Tunicates and Agnatha

Urochordates

Typical Tunicate Life Cycle

Paedomorphosis

Sexual maturity in juvenile form
- Evolution of a switch to maintain juvenile body plan (in an adult form)
- (we think) Important for the evolution of the chordates
- Axelopul- type of salamander (vertebrate) that has this

Chordate characteristics

Notochord
Brain
Dorsal nerve cord
Pharyngeal slits
Endostyle; collects food particles, sorts and sends them on to the pharyngeal
Anus
Muscular, postanal tail (contains someits)

Cephlaochordates

Amphioxus (the lancelet) has some distinct chordate - characteristics:
- Endostyle
- Anus
- Pharyngeal slits
- Notochord
- Dorsal nerve cord

Vertebrate characteristics

same as chordates
+
Vertebral column = "backbone"
Endoskeleton (Bone or cartilage)
Kidneys = better ionic balance
Cranium = skull
Tripartite brain = clearly present

Fish

Highly diverse range of species currently ~ 28,000
Represents > 50% of living vertebrates (total ~55,000 species)
Water is 800 times more dense than air
Water has 1/20th concentration of Oxygen

Ostracoderms

Jawless fish
Covered by bony plates
Bony plates in the mouth acted like jaws.
Notochord was probably the main axial support
Extinct during late Devonian mass extinction

Agnathans were the first...?

Fish

Hagfish & lampreys

Lampreys are more closely related to hagfish than agnathostomes
- Surviving primitive vertebrates
- Do they have a common ancestor?
- Lampreys have more advanced features, did hagfish lose these during evolution -> secondarily primitive -> or never had them?

Hagfish

- Feeding behavior: Feed on dead marine animals, tie themselves in knots to pull at carcass.

- Slime as a defense.

Brain Development

The only known animal with a skull but no vertebral column, possesses a rudimentary thalamus.

Lamprey features

- Single nasal opening
- Rudimentary thyroid gland (Endostyle)
- Has a kidney
- can osmoregulate in freshwater (specialized chloride cells)
- 7 pairs of gill pouches
- Large well developed eyes
- Hagfish may have lost all but rudimentary eyes due to not needing them at depth? Debatabl

Lamprey life cycle

Has a parasitic life-cycle
- Reproduces in streams and produces a filter feeding larva
- Filter feds in the sediment and pops its head out
- Matures and migrates towards lakes where it feeds on fish

Ammocoetes larvae of lamprey

- Looks almost identical to amphioxis (lancelet)
- Originally thought to be a species in its own right
- Remains burrowed for 3 -7 years

Lamprey effect on lakes

Can either have good or bad effects when introduced to new lakes
- Invasive; in great lakes
- In balance; Colombia Basin