Download Lecture 5 Cnidarians - NGHS

Phylum Cnidaria
Hydra, jellyfish, coral, & sea
anemones
General Characteristics
• They are radially symmetrical; oral
end terminates in a mouth
surrounded by tentacles.
• They have 2 tissue layers
• Outer layer of cells - the
epidermis
• Inner gastrodermis, which
lines the gut cavity or
gastrovascular cavity
(gastrodermis secretes digestive
juices into the gastrovascular
cavity)
• In between these tissue layers is a
noncellular jelly-like material called
mesoglea
Cnidarian Body Plans
Polyp form
• Tubular body, with the mouth directed upward.
• Around the mouth are a whorl of feeding tentacles.
• Only have a small amount of mesoglea
• Sessile
Medusa form
• Bell-shaped or umbrella shaped body, with the mouth is directed downward.
• Small tentacles,
directed downward.
• Possess a large
amount of
mesoglea
• Motile, move by
weak contractions
of body
Two Body Forms
Polyp: organisms that
are attached (e.g.
corals and sea
anemones)
Medusa: free-floating,
mouth and tentacles
typically point
downward (e.g. jellies)
Movement
• The cnidarian body is capable of some kind of coordinated movement
• Both the epidermis and the gastrodermis possess nerve cells arranged
in a loose network - nerve net (plexus), which innervate primitively
developed muscle fibers that extend from the epidermal and
gastrodermal cells
• Stimulus in one part will spread across the whole body via the
network
Nutrition
• Cnidarians are carnivores with hydras and corals consuming
plankton and some of the sea anenomes consuming small fishes
• They use they tentacles to capture prey and direct it toward the
mouth so that it can be digested in the gastrovascular cavity via
secretions from gland cells (extracellular digestion); some food is
phagocytized by special cells and digestion occurs intracellularly
• The gastrovascular cavity exists as 1 opening for food intake and
the elimination of waste
• There is no system of internal transport, gas exchange or
excretion; all these processes take place via diffusion
Stinging Organelles
• Prey capture is enhanced by use of
specialized stinging cells called
cnidocytes located in the outer
epidermis.
• Each cnidocyte has a modified cilium cnidocil, and is armed with a stinging
structure called a nematocyst.
• The undischarged nematocyst is
composed of a long coiled thread
• When triggered to release, either by
touch or chemosensation, the nematocyst
is released from the cnidocyte and the
coiled thread is everted
• Some nematocysts function to entangle
the prey; others harpoon prey and inject a
paralyzing toxin
Reproduction
• One of the most amazing adaptations is the
ability of some cnidarians to regenerate lost
parts or even a complete body
• Asexual reproduction is common with new
individuals being produced by budding
• Sea anenomes engage in a form of asexual
reproduction called pedal laceration
• Cnidariand are dioecious
• Fertilization is external, with the zygote
becoming a elongated, ciliated, radially
symmetrical larva - planula larva
Planula larva
U.S. NSF’s Assembling the Tree of Life: An Integrative
Approach to Investigating Cnidarian Phylogeny
Class Hydrozoa
• Includes the solitary freshwater hydra; most are colonial and marine
• Typical life cycle includes both asexual polyps and sexual medusa
stages; however, freshwater hydras and some marine hydroids do not
have a medusa stage
Solitary Hydras
• Freshwater hydras are found in ponds and
streams occurring on the underside of
vegetation
• Most possess a pedal disc, mouth,
hypostome surrounded by 6-10 tenetacles
• Mouth opens to the gastrovascular cavity
• The life cycle is simple: eggs and sperm
are shed into the water and form fertilized
eggs; planula is by passed with eggs
hatching into young hydras
• Asexual reproduction via budding
Class Hydrozoa cont.
Colonial Hydrozoans - e.g., Obelia
• Possess a skeleton of chiton that is
secreted by the epidermis
• All polyps in the colony are usually
interconnected
• Two different kinds of individuals
that comprise the colony: feeding
polyps or gastrozooids (C) and
reproductive polyps or gonozooids
(B)
Class Hydrozoa cont.
Life Cycle of Obelia
• Gonozooids release free swimming medusae
• Zygotes become planula larvae, which eventually settle to
become polyp colonies
• The medusae of
hydroids are smaller
than those of
jellyfishes (C.
Scyphozoa)
• Also, the margin of
the bell projects
inward forming a
shelf-like velum
Class Hydrozoa cont.
Other Hydrozoans
Portuguese man-of-war:
Single gas-filled float with
tentacles
Tentacles house the polyps
and modified medusae of
the colony
Class Scyphozoa
Jellyfish
• The medusae are large and contain
massive amounts of mesoglea
• The differ from the hydrozoan
medusa in that the lack a velum
•Possess four gastric pouches lined
with nematocysts; these are
connected with the mouth an the
gastrovascular system
Scyphozoan Life Cycle - Aurelia
• Gametes develop in
gastrodermis of gastric
pouches; eggs and sperm are
shed through mouth
• Fertilized eggs develop into a
planula larva; settles on
substrate and develops into a
polyp - scyphistoma
• Scyphistoma produces a series
of polyps by budding - strobila
• The polyps undergo
differentiation and are released
from the strobila as free
swimming ephyra
• Ephyra matures into an adult
jellyfish
Class Anthozoa
• Exclusively marine; there is no medusa stage
• At one or both ends of the mouth is a ciliated groove called the siphonoglyph;
generates a water current and brings food to the gastrovascular cavity
• Possess a well developed pharynx
• The gastrovascular cavity
is large and petitioned by
septa or mesenteries;
increase surface area for
digestion or support
• Edges of the septa usually
have threadlike acontia
threads, equipped with
nematocysts and gland
cells
Class Anthozoa cont.
• Solitary anthozoans include sea
anemones
• Most anthozoans are colonial
(e.g. corals) and secrete external
skeletons composed of calcium
carbonate.
• Corals obtain much of their
energy from microscopic
photosynthetic green algae
(zooxanthellae) or
dinoflagellates that live
symbiotically inside the cells of
the coral
Anemone mutualism
Found in the tropical Indo-west Pacific or tropical
Pacific, Indian Ocean, and Red Sea
Anemonefish (e.g. clownfish) receive protection
by living amongst the stinging tentacles
• Fish passes back and forth
through the tentacles to pick
up anemone cells, the
anemone recognizes its own
cells on the fish so doesn’t
sting it
Some Diversity of Medusozoa
Stauromedusae
Cubomedusae
Trachymedusae
Semaeostomeae
Coronatae
Rhizostomeae
Limnomedusae
Actinulida
Narcomedusae
Siphonophora
Leptothecata
Laingiomedusae
Anthoathecata
Some Diversity of Hexacorallia
Actiniaria
Zoanthidea
Ceriantharia
Corallimorpharia
Antipatharia
Scleractinia
Some Diversity of Octocorallia
O. Alcyonacea
O. Helioporacea
O. Pennatulacea
Blue coral
2 families
Sea pens
Soft corals & sea fans
14 families
29 families
Photos: G.C. Williams