Cnidaria

Question 1

Cnidarian bodies consist of _____, a non-living jelly-like substance, sandwiched between ____ layers of epithelium that are mostly ____ cell thick

Answer 1

mesoglea, two, one

Question 2

Many cnidarian species produce ______ that are single organisms composed of ____-like or ____-like zooids or both.

Answer 2

colonies, medusa or polyp

Question 3

How are cnidarian activities coordinated?

Answer 3

A decentralized nerve net and simple receptors, some have simple eyes.

Question 4

What are the major taxa of cnidaria?

Answer 4

Anthozoa (class): Corals, anemones, sea pens

Medusozoa (subphylum): Jellyfish, hydroids

Myxozoa (subphylum): Myxozoan parasites

Question 5

The oldest estimate for the arrival of cnidarians, derived from molecular clock methods is _____

Answer 5

791 million years ago, before the Cambrian explosion

Question 6

In what ways are Cnidarians more complex than sponges?

Answer 6

Cells bound by inter-cell connections and carpet-like basement membranes; muscles; nervous systems; and some have sensory organs

Question 7

How are cnidarians distinguished from all other animals?

Answer 7

Cnidarians are distinguished from all other animals by having cnidocytes that fire like harpoons and are used mainly to capture prey. In some species, cnidocytes can also be used as anchors

Question 8

How do medusae swim?

Answer 8

Medusae swim by a form of jet propulsion: muscles, especially inside the rim of the bell, squeeze water out of the cavity inside the bell, and the springiness of the mesoglea powers the recovery stroke. Since the tissue layers are very thin, they provide too little power to swim against currents and just enough to control movement within currents

Question 9

Describe the skeletons of the different groups of Cnidaria

Answer 9

Medusae: mesoglea.

Hydra and most sea anemones: close their mouths when they are not feeding, and the water in the digestive cavity then acts as a hydrostatic skeleton, rather like a water-filled balloon.

Tubularia: columns of water-filled cells for support.
Sea pens stiffen the mesoglea with calcium carbonate spicules and tough fibrous proteins, like sponges.

In some colonial polyps, a chitinous periderm gives support and some protection to the connecting sections and to the lower parts of individual polyps.

Stony corals secrete massive calcium carbonate exoskeletons.

Question 10

Describe the nervous system of Cnidaria

Answer 10

They have integrative areas of neural tissue that could be considered some form of centralization. Most of their bodies are innervated by decentralized nerve nets that control their swimming musculature and connect with sensory structures, though each clade has slightly different structures

Question 11

Siphonophores and chondrophores sense tilt and acceleration by means of _______, chambers lined with hairs which detect the movements of internal mineral grains called statoliths

Answer 11

statocysts

Question 12

What are ocelli?

Answer 12

Simple eyes that can detect sources of light.

Question 13

What are cnidarian feeding mechanisms?

Answer 13

Predation, absorbing dissolved organic chemicals, filtering food particles out of the water, obtaining nutrients from symbiotic algae within their cells, and parasitism.

Question 14

What do cnidaria give their endosymbionts?

Answer 14

Carbon dioxide, some nutrients, a place in the sun and protection against predators.

Question 15

Digestion is ____ cellular

Answer 15

Intra- and extra-

Question 16

Describe Cnidarian excretion

Answer 16

Indigestible remains of prey are expelled through the mouth. The main waste product of cells’ internal processes is ammonia, which is removed by the external and internal water currents.

Question 17

Describe Cnidarian digestion

Answer 17

Once the food is in the digestive cavity, gland cells in the gastroderm release enzymes that reduce the prey to slurry, usually within a few hours. This circulates through the digestive cavity and, in colonial cnidarians, through the connecting tunnels, so that gastroderm cells can absorb the nutrients. Absorption may take a few hours, and digestion within the cells may take a few days. The circulation of nutrients is driven by water currents produced by cilia in the gastroderm or by muscular movements or both, so that nutrients reach all parts of the digestive cavity.[10] Nutrients reach the outer cell layer by diffusion or, for animals or zooids such as medusae which have thick mesogleas, are transported by mobile cells in the mesoglea.[9]

Question 18

Describe Cnidarian respiration

Answer 18

There are no respiratory organs, and both cell layers absorb oxygen from and expel carbon dioxide into the surrounding water. When the water in the digestive cavity becomes stale it must be replaced, and nutrients that have not been absorbed will be expelled with it. Some Anthozoa have ciliated grooves on their tentacles, allowing them to pump water out of and into the digestive cavity without opening the mouth. This improves respiration after feeding and allows these animals, which use the cavity as a hydrostatic skeleton, to control the water pressure in the cavity without expelling undigested food.[9]

Cnidaria that carry photosynthetic symbionts may have the opposite problem, an excess of oxygen, which may prove toxic. The animals produce large quantities of antioxidants to neutralize the excess oxygen.[9]

Question 19

Describe cnidarian regeneration

Answer 19

All cnidarians can regenerate, allowing them to recover from injury and to reproduce asexually. Medusae have limited ability to regenerate, but polyps can do so from small pieces or even collections of separated cells. This enables corals to recover even after apparently being destroyed by predators

Question 20

Describe the cnidarian complex life cycle

Answer 20

Cnidarian sexual reproduction often involves a complex life cycle with both polyp and medusa stages. For example, in Scyphozoa (jellyfish) and Cubozoa (box jellies) a larva swims until it finds a good site, and then becomes a polyp. This grows normally but then absorbs its tentacles and splits horizontally into a series of disks that become juvenile medusae, a process called strobilation. The juveniles swim off and slowly grow to maturity, while the polyp re-grows and may continue strobilating periodically. The adults have gonads in the gastroderm, and these release ova and sperm into the water in the breeding season.[9][10]

This phenomenon of succession of differently organized generations (one asexually reproducing, sessile polyp, followed by a free-swimming medusa or a sessile polyp that reproduces sexually)[24] is sometimes called “alternation of asexual and sexual phases” or “metagenesis”, but should not be confused with the alternation of generations as found in plants

Question 21

What preys on cnidaria?

