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3A Zoology - Invertebrates > An Introduction to Molluscs, Adaptive Radiation and Torsion in Gastropods > Flashcards

An Introduction to Molluscs, Adaptive Radiation and Torsion in Gastropods Flashcards

1
Q

What class did the first classification of molluscs include?

A

Cephalopods

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2
Q

How species rich are molluscs?

A

Very species rich, second only to arthropods

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3
Q

What class does phylogenetic analysis link molluscs to?

A

Annelids

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4
Q

What is the ‘generalised’ mollusc?

A

The hypothetical ancestral mollusc

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5
Q

Does the generalised mollusc have a coelom?

A

Yes, they are coelomate (have a fluid-filled body cavity)

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6
Q

What symmetry does the generalised mollusc show?

A

Bilateral symmetry

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7
Q

What are the distinctive anatomical features of the generalised mollusc?

A

Muscular foot, visceral mass, mantle, mantle cavity, radula, shell (in many molluscs)

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8
Q

What is housed within the mantle cavity of the generalised mollusc?

A

The gills (ctenidia) or lung
Waste and reproductive organs
Sensory organs

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9
Q

What is the visceral mass of the generalised mollusc?

A

The soft, non-muscular metabolic region of the mollusc that contains the body organs

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10
Q

What is the role of the mantle of the generalised mollusc?

A

Can provide protection and secretes minerals to form the shell (if present)

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11
Q

What is the shell of the generalised mollusc made of?

A

Calcium (calecerous shell) which comes from diet, although can be influenced by habitat and diet and may also contain small amounts of other nutrients

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12
Q

What circulatory system does the generalised mollusc have?

A

Open circulatory system

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13
Q

What is the organ that facilitates gas exchange in most molluscs?

A

Ctenidia (gills)

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14
Q

What molluscs have simple lungs instead of ctenidia?

A

Some terrestrial molluscs, such as certain snails or slugs

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15
Q

What is the mechanism of gas exchange in the ctenidia of molluscs?

A

Water is drawn into the mantle cavity and flows over the ctenidia. Oxygen diffuses across the thin, moist surface into the mollusc’s blood vessels, while carbon dioxide moves out of the blood and into the water.

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16
Q

What is the structure of the ctenidia of molluscs?

A

Comb-like structures composed of filaments arranged in rows.

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17
Q

What is the only class of Molluscs that all lack a radula?

A

Bivalves

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18
Q

What is the radula?

A

The radula is a chitinous ribbon used for rasping food particles, cutting, or drilling holes in prey. It is covered with numerous small teeth made of chitin. A flexible mass made of cartilage supports it, called the odontophore.

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19
Q

Describe the nervous system of the generalised mollusc.

A

Molluscs have a relatively simple nervous system consisting of a pair of cerebral ganglia (brain) located near the mouth, connected to a nerve cord that runs along the ventral side of the body. From the nerve cord, smaller ganglia and nerve fibres extend to various parts of the body, coordinating sensory and motor functions.

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20
Q

How many classes of molluscs are there?

A

7-8, depending non classification

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21
Q

Why is there debate on the number of classes within the phylum of molluscs?

A

Aplacophora can often be split into two further classes.

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22
Q

What are the 7 major classes of Molluscs?

A

Monoplacophora
Scaphopopoda
Cephalopoda
Gastropoda
Aplacophora
Bivalvia
Polyplacophora

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23
Q

Define the term ‘adaptive radiation’.

A

Adaptive radiation refers to a process in evolutionary biology where a single ancestral species rapidly diversifies into a variety of forms, each adapted to exploit different habitats or food sources.

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24
Q

What are the common features of the Aplacophora?

A

Lack a shell and instead have a worm-like body covered with a cuticle or spicules made of calcium carbonate or chitin. The spicules give the body a distinctive sheen. All are marine and benthic.

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25
Q

Why are Monoplacophora sometimes called “living fossils”?

