L8- Taxonomy and Adaptation of Marine Reptiles, Birds, and Mammals
Evolutionary Context and the Transition of Terrestrial Organisms to the Marine Environment
Evolutionary Origin: Marine reptiles, birds, and mammals constitute a group of organisms that evolved in terrestrial systems. This is a fundamental distinction from most other marine life (such as fishes and invertebrates) that evolved within the water medium.
Secondary Adaptation: At various points in evolutionary history, these terrestrial lineages were pushed back or returned to the ocean. This required high-level adaptations to overcome the stressors of a "weird and heavy" medium compared to air.
Medium Stressors: Organisms must adapt to the physical properties of water, such as its density, viscosity, and thermal conductivity, which differ significantly from the terrestrial environments of their ancestors.
Characteristics of Chordates and the Path to Tetrapods
Chordate Attributes: All organisms discussed (reptiles, birds, mammals) belong to the phylum Chordata. To be classified as a chordate, an organism must possess four specific attributes at some stage of its development:
Nodal Cord (Notochord): A rigid structure along the back. In vertebrates, this structure eventually develops into the skeletal backbone.
Dorsal Nerve Cord: A hollow nerve cord running along the dorsal side of the body, allowing signals to pass from the brain throughout the length of the organism. This is distinct from other phyla which may possess ventral nerve cords.
Pharyngeal Gill Slits: Openings or structures in the pharynx. In humans, these develop into ear bones and other facial structures; in fish, they become gills.
Post-anal Tail: A tail that extends beyond the anus. In many species, including humans, this is only present during the larval or embryonic stage.
Vertebrate Distinctions: Chordates that possess a distinct backbone (vertebral column) and a skull are classified as vertebrates.
The Evolutionary Hierarchy towards Tetrapods:
Paired Appendages: Development of paired fins or limbs, seen early in lineages like sharks.
Jaws: Evolution of jaws for more efficient feeding strategies.
Bony Skeleton: Transition from cartilaginous structures (like those in sharks) to ossified bone.
Lobed Fins vs. Ray Fins: Ray-finned fishes possess skinny rays for flexible fins, whereas lobe-finned fishes (like the Coelacanth) share a common lineage with the four-legged Tetrapods.
The Amniotic Egg: A critical evolutionary development for land-dwelling. The amniotic egg is essentially a self-contained "container of water" for the embryo. This allows reproduction to occur away from water bodies.
Amphibian Limitation: Amphibians never developed the amniotic egg, meaning their offspring (e.g., tadpoles) remain dependent on water. Consequently, there are no truly marine amphibians.
Phylogenetic Separation and Modern Identification
Morphological Distinctions:
Mammals: Distinctive for possessing hair and mammary glands.
Turtles: Distinguished by the presence of a shield or shell.
Snakes and Lizards: Characterized by a three-chambered heart.
Crocodiles vs. Birds: Both have four-chambered hearts (similar to humans), but birds are distinguished by feathers, while crocodiles lack them.
Role of Genetics: While morphology is used for basic categorization, genetic analysis is the modern standard for high-precision phylogeny. Morphology can be misleading due to convergent evolution, where unrelated species develop similar traits due to similar environmental pressures.
Convergent and Divergent Evolution in the Marine Environment
Convergent Evolution: Occurs when organisms with different evolutionary backgrounds evolve similar traits to occupy a specific niche or medium.
Flight Examples: Bats (mammals), Pterosaurs (reptiles), and birds all developed wings to access resources or avoid predators.
Swimming Examples: Sharks (fish), Dolphins (mammals), Penguins (birds), and Ichthyosaurs (reptiles) all developed fins, streamlined bodies, and countershading to move efficiently in the water column.
Common Marine Adaptations:
Streamlined Bodies: Facilitates speed for pursuing prey in pelagic (open water) environments.
Countershading: A form of camouflage where the dorsal side is dark and the ventral side is light (advantageous for organisms living in the water column).
Divergent Evolution: The process where related species evolve different traits to avoid competition for resources.
Example (Darwin's Finches): Variation in beak size to exploit different types of seeds.
Example (Marine Birds): Different species of marine birds feed at different depths (e.g., Emperor Penguins can dive to ) or use different hunting techniques (diving vs. skimming) to coexist without competing for the same fish.
Physical Challenges of Returning to the Ocean
Oxygen Availability: Terrestrial ancestors breathed air. Marine descendants must still breathe air, leading to breath-holding adaptations. They are not truly "100% marine" in the sense that they cannot extract oxygen from water and will drown if trapped submerged.
Locomotion and Viscosity: Water is heavier and more viscous than air, requiring streamlined bodies and powerful appendages.
Thermal Regulation: Water conducts heat away from the body much faster than air. Marine mammals and birds are endotherms (warm-blooded), maintaining body heat through high metabolism and insulation like blubber.
