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Biology 320 Invertebrate Zoology Fall 2005

Biology 320 Invertebrate Zoology Fall 2005. Chapter 7 – Phylum Cnidaria. Phylum Cnidaria. Hydra, anemones, stony corals, soft corals, hydroids, and jellyfishes All are marine (approx. 10,000 spp.) or freshwater (approx. 20 spp.), with no terrestrial spp. May be colonial or solitary

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Biology 320 Invertebrate Zoology Fall 2005

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  1. Biology 320Invertebrate ZoologyFall 2005 Chapter 7 – Phylum Cnidaria

  2. Phylum Cnidaria • Hydra, anemones, stony corals, soft corals, hydroids, and jellyfishes • All are marine (approx. 10,000 spp.) or freshwater (approx. 20 spp.), with no terrestrial spp. • May be colonial or solitary • Responsible for the building of coral reefs, which are rivaled in diversity only by tropical rainforests • Posses cnidocytes (cnid = nettle)

  3. General Form and Function • General body structure resembles a gastrula • Posses a cavity known as a coelenteron (gastrovascular cavity) • Have a mouth surrounded by tentacles • Radially symmetrical around an oral-aboral axis • Useful because food, predators, etc. may approach from any angle

  4. Display two body forms, often in one life-cycle • Polyp • Resembles a flower and stem • Has a pedal disc and an oral disc • Sessile and benthic, with a “mouth-up” orientation • Medusa • Umbrella or bell-shaped • Manubrium is more defined and resembles an elephant’s trunk with a mouth at the end • “Mouth-down” orientation

  5. In general, the body wall is composed of three tissue layers • Epidermis - epithelium • Gastrodermis – epithelium • Mesoglea (a gelatinous ECM) – connective tissue • Cnidarians are diploblastic

  6. Colonial Cnidarians • Occurs when a juvenile replicates via budding, however, buds do not separate • Produces zooids • Means “tiny animal” • Pronounced “ZOE-oid” • Resemble the juvenile • Preserves SA:Vol because zooids are small and thus have a large SA:Vol • Predisposition to filter feeding • Multiple mouths and feeding appendages • Broad distribution • Small size

  7. Three main types of colonies, varying in complexity • Stolonate – posses stolons • Coenosarc – posses a coenosarc and solenia • Fruticose – typically upright and branching with a plantlike / feathery appearance • Two types of budding • Fixed-length – as found in Obelia • Axial-polyp

  8. Cnidarian Skeletons • Exceeds the diversity of poriferan skeletons • Exoskeletons of: • Chitinous periderm – some hydrozoans • Calcium carbonate – stony corals • Shell fragments covering the epidermis – some anemones • Endoskeletons of: • Fibers and spicules, similar to those of poriferans – soft corals • Columns of cells containing turgid vacuoles – some hydrozoans • Hydrostatic skeletons – Hydra, many anemones

  9. Musculature and Movement • Posses antagonistic sheets of muscle • Circular smooth muscle – gastrodermis • Longitudinal smooth muscle - epidermis • Medusae posses coronal muscles, encircling the subumbrella. These muscles are antagonized by the elastic mesoglea

  10. Cnidarians perform a wide variety of movements • Shortening, extending and bending in polyps • Constriction of bells in medusae (facilitates swimming) • Inch-worming and somersaulting in polyps • Movement of feeding appendages for prey manipulation • Retraction of the subumbrella in polyps and medusae

  11. Cnidarian Nervous Systems • Two nerve nets • Base of epidermis • Base of gastrodermis • Nets are joined by nerve bridges that span mesoglea • Nerve impulses can travel any direction • Important because of radial sensory structures • Medusa posses nerve rings, musculature, ganglia, and sense organs around bell • Statocysts • Ocelli • Chemoreceptors • Mechanoreceptors

  12. Cnidocytes • For prey capture and defense • Grasp terminology first • Cnidocytes (cells) posses cnida (fluid-filled capsule with tubule) CnidocyteCnidaFunction Nematocyte Nematocyst Sting / release toxins Spirocyte Spirocyst Tubule contains sticky threads used for adhesion

