BIOSCI 108: Lecture 28 - Chordates & Early Vertebrates

Major Characteristics of Chordates

There are some common things they have such as bilateral symmertry, segmented bodies, three germ lines, a well developed coelom, segmented muscles, and a ventral heart.

Some specific characteristics seen with them are:

• Single dorsal tubular nerve cord

• A notochord

• Pharyngeal gill slits

• Muscular post-anal tail

Chordate Phylogeny

It first starts with ancestral deuterostomes with echinoderms branching off

• The next characteristic gained is the notochord with cephalochordata braching off. Everything from this point is a chordate

• Next is vertebrae with myxini (hagfish) and petromyzontida (lampreys) braching off, they are sister groups with a recent common ancestor. These two are cyclostomes (circular mouths)

• Next is jaws & mineralised skeletons with chondrichthyes breaking off, everything from here are gnathostomes

• Next is lungs/lung derivatives with actinopterygii branching off

• Next is cored fins with actinistia and dipnoi branching off

• Next is limbs w/ digits with amphibia branching off

• Next is amniotic eggs with reptilia branching off

• Lastly, is milk with mammalia branching off

Notochord

A cartilaginous rod, and is the “backbone” in some animals. It is an axial skeleton in chordate embryos. In humans, it is the padding/disc between vertebrate.

Ectothermy

Animals that rely on the environment and behaviour to regulate their body temperature (cold-blooded is an outdated term)

Some benefits of it are:

• Allows to focus more energy into other processes rather than investoing it on heating themselves (low cost)

• Have much less needs for resources

• They can be very small, opening up new niches

Some disadvantages of it are:

• Harder to maintain homeostasis

• Restricts distribution of ectotherms to warm environments

Endothermy

Animals that rely on metabolic processes to produce heat and regulate body temperature. The mitochondria, while producing ATP, also produces some heat. Mammals have the UCP-1 protein that uncouples chemiosis and ATP synthesis, allowing electrons to flow to make heat without making ATP

Some benefits of it are:

• Consistent metabolic rates that allow for prolonged activities

Some disadvantages of it are:

• Very energy-intensive even at rest

• Need more resources

Cyclostomes

Hagfish (myxini) & lampreys (petromyzontida), chordates that have circular mouths.

• They have median fins

• Two-chambered hearts (indicating lower metabolic rate)

• Simple digestive system

• Sensory organs focused at the anterior

• External fertilisation

Hagfsih

Eptaretus cirrhatus, has this slime that it is covered with that works as a defense mechanism. When it comes in contact with water and force, it will gelatinise. This can be used to deter predators.

Lamprey

Petromyzontida, their circular mouths allow them to easily bitch and attach to hosts to parasitise them.

Gnathostomes

Chordates that have jaws and mineralised skeletons (bone).

• They have scales and teeth.

• They have paired fins (dorsal for holding position & caudal for propulsion)

• They a lateral line that allows them to sense vibrations from other moving marine organisms

• They have a swim bladder that allows them to adjust their buoyancy

• Gills for respiration and a heart that is situated with a bulbus arteriosus to reduce impact to fragile gills

• A single circulation with one circuit where it flows from the heart, to the gills, to the frest of the body, and back

Condrichthyes

Essentially sharks, have some interesting characteristics

• Two-chambered heart, indicating low metabolism

• Variable detation (teeth)

• Some have nictating membranes to protect their eyes

• Heterocercal caudal fin, a non-equal back/tail fin

• A ventral mouth

• Placoid scales, that are very much like teeth, tough scales that cover skin

  • It is thought that shark dermal detincles (teeth) are homologous to placoid scales

• Cartilaginous endoskeleton

• 5-7 pairs of gills

• No operculum, the bony cover over gills

• Lack swim bladder, relying on a fatty liver

• Internal fertilisation with separate sexes