The Agnatha

Question 1

Extinct agnathans

Answer 1

Ostracoderms…
•covered in odontodes - external skeleton of mineralised tissues
•10-50cm
•varied in features - some most derived had paired fins, something only normally associated with the jawed verts
•have some features like jawed verts (more closely related than modern agnathans)
•plates in mouth could be moved for processing food
•small mouth - fed on soft-bodied animals
•dorsal and tail fin

Question 2

Modern day agnathans…

Answer 2

Cyclostomes - lampreys and hagfish
•molecular and fossil evidence shows 2 are closely related so grouped together
-previously thought lampreys closer to jawed verts
•possible fossil cyclostome - ‘Tully monster’
-has very small V similar to lamps and hags and something like a notochord

Question 3

When did modern day agnathans diverge?

Answer 3

Evidence from haemoglobin
•Alpha and beta global in all verts - except jawless
•500mya ancestral global gene split = 2 new types
•common ancestor of lampreys and hagfish didn’t have split - so diverged before the gene split

Question 4

Lamprey morphology

Answer 4

• sucker-like feature - oral hood, uses to latch onto prey
• chitinous teeth to rasp at flesh and a tongue structure
• cartilage structure to support tongue and musculature to move it
• notochord retains and small V
• single nostril on top of head
• well-developed eyes and spinal eye to detect light
• respiratory tube combined with gut
• velum - prevents water from gill slits entering mouth
• gill slits on side of head - water in and out via tidal ventilation

Question 5

Lamprey lifestyle and larvae

Answer 5

Lifestyle:
•predacious in adult stage
•mainly attack bony fish but marine mammals too - latching on
•feed on soft tissue and will enter fishes gills via the operculum
Larvae:
•filter feeder in FW - gills in filter feeding and respiration
-endostyle produces mucous (homologous with thyroid G)
•thought to be whole different genus
•lasts 5-6 years

Question 6

Lamprey reproduction

Answer 6

Spend adult life in marine, breed in FW - anadromous
•attracted to opposite sex via chemical cues
•F build nest of stones in FW, lays eggs and M releases sperm on them
-adults die after this
•adults attracted to FW via cues by larvae (bile acids), detected at low conc but produced in high quantities
•some adults stay in FW - colonising the Great Lakes of USA - problems though as weaken other fish…

Question 7

Lamprey UK species

Answer 7

1. Brook lampreys - doesn’t feed as an adult, when becomes an adult only reproduce then die
2. River lampreys - normal life cycle, often coastal water for marine part of life
3. Sea lampreys - found in open ocean and return to FW to breed and die

Question 8

Hagfish lifestyle

Answer 8

•mainly scavengers of dead or dying material - enter animal and feed on soft parts
•may be predaceous - produce slime that can clog gills, kill fish and allow them to feed
•nutrients taken up through skin and gills as they burrow into flesh
•can tie themselves in knots to latch onto piece of meat, allowing them to pull of flesh
(also for cleaning off slime)

Question 9

Hagfish morphology

Answer 9

• no dorsal fin, just tail fin
• has barbules - sensory
• single nostril found on top of its head used for smell and intake of water, which will pass over gills
• have gill openings - can have single gill opening hole sometimes
• notochord - provides support for the body throughout life
• rudiments of vertebrae ‘arcualia’
• tongue & chitinous teeth for rasping at flesh
• slime from slime glands

Question 10

Hagfish reproduction and development

Answer 10

• sex ratio F bias - little is known of them but found in deep sea
• some found to be hermaphrodites
• eggs produced are very yolky - no larval stage