  13. Cnidocytes are fairly ubiquitous • Throughout epidermis • On tentacles • Often in gastrodermis • Nematocyst firing • Tubule coiled in capsule • Triggered by a combination of chemical and mechanical cues from prey (rarely fires on accident) • Cytoplasmic water rushes in and ejects tubule • Toxins (proteins) may interfere with Na+/K+ pumps or degrade cell membranes • Hydra discharges 25% of nematocysts eating one brine shrimp • Replaced in 24hr

  14. Nutrition and Internal Transport • Blind gut called coelenteron or “gastrovascular cavity” • Coelenteron may posses septa to increase SA for: • Digestion • Absorption • Gas exchange • Excretion • Reproduction • Hydrostatic skeletal support • Various canals in medusae, that radiate out from central stomach • Radial canals • Ring canals • In polyps and medusae, coelenteron typically branches into each tentacle

  15. When prey is caught, mouth opens and tentacles stuff prey in • Enzymes digest extracellulary • Gastrodermal cells absorb monomers • Larger particles are digested intracellularly • Slow process; may take several days • Wastes ejected through mouth

  16. Many spp. posses photosynthetic endosymbionts • May account for 90% of nutrition in some • Often defined patterns of fluid circulation around coelenteron • Ciliated gastrodermis • Muscular contractions

  17. Gas and Waste Exchange • All gas exchange occurs across general body surfaces • Tentacles • Body wall • Waste exchange (excretion) occurs across body wall • Ammonia is primary waste product (aqueous) • Marine animal physiology does not require water conservation • Terrestrial invertebrates have many structural adaptations for conserving water during excretion (as we will see later)

  18. General Reproduction • Amazing regenerators • Can lose oral end and regrow it • Living anemones can fully recover from dissections • Clonal reproduction is common among polyps, but is less common in medusae

  19. Sexual reproduction in most • Some are monoecious (hermaphroditic) • Most are dioecious (separate sexes) • Germ cells usually develop in gastrodermis • Gametes are exocytosed into coelenteron • Typically extruded = external fertilization • Retained in some spp. = internal fertilization • Zygote typically develops into planula (swimming larva) • Settles (aboral end down) • Develops into a juvenile polyp

  20. Class Anthozoa • “Flower animals” • Sea anemones, corals, sea fans, sea pens, etc. • Largest class, at 6000 marine spp. • Solitary or colonial • All lack medusa phase of lifecycle

  21. Anthozoan Body Form • Long pharynx attaching to coelenteron • Many septa, 6-192 depending on size of species • Some posses acontia • Stringy filaments that are attached near base of septa • Heavily armed • May spew from mouth in anemone deflates • Stony corals lack these structures

  22. Musculature • Mostly epidermal and gastrodermal epitheliomuscular cells • Epidermal musculature controls tentacles and oral disc • Gastrodermal musculature controls body column

  23. Retraction • Controlled by longitudinal septal muscles called retractors • Tentacles and oral disc are deflated of coelenteric fluid • Pulled inside body column • Mesogleal sphincter muscle closes opening like a draw-string • Must remove coelenteric water to retract. Siphonoglyph cilia beat inwards to pump water back in

  24. Diversity of Class Anthozoa • Subclass Zoantharia (Hexacorallia) • Hexamarous symmetry (septa and tentacles in multiples of 6) • Order Actiniaria - anemones • Order Scleractinia – stony corals • Subclass Alcyonaria (Octocorallia) • Octomerous symmetry (septa and tentacles in multiples of 8) • Order Stolonifera – organ - pipe corals • Order Gorgonacea – sea fans, whips, and plumes • Order Pennatulacea – sea pens, feathers, and pansies

  25. Subclass Zoantharia, Order Actiniaria • Sea anemones • 1350spp. • Average size • 1.5cm – 10cm long • 1cm – 5cm diameter • Largest are over a meter wide and a meter tall • Often brightly colored • Some have unique methods of locomotion • Burrowing via peristalsis • Walking on tentacles • Swimming by thrashing tentacles

  26. Methods of nutrition • Carnivores • Suspension feeding - secrete mucus, trap particles on tentacles, and move to mouth • Photosynthate - may posses two sets of tentacles: false for photosynthesizing (exposed during day) and true (exposed during night) • Can reproduce asexually via pedal laceration • Gonads are located in septa

