LECTURE 09 - Spiralia: Mollusca part 1

Question 1

What are Gastropods?

Answer 1

• Gastropods are coelomate bilaterians with a mantle (fold of body wall) enclosing the mantle cavity; ventral creeping foot; terminal mouth with chitinous radula
• Coelom scanty; circulation open; cephalized, nerve cords with nerve ring or brain; gills in mantle cavity
• Mantle secretes calcareous shell
• Extremely abundant and diverse in marine, freshwater and terrestrial environments; wide range of body plans; about 100,000 species

Question 2

What is the radula?

Answer 2

a grasping organ that reduces any prey to small fragments passed to the pharynx
- sometimes compared to a tongue

Question 3

Describe the foot of Gastropods.

Answer 3

• The foot of the main locomotor organ of molluscs
• In very small molluscs, it bears a sheet of cilia, which provides the propulsive force
• In most molluscs, the propulsive force is generated by waves of muscular contraction passing along the ventral surface of the foot
• Typically, wave velocity is bout 3 mm.sec and forward motion is about 1mm/sec
• Direct waves are waves of compression
• The wave begins at the posterior end of the foot by contraction of the dorsoventral and longitudinal musculature of this region
• Contraction of longitudinal muscles displaces the sole forward
• Successively more anterior regions contract, while the posterior end relaxes again as it settles and re-attaches to the substratum
• Retrograde waves are waves of elongation
• They begin when the anterior end of the foot is stretched forward by hemocoelic pressures
• Successively more posterior areas stretch forward while anterior parts contract, settle, and re-attach to the substratum
• the foot also attaches the animal firmly to the substrate, either during locomotion or at rest
• This creates a dilemma
  
  • During locomotion, the foot must be continually detached and re-attached to the substrate, requiring a weak adhesive force
  • At rest, the foot must remain attached despite (say) strong adhesive force
• the dilemma is resolved by secretion of two kinds of mucus
• During locomotion, the foot secretes a gel that provides no strength in shear but enables the animal to adhere to the substrate by suction
• At rest, the foot secretes a different kind of gel that acts as a glue bonding the foot firmly to the substrate
• The gel consists of polymers, sometimes cross-linked, in water
• The attachment force per unit area is 100-500 kPa, comparable with 500-1000 kPa of the solid cements used by mussels and barnacles

Question 4

What can the marsh periwinkle (Littoraria irrorata) produce?

Answer 4

• It can produce adhesive and non-adhesive gels
• These snails forage along mud flats, but when the tide returns, they climb marsh grass stems and glue the lip of their shell down
• In this way, they avoid aquatic predators such as crabs and fish
• When the tide recedes, they break their adhesion and return to the mud flats

Question 5

Describe the mantle and shell.

Answer 5

• A slowly, creeping, soft-bodied animal is vulnerable to attacks from above
• Most molluscs are protected by a shell secreted by the mantle, a fold of the body wall that lines the lowest whorl (body whorl) of the shell
• Most shells have three layers
  
  • The outermost periostracum, a tough organic covering made of chitin
  • A middle prismatic layer of calcium carbonate as calcite
  • An innermost nacreous layer of calcium carbonate as aragonite
• The shell is deposited within a small compartment, the extrapallial space, which is sealed from the environment by the periostracum, a leathery outer layer around the rim of the shell

Question 6

How is the shell formed in Molluscs?

Answer 6

• The current interpretation of shell formation posits a complex process of several stages:
  
  • A protein-based gel first fills the space to be mineralized
  • Chitin fibres are laid down to form an ordered matrix that determines the orientation of mineral crystals
  • The first mineral deposit is amorphous colloidal calcium carbonate
  • Nucleation occurs on the matrix, and crystals grow at the expense of the colloidal phase
  • Aragonite tablets, often with some acidic proteins, grow into the mature tissue

Question 7

What does the shell look like?

Answer 7

• The typical gastropod shell is an elongated cone wound into a spiral around a central axis, the columella
• An elongate uncoiled shell would be impossible to carry because of its high centre of gravity

Question 8

Describe the mantle cavity.

