Class Chondrichthyes and Sharks Flashcards
What are the defining features of a shark?
- prismatic endoskeletal calcification
- pelvic claspers
- placoid scales
Define synapomorphy
cladistic term for a defining character of a clade
Describe Chondrichthyes
- bone absent from cartilaginous endoskeleton
- prismatic calcification of surface layers of cartilage matrix
- bone in teeth and scales
- bone abscence lightens body and increases manoeuvrability
Describe Shark Vertebrae
- simple morphology
- spinal cord in neural canal formed by neural arches
- notochord sheathed by centrum
- solid casing of cartilaginous vertebrae
- neural arches enclose neural canal
- spinal cord dorsal to notochord in neural canal
Describe the exoskeleton of dogfish
early exoskeleton of dermal bone:
* bone forms in skin
* feels rough to the touch due to small placoid scales / dermal denticles
Describe placoid scales
- form in dermis
- project through epidermis to surface
- tooth-like structure: enamel tip, dentin, pulp cavity
- (homologous in structure to teeth)
Describe tooth whorls
- ligamentous band along jaw cartilage beneath flesh
- developing teeth in file behind each functional tooth - conveyor belt system
- tooth replacement ~ weekly in young sharks
What does it mean when sharks have cranial kinesis?
- ability to move skulls
- can protrude upper jaw to get better grip on prey
Define hyostylic jaw suspension
jaw not fused to cranium - linked by cartilaginous hyomandibular brances
What allows the protrusion of the upper jaw in cranial kinesies?
elastic ligaments
Describe gill openings in sharks
separate and uncovered
except chimaeras - soft tissue covering
What are the 2 main ventilation strategies in sharks?
ram ventilation
buccal pump
describe ram ventilation
- when shark is moving: water -> mouth -> gills
- 5 gill pairs
- constant forward movement to ventilate
What is Pelagic ram ventilation?
sharks in deep open ocean won’t rest on seafloor
Which species have obligate ram ventilation?
great white, mako, whale sharks
describe buccal pump ventilation
- when shark at rest: water in via mouth and spiracles, out via gills
- flap valves - no water in via gills
- ## can rest on seabed (benthic)
which species are obligate buccal pumps?
angel, carpet, nurse sharks
How do most sharks ventilate?
mix of buccal pump and ram ventilation
Describe the gill structures
- filaments protrude off gill flaps
- interbranchial septum between gills to allow blood vessels to pass through
Describe filter feeders and a common example
Cetorhinus maximus - basking sharks
- gill filament protrudes
- comb-like gill rakers around 10cm
- cartilaginous structures stop material reaching gills
- basking filter zoo-plankton prey
What is the new 2016 theory on filter feeding gills?
Sanderson et al
- vortical cross-step filtration
- gill arches create vortices
- vortical flow interacts with gill rakers to concentrate particles along slot margins
What are the 3 filter feeding sharks?
- basking sharks
- whale sharks
- megamouth sharks
What are the 2 forms of tribasal pectoral and pelvic fin evolutions?
cladoselache (upper devonian)
hybodus (early triassic to late Cretaceous)
Describe cladoselache fins
traingular basal cartilage
parallel radial cartilages to margin
describe Hybodus fins
stalk of 3 basal cartilages
segmented, shorter cartilages in blade
describe tribasal and pelvic fins in modern sharks
- tribasal structure has mobility and flexibility
- collagenous fibres ‘ceratotricia’ extend to fin margin
- muscles within fin
What is the function of pectoral fins?
- horizontal swimming - no lift from pectoral fins
- to rise: posterior plane of pectoral fins flip up to generate lift
- to sink: posterior plane of pectoral fins flip down
Describe pectoral wings in skates and rays
- dorsoventral flattening of body
- wave-like motion to move, large amplitude
- lateral extension of pectoral fins
- outer half of wing oscillated most
Describe the caudal fin in the modern shark
- heterocercal -> generates lift
- dorsal tail lobe > ventral lobe
Describe the caudal fin in Squalus acanthias
- musculature extends into dorsal lobe of dorsal fin
- sharks swim using wave-like body undulations
- muscles act as caudal fin - changes shape
Describe shark anal fins
- median fin
- not present in all sharks
Give an example of a shark with an anal fin, and a shark without
with: Mesozoic Hybodus
without: Devonian Cladoselache
Describe Median fins
- like dorsal and anal fins balance roll & yaw in locomotion
- may generate additional thrust
Describe the underslung mouth
- evolution of rostral process (snout) due to concentration of packed sensory organs in snout
- Hybodus did not have - separates from modern sharks
What are the hunting senses?
- vision
- olfaction
- hearing
- electroreception
Describe vision for hunting in sharks
- spherical lens focused by change in position not change in shape
- retina rods and cones vary in proportion with habitat
- reflactive tapetum lucidum behind retina to increase sensitivity
Describe olfaction in hunting in sharks
- can smell 1 ppm blood
- nostrils divided externally by flap of skin: water flows in incurrent nostrils and out excurrent nostrils
- detects conc. differents between nostrils, turns towards higher conc.
Describe olfaction in hammerhead sharks
- elongated but flattened olfactory chambers
- folded sensory epithelia lining
Describe hearing in sharks
- low frequencies of struggling prey attract sharks
- fish & water same density therefore only inner ear - no eardrum
- 3 orthogonal D-shaped canals and 2 vestibular sacs
What are the 3 orthogonal D-shaped canals in sharks ears?
