Chapter 7: Cnidarians and Ctenophores

<<Phylum Cnidaria<<

• Ancient group
  • Fossil record dates back over 700 million years
• Over 9000 species
  • Widespread in marine habitats
  • Some occupy freshwater

^^Cnidocytes^^

• Unique and defining feature of cnidarians
• Combined sensory-effector cell
  • Plays a central role in prey capture and defense
• Each house is a cnida
  • Fluid-filled membranous capsule containing a long tubular invagination of the capsule wall
  • 3 general types
  • Most common: nematocyst
    • Often has spines or barbs on surface

^^Nemotocyst^^

• When stimulated, the nematocyst tube everts explosively out of the cell
  • Penetrates skin or sticks to surface
  • Releases toxins
  • Sting
  • Paralysis
  • Trigger = cnidocil
  • Mechanoreceptor

<<Ecological Relationships of Cnidarians<<

• Most abundant in shallow marine habitats
  • Especially tropical regions and warm temperatures
• Other animals feed on cnidarians
  • Rarely serve as food for humans
• Symbiosis Common
• Economic importance
  • Mainly reef-building corals
  • Habitat for fish and other animals
    • Provide substantial food for humans
    • Ecotourism
    • Jewelry and ornaments
    • Coral rock used for buildings and aquaculture

<<Form and Function<<

• Dimorphism and Polymorphism
  • Displayed by many cnidarians
  • Polymorphism
  • Seen in colony that contains several body forms

• Dimorphism
  • Two morphological types
  • Polyp
    • Sedentary or sessile lifestyle
  • Medusa
    • Floating or free-swimming existence

^^Polyp^^

• Oral End
  • Mouth surrounded by tentacles
  • Leads to blind gut
  • Gastrovascular cavity
• Aboral End
  • Usually attached to substrate
  • Via pedal disc
• Asexual reproduction
  • Budding
  • Fission
  • Laceration of pedal disc
  • Torn tissue develops into tiny new polyps

^^Medusa^^

• Bell or umbrella-shaped body
• Tetramerous symmetry
  • Body parts arranged in 4s
• Mouth centered on concave side
• Tentacles extend from rim of bell

^^Locomotion^^

• Colonial Polyps permanently attached
• Hydras and Anemones
  • Move freely by gliding on pedal disc
• Medusae
  • Most move freely

^^Life Cycles^^

• Polyps and medusae play different roles
• Varies among classes
• In general
  • Zygote —> Planula Larva
  • Free-swimming
  • Settles and metamorphoses into a polyp
    • May make other polyps asexually
    • May make medusae asexually
  • Medusae
  • Reproduce sexually
  • Dioecious
  • Individuals either male or female
  • Produce gametes

• Sea anemones and corals
  • All polyps (all the time/no medusa stage)

^^Feeding and Digestion^^

• Cnidarians are predators
  • Prey on variety of organisms
  • Drawn into gastrovascular cavity
    • Extracellular Digestion
    • Via enzymes secreted by gland cells in cavity
    • Intracellular digestion
    • Nutritive-muscular cells phagocytize many food particles
    • Ameboid cells remove undigested particles that are eventually expelled from body
• Body wall composed of 2 layers
  • Epidermis
  • Outer
  • Ectodermal
  • Gastrodermis
  • Inner
  • Endodermal
  • Mesoglea lies between the layers
  • Gelatinous extracellular matrix (ECM)
  • Thicker in medusae
  • Contains cells in some species

<<Gastrodermis<<

• Layer of cells lining gastrovascular cavity
• Composition
  • Nutritive-Muscular Cells
  • Contain myofibrils
  • Form muscular cells
    • Weak
  • Interstitial Cells
  • Can transform into other cell types
    • Undifferentiated stem cells
  • Gland Cells
  • Tall cells
  • Secrete digestive enzymes
  • Cnidocytes
• ^^Hydrostatic Skeleton^^
  • Fluid enclosed within a muscular wall to provide support necessary for muscle action
  • Increase volume of water in gastrovascular cavity

<<Epidermis<<

• Composition
  • Epitheliomuscular Cells
  • Form most of epidermis
  • Functions
    • Covering body
    • Muscular contraction
    • Shortens body or tentacles
  • Interstitial Cells
  • Gland Cells
  • Cnidocytes
  • Sensory Cells
    • Synapse with nerve cells
  • Nerve Cells
    • Form synapses with sensory cells and other nerve cells
    • Response to stimuli

• Nerve Net
  • Diffuse nervous network
  • Occurs at the base of the epidermis and gastrodermis
  • Forms two interconnected nets

<<6 Classes<<

^^Anthozoa^^

• Shallow and deep water
• Tropical – polar seas
• Solitary or colonial
• Great variety in size
• Many have skeletal support

^^Myxozoa^^

• 2180+ species
• Obligate parasites (only can survive with the help of a host)
• Life cycle
  • Two aquatic hosts
  • Fish
  • Annelid worm (or bryozoan)
• Economic impact
  • Damage to commercially valuable fishes
  • Salmon, trout, etc.
  • Causes whirling disease
    • Common in Western and Central US, Canada

