A Fearsome Tiny Weapon: Things to know about Phylum Cnidaria• More highly organized than sponges• Relatively simple animals• Most are sessile• Some that are unattached (i.e.. Jellyfish) swim only feebly• Cannot chase their prey• Very effective predators that are able to kill and eat prey

that are much more highly organized, swift, and intelligent with their tentacles

• It uses it’s Nematocysts (– Tentacles that bristle with tiny, remarkably sophisticated weapons) that is endowed with potential energy to power it’s discharge.

• When a nematocyst is fired, a tiny thread bursts from it achieving a velocity of 2 meters/second and an acceleration of 40,000 x gravity and instantly penetrated it’s prey injecting a paralyzing toxin.

More to know about Phylum Cnidaria:

• Continued from previous slide:– Some nematocyst threads can penetrate human

skin, causing sensations ranging from minor irritation to great pain, even death, depending on the species

– Familiar cnidarians are sea anemones and jellyfishes; also some may know ctenophorans as comb jellies or sea walnuts

– Contains more than 9000 species including: Branching plantlike hydroids; flowerlike sea anemones; horny corals (sea whips, sea fans, and others); and also stony corals who make up coral reefs and coral islands

– Takes it’s name from cells called Cnidocytes– Most common type of cnida is the Nematocyst– Cnidocytes only formed by cnidarians– Ancient group with the longest fossil history of

any metazoan reaching back more than 700 million years

• Some ctenophores, molluscs, and flatworms eat hydroids bearing nematocysts, then store and use these stinging structures for their own defense

• Widespread in marine habitats, and a few in freshwater• Most abundant in shallow marine habitats, especially in warm

temperatures and tropical regions• No terrestrial species• Colonial hydroids are usually found attached to mollusc

shells, rocks, wharves, and other animals in shallow coastal water, some species live at great depths

• Floating and free-swimming medusae occur in open seas and lakes, often far from shore

• Animals such as the Portuguese man-of-war have floats or sails by which the wind carries them

• Quite efficient predators of organisms that are much swifter and more complex

• Sometimes live symbiotically with other animals• Often as commensals on the shell or other surface of their

host• Certain hydroids and sea anemones commonly live on snail

shells inhabited by hermit crabs, providing the crabs some protection from predators

• Algal cells frequently live as mutual's in the tissues of cnidarians

• Some freshwater hydras and in reef-building have this too• Reefs are extremely important habitats for many other

species of invertebrates and vertebrates in tropical waters• Many cnidarians have little economic importance, reef-

building corals are an important exception• Fish and other animals associated with reefs provide

substantial amounts of food for humans, and reefs provide an economic value of tourist attractions

• Classes of Cnidaria– Hydrozoa – most variable class, including hydroids, fire

corals, Portuguese man-of-war, and others – Cubozoa – including cube jellyfishes– Anthozoa – largest class including sea anemones, stony

corals, soft corals, and others– Stauroza – has been proposed because recent phylogenies

show that stauromedusans do not belong within the scyphozoa , do not make medusae but the polyp body is topped by a medusa-like region

Form and Function:dimorphism and polymorphism in

cnidarians• All cnidarian forms fit into one of two morphological types

(Dimorphism) :– Polyp - a hydroid form, which is adapted to a sedentary

or sessile life– Medusa – a jellyfish form, which is adapted for a floating

or free-swimming existence

• Both seem very different, but actually each has retained the saclike body plan characteristic of the phylum

• A medusa is essentially and unattached polyp with the tubular portion widened and flattened into a bell shape

Polyps• Most have tubular bodies• A mouth surrounded by tentacles defines the oral end of the body• Mouth leads into a blind gut or Gastrovascular Cavity• Aboral end of the polyp is usually attached to a substratum by a

pedal disc or other device• May reproduce asexually by budding, fission, or pedal laceration• Budding – a knob of tissue forms on the side of an existing polyp and

develops a functional mouth and tentacles, if detached from the poly that made it, a clone is formed, If bud stays attached to the polyp that made it, a colony will form and food may be shared through a common gastrovascular cavity

• When they do not bud, they are solitary• Others form clones or colonies• Distinction between colonies and clones is sometimes blurred when a

colony fragments• Shared gastrovascular cavity permits polyp specialization• Polymorphism – colonies that include several morphologically distinct

polyps with each specialized for a certain function, such as feeding, reproduction, or defense.

