Phylum Cnidaria
A group of animals including corals, jellyfish, the common lab animal Hydra, sea anemones, and
the lesser known sea fans, siphonophores, zoanthids and myxozoans (about 11,000 species).
Acropora cervicornis
From Brusca and Brusca (2002)
From Brusca and Brusca (2002) E-F
Cnidarians are diploblastic metazoans
Diploblasts vs. triploblasts
Cnidarians are radially symmetrical (hence the name Radiata.)
Phylum Cnidaria – Characteristics (Cont’d)
Cnidarians possess unique stinging or adhesive structures called cnidae (nematocysts)
Ectoderm and endoderm are separated by middle mesoglea--a connective tissue-like
layer of jelly or fibrous material with or without mesenchyme cell constituents
Cnidarians possess a true gut (unlike Porifera) the Coelenteron (gastrovascular cavity;
i.e. with digestive and circulatory function). Coelenteron is derived from the endoderm.
Coelenteron has a single opening (both mouth and anus).
Musculature originates largely from myoepithelial cells, derived from ecto- and
endoderm. Epidermis is derived from ectoderm.
No head, no centralized nervous system, no discrete gas exchange, excretory or
circulatory structures. Primitive nervous system is comprised of a simple nerve net(s).
Life histories exhibit alternate asexual polypoid and sexual medusoid generations.
Cnidarians have planula larvae
Cnidarians are marine, except some hydrozoans, which have invaded fresh waters.
Composed of 4 Classes - Hydrozoans, Scyphozoans, Cubozoans and Anthozoans. The
latest significant change in Cnidarian systematics: The former protistan Phylum
Myxozoa is a highly derived parasitic cnidarian.
Circulatory System (Porifera vs. cnidarians)
From Brusca and Brusca 2002
Cnidaria have two body forms: polyp and medusa
From Ruppert and Barnes (1994)
Gastrovascular Cavity
Exumbrella
True jellyfish, hydromedusae
Corals, hydroids, sea anemones
mesoglia is thin, basement membrane
between epidermis and gastrodermis
Medusa - motile, pelagic, tentacles, oral opening down, umbrella shaped
(jelly fish), mesoglea thick, jelly-like material, makes up bulk of body
mass, hence name, jellyfish
Some species occur in only polypoid form, others only in medusa form,
some in both during the life cycle
Colonial aggregations are most common in polyps
Pencillatus
Montastraea franksi
Body wall Cell Types
Epidermis (ectoderm-derived) is separated from gastrodermis
(endoderm-derived) by mesoglea/mesenchyme (ectoderm-derived)
Mesoglea
Hydrozoans – acellular, gel-like; Scyphozoans and Cubozoans –thick
with cells; Anthozoans thick cellular mesenchyme
Myoepithelial cells – the most common cnidarian cell (the most
primitive muscle cells in Metazoa)in both the epidermis
(epitheliomuscular cells) and in gastrodermis (nutritive-muscular
cells)
A number of cell types within the epidermis and gastrodermis and
some primitive organ precursors-testes, ovaries. Mucus-secreting
cells -found in the epidermis
Myoepithelial cells
Columnar in basic shape, but could be highly modified -Base embedded in mesogleaPossess 1-3 basal extensions with contractile muscle fibrils-Myofibrils oriented parallel to
body stock so shorten the body when contracted-Myofibrils from adjacent cells are
interwoven- Have gap junctions so contract in a coordinated manner because the gap
junctions permit intercellular communication
Interstitial Cells
Located just beneath the epidermal surface, embedded between
musculoepithelial cells. Small rounded cells with large nuclei
Totipotent-give rise to sperm and eggs
Cnidocytes
Located throughout the epidermis
Invaginated within musculoepithelial cells-many cnidocytes may occur
within one musculoepithelial cell, called a battery
Cnidocytes contain organelles called cnidae, which can be everted or
turned inside out
Cnidocytes occur in several forms - most common is nematocyst
(stinging) - Cnidae are of taxonomic importance with respect to
structure
Hydrozoa and Scyphozoa have bristlelike processes called cnidocil
Cnidocytes usually associated with a
neural process (nervous control) and
are anchored to contractile extensions
of musculoepithelial cells
Nematocysts fired as a result of change
in osmotic pressure (Ca++ released
into capsule so that water rushes in
behind and generates hydrostatic
pressure)
The "fired" cnidocil turns inside out
and may embed into prey or enemies
and release toxins (sting of jellyfish or
corals)
Ouchhh!
Gastrodermis - Inner Cell Layer
Similar in basic organization to epidermis-Not homologous to gut lining of
“higher” metazoans, but functions in same way
Nutritive - muscle cells (similar to musculoepithelial cells)-Oriented at right
angles to body axis, so contraction makes body thinner-Only 1 cilium per cell
Most highly developed near hypostome and on tentacles
Enzymatic-gland cells
Secrete digestive enzymes –
extracellular digestion of
large items-Can’t contract
Mucus-secreting cells –
similar to those found in
epidermis, concentrated
around mouth
Symbiotic algae and cnidarians
One of the most important evolutionary innovations!
