Sharks
Recap:
Chondrichthyes – sharks, rays and chimaeras. Became highly specialised for predation – primitive in
some respects, in other ways highly sophisticated.;[-=
Features of sharks and kin:
Shark diversity: 1000 known species. All predators, almost all marine, but exploit a range of
prey and habitats. They sit at the top of the food chain, so sharks are not diverse, or
abundant, but exert a strong selective pressure on species and structure of the marine
ecosystem.
What makes a shark? Compared to bony fish, sharks are primitive in some ways, highly
specialised in others.
Sharks are big: smallest is 21cm (Dwarf lantern shark) and largest is 13m, 20 tonnes (whale
shark).
Unusual life cycle: intensive parental investment. Bony fish may lay millions of eggs,
whereas sharks will lay just a few, large eggs. In some sharks, eggs are retained and hatch
internally – fed via placenta – or on other eggs.
, Intromittent organ and internal fertilisation: bony fish use a spray-and-pray approach to
reproduction – external fertilisation (messy/risky – if you have only a few eggs, might end up
with none fertilised). Sharks have claspers (extensions of pelvic fins) – male sharks can
fertilise eggs inside the female. High chance of fertilisation means less risk to having just a
few (this is convergent on the intromittant organ of amniotes).
Hydrodynamics: sharks have a large, oily liver that helps them to float, but they are denser
than water. To stay up in the water column, a shark must swim. Highly specialised pelagic
species are ram-breathers – ventilate the gills by forcing water through the open mouth.
Reduced skeleton: advanced jawless fish are armoured – bony fish have a bone skull and
skeleton. Sharks lack bone internally – jaws and vertebrae are cartilage. External bony plates
of the skull and body are lost – reduced to a layer of denticles (made of enamel and
dentine). Contribute to reducing the density of sharks.
Drag reduction: denticles of fast-swimming sharks have parallel riblets. Fluid dynamic
studies suggest they control production of vorticles along the body – reducing drag by up to
10%.
High speed: lamnidae have become highly specialised for high speeds – teardrop body,
lunate tail, elongated fins to reduce drag – similar to marlin, dolphins and extinct
ichthyosaurs. Mako have been reliably clocked at 74 km/hr.
Tooth replacement: not unique in sharks – humans replace baby teeth. But sharks have
extremely high rate of tooth replacement – conveyer-belt of new teeth replaces old, dull
teeth. Teeth may be replaced every 9-36 days. Important because fish bones/scales rapidly
dull teeth.
Extensible jaws: jaw cartilage isn’t tightly connected to skull bones, so jaws are highly
flexible. Can even be protruded to seize prey.
Eyeshine: sharks have a tapetum lucidium – a reflective layer. Adaptive significance –
photons that pass through retina without activating photoreceptors bounced back –
widespread in mammals and some fish (coelacanth). Effective way of seeing in low light
conditions.
Ampullae of Lorenzini: sharks are electroreceptive – pit-like organs on the snout detect
electric fields. Some fish are electroreceptive, suggesting this is an ancient feature. But
sharks (e.g. hammerhead) take it to an extreme.
Smell: nostrils are not used for breathing. Flow through: each nostril has an incurrent and
excurrent opening. Pass over folds of chemosensitive tissue – can detect scents down to 1
part per billion.
Shark Ecology: sharks are specialised as hunters – specialised jaws, teeth. Specialised for
hunting in darkness – at night or in deep waters. They can see in low light conditions, or in
no-light conditions using electrical signals and smell (similar to mammals). Specialised
reproductive ecology – few, large eggs and long lifespans. Again, similar to many mammals.
Relationships:
Recap:
Chondrichthyes – sharks, rays and chimaeras. Became highly specialised for predation – primitive in
some respects, in other ways highly sophisticated.;[-=
Features of sharks and kin:
Shark diversity: 1000 known species. All predators, almost all marine, but exploit a range of
prey and habitats. They sit at the top of the food chain, so sharks are not diverse, or
abundant, but exert a strong selective pressure on species and structure of the marine
ecosystem.
What makes a shark? Compared to bony fish, sharks are primitive in some ways, highly
specialised in others.
Sharks are big: smallest is 21cm (Dwarf lantern shark) and largest is 13m, 20 tonnes (whale
shark).
Unusual life cycle: intensive parental investment. Bony fish may lay millions of eggs,
whereas sharks will lay just a few, large eggs. In some sharks, eggs are retained and hatch
internally – fed via placenta – or on other eggs.
, Intromittent organ and internal fertilisation: bony fish use a spray-and-pray approach to
reproduction – external fertilisation (messy/risky – if you have only a few eggs, might end up
with none fertilised). Sharks have claspers (extensions of pelvic fins) – male sharks can
fertilise eggs inside the female. High chance of fertilisation means less risk to having just a
few (this is convergent on the intromittant organ of amniotes).
Hydrodynamics: sharks have a large, oily liver that helps them to float, but they are denser
than water. To stay up in the water column, a shark must swim. Highly specialised pelagic
species are ram-breathers – ventilate the gills by forcing water through the open mouth.
Reduced skeleton: advanced jawless fish are armoured – bony fish have a bone skull and
skeleton. Sharks lack bone internally – jaws and vertebrae are cartilage. External bony plates
of the skull and body are lost – reduced to a layer of denticles (made of enamel and
dentine). Contribute to reducing the density of sharks.
Drag reduction: denticles of fast-swimming sharks have parallel riblets. Fluid dynamic
studies suggest they control production of vorticles along the body – reducing drag by up to
10%.
High speed: lamnidae have become highly specialised for high speeds – teardrop body,
lunate tail, elongated fins to reduce drag – similar to marlin, dolphins and extinct
ichthyosaurs. Mako have been reliably clocked at 74 km/hr.
Tooth replacement: not unique in sharks – humans replace baby teeth. But sharks have
extremely high rate of tooth replacement – conveyer-belt of new teeth replaces old, dull
teeth. Teeth may be replaced every 9-36 days. Important because fish bones/scales rapidly
dull teeth.
Extensible jaws: jaw cartilage isn’t tightly connected to skull bones, so jaws are highly
flexible. Can even be protruded to seize prey.
Eyeshine: sharks have a tapetum lucidium – a reflective layer. Adaptive significance –
photons that pass through retina without activating photoreceptors bounced back –
widespread in mammals and some fish (coelacanth). Effective way of seeing in low light
conditions.
Ampullae of Lorenzini: sharks are electroreceptive – pit-like organs on the snout detect
electric fields. Some fish are electroreceptive, suggesting this is an ancient feature. But
sharks (e.g. hammerhead) take it to an extreme.
Smell: nostrils are not used for breathing. Flow through: each nostril has an incurrent and
excurrent opening. Pass over folds of chemosensitive tissue – can detect scents down to 1
part per billion.
Shark Ecology: sharks are specialised as hunters – specialised jaws, teeth. Specialised for
hunting in darkness – at night or in deep waters. They can see in low light conditions, or in
no-light conditions using electrical signals and smell (similar to mammals). Specialised
reproductive ecology – few, large eggs and long lifespans. Again, similar to many mammals.
Relationships:
