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BIOL 305 > LECTURE 04 - Cnidaria and Ctenophora > Flashcards

LECTURE 04 - Cnidaria and Ctenophora Flashcards

1
Q

What is a zooid?

A

An animal arising from another by budding or division, especially each of the individuals which make up a colonial organism and typically have different forms and functions.

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2
Q

What is an example of a basic zooid?

A

The hydrozoan polyp

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3
Q

What does budding and persistent stolon lead to?

A

budding and persistent stolon lead to a colony of zooids
- compare: cell division + persistent extracellular matrix leads to multicellular individual

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4
Q

What does a colonial growth identify and exploit?

A
  • Colonial growth identifies and exploits favourable sites in a heterogeneous environment
  • A colony is able to occupy a favourable site more effectively than a scattering of individuals
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5
Q

What does colonial growth reduce?

A

Colonial growth also reduces the variance of nutritional status among zooids

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6
Q

What can zooids arising from the stolon do themselves as well?

A

Zooids arising from the stolon may themselves bud, forming a larger colony that extends into the water column

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7
Q

What does colony form depend on?

A

Colony form depends on branching pattern created by budding

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8
Q

What kind of budding does Coryne have?

A

Coryne has hydrorhizal budding in which the polyps arise singly and irregularly from a basal stolon or mat

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9
Q

What does a colony consist of?

A

The colony consists of a main erect stem, the hydrocaulus, secured at its base by spreading stolons and bearing zooids on branches

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10
Q

What is the difference between a monopodial growth and a sympodial growth?

A
  • In monopodial growth, the main stem and all branches bear terminal zooids and continue to elongate by means of a growth zone just beneath each zooid
  • In sympodial growth, the stem terminates in a growing point, rather than a zooid, which elongates indefinitely without differentiation, like the meristem of a plant
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11
Q

What does the differentiation of zooids lead to?

A

Differentiation of zooids leads to a polymorphic colony

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12
Q

Describe the Hydractinia.

A
  • Hydractinia is a colonial hydrozoan which grows on gastropod shells (including those occupied by hermit crabs)
  • The stolon tubes fuse to form a network from which the zooids arise
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13
Q

Name some of the kinds of zooid which are morphologically and functionally distinct.

A
  • Gastrozooid: feeding zooid with tentacles, mouth and gastric cavity
  • Gonozooid: reproductive zooid that forms sexual medusa
  • Dactylozooid: protective zooid with nematocysts but no mouth
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14
Q

What are Siphonophores?

A
  • Compound organisms
  • Siphonophores are distinctive because they show the highest degree of division of labour between the individual zooids of any colonial organism
  • Being a siphonophore is as if you were to bud thousands of conjoined twins throughout your life, some with only legs to move everybody, others with only mouths to ingest food, others with enlarged hearts to circulate the shared blood, and others fully dedicated to the sexual production of new offspring colonies
  • There can be a dozen or more such functional classes of zooids in siphonophore colonies, and they are arranged in precise species-specific patterns
  • This pattern is usually reiterated along a linear stem, with the exact same sequence of specialized zooids occurring over and over
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15
Q

What is the medusa?

A
  • The gonozooid is a reproductive zooid
  • The contrast between the gonozooid and other kinds of zooid is equivalent to the contrast between germ and somatic tissue in multicellular individuals
  • The gonozooid produces dispersive zooids by budding
  • The dispersive zooid is the medusa
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16
Q

In what way is the medusa formed?

A

The medusa is formed vegetatively from the polyp

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17
Q

What type of zooids are polyp and medusa?

A

Polyp and medusa are benthic and pelagic zooids

18
Q

Describe the hydrozoan meduse

A
  • The medusa is a motile zooid that swims by rhythmical contractions of a sheet of striated circular muscle in the subumbrella and velum, activated by motor neurons in the inner nerve-ring
  • Each contraction forces water from the bell, propelling the animal in the aboral direction (mouth trailing)
  • Shape is then restored passively by elastic recoil of the mesoglea
  • The medusa is a sexual zooid bearing gonads
  • Medusae rarely bud (although some do); polyps rarely bear gonads (although some do)
  • The gonads are little more than aggregations of gametes that may arise elsewhere in the body
  • Spermatozoa are of typical flagellate form
  • Oocytes may act as nurse cells, engulfed by the eventual ova
  • Hydrozoans are usually (but not always) dioecious (i.e., having the male and female reproductive organs in separate individuals)
  • Gametes are shed directly into the seawater and fertilization is usually external (although some species have a brood chamber in the subumbrella cavity)
  • Fertilization is facilitated by aggregation of the medusa, by unknown means
19
Q

Describe the development of Hydrozoa

A
  • Cleavage of the fertilized egg is indeterminate, equal and radial
  • It proceeds through a single-layered hollow blastula which gastrulates by unipolar ingression
  • The gastrula develops into a ciliated mouthless larva called a planula
  • After a few hours or days the planula attaches to an object on the sea floor and undergoes metamorphosis into a polyp
20
Q

What is the planula?

