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

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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