Mollusca pt.2 (cephalapods)

Question 1

Class Cephalopoda characteristics

Answer 1

e.g. Cuttlefish, squid, octopods, nautilus
- Circle of arms and tentacles – homologous to anterior of the foot in other molluscs (only squid + cuttlefish have tentacles)
- Defined head (cephalisation)
- Well developed eyes
- Bilaterally symmetrical
- Fulfill same niche as fish

Question 2

Subclass Nautiloidea characteristics

Answer 2

• External, chambered shell for buoyancy
• Pinhole eye
• Up to 90 tentacles
• No chromatophores / ink sac

Question 3

why did shell internalisation develop

Answer 3

• enables more active mode of life
• The shell loses its protective function and becomes solely used for buoyancy
• Mantle is liberated to become a muscular pumping organ - giving much more powerful jet propulsion
• Mantle is also able to develop fin

Question 4

what are the 2 subclasses in class Cephalopoda

Answer 4

Nautiloidea (external shell) + Coleoidea (internal/absent shell)

Question 5

Subclass Coleoidea characteristics

Answer 5

e.g. squid, octopus
- Shell internal or absent
- Typically, with chromatophores and ink sac
- Large brain and eyes
Octobrachia = 8 arms
Decabrachia = 10 arms

Question 6

Order Sepioidea characteristics

Answer 6

cuttlefish
- 8 arms + 2 tentacles (decabrach)
- Internal, chambered shell for buoyancy (Sepia, Spirula)
- Mostly benthic
- Neritic

Question 7

Order Myopsida and Oegopsida characteristics

Answer 7

squid
- 8 arms + 2 tentacles (decapod)
- Internal shell reduced to gladius (pen)
- Large variation in size, form, habit

Question 8

Order Octobrachia characteristics

Answer 8

octopus
- 8 arms (octobrach)
- Internal shell considerably reduced or absent
- include Suborder Cirrata + incirrata

Question 9

Suborder Cirrata + incirrata characteristics

Answer 9

cirrata
- Small, internal shell, two fins
- Pair of cirri with each sucker
- Deep water
Incirrata
- Lacking these structures
- All other octopods – ones we see most often

Question 10

describe locomotion in squid

Answer 10

• best swimmers
• Daily vertical migrations + Seasonal migrations
• all squid use fins to some extent for hovering and manoeuvering
• “Classic” squid depend on jet propulsion at all speeds and cannot even hover without it
• Jet propulsion = whilst extremely useful but inherently inefficient compared with fish – use this to leave the water and fly to a different location (unfold fins and keep squirting out water) - useful for escaping predators
• Ram ventilation

Question 11

buoyancy methods in cephalapods

Answer 11

Useful strategy to save energy on staying in the water collum
- Fish = swim bladder
- Cuttlefish + nautilus = chambered shell
- Muscular Squid = dynamic lift
- Deep sea squid = chemical lift

Question 12

what is dynamic lift

Answer 12

Generating lift from fins + shape of body - requires energy

Question 13

what is static lift

Answer 13

small volume of gas provides sufficient lift - uses less energy
- gas is at a much lower pressure (harder structure) than the hydrostatic pressure of the surrounding water (unlike swim bladders in fish)

Question 14

what is Boyle’s law

Answer 14

p1 V1 = p2 V2
- volume of a gas varies inversely with pressure (at a constant temp)
- Organisms with internal shells don’t need to worry about this (but shells can implode)

Question 15

what are Argonauts

Answer 15

• Group of rarely encountered open- ocean pelagic octopuses
• Female argonauts = brittle ‘paper nautilus’ shell
• Males = dwarfs (females a lot heavier)

Question 16

explain Ventilation in cephalopods

Answer 16

Water comes in behind head + pushed out through funnel

Question 17

explain circulation in cephalopods

Answer 17

• closed circulatory system (only molluscs to have this - more active)
• 3 hearts to reduce the pressure + stop burst of blood vessels

Question 18

explain eyes in cephalopods

Answer 18

• Similar to invertebrates - except for no blind spot
• Solid lens

Question 19

what are chromatophores

Answer 19

• Organs underneath the skin that have a sack of pigment with muscles around it
• Expand and contract
• Main purpose = camouflage + communication (inter- and intra-specific)
• Beneath them are reflecting cells

Question 20

what are the 2 reflecting cells beneath chromatophores

Answer 20

1. Iridophores
   - Complicated structures incorporating stacks of reflective material
   - Different forms in different cephalopods
   - Blues and Greens
2. Leucophores
   - Broad-band reflectors - dominant wavelength
   - Play big role in background matching

**get all sorts of different colours and combinations if the light hits

Question 21

explain ink sac in cephalopods

Answer 21

• Ink gland and reservoir with sphincters
• suspension of almost pure melanin
• Can be used as a smoke screen or to form a “pseudomorph”
• May act as an alarm substance or interfere with fish chemoreception

Question 22

explain Feeding in nautilus, squid and cuttlefish

Answer 22

• Carnivorous
• Powerful beak
• Prey captured with tentacles
• Nautilus has 38 suckered tentacles (some sp. up to 90)
• Squids & cuttlefish = 8 arms (2 long tentacles to catch prey)
• Arms have suckers with serrations or hooks
• Each cup has a muscle fibre to create a vacuum

Question 23

what are the 3 muscle types in cephalopod arms

Answer 23

• longitudinal (axially running along length of the arm)
• Radial (transversal)
• oblique (diagonally surrounding the arm)

Question 24

explain feeding in octopus

Answer 24

• Carnivorous
• Powerful beak – most prey = crustaceans
• 8 arms of equal length
• Arms have suckers but these lack hooks, etc.
• Prey is held and injected with poison + digestive enzymes
  e.g. venomous blue ringed octopus is deadly

Question 25

explain reproduction in cephalopods

Answer 25

• Gonochoristic
• Fertilisation occurs in mantle of female
• Modified arm of male = hectocotylus (used to transfer spermatophore) - may break off in some species
• Fertilised eggs = released or attached to substratum
• Grow fast
• Breed once or few times always within a single yearly period (semelparity)
• Die immediately or soon after one or a few egg layings
• Short life spans (compared with fishes and mammals)