Phylum Chordata (I)

Lecture Series 6

Learning Outcomes • Outline the five distinctive hallmarks about chordates • Identify representatives from the following taxonomic

groups: – Subphylum Cephalochordates – Subphylum Urochordates – Subphylum Craniates

• Superclass Agnatha • Superclass Gnathostomata

– Class Chondrichthyes – Class Actinopterygii – Class Sarcopterygii

• Describe feeding and locomotion mechanisms in the taxonomic groups stated

Fig. 34-2

Lobed fins

Legs

Amniotic egg

Milk

Jaws, mineralized skeleton

Lungs or lung derivatives

Vertebral column

Head

Notochord

Common ancestor of chordates

ANCESTRAL DEUTERO- STOME

Echinodermata (sister group to chordates)

Chondrichthyes (sharks, rays, chimaeras)

Cephalochordata (lancelets)

Urochordata (tunicates)

Myxini (hagfishes)

Petromyzontida (lampreys)

Mammalia (mammals)

Actinopterygii (ray-finned fishes)

Actinistia (coelacanths)

Amphibia (frogs, salamanders)

Dipnoi (lungfishes)

Reptilia (turtles, snakes, crocodiles, birds)

Chordates Evolution (6:58’): http://www.youtube.com/watch?v=ypYesuV3PoI

Derived Characters of Chordates • All chordates share a set of derived

characters • Some species have some of these traits

only during embryonic development • Five key characters of chordates:

1. Notochord 2. Dorsal, hollow nerve cord 3. Pharyngeal slits or clefts 4. Endostyle or Thyroid Gland 5. Muscular, post-anal tail

Fig. 34-3

Dorsal, hollow

nerve cord

Anus Muscular,

post-anal tail

Pharyngeal slits or clefts

Notochord

Mouth

Muscle segments

Endostyle

5 Chordate Hallmarks 1. Notochord • The notochord is a longitudinal, flexible rod

between the digestive tube and nerve cord • It provides skeletal support throughout most of the

length of a chordate • In most vertebrates, a more complex, jointed

skeleton develops, and the adult retains only remnants of the embryonic notochord

2. Dorsal tubular nerve cord • The nerve cord of a chordate embryo

develops from a plate of ectoderm that rolls into a tube dorsal to the notochord

• The nerve cord develops into the central nervous system: the brain and the spinal cord

3. Pharyngeal pouches and slits • Functions of pharyngeal slits:

– Suspension-feeding structures in many invertebrate chordates

– Gas exchange in vertebrates (except vertebrates with limbs, the tetrapods)

– Develop into parts of the ear, head, and neck in tetrapods

4. Endostyle or Thyroid Gland • Functions of endostyle:

– Secretes mucus that traps small food particles brought into the pharyngeal cavity

– Endostyle is present in protochordates and lamprey larvae

– Some cells in the endostyle secrete iodinated proteins. – The cells are homologous with thyroid gland of adult

lamprey and the remainder of vertebrates – In ancestral chordates, an endostyle and perforated

pharynx act as an efficient filter-feeding apparatus

5. Postanal tail • Chordates have a tail posterior to the anus • In many species, the tail is greatly reduced

during embryonic development • The tail contains skeletal elements and

muscles • It provides propelling force in many aquatic

species

Lancelets • Lancelets (Cephalochordata) are

named for their bladelike shape • They are marine suspension feeders

that retain characteristics of the chordate body plan as adults

• Water enters mouth and passes through numerous pharyngeal gill slits

• Bears all 4 hallmarks of chordata

Fig. 34-4

Dorsal, hollow nerve cord

Notochord

Tail

Cirri

Mouth

Pharyngeal slits

Digestive tract

Atrium

Atriopore

Segmental muscles

Anus

2 cm

Urochordata (tunicates)

• 3000 spp • Marine • Most sessile but some free-swimming • They possess a tough non living tunic, test

that surrounds animal • Only larval form shows all hallmarks of

chordata • Adult form does not have notocord, tail

and dorsal nerve cord reduced to single ganglion

• Tunicates (Urochordata) are more closely related to other chordates than are lancelets

• They are marine suspension feeders commonly called sea squirts

• As an adult, a tunicate draws in water through an incurrent siphon, filtering food particles

• Predatory tunicates (0:23’): http://www.youtube.com/watch?v=e8jM94pNssc

Metamorphosis of sea squirt

Adult Tunicates or Sea Squirts

Sea Squirts (0:14’): http://www.youtube.com/watch?v=s95rfGnclX0

Craniates are chordates that have a head

• The origin of a head provided a new way of feeding for chordates: active predation

• Craniates share some characteristics: a skull, brain, eyes, and other sensory organs

• In aquatic craniates the pharyngeal clefts evolved into gill slits

Vertebrates are craniates that have a backbone

• During the Cambrian period, a lineage of craniates evolved into vertebrates

• Vertebrates became more efficient at capturing food and avoiding being eaten

Overview of Vertebrate Diversity Subphylum Vertebrata

Superclass Agnatha Superclass Gnathostomata Class Chondrichythes (cartilaginous fishes) Class Actinopterygii (ray-finned fishes) Class Sarcopterygii (lobe-finned fishes)

