Cnidaria and Ctenophora Lecture Notes

Phylum Cnidaria

Learning Outcomes

  • Distinguishing features of Phylum Cnidaria.
  • Organization of the Cnidarian body; comparison of polyp and medusa stages.
  • Definitions: cnidocyte, cnida, nematocyst, colonial, dimorphism, polymorphism, gastrovascular cavity, nerve net, mesoglea, hydrostatic skeleton, epitheliomuscular cells.
  • Structure of the cnidarian nervous system.

Cnidarian Phylogeny

  • Cnidaria includes Anthozoa and Medusozoa.
  • Anthozoa includes:
    • Myxozoa
    • Staurozoa
    • Scyphozoa
    • Gut with septal filaments
    • Polypodium hydriforme
    • Infective stage in fish oocyte
    • Siphonoglyph
    • Anthozoan pharynx
    • Polar capsule
    • Intracellular cell division
    • Hexaradial and octaradial symmetry
  • Medusozoa includes:
    • Cubozoa
    • Hydrozoa
      • Trachyline-like hydrozoa
      • Man-o-war
      • Hydroids
      • Other hydrozoa
    • Strobilation
    • Complex eyes
    • Velarium
    • Boxlike medusa body
    • Polyp lost
    • Creeping planula without cilia
    • Rhopalium
    • Velum in medusae
    • Polyp reduced or lost
    • Medusae produced by lateral budding; muscles from entocodon
    • Medusoid body form
    • Motor nerve net
    • Some or all stages endoparasitic
    • Primary polyp tentacles hollow
    • Mouth surrounded by solid tentacles
    • Planula larva
    • Cnidocytes
    • Radial, polypoid body form

General Characteristics of Cnidarians

  1. Cnidocytes present, housing nematocysts (stinging organelles).
  2. Aquatic (mostly marine, some freshwater).
  3. Radial or biradial symmetry around a longitudinal axis; no definite head.
  4. Two individual types: polyps and medusae.
  5. Adult body diploblastic: epidermis and gastrodermis derived from ectoderm and endoderm, respectively.
  6. Mesoglea (extracellular matrix) between body layers; may contain cells and connective tissue from ectoderm.
  7. Incomplete gut (gastrovascular cavity), often branched or divided with septa.
  8. Extracellular digestion in gastrovascular cavity; intracellular digestion in gastrodermal cells.
  9. Extensible tentacles encircle mouth/oral region.
  10. Muscular contractions via epitheliomuscular cells (outer longitudinal fibers, inner circular fibers); modifications in hydrozoan medusa (independent ectodermal muscle fibers) and complex cnidarians.
  11. Sense organs: statocysts (balance) and ocelli (photosensitive); complex eyes in Cubozoa.
  12. Nerve net with symmetrical and asymmetrical synapses; diffuse conduction; two nerve rings in hydrozoan medusae.
  13. Asexual reproduction by budding in polyps (clones/colonies); some colonies exhibit polymorphism (different polyp types).
  14. Sexual reproduction by gametes in all medusae and some polyps; monoecious or dioecious; holoblastic indeterminate cleavage; planula larval form.
  15. No excretory or respiratory system.
  16. No coelomic cavity.

Cnidocytes

  • Cnidocyte: specialized cell containing a fluid-filled capsule called a cnida.
  • Located throughout epidermis and in select regions of gastrodermis.
  • Types of cnidae:
    • Nematocysts: stinging.
    • Others: entangle prey, adhesive for attachment/locomotion.
  • Nematocyst structure: coils of inverted thread, cnidocil, operculum.

General Body Plan

  • Saclike body plan.
  • Central gastrovascular cavity.
  • Body wall: outer epidermis, inner gastrodermis, mesoglea in between.
  • Two major body forms: polyp and medusa.

Solitary vs. Colonial

  • Many cnidarians are colonial, composed of interconnected individuals called zooids.
  • Some colonies display polymorphism.
  • Formed via asexual budding.