Answer 8

• The mantle cavity is the space within the body whorl between the mantel and the protrusible part of the body (head + foot)
• The roof of the mantle cavity bears a group of structures called the “pallial complex”: one or two gills; two osphradia (olfactory organs); the hypobranchial gland; and the terminal portions of the digestive, excretory and reproductive systems
• The mantle cavity becomes highly vascularized and acts as a lung in terrestrial gastropods

Question 9

Describe the circulatory system of mollusca.

Answer 9

The upper whorls of the shell are occupied by the visceral mass, including the gut, metanephridium and gonads
- These are not suspended in a spacious coelom; the coelom is restricted to the pericardial cavity and the nephridiocoel
- The respiratory pigment is haemocyanin in most species, dissolved in the haemolymph; there is no equivalent of erythrocytes

• Blood is collected in the auricle then pumped by the ventricle of the heart at high pressure into an aorta
• The aorta splits into an anterior fork (AA) supplying the foot and head and a posterior fork (PA) supplying the viscera
• Unlike vertebrates, there is no highly organized capillary system in either the head/foot or visceral regions
• Instead, blood from the large vessels seeps through tissue spaces and accumulates in several sinuses (cavities)
• Then, it is collected by large veins for return via the short portal vein to the kidney, before entering the gill
• After return through well organized blood vessels in the gill, blood enters the auricle of the heart
• High pressure from the auricle forces an ultrafilterate and soluble materials into the pericardial sac
• From the pericardial sac, a short duct leads ultrfitlrate to the kidney, where soluble materials may be either selectively reabsrobed or excreted to the urine
• Urine is discahrged through a ureter, opening to the mantle cavity

Question 10

Identify the digestive system of Mollusca.

Answer 10

• Regionalized gut
• Mouth
• Style sac
• Anus
• Anal gland
• Rectum
• ‘Coiled gut’
• ‘Thin gut’
• Gonad
• Midgut gland
• Stomach
• Crop
• Salivary gland
• Oesophageal pouches
• Buccal cavity

Question 11

Describe the nervous system of Mollusca.

Answer 11

• The nervous system of gastropods consists of ganglia and nerve cords
• The brain of a gastropod consists of three pairs of ganglia, all located close to the oesophagus
• In some primitive forms, these ganglia are relatively discrete, but in most species they have become so closely bound together as to effectively form separate lobes of a single structure
• The CEREBRAL GANGLIA are located above the oesophagus and supply nerves to the eyes, tentacles, and other sensory organs in the head
• The PEDAL GANGLIA lie beneath the oesophagus, at the forward part of the foot, and supply nerves to the foot muscles
• The PLEURAL GANGLIA lie slightly behind and below the cerebral ganglia, and supply nerves to the mantle cavity
• The PARIETAL GANGLIA innervate the gill
• The VISCERAL GANGLION controls the visceral mass and associated muscles
• In most gastropods, a short pair of nerve cords passes forward from the cerebral ganglia to a pair of BUCCAL GANGLIA located above the back of the mouth
• These supply nerves to the radula and other parts of the mouth

Question 12

Describe the eyes of gastropods

Answer 12

• Every grade of complexity, from simple photoreceptors to image-forming eyes, is found among molluscs

Question 13

Describe the reproductive system of Mollusca

Answer 13

• Most prosobranchs are dioecious
• Gonad is single, in the visceral mass
• FEMALE SYSTEM simple in some types (e.g., limpets): short gonoduct leads from gonad to nephridium
  
  • More complicated in most forms, with separate gonoduct regionally specialized for production of egg capsules
  • After leaving ovary, the oviduct runs along the mantle roof as an expanded tube (in which eggs are brooded in some species), then bear seminal receptacles used to store sperm received at copulation
• NExt region is an albumen capsule gland in species where eggs are enclosed in capsules
• The oviduct then continutes to the female gonopore, with or without a bursa copulatrix
• In the MALE SYSTEM, the gonoduct enlarges after leaving testis to form seminal receptracles for sperm storage; more distal part of duct altered into a prostate gland; duct then runs to base of penis, located on head just behind right tentacle
• Fertilization is interal
• Penis is thrust into some part of female system and sperm ejaculated into the bursa copulatrix or the seminal receptacles

Question 14

Describe Mollusca development

Answer 14

• Early development proceeds by spiral determinate cleavage with gastrulation by invagination leading to a cilitated-band larva

Question 15

What is a Veliger larva?

