not possess the following features, hence, they are Acoelomates:- • Coelom (also called the body cavity) cavity within the body of all animals higher than the coelenterates and certain primitive worms. • Formed by the splitting of the embryonic mesoderm into two layers.  • In mammals it forms the peritoneal, pleural, and pericardial cavities. 

12. Dimorphism & Polymorphism in Cnidarians • A striking & puzzling feature • Dimorphism: existence in polyp & medusa • Polymorphism: colonies of some species have morphologically differing individuals each specialized for certain roles e.g. feeding, reproduction & defense.

13. Task differences in Hydrozoa In Hydrozoans, colonial individuals arising from individual zooids will take on separate tasks. For instance, in Obelia there are: • Feeding individuals referred to as the Gastrozooids; • Asexual reproduction individuals only, referred to as Gonozooids; • Sexually reproducing individuals (medusae) and are free-living, referred to as the Blastostyles • Defensive individuals referred to as Dactylozooids

14. Polymorphism in Cnidaria

15. Differences between Medusa & Polyp Polyp Medusa Adapted for a floating life Free swimming Bell or umbrella-shaped Complex structure Velum present Mouth rectangular Mesoglea highly developed • Sedentary or sessile life • Non free swimming • Has tubular body • Simple body structure • Velum absent • Mouth is circular • Mesoglea poorly developed

16. Attack & Defence Feature • Cnidarians also have cnidocytes (specialized cells that function in defense & capture of prey) • Cnidocytes contain organelles called cnida, which are able to evert (extend or retract). • Cnida that sting are called nematocysts. • These nematocysts can immobilize fish for capture, and they can also be used for defence.

17. Coiled & Uncoiled Nematocysts

18. Types of Cnidocytes • Nematocysts: They have barbs which are used to inject venom into prey. This is common to almost all Cnidarians. • Spirocysts: They entangle the prey and do not penetrate and inject the prey with venom. • Ptychocysts: They have tiny threads used for building protective tubes for the organism to habitate; they are not used for prey capture, this feature is common to tube anemones.

19. Features of a Cnidocyte • Cilium: Afine hair which projects above the surface and acts as a trigger. They are absent in Spirocysts. • Cnida: Atough capsule which houses the thread (payload) and contains mixture of chemicals like venom, adhesives etc. • Finger-like-extensible tube: Found on the wall of the cnida so that when a cnidocyte fires, the finger pops out and a set of barbs anchor the prey to release venom.

20. Features of a Cnidocyte • The thread: An extension of the "finger" and coils round the prey until the cnidocyte fires. (The thread is usually hollow and delivers chemicals from the cnida to the target). • Operculum: A lid covering the end of the cnida. (The lid may be a single hinged flap or three flaps arranged like slices of pie). • Cell body: Produces all the other parts.

21. Firing Mechanism of Cnidocytes Four theories were postulated by Hinde (1998): • Rapid contraction of fibers round the cnida may increase its internal pressure. • The thread may be like a coiled spring that extends rapidly when released. • Chemical changes in the cnida's contents may cause them to expand rapidly by polymerization e.g. Chironex(the "sea wasp"). • Chemical changes in liquid of the cnida increases the concentration, so that osmotic pressure forces water in very rapidly to dilute it e.g. Hydrozoa,

22. Figure Showing Firing Sequence Operculum (lid) opened Barb “Finger” turns out Victim's skin Filament Victim's tissues

23. Stimulus Required to Avoid Wastage in FiringCnidocytes • Their cilia detect contact, and nearby sensory cells "smell" chemicals in the water. This combination prevents them from firing at distant or non-living objects. • Groups of cnidocytes are usually connected by nerves and, if one fires, the rest of the group requires a weaker minimum stimulus than the cells that fire first.

24. The Hydrozoa e.g. Hydra vulgaris • Class Hydrozoa includes hydras and the infamous Portuguese Man-of-War, an organism noted for its potent sting, in the medusa form. • Most Hydrozoans exist in the polyp and medusa forms. • For example, some hydrozoans e.g. Obelia exist as an asexually reproducing polyp that alternates with a sexually reproducing medusa form (see next slide).

