Biology 320 Invertebrate Zoology Fall 2005

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)

4. 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

5. 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

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

7. 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

8. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

17. 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

18. 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

19. 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

20. 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)

21. 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

22. 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

23. 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

24. 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

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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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)

34. 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

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

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

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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

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

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

46. 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

47. 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

48. 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)

49. 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

50. 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