A

Thought extinct until 1952 when they were found in deep waters. They have retained many primitive characteristics found in fossils.

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26
Q

What are the common features of the Monoplacophora?

A

Have a single, cap-shaped shell that covers the dorsal surface of their body. The shell is typically limpet-like, with a low, conical shape and a flattened top. It may be smooth or sculptured with ridges and grooves.

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27
Q

What are the Polyplacophora also known as?

A

Chitons

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28
Q

What are the common features of the Polyplacophora?

A

Dorso-ventrally flattened, 8 overlapping shell plates. Broad foot to cling to rocks. Dull colouration aids camouflage. Several pairs of gills surround foot. Mainly intertidal. Radula often tipped with magnetite for scraping rocks.

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29
Q

How many overlapping shell plates do Polyplacophorans have?

A

8

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30
Q

Where are Polyplacophorans generally found?

A

Intertidal regions

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31
Q

What are radula of Polyplacophorans often tipped with?

A

Magnetite

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32
Q

What are Scaphopoda generally called?

A

Tusk shells

33
Q

What is the method of respiration in Scaphopoda?

A

Water is drawn into the narrow end protruding from the substrate. This flows over the ctenidia which are lined with cilia. These aid flow of water over the ctenidia and enhance gas exchange efficiency. Oxygen is absorbed into the blood vessels and carbon dioxide diffuses from the blood into water.

34
Q

Where do Scaphopods inhabit and what is the mechanism for this?

A

Scaphopods are burrowing organisms, inhabiting sandy or muddy substrates in shallow coastal waters to deep-sea environments. They use their muscular foot to burrow into the sediment, with only the narrow, pointed end of the shell protruding above the surface.

35
Q

What is the feeding mechanism of Scaphopods?

A

Foot surrounded by club like tentacles called captacula used to burrow in benthos. Captacula pass food to radula.

36
Q

What does Bivalvia mean (translation from latin)?

A

Two hinged shell

37
Q

What shells are included in Bivalvia?

A

Mussels, cockles, clams

38
Q

What are the common morphological features of Bivalves?

A

Laterally compressed shells with powerful adductor muscles to open and close hinges. No radula.

39
Q

What is the respiratory mechanism of Bivalves?

A

Gills/ctenidia - water enters the mantle cavity through an incurrent aperture, flows over ctenidia. Oxygen from water is absorbed and enters blood vessels while carbon dioxide from cellular respiration leaves the blood and enters water.

40
Q

What is the feeding mechanism of Bivalves?

A

Filter feeding - Use ctenidia (gills) to filter small particles, plankton and organic matter from water. Cilia on the gills create currents that draw water into the mantle cavity, where food particles are trapped and transported to the mouth for ingestion.

41
Q

What muscles are used to open and close Bivalves?

A

Adductor muscles

42
Q

What can be a predator escape mechanism of Bivalves?

A

Burrowing (using the foot).

43
Q

What Bivalves can burrow through wood or stone?

A

Shipworms

44
Q

How do some species of Bivalves ‘swim’?

A

By ‘clapping’ shell valves together

45
Q

What does the class Cephalopoda include?

A

Octopus, squid, cuttlefish and nautilus

46
Q

What does the modified foot of Cephalopods become?

A

Funnel for jet propulsion and tentacles and/or arms

47
Q

Do Cephalopods have a shell?

A

Shell is generally reduced, although not in the nautilus

48
Q

What is the largest class of Cephalopods?

A

Gastopoda

49
Q

What is the only class with some fully terrestrial species?

A

Gastropoda

50
Q

What are the 3 sub-classes of gastropods?

A

Prosobranchs, opisthobranchs and pulmonates

51
Q

Where are Prosobranchs found?

A

Mainly marine, some freshwater and terrestrial

52
Q

Give an example of a prosobranch.

A

Dog whelk

53
Q

What is the feeding mechanism of a dog whelk?

