This brochure is funded with qualified outer continen-
tal shelf oil and gas revenues by the Coastal Impact
Assistance Program, United States Fish and Wildlife
Service, U.S. Department of the Interior through a
grant award to the Mississippi Department of Marine
The Institute for Marine Mammal Studies
(IMMS) is a non-profit organization established
in 1984 for the purposes of public education, con-
servation, and research of marine mammals and
sea turtles in the wild and under human care.
Located in Gulfport, MS, IMMS is the premiere
organization in the Mississippi-Louisiana-
Alabama sub-region of the Gulf Coast with capa-
bility and expertise to care for sick and injured
marine mammals. Additionally, IMMS pro-
motes public awareness of marine conservation
issues through its involvement in the community.
IMMS encourages the public to be good stewards
of the environment and continuously work to
improve the quality of life on the Gulf Coast.
Institute for Marine Mammals Studies
P.O Box 207
Gulfport, MS 39502
Phone: 228-896-9182
Fax: 896-9183
Cnidarians are a diverse group of organisms
found in marine and freshwater environments
throughout the world. Currently, there are ap-
proximately 9,000 described species with many
more yet to be discovered. This phylum of soft-
bodied animals can be found in the fossil record
over 650 million years ago. Jellies, corals, sea
anemones, siphonophores, and hydras comprise
the Phylum, Cnidaria. While many species may
look very different from others in this group, all
cnidarians share similar life cycles and body
structures. Cnidarians exhibit polymorphism.
This means the animals have two distinct body
shapes, medusa and polyp, which can be either
adult or juvenile, depending on the species. All
cnidarians exhibit radial symmetry that is analo-
gous to a bicycle wheel, with tentacles (spokes)
radiating out
from a cen-
tral body
(hub). Most
cnidarians
are carnivo-
rous (animal
eating), but
some rely on
algae for
nutrients.
Stinging cells called nematocysts which are em-
bedded in tentacles help capture prey. The name
“cnidarian” is derived form the Greek “cnidos”
which means “stinging nettle.” The venom of
some species has been known to harm or even
kill humans but is generally mild in many mem-
bers of this phylum. Most cnidarians are passive
hunters with unlucky prey drifting too close to the
tentacles. Some species, however, actively hunt
fish, crustaceans, or even other cnidarians. Still
other species are known to absorb nutrients di-
rectly from seawater. This group of animals is
preyed upon by sea turtles, fish, and a wide varie-
ty of invertebrates (animals without backbones).
The Basics Anatomy and Physiology
Cnidarians are comprised of as much as 95% water.
They lack a brain, a heart, gills, and blood. The
basic body plan has been compared to a sac, with a
single opening serving as both the mouth and the
anus. Cnidarians are dioblastic, meaning they have
two layers of tissue, the endoderm and the ecto-
derm. In the medusa life stage (free swimming),
the bell is the “body” of the organism. In the polyp
life stage, the animals are generally sessile and una-
ble to move freely. Within the bell, the animal will
have one or more mouths on the central oral sur-
face. Some cnidarians
have arms that will guide
their food to the mouth.
The main source of food
for many cnidarians is zoo-
plankton (small animals
that spend part or all of
their lives floating near the
ocean surface). Regardless
of the lack of sense or-
gans, cnidarians can smell,
taste, and remain balanced
in the water. Sacs located on the bell rim help sta-
bilize the animal in the water. When the bell shifts
too far to one side, the sacs stimulate the nerve end-
ing to contract the muscle to reposition. Humans
have similar structures found in the inner ear.
Chemoreceptors located around the oral arms are
used for the sense of smell and taste. In addition to
being carried by the
ocean currents, cnidari-
ans can use coronal
muscles (a ring of
strong muscles) which
are embedded under the
bell and push water
away from the hollow
bell. As water is being
forced in one direction,
the cnidarian the oppo-
site direction.
Cnidarians
Stinging Beauties
Pacific Sea Nettle (Chrysaora fuscescens)
pinktentacle.com
Prepared by Heather Edwards and
Chris Breazeale, 2012
www.imms.org
Coral Polyps
earthlyissues.com
Dia
gra
m o
f a
Mo
on J
elly
Au
reli
a
cro
no
do
n.c
om
Cnidarian Life Cylce
marinateyourlife.blogspot.com
Diagram of Nematocyst
pznow.co.uk
Class Cubozoa Box Jellies or Sea Wasps
Order Stauromedusae - are sessile, cup-shaped
forms that lack free swimming medusa.
Order Coronatae - are deep-dwelling jellies
which are rarely seen.
Order Semaeostomae - are commonly called
disc jellies.
Order Rhizostomae - lack tentacles and have
eight oral arms instead of four.
