Bivalvia and Cephalopoda

Question 1

Octopus, squid, cuttlefish and nautilus

Answer 1

All marine
Typically 2 - 70cm but also giants Architeuthis dux (giant squid) 18m
Mesonychoteuthis hamiltoni heaviest (500kg) known invertebrate
All active predators, with many prey types
Ammonites dominant in Mesozoic until Cretaceous extinction

Question 2

Evolutionary history

Answer 2

1. Ancestor of all Cephalopoda had shells
2. Modern forms - internal shell i.e. cuttlefish & squid
3. Or the shell is absent i.e. octopus
4. Only Nautilus sp. retains external shell
5. Cephalopod only mollusc with closed circulation

Question 3

Form and function: Nautilus shell

Answer 3

1. Only Nautilus sp. has external shell
2. The shell is divided by transverse septa into internal chambers
3. The septum is perforated in the middle by a cord of tissue called the siphuncle
4. The siphuncle secretes gas into the empty chambers and makes the shell buoyant
5. So why is the shell lost in other Cephalopoda? What are the limitations? Limits speed, adds water resistance

Question 4

Cephalopods nervous sytem

Answer 4

Have a nervous system and sense organs. Dorsal nerve chords, nerve ring, ventral nerve chords
In cephalopods, the ganglia are concentrated and fused to form the brain
A cartilaginous cranium protects the brain
Separate regions of the brain control separate areas
For example, pedal ganglia supply nerves to the funnel; brachial ganglia send nerves to the tentacles

Question 5

Cephalopods - nutrition

Answer 5

The radula is present.
More important are jaws in the buccal cavity.
The beak can bite and tear tissue. Tissue is pulled into the buccal cavity by the radula. A pair of salivary glands empty into the buccal cavity.

Question 6

Nutrition - Octopods

Answer 6

The Octopods also secrete poison and enzymes into prey. Octopods live in dens, they can search for food or lie in wait.
Octopods capture clams, snails, crustaceans and fish and paralyze them with their bite.
Capture with tentacles & variously modified suckers

Question 7

Nutrition - Squid & Cuttlefish

Answer 7

Squid feed on fish, crustaceans and other squid. Cuttlefish swim on the bottom and feed on invertebrates i.e. shrimps and crabs

Question 8

Chephalopods - Eye

Answer 8

Nerve fibres in front of vs behind retina: vertebrates have blind spot
Colour vision different in cephalopods – possibly via chromatic abberation / pupil shape
Development: outgrowth of brain in vertebrates vs. invagination of body surface in cephalopods

Question 9

Cephalopods - Colour change

Answer 9

Chromatophores present in the integument. Small muscles attached to the edge of the Chromatophores
Different species have chromatophores of different colours
The effect is enhanced by iridocytes or reflector cells
It is used for camouflage and communication:
Squid and octopus may darken when they are alarmed
Some colour changes are correlated with behaviour (aggression)
Colour displays are associated with courtship

Question 10

Cephalopods - ink sacs

Answer 10

In Cephalopoda, (not Nautilus) a ink sac opens behind the rectum
The gland secretes a brown or black fluid
The ink is released when the animal is alarmed
The alkaloid ink may also be a deterrent for predators

Question 11

Cephalopods - Reproduction

Answer 11

1. Male display to seduce the female and repel intruders
2. Fertilization takes place in the mantle cavity and involves copulation
3. One of the arms of the male is a modified ‘intromittent organ’
4. During copulation the male receives spermatophores from
   his funnel and inserts into the female mantle (indirect sperm transfer)
5. Development is direct – no larval stages