Mollusca pt.2 (cephalapods) Flashcards
Class Cephalopoda characteristics
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
Subclass Nautiloidea characteristics
- External, chambered shell for buoyancy
- Pinhole eye
- Up to 90 tentacles
- No chromatophores / ink sac
why did shell internalisation develop
- 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
what are the 2 subclasses in class Cephalopoda
Nautiloidea (external shell) + Coleoidea (internal/absent shell)
Subclass Coleoidea characteristics
e.g. squid, octopus
- Shell internal or absent
- Typically, with chromatophores and ink sac
- Large brain and eyes
Octobrachia = 8 arms
Decabrachia = 10 arms
Order Sepioidea characteristics
cuttlefish
- 8 arms + 2 tentacles (decabrach)
- Internal, chambered shell for buoyancy (Sepia, Spirula)
- Mostly benthic
- Neritic
Order Myopsida and Oegopsida characteristics
squid
- 8 arms + 2 tentacles (decapod)
- Internal shell reduced to gladius (pen)
- Large variation in size, form, habit
Order Octobrachia characteristics
octopus
- 8 arms (octobrach)
- Internal shell considerably reduced or absent
- include Suborder Cirrata + incirrata
Suborder Cirrata + incirrata characteristics
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
describe locomotion in squid
- 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
buoyancy methods in cephalapods
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
what is dynamic lift
Generating lift from fins + shape of body - requires energy
what is static lift
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)
what is Boyle’s law
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)
what are Argonauts
- Group of rarely encountered open- ocean pelagic octopuses
- Female argonauts = brittle ‘paper nautilus’ shell
- Males = dwarfs (females a lot heavier)
explain Ventilation in cephalopods
Water comes in behind head + pushed out through funnel
explain circulation in cephalopods
- closed circulatory system (only molluscs to have this - more active)
- 3 hearts to reduce the pressure + stop burst of blood vessels
explain eyes in cephalopods
- Similar to invertebrates - except for no blind spot
- Solid lens
what are chromatophores
- 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
what are the 2 reflecting cells beneath chromatophores
- Iridophores
- Complicated structures incorporating stacks of reflective material
- Different forms in different cephalopods
- Blues and Greens - Leucophores
- Broad-band reflectors - dominant wavelength
- Play big role in background matching
**get all sorts of different colours and combinations if the light hits
explain ink sac in cephalopods
- 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
explain Feeding in nautilus, squid and cuttlefish
- 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
what are the 3 muscle types in cephalopod arms
- longitudinal (axially running along length of the arm)
- Radial (transversal)
- oblique (diagonally surrounding the arm)
explain feeding in octopus
- 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
explain reproduction in cephalopods
- 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)
