Chondrichthyes Flashcards
What are the 5 features that unite chordates?
>notochord >postanal tail >z-shaped muscle segments >pharyngeal slits >dorsal hollow nerve chord
What are the features that unite vertebrates?
What about gnathostomes?
>vertebrae >eye >cranium >brain >tongue >gill arches
> mineralised teeth
1st gill arch forms jaws
paired fins
Why is a shark’s life cycle unusual?
Lay few large eggs w/ big yolks
–>Intensive parental investment
Some sharks e.g. hammerhead:
eggs are retained & hatch internally - fed via placenta or other eggs
How do sharks mate?
Have claspers - extensions of pelvic fins
Males fertilise eggs inside the female
= higher chance of fertilisation –> less risk to having just a few offspring
What are shark claspers convergent to?
The intromittant organ of amniotes
Describe the hydrodynamics of sharks
Large, oily liver helps them float
- but denser than water so must swim to stay up in water column
Don’t have a swim bladder
How do sharks breathe?
Ram-breathers
= ventilate gills by forcing water through open mouth
If stop swimming = drown & suffocate
Describe a shark’s skeleton
Reduced & cartilaginous
= low density
Lack bone internally
- never evolved bone vertebrae
External bony plates of skull, jaws & body lost - reduced to denticles
= made of enamel + dentine
How do sharks reduce drag?
Denticles have parallel giblets
- control production of vortices along body
= reduces drag by 10%
How are sharks specialised for high speeds?
Teardrop body
Lunate tail
Elongated fins
Why do sharks replace their teeth every 9-36 days?
Fish bones & scales rapidly dull teeth
How are shark jaws highly flexible?
What is so special about Goblin sharks?
Jaw cartilage isn’t tightly connected to skull bones
Can protrude jaw to seize prey
What is eyeshine?
What is its importance?
When photons pass through retina w/out activating photoreceptors & bounce back
Increases sensitivity of retina to light
What is are the ampullae of lorenzini?
Electroreceptors
- detect electric fields
Can home in on prey under seafloor w/out replying on light
How do sharks smell?
Each nostril has an incorrect & excurrent opening
Water passes over folds of chemosensitive olfactory tissue
How are sharks specialised for hunting?
> specialised jaws & teeth
can hunt in low light conditions
- smell & electrical signals
What are the 3 main cnrondrichthyes groups?
Holocephali = Chimeras Selachimoprha = Sharks Rays = Batoidea
Give an example of a Holocephali (Chimera)
Describe them
Ratfish (aka rabbitfish)
> Toothplates
- incisor like structures to crush prey
Big eyes for darkness
Swim w/ winglike pectoral fins
Which era are Chimeras from?
Carboniferous
What are the features of Batoidea?
> flattened body
-glide over seafloor
large pectoral fins extend onto head
pavement of small teeth to crush invertebrates
Give examples of Batoidea
Sawfish Torpedos Skates Stingray Manta
What are the features of sawfish?
Long nose w/ spines
= modified denticles
(- NOT teeth)
to club & stun/kill fish
What are the features of torpedos?
Modified muscle cells discharge an electric pulse
- stuns predator/prey
What are the features of skates?
Instead of swimming they walk using finger-like projections of pelvic fins
What are the features of stingray?
Tail spine modified into a long, serrated barb used for defence
What are the features of manta?
Scoops on the mouth funnel food into mouth
filter feeders
What do fossils show about Batoids?
Evolved in the Mezosoic
What are the 2 main groups of sharks?
Squalimorphii (Dogfish, lantern sharks & cookie cutter sharks)
Galeomorphii (requiem sharks, mackerel sharks, catsharks)
What are the features of lantern sharks?
Photophores on underside
- eliminates silhouette when seen against light coming from above
= camouflage
What are the features of cookie cutter sharks?
Small dogfish w/ v large teeth that gouge out hemispherical chunk of prey’s flesh
Prey on larger animals (unusual)
What are the features of horn sharks?
Button-shaped teeth to crush molluscs & crustaceans
Egg-case is spiral-shaped to screw into sediment
What are the features of whale sharks?
Not predators
- filter feeders
- occupied this niche 10s of millions of years before whales
What are the features of goblin sharks?
Swims or drifts until detects prey w/ electroreceptive snout & shoots jaws out to capture
What are the features of thresher sharks?
Elongated upper lobe of tail
- used as a whip to stun fish
What are the features of mackerel sharks?
Can raise body temp well above surrounding water
= endothermic
- lets them maintain high activity levels even in cool water
Who do white sharks prey on & why?
Marine mammals w/ rich fat reserves
To maintain high activity in cold water
When is the oldest shark relative thought to be from?
Devonian
- has cartilage skeleton & lacks bone
Which group is now recognised as shark relatives?
Acanthodians
- have forsal fin spines
- scales resemble denticles
What do Placoderms suggest about sharks?
They has large bony plates on the skull
- suggests ancestor of sharks had extensive body armour
- reduced in sharks & scales reduced to denticles
How has the speed of shark evolution changed since early in their history?
Why has it changed?
Rapid evolution early on
–> slows dramatically
Shark design is as good as it’ll get
Some sharks have evolved to be filter feeders. What does this suggest about evolution?
Things evolve slowly due to competitors prevent them from expanding into new niches
Once competitors are removed evolution speeds up again
