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Marine Biology Notes
PHYLA CNIDARIA and Ctenophora
Part I:
I. PHYLUM CNIDARIA (ny-dar’e-a) (Gr. knide, nettle + L. -aria, like or connected with) (Thread-like) The
name Cnidaria comes from the Greek word "cnidos", which means stinging nettle.
More than 9000 species COELENTERATA (Gr. koilos, hollow, + enteron, gut, + L. -ata. characterized by) is
used less frequently.
A. Cnidaria - takes its name from cells called Cnidocytes, which contain the stinging organelle(Nematocysts)
1. Nematocysts - formed and used by only cnidarians and one species of ctenophore
2. They are very successful and are widespread in marine habitats, and there are a few in fresh water.
3. Most are either sessile, slow moving or slow swimming. They are efficient predators
B. There are four recognized classes of Cnideria
1. Hydrozoa - includes hydroids, fire corals, the Portuguese man-of-war and others
2. Scyphozoa - The "true" Jellyfishes
3. Cubozoa - the box Jellyfishes
4. Anthozoa - the largest class, including sea anemones, stony corals, flexible corals, and others
C. Economic Importance
1. Reef building corals provide animals with a habitat to grow and flourish.
2. Coral Reefs also serve as tourist attractions, bring in tourism dollars.
3. In some island communities, coral rock is used as a building material.
4. Jewelry can also be made from corals (black coral has been over harvested in many areas).
D. Characteristics of Phylum Cnideria
1. Entirely aquatic, some fresh water but mostly marine
2. Radial symmetry or biradial symmetry around a longitudinal axis with oral and aboral ends; no
definite head
3. Two basic body forms: polyp and medusae
4. Exoskeleton or endoskeleton of chitinous, calcareous, or protein components in some.
5. Body with two layers, epidermis and gastrodermis, with mesoglea (diploblastic); mesoglea with cells
and connective tissue (ectomesoderm) in some (triploblastic)
6. Coelenteron or Gastrovascular cavity (gut, stomach) (often branched or divided with septa) with a
single opening that serves as both a mouth and anus; extensible tentacles usually encircling the mouth
or oral region.
II. FORM AND FUNCTION
A. Polymorphism in Cnidarians - In general, all cnidarian forms fit into one of two morphological
types: POLYP OR MEDUSA
1. Polyp or hydroid form - adapted for a sedentary or sessile 1ife
a. tubular body with a mouth at one end surrounded by tentacles.
b. Aboral end - usually attached to a substratum by a Pedal disc or other device
c. May live singly or in colonies (colonies of some species have more than one kind of individual
teach specialized for a certain function such as feeding, reproduction, or defense.
d. Sea anemones and corals (class Anthozoa) are all polyps
2. Medusa - free swimming and have cup-shaped, bell-shaped or umbrella-shaped bodies,
Mouth - usually centered on the concave side (oral surface), and tentacles extend from the rim of the
bell or umbrella margin.
3. The true jellyfishes (class Scyphozoa) are all medusa body as adult, but have a polyp body larval stage
4. Colonial hydroids (class Hydrozoa) sometimes have life histories that feature both the polyp and
medusa stages.
B. Nematocysts: the Stinging Organelles of Cnidarians
1. Nematocyst - a tiny capsule composed of material similar to chitin (a nitrogenous polysaccharide
insoluble in water) which contains a coiled tubular "thread" or filament which is a continuation of the
narrowed end of the capsule. The inside of the un-discharged thread may bear tiny barbs, or spines.
Over 20 different types of nematocysts have been described in the cnidarians so far (they are important
in taxonomic determinations.
a. Cnidoblast - (Ny-doh-blast) - what the cnidocyte is properly called during its development. A cell
in the epidermis of coelenterates in which a nematocyst (cnidocyst) is developed.
b. Cnidocyte - cell that secretes and encloses the nematocyst.
c. Cnidocil - a triggerlike structure of the cnidocyte (a modified cilium) - Not found in the Anthozoa
Class (Tactile stimulation causes the nematocyst to discharge)
d. Operculum- a small lid which covers the end of the stinging cell capsule.
