Animal Diversity lecture 18 Agnathans Flashcards
Agnathans
Jawless fish
What are the extinct Agnathans
Ostracoderms (armoured jawless fish)
- existed 500 MYA
- one of earliest vertebrates
- vary alot in size
- fed on soft bodied animals (v small mouth)
- some had paired pectoral fins (feature similair to jawed verts) - thought to be more derived than the agnathans alive today
What are the present day Agnathans
- cyclostomes
- extant = lampreys + hagfish (very closely related)
how does haemoglobin show us the link between lampreys and hagfish
- 500 MYA ancestral haemoglobin split -> alpha and beta genes
- ALL vertebrates have this split, except jawless vertebrates
- Common ancestor of lampreys+hagfish predates A/B split , diverged before 500 MYA
Lamprey features
1) Initially thought to be primitive, now thought they are very specialised
2) most lampreys are predaceous
3) sucker like feature = oral hood (for feeding)
4) Notochord provides support for the body as they only have minute vertebrae which flank the nerve chord -> Arcualia
5) Use tidal ventilation water is drawn over gills so they can breathe whilst feeding (they latch on)
Describe lamprey reproduction.
Anadromous - spend adult life in sea and return to fresh water to breed.
- chemical cues attract the sexes
- adults die after reproduction
- adults find the streams for reproduction by following bile acids released by larval lampreys.
How do larval lamprey differ from adults
- use gills for filter feeding, spend 5-6 years in larval stage (more time than as an adult)
what happened when lampreys colonised the great lakes
E.g. Great lakes (1920)
- lampreys colonised great lakes, became land-locked
- complete whole lifecycle in the lakes
- problem for fish (higher predation from lampreys)
What types of english lampreys are there
1) Brook Lampreys - never becomes predaceous (does not feed as adult)
2) River Lamprey - found in coastal waters as adults (normal life cycle)
3) Sea lamprey - largest of the 3 + found in open ocean
Features of hagfish
1) mainly scavengers
2) can aquire nutrients through skin and gills
3) Notochord present (rudiments of vertebrae)
4) Only verts with blood isomotic to sea
5) Glands produce slime ( alot!) as defence mechanism + potentially predatory (clog up fish gills)
6) sex ratio female biased
Sensory organs of hagfish
1) Barbels on their head = sensory detectors
2) single nostril is used for smell and intake of water, flows out a single gill opening
How do hagfish deal with excessive slime
Tie themselves into knots and push it off
what are jawed vertebrates called
-Gnathostomes, includes all extant vertebrates except hagfish+lampreys
what did the evolution of the jaw enable
Gnathostomes to feed on a wider variety of prey
did the evolution of the jaw cause the extinction of ostracoderms
No , they co-existed for a long time before ostracoderms went extinct
-misconception that jawed fishes caused their extinction