A
  • The planula is the third kind of distinct hydrozoan zooid
  • It is a polarized animal that consists of a single layer of ciliated ectoderm enclosing a solid endodermal mass
  • It has considerable cellular differentiation, including nerve, muscle, sensory and gland cells, and nematocysts
  • In anthozoans, the planula feeds
  • In hydrozoans, it does not
  • The planula attaches at the aboral end and transforms into a polyp, developing a gastric cavity and a mouth
21
Q

Describe the life cycle of Hydrozoa

A
  • The entire life cycle of Hydrozoa is a complex succession of different kinds of zooid that represents a division of labour between a range of somatic and sexual functions
  • It illustrates two axes of individuality
    • in space: individual - colonial - compound organism
    • in time: direct - larva + metamorphosis - alternation of phases

*Note that the alternation of phases in Hydrozoa is NOT equivalent to the alternation of generations between gametophyte and sporophyte

22
Q

What is the Aglantha digitale?

A
  • A medusa without a polyp
  • A direct-developing holoplanktonic hydromedusa (Hydrozoa) that has no polyp
  • The gonads are visible through the transparent bell
23
Q

What is the armed tentacle?

A
  • The armed tentacle is the crucial innovation that permits the evolution and radiation of benthic and pelagic predators in ctenophores and cnidarians
  • The tentacle is different in these two groups and may have evolved independently
24
Q

What is a cnidocyte?

A
  • The cnidocyte is perhaps the most complex of all metazoan cell types
  • It develops from a totipotent stem cell by assembly of a nematocyst and tubule, the inversion and coiling of the tubule, the packaging of venom and other components, and the construction of an elaborate ciliary apparatus at the apical pole
25
Q

Describe cnidocyte discharge.

A
  • Cnidocyte discharge is stimulated by chemical (prey body fluids) and mechanical (displacement of cnidocil) cues
  • The discharge event is one of the fastest processes in the animal kingdom, characterized by an acceleration of 40,000 G, a terminal velocity of over 15 m/s and a theoretical threat tip pressure of nearly 7 Gpa
  • Cnidocyte discharge is irreversible
  • The capsule stores a large concentration of calcium ions, which are released from the capsule into the cytoplasm of the cnidocyte when the trigger is activated
  • This causes a large concentration gradient of calcium across the cnidocyte plasma membrane
  • The resulting osmotic pressure causes a rapid influx into the cell
  • This increase in water volume in the cytoplasm forces the coiled tubule to eject rapidly
  • Prior to discharge, the coiled tubule exists inside the cell in an “inside out” condition
  • The back pressure resulting from the influx of water into the cnidocyte together with the opening of the capsule tip structure or operculum, triggers the forceful eversion of the tubule causing it to right itself as it comes rushing out of the cell with enough force to impale a prey organism
26
Q

Explain how a prey gets captured by the armed tentacle.

A
  • The armed tentacle of hydrozoan polyps enables them to capture, subdue and eat large living prey
  • Utilizing specialized penetrating nematocysts, cnidarians inject venom that initiates toxic and immunological reactions in the prey
  • These venoms contain enzymes, potent pore-forming toxins, and neurotoxins
  • Enzymes include lipolytic and proteolytic proteins that catabolize prey tissues
  • Cnidarian pore-forming toxins self-assemble to form robust membrane pores that can cause cell death via osmotic lysis
27
Q

Describe the tentacles of Ctenophora.

A
  • Ctenophores possess a pair of retractile tentacles housed in sheaths in the body wall
  • They capture prey with sticky traps called colloblasts; these are found ONLY in ctenophores
28
Q

Describe colloblasts.

A
  • Ctenophore colloblast adhesive is strong, but few colloblasts are simultaneously active, producing a weakly-adhering system
  • This may enable the animal to select prey of suitable size
29
Q

What is one difference between Cnidaria and Ctenophora?

A
  • Cnidarians (99% of them) have cnidocytes but Ctenophora lack cnidocytes
  • Ctenophora instead possess colloblasts
30
Q

What are Ctenophora?

A
  • Non-bilaterian
  • Gelatinous body
  • Marine
  • Predators
  • About 200 species
  • Muscle and nerve
  • Blind gut
  • Comb rows
  • Tentacles with colloblasts
31
Q

What are comb rows in Ctenophora?