Feeding and Digestion: Gastrovascular Cavity

  • Both extracellular and intracellular digestion occur.
  • Extracellular:
    • Enzymes break down food outside the cell.
    • Nutrients absorbed into cells.
  • Intracellular:
    • Food particles engulfed via endocytosis.
    • Enzymes break down food in food vacuoles.
    • Nutrients distributed.

Nervous System

  • Two interconnected nerve nets:
    • One at the base of the epidermis.
    • One at the base of the gastrodermis.
  • Bidirectional transmission at synapses.
  • Medusae also have a nerve ring and marginal sense organs called rhopalia.

Epitheliomuscular Cells

  • Contractile fibers at bases:
    • Epidermal and gastrodermal epitheliomuscular cells
  • Polyps:
    • Sheets of longitudinal and circular fibers
  • Medusae:
    • Coronal muscle around bell margin
    • Radial muscles.

Support Systems

  • Mesoglea
  • Exoskeletons
  • Endoskeletons
  • Hydrostatic skeletons:
    • Fluid within the gastrovascular cavity acts as a support structure

Reproduction

  • Asexual reproduction:
    • More common in polyps but can also occur in medusae
    • Budding, fragmentation
  • Sexual reproduction:
    • Most adult cnidarians are dioecious; some species are monoecious
    • Sexual reproduction can occur in both polyps and medusa depending on the group
  • Life cycles:
    • Most classes of cnidarians (except Anthozoa and Staurozoa) include both polyp and medusa stages

Learning Outcomes 2

  • Distinguishing features of Class Anthozoa
  • Definitions: siphonoglyph, pedal disc, septa, acontia threads, zooxanthellae
  • Describe the anatomy of a reef-building coral
  • Explain what coral bleaching is and why it is problematic.
  • Describe the life cycle of an Anthozoan
  • Distinguish between subclass Hexacorallia and subclass Octocorallia.

Cnidarian Phylogeny

  • Same as before in the lecture notes.

Class Anthozoa

  • All marine
  • Exclusively polyps; no medusa stage
  • Solitary or colonial
  • Gastrovascular cavity is subdivided by septa
  • Three subclasses: Hexacorallia, Octocorallia, and Ceriantipatharia

Reproduction and Life Cycle

  • Asexual reproduction
  • Sexual reproduction
  • Fertilized egg
  • Embryo
  • Planula larva
  • Young anemone
  • Symbionts

Anthozoa (Hexacorallia): Sea Anemone

  • Siphonoglyph = ciliated groove that generates water currents
  • Acontia threads contain nematocysts and gland cells and are protruded through mouth or pores in the body wall to aid in prey capture or defense Pedal Disc

Muscular System

  • Longitudinal fibers in the tentacles and oral disc are located in the epidermis
  • Longitudinal fibers in the body column are within the septa (gastrodermal)
  • Circular muscles are gastrodermal

Anthozoa (Hexacorallia):

Reef Building Corals

  • Sclerosepta
  • Calcium carbonate skeleton

Mutualisms

  • Coral with symbiotic zooxanthellae

Coral Bleaching

  • The central, southern, and northern sections of the Great Barrier Reef were bleached by warmer ocean water

Coral Aggression

  • Corals compete for space and resources through various aggressive mechanisms.

Subclass Octocorallia: Soft Corals & Gorgonians

  • All are colonial & gastrovascular cavities communicate via solenia (gastrodermal tubes)
  • Octomerous symmetry
  • Eight pinnate tentacles
  • Endoskeletal support system composed of limy spicules or horny protein
  • Great variation in colony form

Learning Outcomes 3

  • Describe the distinguishing features of Class Scyphozoa
  • Describe the anatomy of a moon jelly: identify the gonads, gastric pouches, and canals that make up the gastrovascular system
  • Describe the lifecycle of Scyphozoans and contrast it with the lifecycle of anthozoans
  • Define: statocyst, rhopalium, strobila, scyphyistoma, ephyra

Cnidarian Phylogeny

  • (Same as before)

Class Scyphozoa

  • True jellyfish.