25. Body plan of Hydra

26. Body plan of Hydra • Epidermis consist: • Epitheliomuscular cells (can self-reproduce) for covering & muscular contraction • Interstitial cells: differentiate to form the cnidoblast, sex cells, buds, nerve cells. • Gland cells- located around the basal disc and mouth, secrete adhesive substance for attachment • Cnidocytes- contain nematocyst for catching prey • Sensory cells found around the mouth for detection of stiimuli • Nerve cells- synapse with sensory cells & other nerve cells

27. Body plan of Hydrozoa: Hydra

28. Transverse Section through body wall of Hydra

29. Body plan of Hydra • Gastrodermis consist: • Nutritive-muscular cells all tall, columnar, large and vacuolated with flagella at one end & microvilli at the other end • Interstitial cells: can transform to form other kind of cells when the need arises. • Gland cells- surround the mouth to aid feeding & digestion. • Mesoglea lies btw the epidermis & dermis and act like a type of elastic skeleton

30. Biology of Hydra: Hydrozoans • May be solitary or colonial • Found in freshwater • Hydra, unlike other hydrozoa exist as solitary(no medusa or colonial forms). • Cylindrical body shape • Body size depends on level of contraction and extension.

31. Movement in Hydra • Somersaulting- Can be found hanging downward • Walking • Looping • Floating • Climbing • Gliding • Swimming

32. Feeding & Nutrition In Hydra • Use Tentacles to capture & direct the food • Exclusively carnivorous feeds on Cyclops, annelids, crustaceans, insect larvae. • Digestion in enteron (gastrovascular cavity)& is both • Extracellularly- the prey is killed by the action of digestive enzymes from gland cells of gastrodermis • Intracellular- occur in the gastrovascular cavity

33. Feeding & Nutrition In Hydra • Tentacles have many cnidoblasts in them for capturing prey • Nematocysts: Immobilize & paralyze the prey • Inside the gastrovascular cavity, gland cells secrete enzyme to digest the food. • Egestion via the mouth.

34. Feeding process in Hydra

35. Reproduction in Hydrozoa: Hydra • Mostly dioecious(male & female parts separate • Reproduce both sexually & asexually by budding • Zygote undergo holoblastic(complete) cleavage to form the blastula. • Cleavage is indeterminate(regulative) • Fertilization: Cyst formed around the embryo which break loose from parent to form the young hydra.

36. Reproduction in Hydrozoa: Hydra

37. Biology of Obelia • Unlike Hydra, has both colonial & medusa form • Sedentary • It’s a plant-like Hydrozoa • Whitish in colour under microscope • Largely carnivorous • Use tentacles to capture prey • Digestion both extra & intracellular

38. Biology of Obelia • Dioecious • Fertilization is external • Mouth open into manubrium • Zygote forms planular larva • Statocysts are gravity receptor for maintenance of balance

39. Obelia life cycle

40. The life cycle of Obelia, (Hydrozoan)

41. Alternation of Generation in Hydromedusoid forms

42. Cnidaria Larva Types • Planula of Renilla sp. • Planula of Anemone Lebruina • Brooded Planula of Urticinia filina (anemone) • Planula of Zoanthidea • Planktotropic planula M. senile • Early & Late planula of stone coral • Actinula larva Ceriantheria

43. Body plan of Scyphozoa: Aurelia aurita

44. Life Cycle of Jelly fish

45. Class Scyphozoa e.g. Jellyfish • Cup-animals with umbrella or bowl-shape • May attain a bell diameter of more than 2m • Most scyphozoans range from 2 to 40cm in diameter • May be colourless, orange or blue • Jellyfish exists predominantly in the medusa form • Mesoglea unusually thick giving the bell a fairly firmed consistency

46. Biology of Class Scyphozoa e.g. Jellyfish • Predominantly Medusa, polyp form occurs only as small larva during life cycle stage. • Thaumatosyphus hexaradiatus are unusual member because medusa are sessile, attached to seaweed • Movement is by rhythmical pulsation of the umbrella • Tentacles may be few or short as in Aurelia aurita or long as in Cyanea

47. Biology of Class Scyphozoa e.g. Jellyfish • Aurelia aurita is a common member • No velum or nerve rings • Oral arms bears the nematocysts • Have sense organ called rhopalium • Nervous system is made of a nerve net with subumbrella net that control bell pulsations

48. Biology of Class Scyphozoa e.g. Jellyfish • Sexes separate • Sperm carried by ciliary current into gastric pouch of female • Internal fertilization • Reproduction: The medusa sperm + egg = Zygote • Ciliated planular larva emerge (Scyphystoma) (hydra-like form), strobila ephyra

49. Class: AnthozoaSea anemone

50. Class Anthozoa • E.g. sea anemones, corals , and sea fans. • Anthozoans are all polyps, no medusa i.e. Not Dimophic • Symbioses between reef-building corals and the dinoflagellates. • In these relationships, corals provide housing and protection for the protists, and the dinoflagellates provide food for the corals. • Members of all of the Cnidaria classes can respond to external stimuli and can use stinging nematocysts for prey capture and defense.