Lecture 9: Order Chelonia - The Turtles

Taxonomy and Global Distribution of Order Chelonia

• Classification Overview:     * Order: Chelonia.     * Total Families: $14$.     * Total Species: $361$.
• Distribution: Turtles are found worldwide across both tropical and temperate habitats.
• Evolutionary Origins:     * Turtles arose during the Late Triassic period, simultaneously with the dinosaurs.     * Since their appearance, they have remained abundant but have only displayed moderate levels of diversity.
• Prehistoric Giants:     * Stupendemys: A freshwater turtle originating from Venezuela that reached lengths of up to $4\,m$.     * Archelon: A marine turtle from North America that also reached lengths of up to $4\,m$.

Unique Physiological and Anatomical Attributes

• General Characteristics:     * Turtles are unique vertebrates characterized by a slow, often clumsy-looking terrestrial locomotion; however, this belies a highly successful life strategy.     * The Shell: A bony shell provides an armored covering for the body. It consists of the Carapace (dorsum) and the Plastron (ventrum).     * The Beak: They possess a toothless beak used for cropping vegetation and grasping struggling prey.
• Structure of the Rib Cage and Girdles:     * Turtles are the only vertebrates where the limb girdles (shoulder and pelvic) are located inside the rib cage.     * No other vertebrate group can withdraw their head and limbs into the interior of the rib cage for protection.

Major Suborders: Pleurodires vs. Cryptodires

• Suborder Pleurodire (Side-Necked Turtles):     * Neck Retraction: They wrap their necks around the side of the shell.     * Evolutionary Status: Considered the more primitive group.     * Anatomy:         * Some retain the mesoplastron, a skeletal element in the plastron.         * The pelvic girdle is fused to the plastron.         * Jaw adductor muscles pass over a pulley-like surface on the pterygoid bone.     * Distribution: Largely Gondwanan (Southern Hemisphere).     * Example: Podocnemis expansa (Giant River Turtle of South America).
• Suborder Cryptodire (Hidden-Necked Turtles):     * Neck Retraction: They withdraw their heads in a vertical, S-shaped curve.     * Evolutionary Status: Possess a more advanced shell.     * Anatomy:         * Lacks the mesoplastron.         * The pelvic girdle is free from the plastron.         * Jaw adductor muscles pass over a pulley-like surface on top of the otic capsule of the skull.     * Distribution: Largely Laurasian (Northern Hemisphere), though some have significantly invaded southern regions from the north.     * Example: Chelydra serpentina.

Reproductive Biology and Sexual Dimorphism

• General Dimorphism:     * Varies from pronounced to minor depending on species.     * Tail Length: Males of all species have longer tails than females. The male's cloacal opening always exceeds the posterior rim of the carapace on an extended tail.
• Courtship Behaviors:     * Aquatic Emydids: Males may swim backward in front of a female, stroking her face using elongate claws.     * Terrestrial Species: Use headbobbing and shell nudging.
• Fertilization: Internal in all species.
• Size and Anatomy Trends:     * Size Dimorphism: In aquatic species, females are typically larger. In terrestrial species, males are often larger.     * Plastron Shape: Terrestrial males usually have a concave plastron to facilitate mounting.     * Genitalia: Males possess a single midventral penis composed of erectile tissue derived from the cloacal wall.
• Mating Environment: Occurs in water for almost all species except the most strictly terrestrial forms.

Nesting and Hatching Processes

• Nesting Requirements: Turtles are obligate egg layers. They typically nest in loose, sandy soil that is exposed to the sun.
• Construction Process: Includes the creation of a body pit and an egg chamber, followed by the concealment of the nest.
• Parental Care: There is absolutely no parental care or protection provided for the nest.
• Reproductive Constraints: There are no known viviparous turtles.
• Hatching: The process of emerging from the nest is often a cooperative effort among the hatchlings.

Ecology and Evolution of Sea Turtles

• Specialized Adaptations:     * Highly modified for marine life with limbs evolved into paddles.     * Locomotion is described as "underwater flight."     * Land movement is extremely laborious and cumbersome.
• Anthropogenic Threats:     * Exploitation for food (turtle soup, eggs) and the jewelry industry (tortoise shell).     * Losses due to industrial fishing bycatch (trawling) and pollution.