2. Cnidocytes with their nematocysts are produced in folds of the ectodermal cells in some, and in the
gastrodermal cells in others.
3. Cnidocytes can only fire once.
a. Used cnidocytes have to be replaced, which takes about 48 hours.
b. The nematocyst can be triggered by touching the cnidocil and by smell (detection of chemicals
given off by organisms).
c. To minimize wasteful firing, two types of stimulus are generally required to trigger cnidocytes: their
cilia detect contact, and nearby sensory cells "smell" chemicals in the water. This combination prevents
them from firing at distant or non-living objects.
4. Not all nematocysts have barbs or inject poison. There are three functional types
of nematocysts in the hydra:
a. Penetrants - penetrate the prey and inject poison.
b. Volvents - those that recoil and entangle the prey
c. Glutinants - those that secrete an adhesive substance used in locomotion and attachment.
5. The nematocysts of most cnidarians are not harmful to humans, but the stings of the Box Jelly,
Portuguese man-of-war and certain jellyfishes are quite painful and in some cases may be dangerous.
C. Nerve Net (nerve plex) - primitive nervous system
1. Found at both the base of the epidermis and at the base of the gastrodermis, forming two
interconnected nerve nets.
2. Symmetrical synapses – transmission across the synapse in either direction can take place
3. There is no concentrated grouping of nerve cells to suggest a “central nervous system".
4. In Scyphozoan there is a fast conducting system to coordinate swimming movements and a slower
one to coordinate movements of the tentacles
D. Body Structure
1. Gastrovascular Cavity (coelenteron-means hollow intestine) it is the internal cavity of cnidarians
2. Manubrium – throat-like extensions from the mouth (found on oral surface).
3. Oral Lobes - extensions from the oral surface which can bring trapped food to the mouth.
4. Hydrostatic skeleton - water can be brought into the gastrovascular cavity by the beating of the cilia
on the muscular cells.
5. Mesoglea - a gelatinous or jellylike layer. It lies between the epidermis and gastrodermis and
adheres to both layers. Thicker in the medusa, making it more buoyant.
E. Reproduction - most cnidarians are dioecious, and in sexual reproduction many shed their gametes
directly into the water.
1. Zygotes may be retained by the female and brooded for some period.
2. Gonads are epidermal in hydrozoans and gastrodermal in the other groups.
3. Ciliated Planula - free-swimming larva that the embryo characteristically develops into.
4. The most common form of reproduction in cnidarians is Asexual budding,
III. Class Scyphozoa (sy-fo-zo'a) (Gr. skyphos, cup, + zoon. animal) Examples: Aurelia, Cassiopeia.
A. The true Jellyfish - (Aurelia. Cabbage Head, Sea Nettle)
1. Medusae form is the most dominant body form, and the polyp form is generally restricted to the
larval stage.
2. Rhopalium (rhopalia) - a sense organ located around the margin of the bell at each notch.
a. Each rhopalium bears a statocyst for balance, two sensory pits containing concentrations of
sensory cells, and sometimes an ocellus (simple eye) for photoreception.
b. Lappets - lobe like projections located with the rhopallium at each notch.
c. The Aurelia has eight such notches; others may have four or sixteen.
B. The mouth is centered on the sub umbrella side (oral surface)
1. The Manubrium (an extension of the mouth-throat-like) is usually drawn out into four frilly
oral lobes (arms) that are used in food capture and ingestion.
2. The marginal tentacles may be many or few and may be as short, as in Aurelia or long
C. Scyphozoan sexes are separate (dioecious) - fertilization and early development occur in the gastric
pouches or on the frilled oral arms.
1. Planula - a ciliated larval stage that develops after the egg is fertilized and drifts with plankton.
2. Scyphistoma - a little polypoid larva (attached polyp body form) that the planula develops into. It
looks somewhat like a hydra.