A

Carnivorous snail with a frilly mantle that senses prey. They can anchor prey such as mussels using their muscular foot. The whelk scrapes at the shell with teeth on the radula, sometimes taking 2 days to drill through. Secretes digestive juices and begins to feed on prey using radula, consuming mussel alive for the following 2 days. Sometimes use muscular foot to tether prey, causing it to become stuck and starve to death.

54
Q

Where are Opisthobranchs found?

A

Mainly marine

55
Q

What are some examples of Opisthobranchs?

A

Nudibranchs, sea hares, sea butterflies

56
Q

What are some examples of Pulmonates?

A

Slugs and snails

57
Q

Where are Pulmonates found?

A

Terrestrial and freshwater

58
Q

Do gastropods have a shell?

A

Many, but not all

59
Q

What shape is the shell of gastropods?

A

Conispiral (lack bilateral symmetry) rather than ancestral planispiral (bilaterally symmetrical)

60
Q

What is the diet of gastropods?

A

Herbivores - grazers/browsers
although some are carnivores - scavengers or predators

61
Q

Do gastropods have a radula?

A

All have some form of radula

62
Q

What is common to all gastropods?

A

Torsion

63
Q

What is torsion?

A

When the mantle cavity, visceral mass and shell rotate up to 180 degrees from the posterior to the anterior of the animal

64
Q

How many stages does torsion take place in?

A

2

65
Q

When does the first stage of torsion occur?

A

2nd larval stage - veliger stage

66
Q

What occurs during the 1st stage of torsion?

A

Rotation of mantle cavity, visceral mass and shell 90 degrees counterclockwise. Muscular contractions of the foot can generate the twisting motion associated with torsion.

67
Q

What occurs in the 2nd stage of torsion in gastropods?

A

The 2nd stage involves another 90 degree counterclockwise rotation by differential tissue growth.

68
Q

What is the endpoint of torsion in gastropods?

A

The mantle cavity, anus, and other internal structures are positioned anteriorly above the head.

69
Q

What is the only known animal that can rotate 180 degrees using just muscle contraction?

A

Acmaea

70
Q

What is the major issue with torsion?

A

Sanitation issues as anus is near mouth and over gills - excreted waste may lead to fouling of the mouth and sensory organs

71
Q

What are the advantages of torsion?

A
  • Balance - Brings the centre of gravity back to the middle of the gastropod’s body, and helps prevent the animal or the shell from falling over.
  • Predator avoidance - Gastropods can retract into their shell as it is located above the head
  • Ventilation - having gills at the front allows better ventilation as moving in direction of travel and water flows over gills more readily
  • Reduced clogging - less likely to be clogging of sediment as it is stirred up by the foot since pure water enters anteriorly
  • Size - allows gastropods to grow larger
72
Q

What is an adaptation to torsion which means that waste does not pass over the gills? (other than loss of right organs)

A

Water enters anteriorly, passes over the gills then over the anus and nephridiopores and exits.

73
Q

What are the nephridiopores?

A

External opening where waste is excreted

74
Q

What are 2 examples of gastropods that have adapted to torsion by having water pass over gills before anus?

A

Haliotis sp (Abalones) and Keyhole limpets

75
Q

What is an adaptation to torsion to prevent waste from contaminating the gills?

A

Most prosobranchs have lost the right gill, right nepridiopore and associated paired organs. Water enters from the left hand side and exits on the right hand side.

76
Q

What evolved after torsion which aids in the defence of some snails?

A

Operculum - serves as a door covering the aperture of the shell where the soft parts such as the head and foot reside

77
Q
A
78
Q

What is detorsion? Give an example of a class where this has occurred.

A

Some gastropods have evolved either to lose their shell or evolve a shell that has an opening on either side, resulting in the regaining of bilateral symmetry and loss of torsion. For example, in opisthobranchs.

79
Q

Do Opistobranchs have a shell?

A

Only in very few species, the shell is big enough that the animal can withdraw completely. In most species the shell is reduced in size, internalised or lost completely.

3A Zoology - Invertebrates (12 decks)

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