Agnathans Flashcards
What are the cyclostomes?
Hagfish and lampreys
Molecular evidence suggests they should be grouped together
Give an example of an extinct agnathan?
The ostracoderms
Give some features of the ostracoderms
Lived around 500 MYA
Had a dermal exoskeleton
More derived than the cyclostomes
Around 10-50cm in size
Had moveable plates around the mouth
Had a notochord
Had a dorsal fin and some pectroal fins
Coexisted with the gnathostomes for around 50 MYA before going extinct in the late Devonian
What are the present day agnathans?
Cyclostomes
Give some features of the lampreys
Mostly predaceous as adults
Larvae filter feed
Latch onto host and drink blood - rarely kill the host
Have an oral hood to latch onto skin
Have an oral gland to secrete anticoagulant
Have a notochord to support the body - vertebrae are minute
Have tidal ventilation
Have pineal eyes for detecting light
Have a nasohypophyseal opening - acts as a nostril
Have a velum to stop water flowing into mouth
Have an adenohypophysis next to olfactory sac
How to lampreys undergo ventilation?
As larvae:
- take in water through the mouth
- release water through the gills
As adults:
- draw in water through the gill openings
- enters gill chambers
- leaves through the same place
- need to do it this way as mouth is occupied by being stuck onto prey
What does anadromous mean?
Spend adult life in marine envrionment and return to freshwater to breed
Die after mating
How do pheromones play a role in lamprey breeding?
Adults find streams to breed in by detcting pheromones released by larvae
Females use pheromones to detct mature males
Adults can release pheromones that act as alarm cues to avoud certain areas
Can detect cues at low concentrations and these can be produced in large quantities
Cues are made up from bile acids
Give some species of lampreys
Brook lamprey - 20cm long
-doesnt feed as an adult
-found all over the UK but mainly nearer costal regions than inland
River lamprey - 40cm:
-found all over the UK but mainly found more inland
Sea lamprey - 120cm:
- found more northern
Give some features of the hagfish
Live marine in the deep sea
Not sure if they are scavengers or predaceous
Can produce copious amounts of slime - this can clog fish gills so they suffocate
Slime glands along the body
Skin covers eyes so rely on smell
Have an adenohypophysis (this is the anterior pituitary)
Some are hermaphrodites
Have direct development
Have a notochord
Have rudiments of vertebrae - known as arcualia
The only vertebrates whos blood is isotonic to the sea
How do hagfish respire?
Take in water throuh the nostril
Water goes to brachial pouches
Then to the external gill openings to release the water
What are the gnathostomes?
Jawed vertebrates
What did jaws evolve from?
The anterior branchial arch
Spiracle is the remnant of the gill slit
What is the ventilation hypothesis? Give some arguments against this hypothesis
The enlargement of the mandibular arch aided in ventilation and not feeding. The arch functioned in closing and opening the entrance to the pharynx and this improved suction feeding as a secondary
Problems with the hypothesis:
- mandibular arch doesnt form a functional gill arch in any living vertebrate or fossil
- no evidence that the spiracle formed the gill slit
- mandibular arch supports the velum
Give the difference between the gill arches in the cyclostomes and the gnathostomes
Cyslostomes: gill arches are outside the gills
Gnathostomes: gill arches are internal to the gills
What is monorhiny and diplorhiny?
Monorhiny = one nostril
Diplorhiny = two nostrils
What had to change to develop the jaw?
Changing from monorhiny to diplorhiny - need two nasal sacs that are separated from the adenohypophysis
Briefly explain the development of the jaw
Started with monorhiny and a single nasal sac that combines with adenohypophysis opening
Then get two nasal sacs but they are close together
Finally get diplorhiny with double nasal sacs that are separated from the adenohypophysis
What are the different types of fins on a fish?
Spinous dorsal
Soft dorsal
Pectoral
Pelvic
Anal
Caudal
Dorsal, anal and caudal fins are known as midline fins
What else evolved when the gnathostomes evolved? What does these correspond to?
Paired fins and appendages evolved
Corresponds to the duplication of the Hox gene complex
What is pitch and yaw movement in fish? What controls them?
Pitch = up and down movement. Controlled by pelvic and pectoral fins
Yaw = side-to-side. Controlled by dorsal and anal fins
Fish use the fact that water in incompressible so they can push against it for movement
