Cartilaginous fishes (Chondrichthyes)

Question 1

Describe the Placoderms

Answer 1

430-360 MYA
Most extinct in Devonian
Bony shield/plates
Variety of body forms & sizes up to >8 m, Marine / freshwater
Teeth not replaced
Viviparity → internal fert
Claspers found in males
First evidence of copulation 385 MYA

Question 2

Describe the anatomy of the Chondrichthyes

Answer 2

Cartilaginous skeleton
Lipid filled liver, alteration → buoyancy
High blood urea conc. → buoyancy
Internal fertilisation
Most hyostylic jaw suspension
2 groups: holocephalans, elasmobranchs

Question 3

Describe the Holocephalans

Answer 3

E.g rat, rabbit fish
Holostylic jaw suspension where upper jaw is fused to skull
probably due to diet (crustaceans etc.) = crushing
Tooth plates → crushing
Large pectoral fins
Oviparous

Question 4

Whats the difference between the Galeomorphs and Squalomorphs?

Answer 4

→ Galeomorphs (sharks) - anal fin
→ Squalomorphs (dog fish etc.) - no anal fin

Question 5

Describe the anatomy and key features of the Batoidea (skates & rays)

Answer 5

• Dorsoventrally flattened
• Spiracle (primitive gills? debate)
• Mouth and gill slits
• More diverse than sharks
  -Enlarged pectoral fins
• Reducuced placoid scales
• Many are benthic (Exceptions e.g manta ray = filter feeder)
• Jaw protrusible, teeth flat plates (show sexual dimorphism, males have sharper teeth in mating season to hang onto female)
• Electric currents e.g electric ray gill muscles 200v

Question 6

Describe the Mechanoreceptors in cartilaginous fish

Answer 6

sense vibrations detected by neuromast organs

Question 7

Describe the chemoreceptors in cartilaginous fish

Answer 7

Acute sense of smell (1 part in 10 Billion), depends on size of olfactory bulbs
Species dependent: open water sharks tend to have larger olfactory bulbs
Turn to the side that’s stimulated first

Question 8

Describe the electroreceptors in cartilaginous fish

Answer 8

Electrical impulses sensed by ampullary organs
Found on the head of sharks, pectoral fins of rays and rostrum of holocephalans

Question 9

Describe the eyes of cartilaginous fish

Answer 9

Well developed for low light intensities
Rod rich retina
Cells with crystals of guanine (tapetum lucidum) behind retina

Question 10

Describe the brain of cartilagenious fish

Answer 10

• Proportionally heavier than other fishes
• Similar brain-to-body mass ratios for some tetrapods
• Mirror self recognition test in manta rays?

Question 11

Describe reproduction in cartilaginous fishes

Answer 11

• Sophisticated breeding mechanisms
• Internal fertilisation: Males have 2 claspers to lock into female cloaca
• Sperm released through syphon sac contraction
• Males may bite females/ hold fins = to keep her in place while mating → females often have thickened skin in these areas
• Oviparous → egg laying (nutrition from yolk)
  Egg case openings and embryo movements → increase water flow
• Ovoviviparity → retain eggs in oviduct until hatching (lecithotrophy)
• Viviparity - nutrition not only from yolk → matrotrophy