• Hydranths – Feeding polyps• Gonangia – reproductive polyps, typically make medsae

Medusae• Usually free swimming• Have bell or umbrella-shaped bodies• Often exhibit tetramerous symmetry where body parts are arranged in

fours• Mouth is usually centered on the concave side and may be pulled

downward into frilly lobes that extend a long way beneath the umbrella or bell

• Tentacles extend outward from the rim of the umbrella• Have sensory structures for orientation (or Statocysts) and light

reception (ocelli)• Sensory info integrated with motor response by a nerve ring at the base

of the bell; two such rings are present in hydrozoan medusae• Medusae of Class scyphozoa often called Scyphomedusae, where as

those of class hydrozoa are Hydromedusae• Both differ from each other by the presence of a velum (or a shelflike fold

of tissue from the bottom of the bell that extends into the bell• By reducing the cross-sectional area at the bottom of the bell, the velum

increases the exit velocity of water from the bell, making each pulsation more efficient

Life Cycles

• In cnidarian life cycle, polyp’s and medusae play different roles• Life cycle varies among cnidarian classes, but in general, a zygote

develops into a motile planula larva• Planula settles on a hard surface and metamorphoses into a polyp• Polyp may make other polyps asexually, but eventually it

produces free-swimming medusae by asexual reproduction• Polyps by budding, or other specialized methods like Strobilation• Medusae reproduce sexually and are dioecious• A cycle that contains both an attached polyp and a swimming

medusa permits organisms to take advantage of both pelagic (open water) and benthic (bottom) environments

• Occurs in true jellyfishes of class scyphozoa where the medusa is large and conspicuous and the polyps are typically very small

• Most hydroids of class hydrozoa also feature a sessile polyp stage, often in colonies, and a pelagic medusa stage

• There are many variations on the typical pattern

Continuation of Life Cycles….• Some hydrozoans have the polyp colony as not sessile but drifts across

the ocean surface• The Portuguese man-of-war , Physalia, is one such drifter, using an

inflated polyp as a gas-filled float• Other colonies are collections of both polyps and medusae where

pulsating bells propel the colony through the water• Several life cycles do not include medusae• Anthrozoans are presumed to have diverged from an ancestor of the

other cnidarians before the medusa evolved in the latter branch• Other cnidarians, including the hydrozoan Hydra, probably lost the

medusa secondarily• Mechanism of loss is not clear in Hydra but in other hydrozoans a

pattern of loss can be inferred from a comparison of modern forms• Most hydrozoans release medusae that later make gametes• A few forms make medusae without releasing them from the colony• Gametes then form in the gonads of the medusae retained by the polyp

colony• Some species have only a short cuplike form surrounding the gonads• In other gonads develop right on the polyp colony with no trace of a

medusa body• The latter organisms likely represent an extreme form of medusa

retention and reduction

Cnidocytes• Each cnidocyte produces one of over 20 kinds of distinctive

organelles called Cnidae that are discharged from the cell• During it’s development, a Cnidocyte is properly called a

Cnidoblast• Once it’s cnida has been discharged, a cnidocyte is absorbed

and replaced• Type of cnida: Nematocyst• Nematocyst is composed of material similar to chitin and

containing a coiled tubular “Thread” or filament• Operculum – the little lid that covers the capsule on the top of

a nematocyst• Some Cnidae have don’t have barbs or poison• Some spring and clasp their prey• Adhesive cnidae are used in attachment and locomotion,

relying on the creature they are attached to• In Anthozoa, Cnidocytes are equipped with a trigger like

cnidocil (- a modified cilium)• Anthozoan Cnidocytes have a somewhat different ciliary

mechanoreceptor• Some sea anemones ,and perhaps other cnidarians, small

organic molecules from the prey “tune” mechanoreceptors, making them sensitive to the frequency of vibration caused by prey swimming

Feeding and Digestion• Polyps are typically carnivorous• They catch prey with their tentacles, passing them through

the mouth into the gastrovascular cavity for digestion• In Hydra, the tentacles are hollow and the tentacle cavity

communicates with the gastrovascular cavity• Polyps of a hydrozoan colony capture and digest prey

extracellularly• It then passes through a digestive broth into the common

gastrovascular cavity where intracellular digestion occurs• In Hydromedusae, both food type and digestive system are

similar to that of the polyp• But the body is oriented with the mouth facing downward in

the center of the bell; the mouth is at the end of a tube called the Manubrium

• Scyphomedusae are typically lager than hydromedusae, but their basic form is similar

• The mouth edge is extended as a Manubrium, often with four frilly oral arms sometimes called mouth lobes, used in capturing and ingesting prey

• Anthozoan polyps, such as sea anemones, are carnivorous, feeding on fish or almost any animals of suitable size

• A few species feed on minute forms caught by ciliary currents instead of eating large prey

• Corals supplement their nutrition by collecting carbon from their algal symbionts

Class Hydrozoa: Hydroid colonies

• Majority are marine and colonial in form• Typical life cycle includes both and asexual polyp and a sexual medusa stage

