Turtles

Sea Turtles

Evolution

1. Testudines order, all species except leatherback in Cheloniidae family.
2. Species distinguishable by prefrontal scales on the head, the number and shape of scutes on the carapace, and the type of inframarginal scutes on the plastron.
   • Leatherback is only one that does not have a hard shell
     • Only living species in genus Dermochelys
       • Bears a mosaic of bony plates beneath its leathery skin. Lack of bony carapace. Instead of scutes, has thick leathery skin with embedded miniscule osteoderms. Unique among reptiles that their scales lack beta keratin.
       • Largest, as long as 7 feet and over 1,000 pounds at maturity. They weigh 1.6 ox when freshly hatched.
     • Countershading (Thayer’s Law) – turtle’s underside is lightly colored, dorsal surface is dark grey and black. Pattern of animal coloration is a form of camouflage. Reduce the ease of detection of predators and prey by counterbalancing effects of self-shadowing, when light falls on an object and makes it appear solid, acting as visual cue which makes the object easier to detect. Evolutionary theories of countershading include self-shadow concealment which results in improved background matching when viewed from below. Countershading is observed in a wide range of marine animals other than sea turtles, like the grey reef shark.

Behavior and Ecology

Habitat

1. Fairly shallow waters inside reefs, bays, and inlets
2. Attracted to abundance of marine grass and algae
3. Nesting requires open beaches with minimal disturbance

Life Cycle

1. Nesting season varies, occurs nocturnally at 2,3,4 year intervals
2. May lay as many as 9 clutches in a nesting season
   • Clutches range from 75-200 eggs
   • Incubation ranges from 45-75 days
   • Temperature affects incubation range and sex of hatchlings
3. Strong nesting fidelity – green sea turtles
4. 3 types of Life Cycles
   • Entirely neritic
     • All stages of life from juvenile to adult occur near the shore
     • Only Australian flatback turtles
   • Neritic and oceanic
     • Hatchlingand transitional stages occur near shore
     • Early juvenile stage occurs in the open ocean
     • Return near the shore for late juvenile period and adult stage
     • Loggerhead Turtles, green sea turtles, and hawksbill
   • Entirely oceanic
     • Spend entire lifetime after hatching in open ocean
     • Except when leaving the shore after hatching and returning to share for nesting
     • Leatherback Turtles

Diet

1. Hatchlings eat a variety of plants and animals
2. Adults feed almost exclusively on sea-grasses and marine algae
3. Leatherback sea turtles get their nutrition from jellyfish