Answer 15

• Gastropods and bivalves have veglier larvae
• It had the basic topology of a trochophore, but with an expanded ventral foot region with an operculum, a shell secreted to enclose the visceral mass, and an expanded pair of velar lobes with a food groove running along its edge
• The expanded velar lobes greatly lengthen the surface available for food collection

Question 16

What does torsion do in Mollsuca?

Answer 16

• At a specific point during the development of the veglier, the visceral mass is rotated 180° relative to the rest of the animal
• The mantle cavity, gills, anus, gonopore and nephridiopore are now anterior, above the head
• Torsion creates some difficulties, especially in the disposal of waste over the head
• The overwhelming advantage is that it permits a compact body form protected by the shell once the mantle cavity has moved forward
• The twisting of the gut and the crossing of the nerve cords are both produced by torsion

Question 17

Describe the metamorphosis of the veglier larva.

Answer 17

• The veglier lives from a few hours to a few weeks in plankton
• At the end of this period, it sinks to the bottom, casts off its velum and excretory cells and begins to crawl or attach
• The larval heart regresses but other organs are retained
• the larval shell is usually retained as the protoconch at the tip of the shell spire

Question 18

What does Mollusca radiate into?

Answer 18

Limpets
Neritids
Caenogastropoda
Opisthobranchia
Pulmonata

Question 19

Describe Limpets.

Answer 19

• To cling to a rock, the limpet contracts its pedal muscles, lifting the foot and creating negative pressure in mucus below
• This generates a tenacity that requires a force of more than 100 kPa to dislodge the animal
• This strategy has two disadvantages
  
  • Water is liable to cavitate, breaking the bond
  • The force acts normal to the substrate and provides no protection against shear
• Lack of resistance to shear is a serious drawback for an animal exposed to strong wave forces
• It is supplemented, howeve,r by frictional resistance of the shell edge against minute asperities on the substrate
• There is still the drawback that suction adhesion requires continuous muscular efort
• This is overcome by producing a second kind of mucus contianing proteins that crosslink the mucpolysaccharides of the mucus
• This produces a glue-like bond, strogner than suction, that resists shear
• The drawback is that, like all glues, it takes time to set
• Hence, the limpet must alternate between suction adhesion when foraging (high tide) and glue-like adhesion when at rest (low tide)

Question 20

What are Conus?

Answer 20

• The genus Conus contains around 500 different species of predatory cone snails each with its own distinctive, complex and peptide-rich venom
• Venom is used by these carnivorous molluscs not only as the primary weapon to capture prey, but also defensively and competitively

Question 21

Describe flying snails.

Answer 21

• The HETEROPODS have four striking adaptations to the open ocean environment
  
  • First, the bodies and shells are largely TRANSPARENT
  • Second, the foot is laterally-compressed to form a ventral SWIMMING FIN; because of the weight of the dorsal visceral mass, heteropods swim “upside-down,” with the fin directed upward
  • Thirdly, they possess IMAGE-FORMING EYES, with a large, spherical lens and a basal, ribbon-like retina
  • Fourthly, the RADULA has elongate, sickle-shaped lateral and marginal teeth that are used to snare prey after the radula is protruded from the mouth, which is located at the tip of their mobile proboscis

Question 22

What are Opisthobranch molluscs?