Answer 21

Sea slugs, which can incorporate nematocysts into their own bodies for self-defense;[38] starfish, notably the crown of thorns starfish, which can devastate corals;[34] butterfly fish and parrot fish, which eat corals;[39] and marine turtles, which eat jellyfish

Question 22

What is the different between ahermatypic and hermatypic corals?

Answer 22

Whether they are reef-building or not.

Question 23

What is the name of the sac-like central body cavity in porifera and cnidaria?

Answer 23

Coelenteron

Question 24

What are mechanisms of cloning in corals?

Answer 24

a. Fragmentation via mechanical breakage
(e.g. storms, bioerosion, partial predation, … )
b. Fragmentation via dieback of coenosarc (tissues)
and coenosteum(skeleton)
b. Polyp balls (e.g. Goniopora stokesi)
c. Polyp “bailout” Mass detachment of polyps ( plankton)
d. Single polyp detach and walk away ( as “anemones”)
e. Asexual larvae (no meiosis)
(= “true” asexual reproduction

Question 25

Describe heterotrophic coral feeding

Answer 25

• Nematocysts capture prey or particles
• Tentacles capture food & push into
coelenteron for:
– Extracellular digestion
– Phagocytosis
– Intracellular digestion
• Mesenteries and filaments for digestion

Question 26

What are some other heterotrophic feeding strategies?

Answer 26

1. Mucus strands – “fishing”
2. Mucus sheets – particulate feeding
3. Mucus sheets ‐ microbial “culture”
4. Dissolved organic molecules
5. Mesenteric filaments ‐ detritivores

Question 27

What are the advantages of photosynthetic endosymbionts?

Answer 27

Major energy source for corals, increases calcification rate.

Question 28

What are the advantages of photosynthetic endosymbionts?

Answer 28

Major energy source for corals, increases calcification rate.

• Zoox get CO2 & nutrients from coral
• up to ~95% of sugars to corals
• up to ~98% of coral’s energy
(triglycerides cetyl palmitate)
• accelerate coral growth
• increase calcification (5–20 X)
• enable dominant reef‐builders

Question 29

What marine organisms have photosynthetic endosymbionts?

Answer 29

Clams, anemones, corals, etc.

Question 30

When did photosynthetic symbionts arise in corals?

Answer 30

Around Triassic, 220 mya.

Question 31

What are the major clades of coral symbionts and their environmental preferences?

Answer 31

– Types A and B –tolerant of high light
– Type C – shade loving
– Type E – relatively common on nearshore reefs

Question 32

How do corals get zooxanthellae?

Answer 32

Vertical transmission
= zooxanthellae in egg or brooded larvae

• Horizontal transmission
= gained from seawater when feeding (via phagocytosis)

Question 33

What is phagocytosis?

Answer 33

In cell biology, phagocytosis is the process by which a cell—often a phagocyte or a protist—engulfs a solid particle to form an internal compartment known as a phagosom

Question 34

What is the adaptive bleaching hypothesis?

Answer 34

May switch genetic type in severe environmental stress

Question 35

What are the benefits for zooxanthellae of the symbiosis?

Answer 35

• Supply of N (from coral’s heterotrophy)
• Fixed position in water column and favorable light regime
• Protection from planktonic grazers
• CO2 from coral respiration
• CO2 from water
• Protection from UV

Question 36

What are the specific benefits for the host coral?

Answer 36

• Photosynthate (carbohydrates & lipid precursors)
• Energy reserves (cetyl palmitate = wax ester)
• N recycling and conservation
• (?) O2 for metabolism
• Enhanced growth
• Enhanced calcification

Question 37

Describe broadcast spawning.

Answer 37

• Gametes shed into water
• Fertilization external
• Zygote develops in plankton

Sperm can detect & move to
egg in water
• Development fast (48‐72 hr)
• Long‐lived larvae (>100 days)
(can settle ~ 2 day ‐ >3 mon)
• Wide dispersal potential
(Settle locally or far away)
• Common all sexuality modes

Question 38

Describe brooder spawning.

Answer 38

• Fertilization internal
• Zygote develops in coelenteron
• Mature larva released

Release only sperm to water
• Retain eggs in coelenteron or mesentery
• Internal fertilization
• Brood larvae for <4 weeks to >15 months
• Larvae “competent” to settle
immediately
• Both hermatypic and ahermatypic
• Both zooxanthellate and azooxanthellate
• Both solitary and colonial
• Often have vertical transmission of
zooxanthellae (mother to larvae)
• Local and long‐distance dispersal
• Some brood asexual larvae
• Is self‐fertilization possible?

Question 39

Describe sexual reproduction in corals.

Answer 39

Gametogenesis
• Many corals are gonochoric (separate sexes)
• Most corals are hermaphroditic (both sexes)
Development of zygote  planula larva
• May be internal (brooding)
• May be external (planktonic)
• All larvae potentially have planktonic phase
Settlement and metamorphosis
• Many corals need coralline algae or bacterial film
Some corals have both sexual and asexual larvae