Turtle Biology: Species, Traits, and Habitats

• Testudines are turtles, terrapins, and tortoises
• There are 2 suborders of turtles

Cryptodira

• The "hidden necks” suborder bends its neck in a vertical S-shape to retract
• It comprises most of the extant turtle taxa at 11 families
• This suborder consists of freshwater turtles, snapping turtles, tortoises, softshell turtles, and sea turtles

Pleurodira

• The "side necks" suborder bends its neck laterally to retract
• It consists of 2 families of extant taxa found in Australia, South America, and Africa/Madagascar
• The diversity is at 356 species in 13 families
• Species diversity is highest in SE Asia and the SE US
• The size ranges from ~7 cm-2.2 m in length
• Turtles can be found in marine, freshwater, & terrestrial habitats on all habitable continents, except at high elevations and high latitudes

Size variation

• The smallest musk turtles (Sternotherus odoratus) are 5-12cm (2-5in), and common in Eastern U.S. shallow waters
• The speckled tortoise or speckled Padloper (Homopus signatus) is the smallest tortoise
• Males measure 6-8cm (2.4-3.1in) in carapace length, and females measure up to almost 10cm (3.9in)
• Their weight is 95-165g (3.4-5.8 oz); endemic to South Africa and southern Namibia

Largest Turtle

• The largest is the leatherback (_Dermochelys coriacea)
• It is the only sea turtle species that lacks scales and a hard shell, which it is named after for its tough rubbery skin
• Some swim over 10,000 miles a year between nesting and foraging grounds
• Leatherbacks dive to nearly 4,000 feet
• This turtle weighs up to 1,000lbs, 130-183cm (4-6ft)
• The largest ever recorded was almost 10 feet (305 cm) from the tip of its beak to the tip of its tail and weighed in at 2,019 pounds (916 kg)

Eunotosaurus africanus

• Eunotosaurus africanus is a South African species from about 260 million years ago
• It had distinctively broadened ribs, along with other traits that are unique to turtles
• A later study found that this species was a diapsid, which links it to other reptiles
• Modern turtles are anapsids

Odontochelys semitestacea

• Odontochelys semitestacea - "toothed turtle in a half-shell"
• This turtle occurred in China about 220 MYA and was 35cm from snout to tail
• Its shell consisted of just a plastron, not a carapace
• Studies suggest that the shell was a two-step innovation, with the plastron evolving first and the carapace coming later
• As turtle embryos mature, the plates of their plastron harden before those of their carapace
• This has structures that look like the very earliest stages of carapace evolution
• The turtle's upper ribs had started to expand, and its backbone had bony outgrowths at its flanks called neural plates
• Widening ribs merge with each other and the neural plates, to complete the second half of the distinctive shell
• Turtle embryos do the same thing as they grow

Proganochelys

• Proganochelys occurred about 210 MYA
• Fossils have been found in Germany and Thailand
• It had a fully formed, massive, spiky shell
• It formed by bony plates fused to the ribs
• Its head could not retract underneath the shell
• Its neck had a covering of spines that would have made it more difficult for a predator to close its mouth
• The tail of was quite long with a covering of spines, and ended with a small club

Cryptodira: "Hidden-Necked" Turtles

• Cryptodira makes up 3/4 of species
• It is common is freshwater turtles, land tortoises, & sea turtles
• This turtle has an S-shaped bend in the spine when they retract their heads
• It primarily uses vertical movements and restricted lateral movements outside of the shell
• Their Cervical vertebrae are more distended, and their zygapophyses are much more widely spaced

Pleurodira: Side- and Snake-Necked Turtles

• Pleurodira is found in South America, Australia, & Africa/Madagascar
• The neck is bent in the horizontal plane, drawing the head into a space in front of one of the front legs
• A larger overhang of the carapace helps to protect the neck, which remains partially exposed after retraction

Synapomorphies for Testudines

• Turtles have 2-part shells and 10 vertebrae in the trunk and 8 in the cervical
• The Carapace is the dorsal half, dermal bony plates solidified to most of the vertebrae & all ribs
  • This inner layer layer is formed by the expansion of the ribs
• The Plastron is a ventral component and dermal plates fused to clavicles, interclavicles, & gastralia (abdominal ribs)
• Shells are usually covered with epidermal scutes - a thickened horny or bony plate on a turtle's shell
• The Carapace has 5 central scutes bordered by 4 lateral scutes on each side
• The shell has 10-12 marginal scutes on the outside
• The Plastron has a series of 6 paired scutes
• Girdle placement requires that pectoral & pelvic girdles be shifted inside of the rib cage in embryogenesis
• There are No teeth (in extant species)
  • Jaws covered with keratinized beak-like structure called tomium