3. Strobila stage - a series of juvenile, saucer-shaped medusae that are stacked one on top of the
other. Develops from the Scyphistoma stage. called Ephyra.
4. Ephyrae – these are the juvenile medusa which break loose from the stack and swim away to
grow and mature into the sexual adult form.
IV. Class Cubozoa (ku’bo-zo'a) (Gr. kybos, a cube, + zoon, animal) Examples: Tripedalia, Carybdea,
Chironex, Chiropsalmus
A. The Box Jellyfishes
1. The bell (umbrellas or cup) is almost square
2. A tentacle or group of tentacles is found at each corner of the square at the umbrella margin, and
are attached to the Pelalium.
3. The Pedalium - a flattened tough blade found at the base of each tentacle that connects to the
body of the box jelly.
4. Sub umbrella edge turns inward to form a velarium, similar to the velum of hydromedusae.
5. Cubomedusae are strong swimmers and voracious predators, feeding mostly on fish.
6. The Sea Wasp (Chironex fleckeri) - Its stings are quite dangerous and sometimes fatal, with death
occurring rapidly (usually within a few minutes).
7. Most of the fatalities resulting from stings have been reported from tropical Australian waters.
Marine Biology Notes
PHYLA CNIDARIA and Ctenophora
Part 2
V. Class Hydrozoa (hy-dro-zo'a) (Gr. hydra, water serpent, + zoon. Animal)
A. Marine Hydrozoans – the majority are found in colonies and the life cycle includes both the asexual
(polyp body) and the sexual (medusa body) stages (freshwater hydra does not have medusa stage).
1. In this class or organisms, the Polyp body stage is their dominant form.
2. Some marine hydroids do not have free medusae stages.
2. Some hydrozoans only occur as medusae and have no polyp.
B. Hydroid Colonies - Both polyp and medusa stages exist in the life cycle of most.
1. Obelia - the typical hydroid type has a base stalk and one or more terminal polyps (zooids).
2. Attached to the hydrocaulus are the individual zooids, most of which are feeding polyps.
a. Hydranths (gastrozooids) - feeding tentacles which may be tubular, bottle shaped, or
vase like.
b. Some polyps are naked, but others, as in the Obelia have a protective cup into which
the polyp can retreat for protection.
3. Food for one is food for all, in a Common Gastrovascular cavity where intracellular digestion
occurs for the colonial hydroid.
C. Other Hydrozoans
1. Physalia (Portuguese man-of-war) - form floating colonies which produce several]
types of modified medusae and polyps
a. Dactylozooids- defensive polyps that bear the nematocysts
b. Gastrozooids - captures and digests for the entire colony
c. Gonozooids - reproductive polyps which produce a medusa form for sexual reproduction.
d. The rainbow-hued float is a mixture of gases similar to air, and secreted by an oval disc at the
base of the float.
D. Hydrocorals (false corals) - Hydrozoans which secrete massive calcareous skeletons that resemble
true corals
1. Some occur commonly in caves and crevices in reefs (Stylaster roseus).
2. These fragile colonies may be white, pink, purple, red or red with white tips.
3. Fire Coral (order Milleporina) - Contain powerful nematocysts that produce a burning
sensation on human skin. They form branching or platelike colonies and often grow over the
horny skeleton of some alcyonarians.
VI. Class Anthozoa (Gr. Anthos, flower + zoon, animal) (an-tho-zo’a)
A. General Characteristics
1. All are marine and are found all over the world in both deep and shallow water
2. They may be solitary or colonial, and vary greatly in size
a. Many forms are supported by skeletons
b. All polyps, no medusa body forms as adults
3. Septa (septum) or mesenteries - Inward extensions of the body wall which partition the large
gastrovascular cavity and divide the corallite of the coral.
a. The walls and septa contain both circular and longitudinal muscle fibers
b. There is a general tendency toward biradial symmetry in the septal arrangement.