  27. Interesting ecological relationships • Hermit crabs will wear anemone on shell; will transfer to new shell if anemone doesn’t transfer itself • Anemone gets substrate, transportation to food, protection from predators, and access to mates • Crab gets camouflage, nematocyst protection • Clown fish has surface mucus that lacks nematocyte-triggering compounds • Anemone gets food attracted by fish, and removal of sediment and necrotic tissue • Clownfish gets protection and food scraps

  28. Subclass Zoantharia, Order Scleractinia • Stony corals • 3600 spp., closely related to anemones • Secrete a CaCO3 exoskeleton • Can weigh tons • Produce cups known as corallites that they can retract into • Puffer fish are coral specialists • Most are colonial with polyps 1mm to 3mm in diameter

  29. Coral can be very colorful due to photosynthetic endosymbionts • Algae are often released in conjunction with gametes • Coral reefs are in danger • Coral bleaching (algae partially or completely expelled) may occur under stressful environmental conditions • Incorrect light intensity (including UV) • Salinity • Temperature (even 1°C)

  30. Subclass Alcyonaria, Order Stolonifera • Most octocorallians are soft corals, and do not produce a CaCO3 shell • Most lack nematocysts and produce noxious chemicals to deter predators • Most are more tolerant of environmental fluctuations • Organ – pipe corals are included in Order Stolonifera

  31. Subclass Alcyonaria, Order Gorgonacea • Plantlike sea whips, fans, and plumes • Highly branched • Endoskeleton is an axial rod made of gorgonin (highly cross-linked collagen)

  32. Subclass Alcyonaria, Order Pennatulacea • Sea pens, feathers and pansies

  33. Medusozoa • Medusa phase present in life cycle • Planula polyp medusa • Tetramerous (multiples of four) radial symmetry • Cnidae are all nematocysts • Two major classes • Scyphozoa – large jellies • Hydrozoa – small jellies, Hydra, and hydroids

  34. Class Scyphozoa, Body Form • 200 spp. of large jellies • Polyps are small and funnel shaped • Known as scyphistomae • Coelenteron is divided by four septa • Also have four septal funnels • Circulate water to gonads in adults

  35. Medusa bells • Typically 2-40cm in diameter • Some greater than 2m • Some brightly colored • Manubrium is divided into four oral arms • Tentacles are located around the periphery of the bell

  36. Coelenteron is divided into four gastric pockets by septa • Four pairs of gonads in septa • Four septal funnels • Many have radial canals and marginal canals • Gastrodermal cilia circulates water

  37. Lappets – rounded lobes of umbrellar margin • Rhopalia – sensory organs found in grooves between lappets • Statocyst • General mechanoreceptor • Possibly a chemoreceptor • Sometimes a photoreceptor • Jellies have a nerve net and nerve ring

  38. Scyphozoan reproduction • Scyphistomae undergo asexual reproduction • Differentiate into a strobila from which stacked miniature medusae separate via transverse fission • Process is known as strobilation • Juvenile medusa are known as ephyra • Adult medusae sexually reproduce

  39. Diversity of Class Scyphozoa • Five orders Semaeostomeae – Aurelia Rhizostomeae - Stomolophus

  40. Order Coronatae - Linuche Order Stauromedusae - Haliclystus Order Cubomedusae - Chironex

  41. Class Hydrozoa • Hydra and hydroids • Other examples: fire coral and Portuguese man of war • 3000 spp. • Mainly colonies consisting of polyp zooids and medusa zooids • Oftentimes, medusa form as buds, but fail to be liberated from colony • Hydra lifecycle does not include medusa phase

  42. Hydrozoan Body Plans • Two forms of polyps • Athecate (A form) • Thecate (L form) • Zooids are usually 1mm, or less, in length • Large SA:Vol • Lack gastrodermal septa

  43. Medusae • Arise as lateral buds from colony, rather than via strobilation • Many posses a velum, an iris diaphragm on subumbrellar margin that aids in swimming • Colonies • Sessile, benthic, colonies are called hydroids • Typically resemble plants or seaweeds • May be any of the three colony forms (stolonate, coenosarcal, or fruticose)

  44. A-form colonies • Athecate – lack a theca (extension of the periderm that acts as protective cup) • Periderm ends at attachment point of zooids • Typically grow via axial-polyp budding

  45. L-form colonies • Periderm forms a wine-glass shape theca • Hydranth (water – flower) may retract into theca • Theca may have a hinged lid (operculum) • Grow via fixed-length budding

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