Osmotic Pressure and Salinity: Marine environments are hypersaline compared to animal blood. Organisms face the risk of dehydration as water is drawn out of their tissues. Many developed specialized salt glands to excrete excess salt.
Marine Reptiles
Metabolism: Most are ectotherms, meaning they rely on external heat and are typically limited to tropical waters.
Sea Snakes: Approximately species. They are generally around in length. While highly poisonous, they are typically non-aggressive. Some lay eggs on land, while others give birth to live young at sea.
Marine Iguana: Endemic to the Galapagos Islands. Described by Darwin as "disgusting clumsy lizards." They eat seaweed and can dive to . They face specialized predators like the Galapagos Hawk.
Crocodiles: Include the Saltwater Crocodile and the American Crocodile.
Safety: Saltwater crocodiles are estimated to be more deadly to humans than sharks.
Behavior: They are ambush predators often found in murky river mouths but have been tagged offshore.
Sea Turtles: to species globally. Unlike land turtles, they cannot retract their heads or limbs into their shells.
Leatherback Turtle: Massive (). Lacks a hard shell, featuring small embedded bones instead. It can regulate body temperature slightly, allowing it to live in colder waters and dive to .
Reproduction: They are tied to the land to lay eggs (amniotic system). Nest temperature determines the sex ratio (warmer temperatures produce more females).
Environmental Threat: Some species eat jellyfish, leading to accidental ingestion of plastic bags that mimic the appearance of jellyfish.
Marine Birds
Diversity: Around marine species globally. New Zealand is a major biodiversity hotspot.
Adaptations:
Light/Hollow Bones: For flight.
Waterproofing: Use of an oil gland near the tail to groom and waterproof feathers, preventing them from becoming waterlogged.
Nesting: Must nest on land. Unlike turtles, many nest on cliffs to avoid predators like rats, pigs, and humans.
Penguins: Flightless marine birds found only in the Southern Hemisphere (a biological distinction often missed in pop culture, where they are paired with polar bears).
Living Conditions: Can live in massive colonies of over individuals.
New Zealand Species: species observed, breeding, found on the mainland (Little Blue Penguin, Yellow-eyed Penguin, and Fiordland Crested Penguin).
Marine Mammals: Order Carnivora and Order Sirenia
Mammalian Traits: Hair (made of keratin), mammary glands (milk production), four-chambered hearts, and elevated intelligence.
Sea Otters (Order Carnivora):
Relatively young evolutionary group ( years old).
Live in kelp forests (shallow waters up to ).
Keystone Species: Crucial for maintaining ecosystem balance by eating sea urchins.
Tool Use: Use rocks to break open shells.
Polar Bears (Order Carnivora): Closely related to brown bears. Spend much of their time on Arctic sea ice hunting seals. Climate change and melting ice make it difficult for them to reach breathing holes used by seals.
Sirenians (Manatees and Dugongs):
Large () herbivores related to elephants.
Steller's Sea Cow: A tragic example of human-induced extinction. Discovered in in the Arctic and hunted to extinction by ( years after discovery).
Order Pinnipedia: Seals and Sea Lions
Distinguishing Features:
True Seals (Phocids): Lack external ears, have short necks, and hind limbs that only face backward (clumsy on land).
Sea Lions/Fur Seals (Otariids): Possess external ears, long necks, and flippers that can rotate, allowing them to "walk" or climb rocks on land.
New Zealand Species: New Zealand Fur Seal, Sea Lion, Leopard Seal, and Elephant Seal.
Order Cetacea: Whales and Dolphins
Classification: Closely related to hippos (separated over years ago).
Baleen Whales (Mysticeti):
Lack teeth; possess baleen plates made of keratin to filter prey (e.g., krill).
Feeding Types: Skim feeders, benthic suction feeders (Gray Whale), and engulfment feeders (Faro whales with flexible throat furrows).
Toothed Whales (Odontoceti):
Includes dolphins, orcas, and sperm whales.
Possess conical teeth to catch and hold fish.
Have a single blowhole (Baleen whales have two).
Diving Adaptations:
Sperm Whale/Cuvier's Beaked Whale: Can dive to approximately and stay submerged for over an hour.
Physiological Tricks: High concentrations of red blood cells and myoglobin (oxygen storage in muscles), slowing heart rates, and shutting off blood flow to non-essential limbs to prioritize the brain.
Echolocation: High-frequency clicks produced in the nose and focused through a fatty structure called the melon. The sound bounces off prey and is received through the lower jaw. This is another example of convergent evolution with bats.
Questions & Discussion
Whale Watching: New Zealand (specifically Kaikoura) is a hotspot for whale watching due to the migration patterns of whales moving from polar feeding grounds to tropical breeding grounds.
Baleen Feeding Incident: Reference was made to a naturalist photographer almost being engulfed by a whale during a feeding event; he was spit out as he was too large for the whale's throat.
Personal Side Note: Discussion briefly touched on student family history with the university, mentioning a student being a possible third-generation attendee, though their father was an electrician rather than an academic.