(i.e. Obelia)• Typical hydroid consists of a base, a stalk, and one or more terminal zooids• The base by which colonial hydroids attach to the substratum is a rootlike

stolon, or Hydrorhiza • Hydrocauli – one or more stalks that are given to rise• Tubular Coenosarc – the living cellular part of the hydrocaulus• Perisarc – a nonliving chitinous sheath that protectively covers the

hydrocaulus• Hydranths – most zooids that are feeding polyps, also known as Gastrozooids• Thecate – the perisarc continues as a protective cup around the polyp into

which it can withdraw for protection• Athecate – where the polyp is naked• Gonangium – a reproductive polyp from the medusae bud in Obelia• Entocodon – a unique derivative of ectoderm

Feeding, Digestion, and Epidermis

• Basal – a pedal in which a slender stalk ends• Disc – part of said pedal, or Basal• Hypostome – the conical elevation in which the mouth is located• Nutritive-muscular cells – usually tall columnar cells and have

laterally extended bases containing myofibrils• Hydrostatic skeleton – water in the gastrovascular cavity• Epitheliomuscular cells – compose most of the epidermis and serve

both for covering and for muscular contraction• Interstitial cells – undifferentiated stem cells found among the bases

of the epitheliomuscular cells• Gland cells – tall cells, located around the basal disc and mouth, that

secrete an adhesive substance for attachment and sometimes a gas bubble for floating

• Cnidocytes – occur throughout the epidermis• Sensory cells – scattered around the other epidermal cells, especially

near the mouth and tentacles and on the basal disc• Nerve Cells – generally multipolar, although in more highly organized

cnidarians the cells may be bipolar

Reproduction

• Reproduce sexually and asexually• In asexual reproduction, buds appear as out

pocketings of the body wall and develop into young hydras that eventually detach from the parent

• Most are dioecious• Temporary gonads usually appear in autumn• Testes or ovaries, when present, appear as rounded

projections on the surface of the body• Eggs in the ovary usually mature one at a time and

are fertilized by sperm shed into the water• Dactylozooids – fishing tentacles• Gonophores – a little more than sacs containing

either ovaries or testes• Pneumatophore - a float believing to have expanded

from the original larval polyp

Class Scyphozoa• Includes most of the larger jelly fishes, or Cup animals• May attain a bell diameter exceeding 2 meters and tentacles

60-70 meters long, but most range from 2 to 40 cm in diameter

• Most drift or swim in the open sea• Some even at depths of 3000 m.• Movement achieved by rhythmical pulsations of the bell• Bells differ in depth from a shallow saucer shape to a deep

helmet or goblet shape, but a velum is never present• The margin of the umbrella that is scalloped, usually with

each indentation, or notch bearing a pair of Lappets• Rhopalium – a sense organ• Nervous System - a nerve nest with a subumbrella net that

controls bell pulsations and another, more diffuse net that controls local reactions such as feeding

• Oral arms – used in capturing and ingesting prey

Class Scyphozoa

• Gastric Pouches – gastrodermis extends down in little tentacle-like projections called Gastric Filaments

• Radial Canals – a complex system that branches outward from the pouches to a Ring Canals in the margin and forms a part of the gastrovascular cavity

• Scyphistoma – a hydralike form that may bud to produce a polyp clone

• Strobilation – As process of the scyphistoma of Aurelia forming into a series of saucerlike buds

• Ephyrae – saucerlike buds• Stobila – what's left of the saucer like buds

Class Cubozoa

• Once were considered an order of scyphozoa

• Medusoid is the predominant form• Pedalium – a flattened, tough blade• Velarium – functions as a velum does in

hydrozoan medusae, increasing swimming efficiency, but it differs structurally

• Stings of some species can be fatal to humans

• The polyp is tiny, solitary, and sessile

Class Anthozoa

• Known as flower animals, they are polyps with a flowerlike appearance

• There is no medusa stage• All marine and occur in both deep and shallow water and in

polar seas as well as tropical seas• Hexacorallia – known as zoantharia contains sea anemones,

hard corals, and others• Ceriantipatharia – containing only tube anemones and thorny

corals• Octocorallia – also known as Alcyonaria containing soft and

horny corals, such as sea fans, sea pens, sea pansies, and others

Sea Anemones

• Polyps are larger and heavier than hydrozoan polyps• Some are quite colorful• Cylindrical in form with a crown of tentacles arranged in

one or more circles around the mouth of the flat oral disc• Pharynx – the part that the slit shaped mouth leads into• Siphonoglyph – extends into the pharynx• Monoexious species are Protandrous• Pedal laceration – by longitudinal fission, occasionally by

transverse fission or by budding