Gastroderm may contain symbiotic algae
Hydra-zoochlorellae (so color is green)
In marine Cnidarians-zooxanthellae (so color is yellow brown)
Acropora cervicornis with
white band disease
Gas exchange/waste excretion occurs across the general body surface only 2 cell layers so wastes can be discharged directly to environment or
to G-V cavity, then expelled
Receptor and Nerve Cells
Receptors may be modified cilia, project from between musculoepithelial cells, give rise to
neural processes at the base of the epidermis for communication
Neurons are superficially similar to multipolar neurons of other animals, located next to the
mesoglea. Nerve Cells arranged in a nerve net (no central system) 1 net fires rapidlybipolar neurons 1 fires slowly-multipolar neurons. Transmission of action potentially
bidirectional
Reproduction in Hydras- Asexual
Budding occurs by evagination of body wall, new tentacles and hypostome
form distally and new individual eventually detaches
Cells of gastrodermis and epidermis retain totipotency so that if hydra is
injured or disrupted, complete regeneration can occur
Hydra can be turned inside out and then epidermis and gastrodermis will
reorganize themselves
Cell division within hydra is continuous, greatest at oral region
New cells migrate out to tentacles and down body stalk
Hydra never grows old
Polarity of cell orientation remains
Sexual Reproduction - in response to poor environmental conditions
Cnidarians are diploid, produce gametes by meiosis, most are dioecious,
although some are hermaphroditic
Reproductive cells (ovaries and testes) develop from interstitial cells in
epidermis in hydrozoans
Egg cell erupts through the epidermis and is fertilized by sperm released to
the external environment from nipple-like testes
Fertilized egg undergoes mitosis and forms a chitinous shell-protective
coating
Small hydra emerge from egg
Actinula larvae
Planula larva
Movement
Effected by contraction of musculoepithelial cells in epidermis, opposed by
nutritive muscle cells of gastrodermis (in general)
Forces transmitted by hydrostatic skeleton (water trapped in gastrovascular
cavity)
Nutrition
Almost all are carnivorous although symbionts may contribute some excess
photosynthate
Feed mainly on small crustaceans and other plankton which are captured by
the tentacles
Mouth is distended for swallowing, mucus lubricates passage of prey through
mouth which may be much smaller than prey item
Enzymatic gland cells secrete proteolytic enzymes that reduce prey to
particles that can be engulfed and digested completely within cells
Benthic locomotion in cnidarians
Cnidaria Feeding
Cnidarians feed on zooplankton but some suspension feeders. Prey is caught with
the tentacles and immobilized by cnidocytes
Brusca and Brusca 2002
From Barnes and Ruppert 1994
Class Hydrozoa
About 2700 species, only class with freshwater members
Many species have both polypoid and
medusoid forms in life cycle that
alternate
Some have only medusae (Aglaura),
some only hydroid (Hydra)-most
derived form, although as a class, most
primitive of Cnidarians
Acellular mesoglea
No cnidocytes in the gastrodermis
Gametes produced in the epidermis
with a few exceptions; where gametes
are gastrodermal but shed directly to
exterior-never fertilized in the G-V
cavity
Most hydroids are colonial, produced
by budding-buds remain attached
•Different branching patterns
•Physalia-Man-o-War is upside down
hydroid colony suspended from float
G-V cavity and tissue layers of
attached polyps are continuous
Colonies usually polymorphic functional polyps or Zooids
Oral end of the polyp is called the
hydranth
Stalk is called the hydrocaulus
Some hydroids have a secreted shell
or perisarc (theca over hydranth)
Obelia sp.
Aglantha digitale, a directdeveloping hydromedusa no
polyp. Gonads visible through
the transparent bell
Obelia life cycle
Many hydroid colonies are quite small (5-15 cm) but some reach sizes up to 2 m
Gastrozooids-feeding polyps, usually
armed with nematocysts and long
tentacles for catching prey
•Extracellular breakdown of planktonic
prey occurs in gastrozooid, then
material is passed to other zooids
through common G-V cavity
•May be defensive
Gastrozoid
Dactylozooids-specialized defensive
polyps with many cnidocytes and and
adhesive cells
Physalia
Gonozooids-reproductive polyps,
usually lack mouth and tentacles-give
rise to medusae by budding which
then produce gametes
Many hydroids do not release their
medusae-remain attached, sexual
reproduction occurs and the larvae
may develop to planula stage, or to
actinula which is then released to
settle as new hydroid polyp
If budded medusae are reduced to
gonadal tissue, then called sporosac
Medusa Structure in Hydrozoa
Called hydromedusae
Upper surface called exumbrella
Lower surface, subumbrella
Have an inward projecting shelf
on underside called velum- aids in
restricting opening for jet
propulsion
Downward hanging oral opening
is called manubrium-may be
highly folded in some species
Hydromedusae
Class Scyphozoa (about 200 spp.)