Answer 22

• Nudibranchs are secondarily detorted and thus externally (but not internally) bilaterally symmetrical
• Shell and mantle cavity are reduced, modified or (usually) absent, with respiratory structures developed externally from the dorsal body wall
• Dorsal projections also contain gut diverticula
• Head with eyes and prominent tentacles
• Creeping foot, often with expanded lateral flaps that can be used in swimming
• radula present; generally carnivorous, feeding on sponges, cnidarians, tunicates, bryozoans, etc.
• Hermaphroditic, with complete female and male systems; internal fertilization

Question 23

How does mimicry work in nudibranchs and polyclad flatworms?

Answer 23

• Some nudibranchs lack dorsal projections and resemble polyclad flatworms (for which they were originally mistaken)
• Many are able to sequester repellant chemicals from the sponges they eat and sequester them in glands on the edge of the mantle
• Such species are often conspicuously coloured to warn potential predators that they are distasteful (aposematism)
• Other species have similar colouration but are not repellent (Batesian mimicry)
• Polyclad flatworms living in the same place may have very similar colouration, although it is not clear whether they are distastful (Mullerian mimicry) or edible

Question 24

How do pulmonate gastropods breathe?

Answer 24

• Pulmonates are the familiar snails and slugs, which are extremely diverse and abundant in terrestrial and freshwater habitats throughout the world
• Mantle sac fused with neck and converted into a pulmonary sac communicating with the air via a contractile opening, the pneumatophore

Question 25

What are Bivalvia?

Answer 25

• Headless molluscs without jaws or radula, enclosed by bivalve shell clamped shut by adductor muscles
• Digging foot (e.g., in clams); or foot reduced in attached forms (e.g., oysters)
• Buried forms communicate with surface through siphons
• Two halves of mantle fused at edges, with anterior opening for foot and posterior opening excurrent siphon
• The incurrent siphon is also posterior, below the excurrent

Question 26

What does the mantle cavity in Bivalves look like?

Answer 26

• The mantle cavity contains a pair of large ciliated gills; ciliary beat draw water in ventrally through incurrent siphon and over gill; food particles move over edges of gill to labial palps, where they are ingested; water flows out dorsally through excurrent siphon
• Water stream used for feeding, respiration and gamete discharge

Question 27

What is suspension feeding in Bivalves

Answer 27

• Bivalves are filter feeders
• Tubular extensions of the mantle form a pair of siphons, one for the entry of a current of water, the other for its exit
• This current is maintained by the beating action of hair-like cilia with which the gills are liberally covered
• Water is drawn in through tiny pores in the gills and eventually ejected by way of the discharge siphon
• Particles of food are strained out and trapped in a layer of sticky mucus, a substance similar in consistency to human saliva, that cover the gill surfaces

Question 28

What do bivalves not have?

Answer 28

Bivalves have no head, brain or central nervous system
- the only remnant of the nervous system is three paired ganglia connected by nerve cords

Question 29

Describe bivalve eyes

Answer 29

• As light enters into the scallop eye, it passes through the pupil, a lens, two retinas (distal and proximal), and then reaches a mirror made of crystals of guanine at the back of the eye
• The curved mirror reflects the light onto the interior surface of the retinas, where neural signals are generated and sent to a small visceral ganglion, or a cluster of nerve cells, whose main job is to control the scallop’s gut and adductor muscle

Question 30

What are chemoautotrophic bivalves?

Answer 30

• Giant clams living close to hydrothermal vents harbour sulphur-oxidizing bacteria in specialized cells in their gills
• These bacteria use the upwelling hydrothermal fluid as a source of energy
• The clams rely on the bacteria for metabolism and are no longer able to feed themselves; the gut has almost entirely disappeared
• The bacteria are transmitted vertically via the eggs of the clam, so true symbiosis can evolve

Question 31

What are Scaphopoda?

Answer 31

• Animal occupies slightly curved tubular shell, open at both ends
• Burrow wth retractile foot and lies completely buried head-down in sand or mud
• Inconspicuous head bears thin ciliated tentacles which probe for food, which is brought to mouth and crushed by large radula
• Gills lost; water is circulated in mantle cavity by cilia and respiration is through the mantle tissue
• Dioecious with external fertilization
• Free-swimming trochophore and veliger larvae
• Primarily deepwater group