Ch. 14: Cnidarians and Ctenophores

A look into the phylogeny of corals, sea anemones, jellies, and others.

radiates

animals displaying radial or biradial symmetry

diploblastic (ectoderm and endoderm)

no cephalization

The radiates clade of animals contains…

phylum Cnidaria and Cnetophora

phylum Cnidaria

sea anemones, jellyfish, corals, etc.

have cnidocytes for defense and/or offense

ancient group of animals, with fossils from 700 mya

mostly marine and sessile

sometimes lives symbiotically with other organisms

cnidocytes

defensive cells seen in phylum Cnidaria

has discharging organelles

cnidocyte type example

nematocyst

nematocyst

a type of cnidocytes

injects toxin for prey capture or defense through hollow filament

sometimes has barbs to latch into victim

usually triggered by cnidocil

Are nematocysts single use or reuseable structures?

single-use

once they have been launched, they cannot go back in

cnidarians can shed these cells and make new nematocysts

phylum Cnidaria classes and description of what’s included

Hydrozoa - hydras

Scyphozoa - the true jellies

Cubozoa - box jellies, sea wasps

Anthozoa - anemones, corals, and other related things

Staurozoa - very few species of medusa-like animals (not much focus in this course)

Myxozoa - highly modified parasitic forms (very different from rest of the phylum)

phylum Cnidaria body plans

dimorphic: polyp and medusa forms

physical relationship of polyp and medusa form

polyp is the sessile, “upright” form

medusa is the free moving, “downward” form

can show with your hand: polyp form is palm facing up, medusa form is palm facing down, and your fingers are the tentacles.

dimorphic

an animal having two separate forms

polyp

typically tubular

blind gut

sessile

reproduces asexually

colonies may have several morphologically distinct polyps

medusa

the “jelly” form

bell or umbrella shaped

usually free-swimming

mouth is directed downwards

tentacles may extend down

sensory structures for sensing orientation and light

sexually reproduce to make polyps

* linked to motor response via nerve ring at the base of the bell

Are medusa forms dioecious or monoecious?

dioecious

general life cycle of cnidarians

zygote → motile planula larva → planula larva settles, metamorphosis → polyp → asexually form medusa → mature medusa → sexually reproduce with other medusae and form a zygote

Polyps may form other polyps via…

asexual reproduction

planula larva

the larval stage between zygote and polyp in cnidarians

ephyra

a baby medusa that still needs to grow into a full size medusa

variations in life cycle in cnidarians

drifting polyp colony, or polyp/medusae colony

life cycles without medusae

cnidarian cell layers

two cell layers:

* inner layer - gastrodermis
* outer layer - may have cnidocytes with nematocysts

has mesoglea between layers

gastrodermis

AKA inner layer or endoderm

digestion layer on the inside of the cnidarian

epidermis

AKA ectoderm

may have cnidocytes with nematocysts in cnidarians

mesoglea

gelatinous, buoyant filling between cell layers

supports the body

organ systems of cnidarians

feeding and digestion

nerve net

feeding and digestion in cnidarians

both stages carnivorous

coral also get carbon from algal symbionts

nerve net

example of diffuse nervous system, *not* central

in medusae, nerve net and nerve ring function like a central nervous system, but not quite

important landmark in evolution of nervous systems

nerve ring

ring of nerves near the end of the bell

Class Hydrozoa

mostly marine, colonial (freshwater forms do exist!)

has polyp and medusa forms

typical hyzroid colonies look like:

* base, stalk, one or more terminal zooids
* hydranths
* gonangia

may have perisarc

zooids

individual polyps found in hydroid colonies

hydranths

feeding zooids that capture prey in a hydroid colony

gonangia

reproductive polyps that form medusa buds in hydroid colonies

perisarc

non-living chitinous covering found “shrink-wrapped” around hydroids

hydroid medusae

small (mm or cm across)

velum

gastrovascular cavity - continuous from mouth to tentacles

bell margin has:

* many sensory cells (detecting light and other things)
* nerve rings

velum

shelf-like lip in the medusa form of hydroids

helps with trapping water in order to propel itself farther

freshwater hydras

solitary polyps

can flop around and move

no medusa stage

nematocysts capture prey

some species with symbiotic algae

sexual and asexual reproduction

other examples of hydrozoans

*Physalia* (Portuguese man-of-war)

polymorphic colonies with both polyps and medusae

acts as one individual

mutualistic relationship with some fish

Class Scyphozoa

most of the larger jellyfish in this class

mostly in open sea

bells vary in shape and size

most less than 1/2 m across

scalloped margin

true jelly body plan

no velum

many nematocysts

thick layer of mesoglea

dioecious

internal fertilization

examples of true jellies

moon jellies

* cosmopolitan distribution

upside down jellyfish

* tissues have symbiotic dinoflagellates
* acts like a polyp so the dinoflagellates make sugar
* uses bell to suction cup itself to the bottom

