CHAPTER 13
Phylum Cnidaria
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Phylum Cnidaria
Fossil specimens dated to over 700 million
years ago
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General Characteristics
Soft bodied
 Carnivorous
 Stinging tentacles
 Examples: Jellyfish, sea anemones
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Phylum Cnidaria
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Classes of Cnidaria
 Hydrozoa (Portuguese Man-of-War, Hydra)
 Scyphozoa (True Jellyfish - Aurelia)
 Anthozoa (Sea Anenome, Corals)
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Phylum Cnidaria
Characteristics of Phylum Cnidaria
 All are aquatic and mostly marine
 Symmetry
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Two body types
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Radial
Free-swimming medusa
Sessile polyps
Acoelomates - No coelom
Diploblastic
Blind (Incomplete) Gut
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Phylum Cnidaria
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Tentacles usually encircle mouth or oral region
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Mechanism of Nematocyst Discharge
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Equipped with specialized cells: cnidocytes
 Cnidocytes: stinging cells located on their tentacles
 Nematocyst: stinging organelle, poison-filled structure that
contains a tightly coiled dart.
 Tactile stimulation of cnidocil, causes nematocyst to discharge
 After discharge, cnidocyte is absorbed and another develops
When stimulated, water rushes into the capsule
The operculum (cap) opens and rapidly launches the filament
Barbs inject poison into prey
Only a few jellyfish - and the Portuguese man-of-war can seriously
harm humans
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Cnidarian Phyla
Topical
acidic
Solutions if stung:
(vinegar)
Urine - only if urine is acidic?? Most urine is sterile.
Apply heat or ice for pain (also denatures toxins)
Salt water should be applied to remove any excess toxins.
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Discharged
cnidae
recoiled
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Phylum Cnidaria
OTHER CNIDARIAN CHARACTERISTICS
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Sense organs for balance (statocysts) and
photosensitivity (ocelli)
Nerve net
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Asexual reproduction
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Budding in polyps
Sexual reproduction
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Simple nervous system, no brain
By gametes in all medusa and some polyps
Monoecious or Dioecious
No excretory or respiratory system
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Phylum Cnidaria
Life Cycles
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Typically, zygote develops into a motile planula larva
Planula settles, and metamorphoses into a polyp
 Produce other polyps asexually
 Polyps eventually produce a free-swimming
medusa by asexual reproduction (Budding)
Hydrazoan
Life Cycle
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A.
B.
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Hermit crab uses cnidarian as protection
Showing growth on host shell
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Phylum Cnidaria
Form and Function
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Cnidaria have two basic body plans:
polyp and medusa
 Polyp
Adaptation to a sedentary life
 Tubular body with the mouth directed upward
and surrounded by tentacles
 Mouth leads into a blind-gut cavity
 Reproduce asexually by budding, fission, or
pedal laceration
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Phylum Cnidaria - Medusa
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Medusa
Bell or umbrella-shaped
 Usually free-swimming
 Mouth directed downward
 Tentacles may extend down from rim of
umbrella
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Phylum Cnidaria
Body Wall
 Cnidarian body
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Outer epidermis
Inner gastrodermis (gastrovascular cavity = gut)
Layers separated by mesoglea
Mesoglea
 Gelatinous (at least 95% water) - jellylike
 Continuous, extending through body and
tentacles
 Supports body
 Thickness varies
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Phylum Cnidaria
Class Hydrozoa
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Most marine and colonial with both polyp and
medusa forms (polyp form dominates)
Colonial Obelia
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Typical hydroid has a base, a stalk, and one or
more individual polyp animals
Feeding polyps with tentacles around mouth=
hydranths
In Obelia, the medusa buds are formed by a
reproductive polyp called a gonangium
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Obelia Life Cycle
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Hydroids
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Bell medusa
(Hydroid)
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Medusa
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D. Developing Polyp from Planula: Frustule = bud
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Freshwater
Hydrozoan
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Phylum Cnidaria
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Class Hydrozoa: Animals
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Hydra
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Portuguese Man-of-War
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Found in fresh water
Worldwide distribution (16 species in North America)
Polyp body form
Ring of 6–10 hollow tentacles encircles mouth
Floats on surface (air bladder)
Tentacles reach up to 10 meters
Contains polyp colonies and medusa
Air bladder contains gases released
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Hydra catching a water flea
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Hydra w/ developing bud and ovary/egg
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Portuguese man-of-war Colony
Secrete powerful neurotoxin - Class Hydrozoa (Many Polyps and medusa in colony)
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Differences between Hydrozoa and Scyphozoa
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Hydrozoa
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Colonial Stages
Mostly Polyp life form
Medusa, if present, are
usually small
Small amounts of mesoglea
Less tentacles
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Scyphozoa
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Large Medusa forms
Cells present in mesoglea
More complex
More tentacles
Large amount of mesoglea
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Phylum Cnidaria
Class Scyphozoa
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Most of the larger jellyfishes belong to this
class
Nearly all float in open sea
Bells vary in shape and size
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Composed mostly of mesoglea
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Class Scyphozoa
Giant jellyfish
-exceeds 2 Meters
in diameter
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Moon
Jellyfish
(Aurelia)
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Phylum Cnidaria - Scyphozoa
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Reproduction methods
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Asexual - Similar to other cnidarians. Budding
But also use Strobilation - asexual budding of
saucerlike buds that develop into medusa.
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Life Cycle of Aurelia, marine scyphozoan
Strobilation
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Phylum Cnidaria
Class Anthozoa
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Lack a medusa stage
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All marine, in both deep and shallow water,
and vary in size
Examples: sea anemones, sea corals
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Sea Anemones
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Structure of Sea Anemone
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Phylum Cnidaria - Anthozoa
Sea Anemones
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Polyps larger and heavier than hydrozoan
polyps
Attach to shells, rocks, timber, etc. by pedal
discs
Crown of tentacles surrounds the flat oral
disc
When in danger, water rapidly expelled through
pores as the anemone contracts to a small size
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Sea Anemone being attacked by a Sea Star
Can Sea Anemones run?
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Tube Anemone
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Phylum Cnidaria - Anthozoa
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Reproduction of Sea Anemones
Some dioecious, some monoecious
 Monoecious species - produce sperm first and
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eggs later
Fertilization is external or in gastrovascular
cavity
 Zygote becomes a ciliated larva
 Pedal laceration, small pieces of pedal disc
break off and regenerate a small anemone
 Budding can occur
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Closed during day, open to feed at night
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Polyp of
Coral
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A Thorny Coral
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A soft coral on Pacific Coral Reef
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Horny Coral Types
Red Gorgonian
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Sea Fan
Red Whip Coral
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Phylum Cnidaria
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Coral Reefs
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Great diversity of organisms (under rainforest)
Plants and animals are limited to top layer
 Above the calcium carbonate deposits
Reef forming corals and algae precipitate calcium
carbonate to form most coral reefs
 Require warmth, light, and salinity of undiluted sea
water (contains calcium)
 Photosynthetic zooxanthellae live in their tissues
 Provide food for corals, carbon for calcium
carbonate, and recycle phosphorus and
nitrogenous wastes
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Phylum Cnidaria - Human Impact
HUMAN IMPACT:
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Nutrients from fertilizer and sewage threaten coral
reefs with excessive algal growth - or by killing
zooxanthellae, which leads to coral bleaching.
Persian Gulf reefs have withstood surprising
amounts of oil pollution
Coral reefs in many areas are threatened by factors
mostly of human origin
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