Sea Turtle Species Profiles

• Green Turtle (Chelonia mydas):     * Dimensions: Carapace to $122\,cm$ ($48\,"$); weight up to $204\,kg$ (450\,#).     * Diet: Strictly herbivorous, feeding on sea grasses.     * Identity: Named for the greenish color of its body fat; primary ingredient in turtle soup.     * Scaling: Two prefrontal scales and four costal scutes; shell is brownish or olive.     * Conservation: Recovered significantly due to efforts by Archie Carr and the Caribbean Conservation Corp (CCC).
• Black Turtle (Chelonia agassizi):     * Often considered a subspecies of the Green Turtle; occurs in the eastern Pacific.     * Distinctions: Darker shell and skin; shell is notched above the hind flippers; scales on fore-flippers are much smaller than in C. mydas.
• Flatback Turtle (Natator depressus):     * Dimensions: Carapace to $99\,cm$ ($39\,"$); weight to $90\,kg$ (198\,#).     * Range: Nesting beaches exclusively in northern Australia.     * Physiology: Depressed shell with a soft, leathery covering instead of hard keratinous scutes.     * Predation: Preyed upon by saltwater crocodiles on nesting beaches.
• Hawksbill (Eretmochelys imbricata):     * Dimensions: Carapace to $88\,cm$ ($35\,"$); weight to $75\,kg$ (165\,#).     * Ecology: Sponge specialists; play a critical role in reef ecology.     * Physical Traits: Four prefrontal scales and four imbricate (overlapping) costal scales; classic tortoiseshell pattern.
• Loggerhead Turtle (Caretta caretta):     * Dimensions: Carapace to $124\,cm$ ($49\,"$); weight to $200\,kg$ (440\,#).     * Physical Traits: Reddish-brown color; four or five prefrontal scales; five costal scutes (first is smallest); three to four inframarginal scutes lacking pores.     * Feeding Behavior: Specialist in shellfish (mollusks/crustaceans). Uses "infaunal mining," digging trenches with mouths and flippers to extract buried prey.
• Ridley Turtles (Genus Lepidochelys):     * Common Traits: Small size; four prefrontal scales; five costal scutes; four or more inframarginal scutes with a pore along the suture.     * Kemp’s Ridley (Lepidochelys kempii): Carapace to $76\,cm$ ($30\,"$); weight up to $45\,kg$ (100\,#). Found in the Gulf of Mexico and Atlantic.     * Olive Ridley (Lepidochelys olivacea): Carapace to $76\,cm$ ($30\,"$); weight to $43\,kg$ (95\,#). Found in the Pacific, Southern Atlantic, and West Indies.     * Breeding (Arribada): Perform coordinated, mass beach landings during the day involving thousands of individuals.
• Leatherback Turtle (Dermochelys coriacea):     * Family: Dermochelyidae.     * Dimensions: Carapace to $178\,cm$ ($70\,"$); weight to $907\,kg$ (2000\,#).     * Physiology: Lacks a normal shell; possesses a thick leathery shell with internal bony ossicles and seven longitudinal ridges.     * Thermoregulation: Capable of generating enough body heat to remain significantly warmer than their surroundings.     * Diet: Feeds almost exclusively on jellyfish.

Longevity and Notable Specimens

• Challenges in Documentation: Official records are few because documentation rarely lasts multiple human generations.
• Documented Lifespans:     * Macroclemmys temmincki: $70$ years.     * Galapagos tortoise: $62$ years.
• Famous Individuals:     * Lonesome George: A Pinta Island Tortoise; hatched approx. $1910$, died June 24, 2012. He was the last of his kind.     * Harriet: A Galapagos Tortoise believed to be collected by Charles Darwin in the $1830s$. Lived in the Australia Zoo and was estimated at $175$ years old when she died on June 23, 2006.     * Tui Malila: A Madagascan radiated tortoise that lived in captivity for $188$ years.     * Addwaita: A captive Aldabra tortoise who lived for at least $150$ years (possibly up to $255$). Died in Kolkata Zoo in March of $1966$; rumored to have belonged to Robert Clive.

Feeding Mechanisms and Specializations

• General Strategy: Most turtles are toothless omnivores that employ an active searching foraging mode.
• Dietary Specialists:     * Vegetarians: Large tortoises, Central American River Turtle (Dermatemys mawi), and Green Turtles.     * Mollusk Specialists: Map turtles (Genus Graptemys).     * Sponge Specialists: Hawksbills.     * Jellyfish Specialists: Leatherbacks.
• Lingual Luring: Alligator Snapping Turtles possess a bright pink, vermiform (worm-like) lure on the floor of the mouth to attract fish.

Predator Defense

• Primary Defenses:     * The bony shell acts as a physical barrier.     * Large body size serves as a deterrent.     * Powerful bites: Species like the alligator snapper can inflict significant damage (e.g., shortening human fingers) and use their bite as a defense against various predators.