Four orders comprise class, Scyphozoa. Often
called “true jellies,” these organisms can be
found in most marine habitats. Known by beach-
goers throughout the world, these jellies can de-
liver a powerful sting. There are approximately
200 described species of scyphozoans, but scien-
tists believe that many more may soon be discov-
ered. These jellies propel themselves through
the water by
relaxing and
contracting
muscle fibers
found in the
mesoglea.
The meso-
glea is a ge-
latinous ma-
terial that helps
give structure to
these soft-bodied animals much in the way that
skeletons give structure to vertebrates. Most scy-
phozoans measure two-20 centimeters in diame-
ter, but larger species, such as the Lion’s Mane,
can grow as large as two and one-half meters
across with tentacles extending more than 35
meters. Many scyphozoan species are the pre-
ferred diet of sea turtles.
Class Scyphozoa
True Jellies
Class Anthozoa
Sea Anemones, Corals, and Sea Fans
Most hydrozoans are marine organisms and can
be found in virtually every marine environment
with some even found in freshwater. Hydrozo-
ans can be found on all continents except Ant-
arctica. They are most abundant however, in
warm, shallow saltwater. Approximately 3,700
described species show a great deal of diversity
but all have both medusa and polyp life stages.
Hydrozoans differ from other cnidarians be-
cause the medusas grow from buds and they
have complex life cycles with two to three dif-
ferent stages. Many hydrozoa form large colo-
nies of individuals, the best known of which are
the siphonophores, like the Portuguese man-of-
war (Physalia physalis). These colonial polyps
often show a division of function, with some
polyps responsible for feeding, some for diges-
tion, and others for locomotion. Hydrozoans
show a wide variety of feeding behaviors. Some
filter-feed plankton from the water; some active-
ly hunt small fish and other cnidarians; and oth-
ers host symbiotic zooxanthellae. They also
display a wide variety of reproduction methods
and life styles. Some hydrozoans are free-
swimming; others are
sessile; some release
sperm and eggs into the
water; and others re-
produce asexually.
These incredibly di-
verse organisms will
continue to be studied
by scientists for many
years.
Anthozoans include corals, sea anemones, and
sea fans. They differ from other cnidarians be-
cause they are found in the polyp stage as adults.
These animals have been popularized in film
and are a favorite of SCUBA divers and home-
aquarium enthusiasts
due to their natural
beauty. Some corals
build large reefs
(some large enough
to be seen from
space). Most corals
are found in warm,
shallow water, alt-
hough a few species live in deep, cold water.
Reefs are constructed of the tiny coral cups, or
skeletons, left over from dead polyps. Many
anthozoans have a mutualistic symbiotic rela-
tionship with tiny algae called zooxanthellae.
These single-celled plants grow inside the tissue
of the polyps and provide nutrients in time of
need. When water becomes too hot, too deep, or
too polluted, the zooxanthellae will leave the
polyp in a process known as coral bleaching.
Other organisms that can be found near reefs are
sea fans. Often confused for plants, these sessile
(immobile) organisms are at the mercy of the
currents to deliver
plankton for food.
Sea anemones are
often called
“flowers of the
sea” even though
they are truly ma-
rine animals.
Members of the class, Cubozoa, are often called
box jellies due to their cuboidal shape. Some-
times referred to as “sea wasps” because of their
painful sting, box jellies are among the most ven-
omous creatures in the animal kingdom. There
are approximately 20 species in this class with
the largest member growing to the size of a bas-
ketball and tentacles as long as 10 meters. Box
jellies differ from other cnidarians in that they
possess true eyes,
complete with reti-
nas, corneas, and
lenses. They also
have a nerve ring
around the base of
the bell that helps to
coordinate move-
ment. This nerve
ring works in con-
cert with the velari-
um, or flap, to in-
crease the flow of
water from the un-
derside of the bell. This allows the box jelly to
move at speeds “up to” six meters per minute,
much faster than other jellies. Cubozoids are
found throughout the world in tropical and sub-
tropical waters. The most dangerous species, the
box jelly Chironex fleckeri, however, is restricted
to parts of the Indian Ocean, the Pacific Ocean,
and the Great Barrier Reef. Divers and swim-
mers in these areas often wear protective layers
over their skin to prevent being stung. An esti-
mated 20-40 people worldwide die annually from
being stung by the box jelly.
Class Hydrozoa
Hydras, Fire Corals, and Siphonophores
Box Jelly (Chironex fleckeri)
goseeaustralia.com.au
Lion’s Mane Jellyfish (Cyanea capillata)
storiesfromoceansedge.blogspot.com
White Sea Anemone (Epiactis prolifera)
Photograph by Jens Troeger
Black Sea Fan (Clypeaster Rosaceus)
naplesseashellcompany.com
Portuguese Man-of-War
bioweb.uwlax.edu
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