B. There are three subclasses of the Class Anthozoa: Subclass Zoantharia, Sub-class Alcyonaria
and Subclass Ceriantipatharia
1. Subclass Zoantharia (zo’an-tha’ri-a) - Sea anemones, true stony (hard) corals (order Scleractinia),
and
others. Examples: Metridlum. Adamsla. Astrangla. Acropora
a. Hexamerous plan - (Of six or multiples of six), or polymerous symmetry and have simple
tubular tentacles arranged in one or more circlets on the oral disc
b. Sea Anemones (order Actiniria) - larger and heavier than hydrozoan polyps
 Attach to the substrate, or to shells, rocks, timber or any submerged substrata they can find
with their pedal discs
 some can burrow in the bottom mud or sand
 Oral Disc - Flat surface where a crown of tentacles are arranged in one or more circles around
the mouth.
 Pharynx - gullet where slit shaped mouth leads
 Siphonophore (Siphonoglyph) – The cilia along this groove, help to circulate water through the
gastrovascular cavity or pharynx of sea anemones and some corals. These water currents are
important for:
1. respiration or gas exchange,
2. maintenance of internal pressure for hydrostatic skeleton
3. to move wastes out.
4. Gastrovascular Cavity - leading from the pharynx, this large cavity is divided into radial
chambers, which communicate with each other, by pairs of septa.
5. Sea anemones are carnivorous, feeding on fish or anything of suitable size.
6. They can be either dioecious (separate sexes) or monoecious (hermaphroditic), gonads are
arranged on the margins of the septa. Fertilization can be either external, or take place within
the gastrovascular cavity
7. Zygote develops into a ciliated larva
8. Pedal Laceration – a common form of asexual reproduction where small pieces of the pedal disk
are scraped off as the animal moves, and each of these can regenerate a small anemone. The
pedal disc contains gland cells which can secrete adhesive material or mucus. The Muscular
system is advanced and can expel water for rapid shrinkage.
• Mutualistic symbiosis with clown fish, anemone fish and hermit crabs. The anemone
supply the fish and crab protection and small bits of food, while the fish keep the anemone
clear of debris and also lure prey into its stinging tentacles.
9. Coral Reefs - among the most productive of all ecosystems, and have a biodiversity
rivaled only by the rain forests. They provide a home for twenty-five percent of all marine
species, including fish, molluscs, echinoderms and sponges.
a. Reef-building corals and coralline algae are the two most important organisms that take
dissolved calcium and carbonate ions from seawater and secrete it as limestone to form reefs.
b. The Zoantharia corals are called Stony corals because they are the coral animals which secrete
the limestone that forms the coral reefs.
 The polyps can be either solitary or colonial (in the colonial corals the skeleton may become
very large and massive, building up over many thousands of years)
 The corallite is the cup shaped skeleton secreted by each stony coral polyp
 The gastrovascular cavities of the polyps are all connected (food or disease for one is food or
disease for the entire colony)
 Zooxanthellae (mutualistic algae) live in the tissue of the coral polyp, and give the coral its
color, as well as food and oxygen.
c. The three most important Reef requirements required by reef building stony corals:
• Warm waters (68 - 85 degrees Fahrenheit),
• Light (most reef building corals stop growing at a depth of about 75 to 125 feet)
• A stable salinity level (35 ppt.)
d. Most coral reef formations are found between 30 degrees north and 30 degrees south latitudes
 reefs do not normally occur near the mouths of river or where upwelling’s bring cold water to
the surface
 cold waters limit the secretion of calcium carbonate
 Brain and Boulder coral grow approximately 1/4 to 2 inches per year.
 Branching corals (elkhorn, staghorn, etc.) can grow up to 6 inches a year, depending on the area
they inhabit. They are more susceptible to storm wave damage.
e. There are 3 main types of Coral Reefs: Fringing, Barrier and Atolls
 Fringing reefs form borders along the shore acting as an extension of the coastline, and can
surround islands.
 Barrier reefs are found far from shore and are separated from the coast by large lagoon or sea
known as a channel. The outermost part of the reef is normally a wall.