Shell Morphology

• Aquatic turtles have low carapaces that don't create drag in the water
• They have large feet with extensive webbing, reduced ossification of the shell, and ribs embedded in soft connective tissue
• The Carapace and plastron is covered with skin
• Many aquatic turtles lie in wait on the bottom of a pond and use their long neck to grab passing prey
• Marine turtles have forelimbs modified into flippers and are mostly carnivorous
• They can survive far away from land (leatherbacks and sea turtles)
• Terrestrial turtles (tortoises and box turtles) have dome-shaped carapaces
• Some have hinges that allow them to close their shell
• Tortoises block their head with their forelimb and retract their hindlimbs
• Some tortoises (African hinge backed tortoises) have a hinge across the carapace that allows them to close off the rear opening of their shell

Turtle Lungs

• Ribs are fused to their shell
• Lungs are attached to the carapace and connective tissue
  • To inhale, turtles expand the gut downward to bring air into the lungs (inspiration)
  • To exhale, turtles contract the gut upward compresses the lungs and expels air (expiration)
• Aquatic turtles exchange oxygen and CO2 with water through the pharynx and cloaca

Turtle Heart

• The turtle atrium is divided, but the ventricle is not
• The ventricle divides into 3 compartments when it contracts
• Oxygenated blood from the left atrium enters the cavum arteriosum
• Deoxygenated blood from the right atrium enters the cavum venous, then crosses the muscular bridge and enters the cavum pulmonale
• When the ventricle contracts, deoxygenated blood travels through the cavum pulmonale to the pulmonary artery to the lungs
• The valves contract and oxygenated blood moves out through the aortas

Temperature Regulation

• Turtles are ectotherms
• Many small turtles bask in the sun to raise their body temperature to speed up digestion, growth, and the production of eggs
• Turtles move into the shade when it gets too hot
• Large tortoises have thermal inertia and are prone to overheating
• Marine sea turtles (like green sea turtles) use countercurrent exchange and fat around the neck to maintain an increased body temperature in cold water

Social Behavior and Courtship

• Many pond turtles have colorful markings that allow species to find mates
• Tortoises vocalize when they are mating
• Turtles can have glands that are enlarged during the mating season, and they rub the pheromones on their front legs
• Tortoises also have social hierarchies and move accordingly
• The plastron is concave to allow mounting

Reproduction

• All turtles are oviparous
• Females dig nests in sand or soil and deposit eggs
• Small species lay 4-5 eggs and the largest sea turtles laying >100
• Most turtles have soft-shelled eggs, but some have rigid-shelled eggs
• Gestation is usually 40-60 days
• Moisture is important -- Larger hatchlings in moist conditions than in dry conditions, and water is needed to metabolize the yolk

Sex Determination

• Mammals and birds generally have GSD (genetic sex determination)
• Turtles, crocodilians, some lizards, and 1 group of birds have TSD (temperature-dependent sex determination)
  • Type 1a - males at low temperatures, females at high temperatures
  • Type 1b - females at low temperatures & males at high temperatures
  • Type II - females at low & high temperatures & males at intermediate temperatures
• Incubation temperature may be important for size,growth rate, or swimming and running speeds, and sex determination is just a byproduct
• Temperature during the middle third of the cycle is the most important factor
• High temperature during that time usually determines sex

Parental Care

• There is no male parental care
• There is very limited female parental care
  • Some female tortoises (e.g. desert tortoises) guard their nest and attack egg predators
  • A river turtle in South America nests communally
• Embryos vocalize in their eggs to synchronize hatching
  • Females wait near the shore and vocalize in the water
  • The hatchlings go to them and then migrate to flooded forests

Sea Turtle Hatchling Emergence

• All eggs hatch within a few hours in a nest up to 50cm beneath the surface of the sand
• ~100 baby turtles pile on top of each other and the ones at the top start digging upwards
• The turtles wait until nighttime to emerge from the soil, then 100s - 1000s of baby turtles migrate to the water together
• There is high mortality due to terrestrial (crabs, foxes, raccoons, birds, etc.) and aquatic (sharks and bony fish) predators
• It is unknown where the turtles go until they become adults, 3 years later
• They might float on ocean currents and feed on algae and fauna that live in them

Long Distance Migration

• Marine turtles perform oceanic voyages
• Loggerhead hatchlings use light, wave direction, and magnetism as cues for orientation
  • They move toward light because the sky looks lighter over the ocean than land
  • They swim into the waves so they move away from shore and into the Gulf Stream and the North Atlantic Gyre
  • The turtles use earth's magnetic fields to go back south instead of north of the coast of Portugal

Important Life History Traits

• Maturity occurs late in life, can be 7-8 years in small species
• Turtles have longevity and indeterminate growth
• Clutch size increases with body size
• Delayed reproductive senescence
• Low replacement rates due to high juvenile mortality and low adult survivorship
• Turtles are often caught and killed as by-catch
• Pollution can reduce or eliminate recruitment
• Turtles threatened by humans as food, traditional medicine, and exotic pet trade

Explore turtle diversity, focusing on species identification based on geographic location. Investigate the differences between Cryptodira and Pleurodira turtles. Learn about turtle characteristics in Southeast Asia and habitats of musk turtles.