A
  • Each comb is a blade formed by many cilia fused edge-to-edge
  • The combs are arranged in rows and are mechanically coupled to beat in a coordinated (metachronal) sequence, propelling the animal through the water
  • There are eight comb rows running longitudinally along the outside of the body
  • Ctenophores are the largest animals propelled by cilia
32
Q

Describe the general anatomy of Ctenophores.

A
  • Two cell layers: epiderm and endoderm, separated by an acellular mesoglea
  • Muscle cells are located in the mesoglea, so it has been argued that the mesoglea is a mesoderm
  • Epidermal body wall with basement membrane
  • Mouth leads to blind gut with four branches (diverticula)
  • Anal pores may eject small particles from gut, but most waste matter is regurgitated through mouth
  • Anal pores may facilitate osmoregulation
33
Q

Describe the nervous system of Ctenophores.

A
  • Ctenophores have nerve cells with synapses
  • Neurons are arranged as a nerve net enveloping the body, with concentrations of neurons beneath the comb rows and around the mouth
  • Rapid locomotion, escape behaviour and tentacle retraction are under neural control
  • There is no brain, central nervous system or definite nerve cords
  • Ctenophores lack many bilaterian genes for neural development and neurotransmission; it has been suggested that the ctenophore system evolved independently of the bilaterian system
34
Q

In the sensory system of Ctenophore, what does the apical organ accomplish?

A
  • Ctenophores have a complex statocyst-based organ at the aboral pole that sense gravity and controls movement, including ciliary beating
35
Q

Describe the musculature of Ctenophores

A
  • Ctenophores have smooth muscle arranged in longitudinal and circular bands
  • It can be used to close the mouth forcibly to capture prey or move rapidly
36
Q

Describe the development of Ctenophores.

A
  • Most ctenophores are cross-fertilized hermaphrodites
  • Gametes are produced in gonads underlying the comb rows and released through pores in the epidermis
  • Fertilization is external in most species
  • Cleavage is biradial and the 8-cell stage is a curved plate of cells, the long axis of which becomes the future tentacular plane
  • Cleavage is determinate and development is mosaic and results in a free-swimming cydippid larva
  • This larva resembles the Cydippida adults and undergoes a variable degree of metamorphosis
  • Gastrodes has a planula larva
  • There appear to be no Hox genes in ctenophores
37
Q

What are the two basic ctenophore body plans?

A
  • Most ctenophores capture prey by armed tentacles or muscular oral lobes; these two body plans have both evolved independently in different groups and do not constitute clades
  • CYDIPPID: egg-shaped body with pair of long tentacles
  • LOBATE: pair of lobes extending beyond mouth; tentacles reduced
38
Q

What did the fact that cydippid and lobate ctenophores were polyphyletic suggest?

A

Suggested independent loss of both cydippid larval stage and tentacle apparatus as well as independent development of bilateral symmetry in benthic ctenophores, Vallicula and Coeloplana

39
Q

What are Cestida?

A
  • Unlike other comb jellies, the body of cestids is greatly flattened, and drawn out into a ribbon-like shape
  • The two tentacles are greatly shortened, and two of the four ciliated comb rows are reduced
  • The unusual body form allows the animals to swim by means of muscular undulation, as well as by using their cilia
40
Q

What are Platyctenida?

A
  • Platyctenida is the only benthic group of Ctenophora
  • They are small (< 15 cm) and have dorsoventrally flattened, overall bodies
  • Platyctenids look very much like nudibranchs or flatworms and are often confused for them
  • All but one species lack the ciliated comb-row that distinguishes Ctenophora but they still possess the pair of tentilla-bearing tentacles and adhesive collocytes in pores along the dorsal surface
  • They cling to and creep on surfaces by everting the pharynx and using it as a muscular “foot”
41
Q

What are predatory beroids?

A
  • Beroid ctenophores are a remarkable group that preys voraciously on other comb jellies
  • Unlike their cydippid relatives, Beroe lack tentacles throughout the life cycle
  • The body resembles a swimming sac, with a large forward directed mouth
  • Instead of sticky tentacles or large oral lobes, the cavernous mouth is used to engulf prey whole
  • Small Beroe use modified mouth cilia (the macrocilia) to bite off pieces of comb jelly prey; larger individuals also use the macrocilia to grab and swallow whole prey
  • Using the eight comb rows for propulsion, Beroe are relatively strong swimmers
  • They swim constantly in search of prey, which are encountered blindly
42
Q

Describe early Cambrian ctenophores.

A
  • The Chengjiang armoured ctenophores lived about 520 MYA and were sessile animals
  • The earliest ctenophores were polypoid animals with tentacles bearing rows of compound cilia on pinnules

BIOL 305 (25 decks)

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