2 major extant groups of chondrichthyes
- elasmobranchii (sharks, rays, skates)
- holocephali (ghostfish/ratfishes, chimaerans)
break down elasmobranchii meaning
elasma = plate
branchia = gills
how many gill openings do elasmobranchii have? where are they?
5-7 on each side of the head
what groups are included in the elasmobranchii
extant shark, skates and rays
what jaw suspension do elasmobranchii have?
HYOstylic upper jaw suspension
hyostylic upper jaw suspension
hyo -> shaped like the letter u
- upper jaw not fused to chondrocranium, permits it to be projected and retracted
jaw suspension in rays vs sharks
both:
- ligament connects upper jaw to bone
- ligaments are humongous
sharks:
- jaw is outward, allowing for manipulation of prey
- hyomandibula supports upper jaw from back
- ceratohyal supports lower jaw from back
- basihyal
rays:
- hyomandibula is connected only to lower jaw
- ceratohyal is parallel to hyomadibula
- no basihyal
break down holocephali meaning
holo = whole
cephalo = head
whole body is together
how many gill openings do holocephali have? where are they?
single gill opening on either side (right and left), right in front of pectoral fins
what groups are included in the holocephali
ratfishes/ghostfishes and chimaerans
what jaw suspension do holocephali have?
HOLOstylic upper jaw suspension
holostylic upper jaw suspension
upper jaw is fused to the chondrocranium
jaw suspension in chimaera
- jaw is fused to chondrocranium (HOLOcephali)
- lower jaw acts like nutcracker to crack open prey
- has nostrils, hyomandibula and ceratohyal
what are selachii
sharks
what are batoidea
skates and rays
distinct characteristics of chondrichthyes
- placoid scales
- cartilaginous skeleton
- teeth whorls (sharks)
- internal fertilization
- lipid-filled liver
placoid scales
- composed of enamel and dentine with a pulp activity (similar to human molar)
- apart of dermis
- appear scale-like but since they have blood supply, not actual scales
- efficient for moving through water
- run your hand one way, smooth, run opposite, rough
cartilaginous skeleton
- ancestral condition of vertebrates
- mineralized with hydroxyapatite (bone of osteichthyans also mineralized w. hydroxyapatite but deposited diff)
- not necessarily weaker than bone (can be compressed but not stretched)
- more buoyant than bone
teeth whorls of sharks
- composed of dentine, enameloid, and traces of bone
- not embedded in jaw (sit on top)
- constantly being shed and replaced
internal fertilization
- males have pelvic claspers for internal fertilization, insert into female cloaca
- males grip female pectoral fin in their jaws to avoid slipping
lipid filled liver
- large in chondrichthyes
- less dense than other body tissues and may be large
- facilitates buoyancy
- skates and bottom dwelling species have smaller livers (sink to sea floor)
clade selachii characteristics
- highly diverse group
- marine apex predators (top of food chain)
- many derived locomotor (methods of swim), sensory and behavioural adaptations
selachii vision
- well developed permits vision at low light
- rod rich retina (good for low light, rather than cone cells - for colour)
- platelike crystals of guanine - tapetum lucidum: acts as mirror by amplifying light in low light (cat in headlights)
- lens can move forwards and backwards for focus
selachii smell
- chemoreception of sharks is extremely acute
- electroreception to weak electric fields via ampullae of Lorenzini (pores on shark snout)
