Biology 320 Invertebrate Zoology Falls 2005 PowerPoint Presentation

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Biology 320Invertebrate ZoologyFall 2005 Chapter 7 – Phylum Cnidaria

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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)

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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

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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

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

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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

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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

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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

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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

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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

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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

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

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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

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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

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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

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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

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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)

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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

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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

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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

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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

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

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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

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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

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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

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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

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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

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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

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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)

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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

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

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Subclass Alcyonaria, Order Pennatulacea Sea pens, feathers and pansies

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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

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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

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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

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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

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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

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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

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

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

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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

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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

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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)

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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

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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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Zooid Terminology Can be a bit confusing, so spend some time with this Monomorphic colonies Only consist of gastrozooids (feeding hydranths) Reproduce by releasing medusa buds Polymorphic colonies Posses other types of zooids, in conjunction with gastrozooids Gonozooids – modified gastrozooids that bud gonophores (permanently attached medusae that produce gonads) Dactylozooids – have nematocysts for protection. May capture food and transfer to gastrozooids

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Typical Life Cycle

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Diversity of Class Hydrozoa Order Anthoathecatae Athecate (A-form) Examples: Hydra – gonochoric, also reproduce asexually, but lack medusa phase Millepora – fire corals. Colonize axial rods of dead gorgonians. Reef builders that have a nasty sting that burns like fire Velella – by-the-wind sailor has a float with a sail, displays extreme polymorphism, with zooids that are suspended mouth-down

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Male and female Hydra. Life cycle lacks medusa phase Millepora Velella

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Order Siphonophora Polymorphic A-form hydrozoans Pedal end of polyp is a gas-filled float known as a pneumatophore Buds arise from column of polyp in sets called cormidia. Oldest is near float, and consist of: Gastrozooid Gonozooid Dactylozooid – long tentacle with nematocysts Bract – fleshy overhang that protects zooids Prime example is Physalia, the Portuguese man of war

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Order Leptothecatae L-form hydroids Feather-shaped colonies Obelia is the most famous representative

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Order Limnomedusae L – form Polyp and medusa phase Gonionemus is prime example One additional order, Trachylina Life cycles are devoid of the polyp phase in all members of this order Therefore, neither A-form or L-form