Scyphozoa life cycle

zygote → cilicated planula larva → planula attaches to form scyphistoma → scyphistoma under goes strobilation → strobila forms ephyrae → break loose to form

scyphistoma

the name of strobilating polyps in class Scyphozoa

strobilation

the asexual reproduction of medusae

Class Staurozoa

no medusa stage

solitary polyp on a stalk

polyp top resembled a medusa

reproduce sexually

very few species

not much is known about this class

Class Cubozoa

box jellyfish, sea wasps

dominant medusa form

* polyp form inconspicuous or unknown

voracious predators

box jelly body plan

mostly small (2-3 cm)

umbrella square, tentacles at the corners

pedalium at base of each tentacle

has velarium at edge of umbrella

has potent toxins: can kill a human in a couple of minutes

pedallium

flat blade at the base of each tentacle in class Cubozoa

velarium

umbrella edge turns in in class Cubozoa

increases swimming efficiency

like the velum in hydrozoa

Class Myxozoa

newer class in Cnidarians

obligate parasites

structurally, are highly reduced cnidarians

* extremely small genome for an animal

Class Myxozoa body plan

tiny - just a couple of cells big

has polar capsules

Class Myxozoa examples

those that cause whirling disease

annelid worms

polar capsules

found in class Myxozoa

homologous to nematocysts

Class Anthozoa

no medusa stage

all marine

large gastrovascular cavity and divided into different parts

has three subclasses

Anthozoa subclasses

Hexacorallia

Ceriantipatharia

Octocorallia

subclass Hexacorallia

part of class Anthozoa

sea anemones, hard corals

hexamerous (6) body plan

polyps larger, heavier than hydrozoan polyps

resides in costal areas

glide on pedal discs

oral discs

carniverous

some can swim

pedal disk

found in subclass Hexacorallia

lets it attach to shells, rocks, etc.

oral disk

found in subclass Hexacorallia

surrounds the open mouth and where the tentacles attach to

sea anemone behavior

can contact and withdraw tenacles into their oral disk

mutualistic relationships:

* many harbor symbiotic dinoflagellates (algae)
* some live on crab shells
* some provide shelter for anemone fish

sea anemone reproduction

sexual reproduction:

* some dioecious, some monoecious
* monoecious species are protandrous

asexual reproduction:

* occurs via pedal laceration
* occurs via longitudinal fission, but also transverse fission and budding

protandrous

produce sperm first, then eggs later

found in sea anemones

pedal laceration

type of asexual reprduction

pieces of pedal disk break off and regenerate

found in sea anemones

true (stony) corals

looks like tiny sea anemones in calcareous cups

secretes exoskeleton

in colonies, exoskeleton can becomes massive, but the living coral forms thin layer over that exoskeleton

subclass Ceriantipatharia

part of class Anthozoa

few species

tube anemones

* solitary, buried in soft sediments

thorny corals

* colonial, attach to form substrates
* tough, spiny exoskeleton

hexamerous (6) body plan

subclass Octocorallia

part of class Anthozoa

soft and horny corals (sea fans, sea pens, sea pansies, etc.)

octomerous (8) body plan

* eight pinnate tentacles
* eight complete septa

all are colonial, with some varied forms

soft & horny coral body plan

gastrovascular cavities communicate through solenia

have coenenchyme

skeleton secreted within the coenenchyme

solenia

tubes found connecting gastrovascular cavities in soft and horny corals

coenenchyme

tissue consisting of mesoglea and solenia, connecting cavities together

found in soft and horny coral

coral reefs

productive, diverse ecosystems

limestone deposited over 1000s of years

* living plants and animals limited to the top layer

Coral reefs need _________________ corals for reef formation, but other species may be involved.

hermatypic (hard)

(class Hexacorallia)

coral reef requirements

warmth, light, salinity of undiluted sea water (basically can’t be next to a river mouth or other body of fresh water)

limited to shallow waters between 30 N and 30 S

relationship of zooxanthellae and coral tissues

\
zooxanthellae live inside the cells of coral, and gives food to coral during the day

coral gives zooxanthellae a place to live, and the zooxanthellae can take in phosphorous and nitrogen excreted by the coral

threats to coral reefs

nutrients from fertilizer and sewage → excessive algae growth

overfishing of herbivorous fishes → excessive algae growth

pollution (pesticides, oil, sediment, etc.)

global warming → warm water causing coral bleaching

higher atmospheric CO2 → acidifies oceans water, making precipitation of CaCO3 by corals more difficult

coral bleaching

caused by global warming

coral expels zooxanthellae, causing the coral to turn white

usually results in the death of the coral

phylum Ctenophora

comb jellies

about 150 species

all marine, mostly preferring warm waters

8 rows of comb-like plates for locomotion

nearly all free-swimming

biradial symmetry

comb jelly body plan

some relatively large (1 m)

no definite head, but do have oral and aboral ends

translucent body with gelatinous layer

most have two extendable tentacles

comb jelly behavior

some feed on cnidarians, incorporating cnidocytes as defensive mechanisms

bioluminescent

comb jelly reproduction

monoecious in most species

cydippid larva somewhat resembles adult

medusae and polyp are both monoecious, not different like other cnidarians

cydippid larva

comb jelly larva

phylogeny of the diploblasts

ctenophores and cnidarians have typical diploblastic characteristics,

but the cells within the gelatinous layer are problematic…

are they really triploblastic organisms?

in short - diploblastic with some added features (from what we know)