Definition of Chordates
- Chordates are a group of animals known for possessing certain key characteristics during their life cycle.
Key Characteristics of Chordates
Chordates must exhibit the following four characteristics at some point between fertilization and death:
- Notochord
- A flexible rod-like structure that provides support. In vertebrates, it evolves into the backbone.
- Dorsal Hollow Nerve Cord
- Forms the central nervous system, connecting the brain to the peripheral nervous system.
- Pharyngeal Slits
- These structures appear during embryonic development. In some species, they develop into gills (e.g., fish) while in others, they may become parts of the lungs.
- Post-anal Tail
- A tail that extends beyond the anus, seen in the larval stages of many chordates.
Invertebrate Chordates
Urochordates (Tunicates/Sea Squirts)
Adult Appearance
- Tunicates appear less chordate-like compared to their larvae.
Larval Characteristics
- Tunicate larvae resemble tadpoles and display the defining chordate structures:
- Notochord
- Pharyngeal gill basket
Anatomy
- Tunicates are filter feeders with a complex anatomy:
- Covered by a tunic
- Water enters through an incurrent siphon, passes through pharyngeal slits where food is filtered, and exits through the excurrent siphon.
- The endostyle produces mucus that helps trap food particles for digestion.
Ecological Importance
- Tunicates serve as filter feeders and are also associated with potential antibacterial and anticancer drug properties.
Cephalochordates (Lancelets)
Characteristics
- Lancelets are small, fish-like organisms that live buried in sand.
- Exhibit defining chordate features including:
- Gill slits
- Notochord
- Dorsal nerve cord
- Clear signs of cephalization (the concentration of sensory organs at the anterior end).
Reptiles (Class Reptilia)
Taxonomy
Class Reptilia within Phylum Chordata.
Major Orders within Reptilia:
- Order Squamata (Lizards and Snakes)
- Order Testudines (Turtles, Tortoises, Terrapins)
- Order Crocodylia (Crocodiles and Alligators)
Order Squamata
Common Examples
- Includes snakes and lizards such as anoles and curly tails.
Adaptations in Marine Species (e.g., Sea Snakes)
- Nostrils can close to prevent water entry.
- Compressed tail acts like a rudder.
- Highly neurotoxic venom for hunting.
Marine Iguanas
Geographic Distribution
- Indigenous to the Galapagos Islands.
- Exhibit distinct adaptations:
- Dark coloration to blend in with volcanic rocks.
- Adapted diet focused on algae.
Order Testudines
Turtle Anatomy and Reproduction
- Shell is part of their body, growing with them; cannot change shells.
- Males have a notch in the shell for successful mating.
Cloaca
- An all-purpose orifice used for reproduction, defecation, and excretion.
Importance of Beach Habitats
- Reproductive habits include laying eggs on beaches that are crucial for population sustainability; affected by environmental changes such as beach renourishment projects.
Species of Sea Turtles in Florida
Green Turtle (Chelonia mydas)
- Primarily herbivorous, feeding on seaweed and seagrass.
- Federally protected; historically hunted for their meat.
Hawksbill Turtle (Eretmochelys imbricata)
- Carnivorous, primarily feeding on sponges and crustaceans.
- Sought after for tortoiseshell products; now protected.
Loggerhead Turtle (Caretta caretta)
- Omnivorous; most commonly nests on Florida’s beaches.
- Recognizable by a larger, heavier structure compared to others.
Leatherback Turtle (Dermochelys coriacea)
- Largest of the sea turtles; primarily feeds on jellyfish.
- Unique among turtles due to lack of a hard shell, covered instead by leathery skin.
Crocodilians (Order Crocodylia)
Comparison with Other Reptiles
Unique adaptations include a powerful circulatory system with a four-chambered heart similar to mammals.
Both alligators and crocodiles require a land environment for egg laying, with specific physical traits distinguishing them:
- American Alligator (Alligator mississippiensis)
- Dark-colored, broader snout; teeth typically visible only from the upper jaw.
- American Crocodile (Crocodylus acutus)
- Lighter, more tapered snout; visible teeth from both upper and lower jaws.
Notable Examples
- American crocodiles are found in the southern tip of Florida, notably in the Everglades, where both species coexist.
Introduction to Fishes
Diversity of Fish
Fishes represent the most diverse vertebrate group, with estimates of 33,000 to 35,000 species.
They vary widely in size and habitat, including both freshwater and marine environments.
The diversity of fish outnumbers that of all other vertebrate classes combined.
Classification of Fishes
Superclass Agnatha:
- Jawless fishes (e.g., lampreys, hagfish); known for their suction mouth structure.
Class Chondrichthyes:
- Cartilaginous fishes including sharks, rays, and skates.
- The skeleton is made entirely of cartilage; only the teeth are coral.
Class Osteichthyes:
- Bony fishes with skeletons composed of bone tissue.
Jawless Fishes (Agnatha)
Features of Agnatha
- Include lampreys (parasitic) and hagfish (scavengers).
- Lampreys possess a disc-shaped mouth with sharp barbs; significant impact on fish populations due to parasitic behavior.
Importance of Control Measures
- The introduction of canals has allowed lampreys to spread to new habitats, leading to declines in native fish populations, especially trout.
Cartilaginous Fishes (Chondrichthyes)
Key Characteristics
- Entirely composed of cartilage.
- Include sharks, rays, and skates; to be discussed in greater detail in future sessions.
Summary and Conclusion
A comprehensive understanding of the classes within the phylum Chordata aids in grasping the evolutionary relationships among vertebrates. The focus will continue on fishes in the subsequent lecture sessions.