Orientation

1. Offshore migration
   • Use of wave cues: turtles are in the water on the Florida coast, need to head east to Gulf Stream. When they are tiny and have to swim, they are easily led by waves. They have to swim fast because they are living off of the yolk sac that fuels three days of swimming. Typically feed on small animals in the seaweed. Need to get away from predators, birds and fish are closer to land and have no defense against them.
   • Turtle swimming into oncoming waves brings them away from land. Watched the turtle by the line of its body axis on the shell to see where it is trying to go, and measured wave direction. The compass bearing of each turtle when it disappeared from view, “vanishing bearing.” Waves typically approach the east coast of Florida from the east.
     • When the wave direction changed, the turtles swam in that direction. Direction of wave approach predicted the average orientation angle of the turtle, good evidence that the turtles were swimming into waves.
   • Hatchlings cannot visibly see a wave train to determine which way the waves are moving. Experiment: tank had a wave making machine with turtle capable of swimming into the waves with the lights off.
     • The forces below the surface that produce the wave moves in a column of circles. The turtle would feel the forces of going up, back, down, forward. Could a turtle distinguish between the two scenarios? Wave motion simulator was a motor and rotating arm that caused the water near the turtle to move in circle.
     • Experiment was done in the air, eliminating any other hydrodynamic factors, able to tell visually which way the turtle wanted to swim. Suspended in air without touching anything, must be time to swim, but will crawl when touching ground sand.
       • fixed action pattern to swim and breathe periodically.
     • When the hatchling’s left flipper is out, wants to turn left. If it does not want to turn, tucks both flippers behind it. If waves were coming on their left side, they would try to turn left to face the wave. If the wave was coming on their right side, they would try to turn right. If waves are coming from exactly behind them, some turtles chose to turn left, or turn right, or both to turn around 180 degrees
     • Hatchling sea turtles can detect the direction of wave movement by monitoring the sequence of accelerations that occurs under water as waves propagate.
2. Use of magnetic compass
   • Experiment of “Rubens coil field” with square frames with copper wires hooked up to the right power source to create a magnetic field and change the earth’s magnetic field to see if the turtles pay attention.
     • Light bulb was used to encourage swimming. Hatchlings were collected and experimental lever arm monitored their movements in a Velcro harness. Geomagnetic field with light in the east, then light is turned off, turtle adjusts to geomagnetic field and to reversed field when the coil is turned on. Turtle swims in circles while looking for direction to go.
     • Earth’s force field in the north cued turtles to go northeast. The reverse, synthetic magnetic field was going south and the turtles went southwest, ignoring magnetic north.
     • How do they know how to use the magnetic compass to go east? They were oriented to swim east toward the light and become accustomed to that direction of the magnetic field.
   • Under non experimental conditions, turtles pay attention to light while crawling on the beach. Perhaps turtles in nature use light to set their magnetic compasses for the correct course? The artificial beach crawled down a runway to the light and physically picked up and moved in orientation. When crawling east with light, swam northeast. If they crawled towards west with light, swam west. If they crawled east with no light, their swimming direction had no significant orientation.
     • Turtles go from visual cues of light, to wave orientation when in ocean near shore, to using magnetic orientation when swimming in the open ocean. If they used local environmental cues, they wouldn’t be able to find the current.
3. Do the baby turtles recognize where they are in the Atlantic Ocean? It was believed that they passively drifted in the current, but we now know that it is done on purpose
   • Observational evidence that the babies don’t seem to be going around every two years because they are actively dropping out of the current to hang out on certain islands
   • Turtles use positional information from earth’s magnetic field. inclination angle increases as you move from equator to the pole, and can realize how north or south you are related to the hemispheres
   • Isoclinics are lines of equal magnetic inclination, the angle gets less farther south and higher farther north. 0 runs through South America and Africa, 60 in NC, -85 in Antarctica
4. Hatchling life stage, crawl to the sea and start offshore migration to Gulf Stream Current. Pelagic Juveniles or Post Hatchlings have many years in the North Atlantic Gyre
   • Coastal Juveniles establish coastal feeding sites, able to home back to them when displaced, and some show seasonal migrations between summer and winter sites
   • Experiment: test whether a turtle can detect inclination angle by seeing if it has a compass sense. Coil around the experimental arena and turtle pulls a rotatable arm
     • With 60 degrees, turtles went mostly south. Coincides with the current system of the ocean, any turtle that doesn’t stay in the southern part of the current risks getting picked up by the wrong, northern current where it is too cold and they die, heading towards England and Ireland
     • With 30, swam northeast. Current is going west, turtles are swimming north
5. North Atlantic Gyre
   • Turtles respond to inclinations and intensity, but these variables were tested separately with other parameters constant. This meant that turtles were experiencing experimental magnetic fields that don’t exist in the real world
     • Experiment: testing both isodynamics and isoclinics at the same time to mimic the parameters of Florida or off the coast of Africa
   • Navigation by juvenile turtles and the possibility of a magnetic map
     • Older adults navigate to specific locations such as feeding sites and nesting areas. juveniles are not specific when navigating
     • when being transported from Nicaragua to Florida, turtle escaped from the boat during a tropical storm and swam back and was captured in the same place two weeks later, initials carved into its shell
     • Jekyll Island, Georgia magnetic field had turtles swimming south, and at the tip of Florida they swam northeast. Experiment to conclude that they detect the magnetic field via geomagnetic positional system (GPS)

Relationships with Humans

1. Leatherback – Endangered
2. Loggerhead – threatened
3. Olive Ridley - endangered in Mexico, threatened everywhere else
4. Kemp’s Ridley – Endangered
5. Hawksbill – endangered
6. Green – endangered in FL and Mexico, threatened everywhere else

Importance in Ecosystems

1. Maintaining healthy seagrass beds and coral reefs
   • Increase productivity and nutrient content of seagrass bladeswhen grazing
   • Decreasing supply of nitrogen
   • Hawksbills especially play a key role in managing the diversity of coral reefs by limiting the growth of sponges
2. Balancing marine food webs
   • Carry barnacles, algae, and epibionts which provide food for shrimp and fish
     • Some fish species eat only epibionts from turtles
3. Facilitating nutrient cycling
   • Nutrient recycling from shelled marine life by speeding up disintegration
   • Affect aeration, compaction, and nutrient distribution of the sediment

Threats

1. Desired for flesh and shell
2. Fishing methods
   • Bycatch
   • Long-lining
   • Shrimp trawling
3. Oil spills and marine debris
   • Fibropapillomatosis – tumor causing disease that can inhibit the turtle’s ability to swim, eat, and potentially cause other dangerous health problems
     • Linked to environmentally disturbed ocean habitats
     • Heavy pollution and high human population density
4. Beach Development – decreases nesting beach availability
   • Turtles are habitual when it comes to nesting so removing their nesting locations is detrimental to reproduction
   • Lighting – discourages females from nesting on developed beaches
   • Erosion and armoring – these have affected the available space for nesting
   • Use of turtle nesting habitat for leisure – discourages females from using that area for a nest – beach driving, furniture on beaches
5. Climate change
   • Changes in sea level lead to less beach area for nesting – nest crowding
   • Nesting has started to move north due to temperature changes
   • Currently protected areas may not overlap with the changing location of nests
6. Predators
   • Nests – raccoons, ants, and crabs
   • Hatchlings – birds, crabs
   • Adults – occasional shark attacks

Conservation Efforts

1. Hard to monitor the effectiveness of conservation efforts because only nests are being observed – no idea of whole population status due to wide range and limited ability to track populations aside from nesting
2. US Endangered Species Act – prohibits hunting of sea turtles and reduces incidental losses from shrimp trawling and development
3. Turtle Excluder Devices – a grid of bars with an opening at either end of the net, catches larger animals and ejects them through the opening while still catching the smaller animals ie: shrimp