Lecture Five: Chondrichthyes Flashcards
February 6
Chondrichthyes tend to live in shallower water than osteichthyes despite having more flexible skeletons. Why might this be?
Are sharks relatively unchanged in the last 250 million years?
No! They are existing alongside animals that have not been around that long. Some sharks’ primary diet is made up of mammals (only existed for about 45 million years).
What benefits do jaws allow?
Active predation
Suction to pull prey in
Grabbing capability
Reproduction (holding on)
Chondrichthyes are considered less complicated than Osteichthyes. What is one aspect that is more more complicated for Chondrichthyes than Osteichthyes?
Reproductive strategies.
Bony fish are broadcast spawners (release eggs and sperm into water).
Cartilaginous fish have internal fertilization, causing a complex social structure to arise. Must be in contact.
Where were jaws located in the early Chondrichthyes compared to more modern radiations?
Early: Terminal (front of head).
Modern: Subterminal (underside) – allows snout to be in front of jaws.
What changes occurred in the caudal fin between the early Chondrichthyes compared to more modern radiations?
Early: Homocercal tail (equal top/bottom).
Modern: Heterocercal tail (longer top, shorter bottom)
Describe placoid scale composition.
Similar to teeth in composiotion. Can fall off and be replaced.
What do placoid scales reduce? How?
Hydrodynamic drag. Makes water flow much smoother across body.
Why is it important for fish to have scales?
Must have an external coating since they have to isoregulate. Limits only certain parts of the body to be able to exchange with the environment.
(Mucus or scales)
What are the two types of jaw suspension in Chondrichthyes. Describe them.
- Holostylic: Upper jaw fused to cranium.
- Hyostylic: Connected to cranium by ligaments. Supported by hyomandibular bone.
Ampullae of Lorenzini.
Pick up chemical ques and electrical signals from environment.
Why is it beneficial for sharks to extend their jaws beyond their snout?
Avoids contact with the nose. Ampullae of Lorenzini make the snout very sensitive.
Tonic immobility.
What benefit does this provide?
Coma like state induced by being upside down.
Assists during mating so the female doesn’t thrash and cause damage.
When did the first bone appear?
Ostracoderms. Head plate.
What organisms make up Chondrichthyes?
Holocephali (chimeras, paraselachians), Elasmobranchi (sharks, skates, rays).
What type of teeth do Chondrichthyes have?
Tooth whorl.
Is cartilage weaker than bone?
No, it is less mineralized and more flexible. Not weaker.
What is the difference in vasculature between cartilage and bone?
Cartilage has no blood supply/vasculature.
Bone has a blood supply.
What are the strategies for development of young in Chondrichtyes?
- Lay eggs (oviparous).
- Eggs are retained and hatch in the body. They feed on the unhatched eggs then are birthed live (ovoviviparous).
- Young are supported by a placental-like system and are birthed live (viviparous).
What is the benefit of being having a heterocercal caudal tail?
A larger upper lobe forces the head down.
Do Chondrichthyes possess a swim bladder?
No.
What anatomical features aid in buoyancy for Chondrichthyes?
- Cartilaginous skeleton.
- Fixed placement of pectoral fins AND shape of caudal fin (pectoral fins provide lift when head is forced down by heterocercal tail).
- Oily liver.
What major changes occurred between ancestral and more modern Chondrichthyes?
- Terminal to subterminal jaw.
- Hyostylic to holostylic jaw.
How many gill slits do Chondrichthyes have?
Mostly five, especially in more modern species.
Some have seven, especially the older species.
Holocephali have ONE gill slit.