 Atolls are a somewhat circular chain of reefs and or islands (cays) that surround a central
lagoon. These are the most spectacular type of coral reef.
f. In the Florida Keys, the following types of reef formations are found
 Outer bank spur and groove reefs (Barrier Reef) – Spurs of coral separated by grooves of sand
between them, are what you find in the Florida Keys They are also found off shore, and are
separated from the coastline by a channel.
 Patch reefs – small l patch found normally between the shore and the outer reef structure.
g. Coral feeding - polyps are nocturnal carnivores, feeding on all types of drifting
planktonic organisms and detritus.
● Most of the time, the coral animal absorbs nutrients from the Zooxanthellae, the algae that
lives in their tissue.
3. “The flexible Corals “ Examples: Sea pans, sea pens, sea pansies Sub-class Alcyonaria - (al’cy-ona’ri-a) (Gr. alkonion, kind of sponge resembling nest of kingfisher + L. aria, like or connected
with) (Tubipora, Alcyonium, Gorgonia, Renilla).
a. Flexible corals have an Octomerous body plan (built on a plan of eight) and always have eight
pinnate (featherlike) tentacles arranged around the margin of the oral disc, and eight unpaired,
complete septa.
b. They are often referred to as octocorals or Gorgonians because of their strict octomerous
Symmetry and their skeletons are formed from a flexible, horny substance called gorgonin.
c. They are all colonial, and the gastrovascular cavities of the polyps communicate through a
system of gastrodermal tubes called solenia.
d. These beautiful graceful corals in hues of yellow, red, orange, and purple create the
"submarine gardens"
4. Sub Class Ceriantipatharia (se-ri-an-tip’a-tha’ri-a) (Gr. keras. horn. + anthos. Flower; antiagainst, + pathos suffering, + aria, like, connected with)
a. Tube anemones and black or thorny corals. Examples: Cerianthus, Antlpathes, Stichopathes.
b. Simple unbranched tentacles; mesenteries unpaired
VII. Phylum Ctenophora
A. General Characteristics
1. Ctenophora (te-nof’o-ra) (Gr. kteis, ktenos, comb, + phora, pl. of bearing) comprise a small
group of fewer than 100 species.
a. Occur in all seas, especially warm waters, all are marine, and except for a few creeping forms,
they are all free swimming.
b. They take their name from the eight rows of comblike plates (ctenes) they bear for locomotion.
c. Common names are “sea walnuts” and "comb jellies"
2. Only one species has nematocysts
3. There are no definite organ systems just as with cnidarians
4. Ctenes - ciliated comb plates which they use to propel themselves mouth-end forward.
a. Cestum - a highly modified form uses its sinuous body movements as well as their comb plates
in locomotion.
5. Their fragile transparent bodies are easily seen at night when they emit light (luminescence)
B. Form and Function
1. Pleurobrachia (Gr. pleuron, side + L. brachia, arms), bears on its surface eight longitudinal rows
of transverse plates bearing long fused cilia called comb plates.
2. Beating rows of cilia in unison start at the aboral end and proceed to the oral end, propelling the
animal forward (cilia can reverse direction).
10. Colloblasts - specialized glue cells which secrete a sticky substance that is used in prey
capture
is found on the two long tentacles.
4. When the tentacles are full of food, they contract and are wiped off on the mouth.
5. The gastrovascular cavity consists of a pharynx, stomach and a system of gastrovascular canals
a. Rapid digestion occurs in the pharynx but basically they digest intracellularly
b. Waste products are expelled through the aboral end
6. Statocyst - Organ of equilibrium located at the aboral pole and it is also concerned with the
beating of the comb rows.
a. A nerve net system similar to that of cnidarians includes a sub-epidermal plexus
concentrated under each comb plate.
b. The nerve net is under control of the statocyst, and control rhythm of comb plates
7. Reproduction - All ctenophores are monoecious (hermaphroditic), bearing both an ovary and a
testis.
a. Gametes are shed into the water, except in a few species that brood their eggs.
b. Larvae is free swimming