Scyphozoa Anatomy

  • Subumbrellar surface
  • Manubrium
  • Gonad in gastric pouch
  • Oral arm
  • Tentacles
  • Rhopalium
  • Ocellus
  • Sensory lappet
  • Statocyst

Statocysts

  • Statocyst of Liriope tetraphylla
  • A longitudinal section of pendant trachymedusan statocyst, after electron-optic images, the stiff sensory cilia serve to sense the motion of the organ, note that the core is derived from entodermal tissue (green). The whole structure can be homologized to a modified tentacle.

Alternative Feeding Strategies

  • Some jellyfish actively hunt, while others are passive predators.
  • Some species also utilize symbiotic relationships.

Life Cycles

  • Planula Larva
  • Scyphistoma
  • Strobila
  • Ephyra Larva
  • Adult Medusa

Learning Outcomes 4

  • Describe the distinguishing features of Class Cubozoa
  • Describe the rhopalia and the function of eyes

Cnidarian Phylogeny

  • (Same as before)

Class Cubozoa

  • Box jellyfish.

Cubozoa Anatomy

  • Gastric filaments
  • Manubrium
  • Radial pouch
  • Gonads
  • Circular canal:
  • Rhopalium
  • Pedalium
  • Tentacle

Cubozoan Rhopalium

  • Bell
  • Rhopalium
  • Pedalial Canal
  • Pedalium
  • Tentacle

Cubozoan Rhopalium

  • The visual system of the cubozoan Tripedalia cystophora comprises four sensory structures called rhopalia.
  • Each rhopalium carries six eyes of four morphological types (lower lens eye LLE, upper lens eye ULE, pit eye PE, and slit eye SE) and a light-sensitive neuropil (NP, red broken line).
  • The eyes are responsible for the image formation in the animal, and the light-sensitive neuropil is thought to be involved in diurnal activity.

Cubozoan Life Cycle

  • Mature medusae mate via spermcasting aggregations and internal fertilization.
  • Embryos released into the water.
  • Planulae hatch from blastocysts with perisarc.
  • Sessile polyps adhere to the substrate and bud additional polyps asexually.
  • Polyp tentacles retract, and the apical type metamorphosis into a medusa.
  • Juvenile medusa is released.

Injuries from Cubozoa

  • Chironex fleckeri is one of 51 known species of box jellyfish, whose venom is among the world's deadliest.

Learning Outcomes 5

  • Describe the distinguishing features of Class Hydrozoa
  • Give examples of organisms found within this class
  • Describe the structure of a hydrozoan colony
  • Describe the lifecycle of a hydrozoan colony and compare and contrast it with the Scyphozoan and Anthozoan life cycles
  • Explain what is unique about the life history of the immortal jellyfish

Cnidarian Phylogeny

  • (Same as before)

Class Hydrozoa

  • Diverse group including colonial and solitary forms, with both polyp and medusa stages.

Colonial Hydrozoa

  • Typical colony has a base, a stalk, and one or more terminal zooids.
  • Perisarc:
    • Protective, nearly transparent, non-living chitinous sheath that helps to support the colony.
  • Zooids:
    • Polymorphic:
      • Gastrozooids (Hydranths)
      • Gonozooids (Gonangia)

Colonial Hydrozoa: Reproduction

  • Alternating Asexual and Sexual Generations
    • Asexual: Budding
      • Generates new feeding polyps (hydranths) or medusa buds (produced in gonangia)
      • Increases the size of the colony
    • Sexual
      • Young medusae leave the colony as free-swimming individuals
      • Produce gametes that develop into planula larvae
      • Settles out and starts a new colony

Class Hydrozoa: Order Siphonophora

  • Colonial hydrozoans with specialized zooids forming complex floating colonies like the Portuguese man-of-war.

