Chondrichthyes - the Cartilaginous Fishes

Question 1

Class Chondrichthyes

Answer 1

“cartilage + fish”
>1300 spp; Devonian speciation (415 mya)
- eg. chimaeras, sharks, skates, rays
- Other than whales, sharks are the largest living vertebrates
Well-developed sense organs
Powerful jaws - predator lifestyle
Powerful swimming musculature
Cartilaginous skeleton
- Heavily calcified (i.e., mineralized tissue) but not true bone
- But phosphatized mineral tissues similar to bone retained in teeth, scales, and spines

Question 2

What does it mean to have no bones?

Answer 2

You can have calcified cartilage without having bone
- Bones are organs: blood flow and cells

Question 3

Class Chondrichthyes -> Subclass Holocephali

Answer 3

• Only ~ 55 spp.
• Ratfish, rabbitfish, chimaeras
  (= mythical monster w. parts from many animals)
• Benthic, deep water
• Mechano- and electroreceptors on rostrum/snout
• Fused crushing tooth plates
• Oviparous (shallow water eggs)

Question 4

Class Chondrichthyes -> Subclass Elasmobranchii

Answer 4

• “plate + gills”
• ~1277 living spp. (200 in last decade!)
• Sharks, skates and rays
• 5-7 pairs gill slits, usually w. spiracle behind each eye
  
  • water passage for gills
• Streamlined fusiform body w. large rostrum (sharks)

Question 5

Locomotion

Answer 5

• Torpedo-shaped (sharks)
• Thrust and lift provided by an asymmetrical heterocercal tail
  
  • vertebral column turns upward and extends into dorsal lobe of caudal fin
• Paired pectoral and pelvic fins

Question 6

Organ Systems

Answer 6

• Have all major organs
• Spiral valve in (short) intestine
  - slows passage of food and
  increases absorptive area
• No swim bladder (sink or swim)
  -> large fatty liver

Question 7

Sensory Organ Systems

Answer 7

Predation!
- Keen sense of small -> “swimming noses!” (i.e., blood)
- Neuromasts
- Sensitive to vibrations/sounds
- lateral line system
(mechanical)
- Ampullae of Lorenzini
- detect bioelectric fields of
animals
- Good vision, especially in low light

Question 8

Jaw Modifications

Answer 8

Jaw protrusion in feeding sharks
- Hydrostylic jaw suspension
- Lower jaw + upper jaw attached to chondrocranium via hyoid arch
- Swings forward and out

Question 9

Scales and Teeth

Answer 9

• Placoid scales: (three prong, face backward), dentine + enamel
• Dermal origin
• Modified anteriorly for form replaceable rows of teeth

Question 10

Reproduction

Answer 10

• Fertilization is internal
• Pelvic claspers
  
  • Modifications to male pelvic
    fins for copulations
    Maternal care variable
• Oviparous (egg hatch)
• Viviparous (nourished by placenta; live birth)
• Ovoviviparous (AKA aplacental viviparity– nourished by egg; live birth)
  No care after young are born

Question 11

Sibling Conflict..

Answer 11

Sand tiger sharks
- embryos cannibalize siblings
up to 12 embryos but only 1-2 live births

Question 12

Galeoid Sharks

Answer 12

~370 spp
- Shallow warm water, pelagic
- Streamlined, muscular, fast
- High metabolism, high food (large carnivores)
- Great whites, hammerheads

Question 13

Squaloid Sharks

Answer 13

~200 spp
- deep, cold water
- Long, eel-like, (usually) smaller than Galoids; slow swimming
- No anal fin
- Low metabolism, low productivity, low food

Question 14

Batoids: Skates and Rays

Answer 14

~700 spp (> 1/2 of all elasmobranchs; more diverse than sharks!)
- Dorso-ventrally flattened
- dorsal eyes and spiracles
- Ventral gills
- Ventral protrusible mouth and
crushing teeth
- Benthic, some FW
- Large pectoral fins
- Some can stun (electric currents) or with venemous barb/sting

Question 15

Skates vs Rays

Answer 15

Skates:
- most w. thic tail and 2 dorsal fins that lack spines; oviparous; small teeth
Rays:
- Most w. long whip-like tail; dorsal fins absent/reduced; w. venomous barbs/sting; viviparous, plate teeth