^^Staurozoa^^

• No medusa phase in life cycle
• Anatomy
  • Stalked body
  • Basal adhesive disc to attach to a substrate
  • White spots = clusters of nematocysts
• Reproduction
  • Sexual
  • Creeping planula larvae

^^Scyphozoa^^

• Includes most of the larger jellies
  • Largest: Lion’s mane jellyfish (Cyanea capillata)
  • Bell diameter 2+ meters
• Medusae
  • No velum
  • Variety of bell shapes
  • Shallow saucer, helmet, goblet
  • Many have scalloped margin
  • Each notch bears:
  • Rhopalium (sense organ)
    • Has statocyst
    • Balance
    • 2 sensory pits with sensory cells
    • Some: Ocellus
    • Simple photoreceptor
• Reproduction
  • Separate sexes
  • Internal fertilization
  • Sperm carried by ciliary currents into female
  • Zygote development
  • In seawater
  • Brooding in folds of oral arms
  • Planula larva
    • Develops into scyphistoma
    • Hydra-like form
    • May bud to make other polyps
    • Forms ephyrae and becomes strobila
    • When ephyrae break free, they grow into mature jellies

^^Cubozoa^^

• Predominant body form = medusa
  • Bell is box-shaped
  • Tentacles occur at each corner of the square of the umbrella margin
• Rhopalia
  • Each houses 6 eyes
  • 3 different types
  • Some are image-forming
• Strong swimmers and voracious predators
  • Main food: fish
• Some species exhibit complex mating behaviors
  • Intertwine tentacles
  • Male passes spermatophore to female

^^Hydrozoa^^

• Most are marine and colonial in form
• Typical life cycle includes both asexual polyp and sexual medusa stages
  • Exceptions
  • Hydra
    • No medusa stage
  • Certain marine hydroids do not have free medusae
  • Some hydrozoans occur only as medusae
• Nerve net
  • Usually concentrated into two nerve rings at the base of the velum
  • Bell has lots of sensory cells
  • Statocysts — small organs of equilibrium
  • Ocelli – light-sensitive organs

• Siphonophores
  • Form floating colonies
  • Example: Physalia
  • Portuguese Man-of-War
  • Colonies include:
    • Several types of modified medusae and polyps
    • Rainbow-hued float (modified polyp)
    • Air sac filled with secreted gas
    • Carries the generations of individuals that bud from it and hang suspended in the water
      • Several types of individual polyps
      • Feeding
      • Reproductive
      • Long stinging tentacles
      • Jelly polyps

<<Class Anthozoa Subclasses<<

• ^^Zoantharia/Hexocorallia^^
  • Sea anemones, hard corals, and others
  • Hexamerious (body plan)
  • Based on 6, or multiples of 6
  • Simple tubular tentacles
  • Order: Scleractinia
  • Subclass Zoantharia
  • Stony corals
  • Resemble miniature sea anemones
  • Live in calcareous cups
    • Made of calcium carbonate
    • They secrete it
    • Provide safety when polyps not feeding
    • Exoskeleton
    • Secreted below living tissue
    • May become massive
    • Living coral forms sheet of tissue over the surface
• ^^Ceriantipatharia^^
  • Tube anemones
  • Thorny corals
• ^^Octocorallia^^
  • Soft Corals
  • Sea pens
  • Sea fans
  • Sea pansies
  • Other gorgonian corals
  • Octomerous symmetry
    • 8 tentacles
  • Almost all colonial
    • Gastrovascular cavities of polyps communicate through solenia
    • System of gastrodermal tubes
    • Run through extensive mesoglea
  • Contain gorgonin
    • Stiff/flexible/protein
    • Chemically similar to keratin and collagen
    • Role: structural support
  • Octomerous
  • Built on a plan of eight
    • 8 tentacles arranged around oral disc

<<Coral Reefs<<

• Among the most productive of all ecosystems

• Large formations of calcium carbonate

  • Laid down by living organisms over 1000s of years

• Mutualism

  • Zooxanthellae
  • Algae
  • Live in coral tissues
  • Produce food for themselves and coral
  • Coral provide shelter for algae

• Tremendous intrinsic and economic value

  • Under threat—mostly from humans
  • Overenrichment
    • Fertilizer
    • Sewage
  • Overfishing
  • Pesticides/herbicides
  • Oil spills
  • Tourists
  • Global climate change
    • Coral bleaching
    • Corals become white & brittle
    • Expel zooxanthellae when under stress

<<Phylum Ctenophora<<

• ~150 species
• All marine
• Ctenes
  • 8 longitudinal rows of transverse plates bearing long, fused cilia
  • Beating of cilia in unison aids in locomotion
• Tissue level of organization (like cnidarians)
  • Diploblastic
  • No true organs
  • Do have true muscle layer (develops from endoderm)
• No nematocysts
• Bioluminescence
• Nerve Net
  • Similar to cnidarians
• Statocyst
  • Organ of equilibrium
• Monoecious
  • Individuals have both an ovary and a testis
  • Spawn or brood eggs
  • Free-swimming larvae