Hydrozoan Medusae

  • Radial canal
  • Manubrium
  • Gonads
  • Gastrodermis
  • Nerve rings
  • Mesoglea
  • Gastrovascular cavity
  • Exumbrella
  • Mouth
  • Subumbrella
  • Ring canal
  • Tentacular bulb
  • Oral lobe
  • Adhesive pad
  • Velum
  • Statocyst

The Immortal Jellyfish

  • Ontogeny Reversal vis Transdifferentiation
  • 'Ontogeny reversal' in Turritopsis dohrnii
    • Immature medusa
    • Ball of tissue
    • Mature medusa fertilizing eggs
    • Polyp
    • Young polyp
    • Planula

Class Hydrozoa: Hydra

  • Solitary, freshwater polyp
    • Lives under submerged vegetation in cool, clean freshwater pools and streams
    • Found throughout the world with several species in North America
    • Small: can extend to 25-30mm
  • Typical polyp body plan
    • Long slender tube
    • Oral end has a mouth is located on a small cone (hypostome) surrounded by tentacles
  • Tentacles are hollow, and part of the gut cavity extends into each of them
    • Aboral surface has a basal (or pedal) disc for attaching to the substrate

Learning Outcomes 5

  • Describe the distinguishing features of Phylum Ctenophora
  • Compare and Contrast Phylum Ctenophora and Phylum Cnidaria
  • Describe locomotion within this phylum

Phylum Ctenophora

  • Comb jellies.

General Ctenophora Characteristics

  1. Eight rows of combs (ctenes) arranged radially around the body
  2. Colloblasts, adhesive cells used in prey capture, present in most
  3. Entirely marine
  4. Symmetry biradial; arrangement of internal canals and position of the paired tentacles change the radial symmetry into a combination of radial and bilateral
  5. Body ellipsoidal or spherical in shape with oral and aboral ends; no definite head
  6. Adult body with gelatinous middle layer containing muscle cells; the derivation of the middle cellular layer is controversial (ectodermal vs. endodermal) affecting status as diploblastic or triploblastic
  7. Complete gut; the mouth opens into the pharynx; the gut has a series of branching gastrovascular canals; the gut terminates at the anal pore; wastes exit via the anal pore and mouth
  8. Extracellular digestion in the pharynx
  9. Two extensible tentacles occur in most
  10. Muscular contractions via muscle fibers (cells), not epitheliomuscular cells
  11. Nervous system consisting of a subepidermal plexus concentrated around the mouth and beneath the comb plate rows; an aboral sense organ (statocyst)
  12. Reproduction monoecious in most; gonads (endodermal origin) on the walls of the digestive canals, which are under the rows of comb plates; mosaic or regulative cleavage within embryos; cydippid larva
  13. No respiratory system
  14. No coelomic cavity

Anatomy

  • Tentacle
  • Comb row
  • Tentacle
  • sheath
  • Statocyst
  • Anal canal
  • Aboral canal
  • Gastrovascular
  • Canals
  • Stomach
  • Tentacle
  • Adhesive granule
  • Nucleus
  • Helical thread
  • Nerve cell
  • Root
  • Collenchyme
  • Paragastric canal
  • Pharynx
  • Mouth
  • Statolith
  • Comb row
  • Comb plates

Locomotion: Ctenes (comb rows)

  • Ctenophores use ctenes, rows of fused cilia, for locomotion.
  • The coordinated beating of the ctenes propels the animal through the water.
  • Different body shapes & arrangements of ctenes exist within the phylum.

Feeding

  • Ctenophores are carnivorous, using colloblasts on their tentacles to capture prey.
  • Some species engulf prey directly with their mouths.

Invasive Species

  • The invasive ctenophore Mnemiopsis leidyi overwinters in high abundances in the subarctic Baltic Sea.