Posterior, horizontal and anterior canals
What are the 2 vestibular sacs in sharks ears?
utriculus
sacculus
How is displacement sensed in the inner ear?
- calcium carbonate otolith and neuromast hair cell clusters
- deflection of hair cell: in one direction decreased nerve-cell discharge rates, in opposite increase discharge rates
How is sound and movement detected in shark hearing?
low frequencies only detected, by particle motion
Describe the lateral line in shark hearing
- neuromasts also in canals below skin
- detects water displacement through pores
- neuromasts wafted directly through water
- most visible in chimaeras
Describe electroreception in sharks
- primary mode of prey detection - rostral process
- sense weak electric fields generated by other animals - esp blood
- terminal metre of attack
- known as Ampullae of Lorenzini
Describe the Ampullae of Lorenzini
- pores on head of shark have canals leading to jelly filled sacs
- canals allow voltage at pore surface to extend to ampulla
- high resistance walls filled with conductive jelly
- modified hair cells respond to electric potential
Describe electoreception in embyronic sharks
- ‘Bamboo shark’ embryos in egg cases detect electric fields
- cease respiratory gill movements to avoid detection
Describe the coelomic cavity
- contains internal organs
- subdivided by transverse septum
- lined by tissue sheets pericardium and peritoneum
What is in the anterior pericardial cavity?
heart
What is in the posterior peritoneal cavity?
- oily liver
- underneath liver is u-shaped stomach
- spiral valve
- excretory organs
Describe the single circulation system in sharks
- blood passes through heart once in each circuit
- heart chambers in series
- ventral aorta: from ventricle to gills
- dorsal aorta: from gills to body
Describe aortic arches from the ventral aorta?
afferent arteries to the gills
Where do efferent arteries lead to?
from the gills to dorsal aorta and carotid artery
Describe the oily liver
- most of body cavity
- up to 25% body weight
- buoyancy organ
- fatty reserves - energy store
- oil utilised for humans
How does the liver act as buoyancy organ?
the oils are lighter than water
Why do sharks not have bone marrow?
blood cells produced in spleen and epigonal organ
Describe the excretory organs
- ribbon-like kidneys each side of dorsal midline along length of body cavity
- rectal gland associated with rectum
Describe osmoregulation in sharks
- osmoconformers
- slightly hyperosmotic
- reabsorption of urea by kidney tubules
- urea denatures proteins but countered by accumulation of TMAO
- rectal gland excretes excess Na+ and Cl - lower salt conc than seawater
What are osmoconformers?
- internal osmolarity is close to seawater
- use urea not salt to keep osmolarity high
Why are sharks slightly hyperosmotic?
water influx via gills due to use of urea
Describe the urogenital system in sharks
- excretory and reproductive systems from similar embryonic tissues
- some common ducts
- cloaca
What is a common duct in sharks?
archinephric duct - ancestral kidney drainage
also for sperm in jawed vertebrates
evolved into part of urogenital system
Describe the cloaca
- posterior orifice
- common opening for digestive, urinary and reproductive tracts
- in sharks, lobe-finned fish, amphibians, birds, reptiles
- chamber with 3 compartments separated by folds in wall
What are the 3 comparements of the cloaca?
- faeces from intestine
- urine from kidneys
- sperm from male sex organs
Describe the male shark genital system
- testes - paired structures at anterior dorsal end of coelom
- spermatozoa mature in epidydimis ducts
- sperm stored in seminal vesicles for several weeks before mating
Describe the female shark genital system
- ovaries - only 1 in dogfish
- 2 oviducts consisting of: funnel, shell gland, isthumus, uterus
Describe claspers
- earliest copulatory organs
- inserted into female cloaca
- elongated portions of male pelvic fins - so 2 claspers
- clasper tip spreads to lock within female urogenital system
Describe copulation in sharks
- small flexible sharks - male coils around female
- large stiff bodied sharks - male lies parallel to female
- biting of female by male often observed
Describe copulation in Paleozoic sharklife forms
- Carboniferous 318 mya
- fossil claspers
- appendage on head of male - held by female in possible fossil copulation
Describe oviparity in sharks
- shell gland enlarges post-fertilization
- secretes albumen & collagenous fibrils as eggs pass through
- eggs - relatively short time in uterus, laid on substate or bottom structures
- varied egg shapes
- placental viviparity in half of carcharhiniforms order
What is viviparity?
giving birth to live young
What is an example of egg shape?
- ## spiral to wrap around seaweed
Which species tend to lay eggs?
- smaller, benthic and reef forms
- Catsharks - Scyliohinidae
- Bullhead sharks - Heterodontidae
- Wobbegongs - Orectolobidae
What are some examples of species that display viviparity?
- deeper water sharks
- Hammerhead - Sphyrnidae
- Weasel - Hemigaleidae
What are the 3 sections of ovoviviparity?
- lecithotrophy
- histotrophy
- oophagy
Describe lecithotrophy
- embryos retained in uterus but soley depend on yolk deposited at ovulation
- 2 orders of rays and squalomorphii shark superorder
Describe histotrophy
- more specialised - 1 order, stingray
- embryo obtains protein and lipid rich secretions from trophonemate in uterus
Describe oophagy
- embryo consumes later ovulated eggs
- precocious dentition
- mackeral shark order - includes great whites