Class Amphibia Class Reptilia Class Aves Class Mammalia

1. Fish

2. Tetrapods

Jawless Fish Lampreys Hagfish

–Superclass Agnatha • Class Myxini (hagfishes) • Class Petromyzontida (lampreys)

–Superclass Gnathostomata • Class Chondrichythes (cartilaginous fishes)

–Subclass Elasmobranchii (sharks and rays) –Subclass Holocephali (chimeras)

• Class Actinopterygii (ray-finned fishes) –Subclass Chondrostei (chondrostean ray-

finned fishes) –Subclass Neopterygii (modern bony fishes)

• Class Sarcopterygii (lobe-finned fishes)

Superclass: Agnatha (Jawless fishes)

• Lack jaws • No paired fins • Taxonomy

– Class Myxini (hagfishes) – Class Petromyzontida (lampreys)

Myxini (Hagfishes) • Lack jaws & lack vertebrae • Only 43 marine species • Detritivore • Once dead body is found, it latches via

toothed plates and uses its tongue to rasp off bits of tissue

• For more leverage, it will tie a knot in their tail then slowly bring it forward so that it presses against the side of prey

• Hagfish predatory & slime defence (3:30’): http://www.youtube.com/watch?v=F8aVgSIDJjM

• Mucus Man (2:08’): http://www.youtube.com/watch?v=zdGGZJTmqEU

Lampreys

• Oldest living lineage of vertebrates but still lack jaws

• Marine or freshwater • Parasitic and non-parasitic forms

– Non-parasitic forms do not feed and die after spawning

– Parasitic forms attach to fish with sucker-like mouth, rasp off flesh with teeth and suck blood

• Fish attacked by lampreys

"Silent Invaders" Sea Lamprey 2013 (2:58’): http://www.youtube.com/watch?v=9JQ6oHjpeqU

Superclass Gnathostomata

• Jaws present • Paired limbs • Taxonomy:

– Class Chondrichythes (cartilaginous fishes) – Class Actinopterygii (ray-finned fishes) – Class Sarcopterygii (lobe-finned fishes)

• Today, jawed vertebrates, or gnathostomes, outnumber jawless vertebrates

Fig. 34-13-3

Skeletal rods

Cranium Gill slits

Mouth

Jaws evolved from the skeletal rods of the anterior pharyngeal slits. The remaining gill slits maintained their function in gas exchange.

• Cartilaginous skeleton • Bone entirely absent • Taxonomy:

– Subclass Elasmobranchii (sharks and rays) – Subclass Holocephali (chimeras)

Class Chondrichthyes (sharks, rays chimera)

Different species of sharks

Subclass Elasmobrachii (sharks and rays)

• Sharks – Predaceous – 5-7 gill slits – Tracks prey using lateral line and well

developed olfactory organs – Sharks have heterocercal tail which provides

lift as they swim – Placoid scales on skin which are modified to

form rows of teeth at mouth

Lateral line in shark

Rows of teeth

A. heterocercal - B. protocercal - C. homocercal - D. diphycercal

Lion of the ocean

• Great White Ambush (2:31’): http://www.youtube.com/watch?v=AOwdkMTwp6w

• Planet Earth Shark Jump (1:06’): http://www.youtube.com/watch?v=n-t2ayKadD0

• Rays – Dorsalventrally flattened – Enlarged pectoral fins used for swimming – Spiracles on top where water enters – Mouth and gill slits on ventral side

Subclass Holocephali (chimeras)

• 30 species • Fossil chimeras

appeared 120-50 million years ago where they were more abundant

• Mixed diet of seaweed and other marine prey

Ray-Finned Fishes and Lobe-Fins

• The vast majority of vertebrates belong to a clade of gnathostomes called Osteichthyes

• Osteichthyes includes the bony fish and tetrapods

• Nearly all living osteichthyans have a bony endoskeleton

• Most fishes breathe by drawing water over gills protected by an operculum

• Fishes control their buoyancy with an air sac known as a swim bladder

Fig. 34-16

Intestine

Adipose fin (characteristic of trout)

Cut edge of operculum

Swim bladder Caudal

fin

Lateral line

Urinary bladder Pelvic

fin

Anus

Dorsal fin Spinal cord

Brain

Nostril

Gills

Kidney Heart

Liver

Gonad

Anal fin

Stomach

Ray-Finned Fishes

• Class Actinopterygii, the ray-finned fishes

• The fins, supported mainly by long, flexible rays, are modified for maneuvering, defense, and other functions

Butterfly fish

Fig. 34-17

(a) Yellowfin tuna (Thunnus albacares)

(b) Clownfish (Amphiprion ocellaris)

(c) Sea horse (Hippocampus ramulosus)

(d) Fine-spotted moray eel (Gymnothorax dovii)

Lobe-Fins

• The lobe-fins (Sarcopterygii) have muscular pelvic and pectoral fins

• Three lineages survive and include coelacanths, lungfishes, and tetrapods

Fig. 34-18

Moving to land…