Cassiopea frondosa
“True jellyfish”
Pelagic cnidarians
Medusa is the dominant form
Manubrium (highly developed) may be
drawn into long, frilly arms, each with
nematocysts
Cnidocytes in gastrodermis as well as
epidermis
Mesoglea is partly cellular
No velum, although similar structure
(velarium) serves the same function
Margin of the bell may be scalloped into
lappets
Aurelia aurita
G-V cavity is divided into gastric
pouches by septa
Septa may be drawn out into gastric
filaments with cnidocytes and gland
cells (paralyzing prey and digestion)
Prey range from zooplankton to fish
Small food items trapped in mucus
and brought to oral opening by
ciliary movement
Peripheral nerve ring innervates
Rhopalia-sensory structures
•Light sensors can be complex in
some
•Statoliths aid balance as in
Hydrozoans
Reproduction
Dioecious, gastrodermal gonads in
gastric pouches
Fertilization is usually “internal”,
followed by development to at least
planula larva
Larva becomes planktonic, settles,
changes to scyphostoma (hydra like
except for septa), buds asexually
Some produce medusae by
strobilization
Immature medusae called ephyrae
Ephyrae grow into mature medusae
Ephyra Strobila
Brusca and Brusca (2002)
Scyphozoan Life Cycles
Brusca and Brusca (2002)
Class Cubozoa (sea wasps, box jellyfish)
35-40 species, almost all tropical, sting very toxic
The cubozoan bell is square in horizontal cross section. Inside the bell are the
manubrium and mouth. A flap of tissue called the velarium is located along the underside
of the bell. Muscular fleshy pads called pedalia are located at the corners of the bell.
The gut, or stomach cavity, is partitioned by septa and extends to the tentacles through
pedalial canals. On the bell, located midway between the pedalia, are four sensory
structures called rhopalia
In the close-up of a rhopalium (far right), there are 6 reddish spots, sensitive to light.
The 4 smallest spots are relatively simple. The 2 larger regions contain lenses, corneas,
and retinas, rather similar to those in our eyes.
Rhopalium
Rhopalium close-up
Inside each rhopalium, located below the eyes, is the statocyst.
Inside each statocyst is the statolith (calcium sulfate).
Statoliths appear to have daily growth rings.
The statocysts are sensitive to orientation (used for orientation)
Stinging Cells
Notice how the barb is
coiled up inside a capsule.
Nematocysts are concentrated in rings
on the tentacles of cubozoans
Rhopalium
Cubozoan Life Cycle
In some species, the male puts his tentacles into the bell of the female
and pass packets of sperm. Another species has been observed in large
mating aggregations. Fertilization takes place inside the females.
Mating appears to occur once every year.
In some species the fertilized eggs are released into the water column where they develop
into planulae, while in others development into planulae occurs inside the female.
Planulae swim in the water column for a few days and then settle on to the substrate.
The settled planulae grow into polyps. The polyps are mobile, and they frequently bud off
more polyps.
After a few months of feeding, the polyps are mature and metamorphose into a single
juvenile medusa (see picture).
During metamorphosis, the polyp tentacles are resorbed and four new tentacles and four
rhopalia are formed.
With a couple of contractions, the entire individual becomes detached and swims away as a
juvenile medusa.
There are two main groups of cubozoans, Chirodropidae and Carybdeidae.
• In carybdeids, each tentacle is connected to a single pedalium. Usually there are four
pedalia each with a tentacle, however, in Tripedalia species, each corner of the bell has
two or three tentacles each connected to a single pedalium
•Chirodropids always have four pedalia, one at each corner, with multiple tentacles.
Carybdeid
Phylogeny of Cubozoans
They were originally considered to be an order (Cubomedusae Haeckel 1877) in the Class
Scyphozoa (e.g. Mayer 1910; Kramp 1961). See Figure A below
Recently, they were given the status of a class (Cubozoa in Werner (1975); see also
Mianzan & Cornelius 1999) on the basis of morphological differences (mainly based on the
observation that the cubozoan polyp and life cycle were different than those of
scyphozoans; See Figures B & C)
The latest molecular data suggest that cubozoans are most appropriately viewed as an
order within the Scyphozoa (e.g. Collins 2002).
A
B
C
A big sting from Chironex fleckerican easily kill a
human, in as little as 3 minutes. There have been
roughly 100 deaths due to Chironex stings during
the past 100 years in northern Australia. The
sting is not always lethal. Nematocysts are absent
from the bell.
Chironex fleckeri
Carybdea sivickisi