Marine Reptiles

Lecture Notes

 

These primitive vertebrates successfully adapted to life in the sea. 4 groups of Marine Reptiles are SEA TURTLES, SEA SNAKES, SALTWATER CROCODILE, AND MARINE LIZARDS. They live in primarily shallow coastal waters of tropical or subtropical oceans, but some turtles move into cooler waters to search for food. The reptiles are usually restricted to warmer waters because they are cold blooded and depend on the external temperature of the water to control their metabolic rates. Their biochemical reactions slow down in cooler water.

 

Marine reptiles are equipped with SALT GLANDS which remove excess salts (unlike their land and freshwater relatives). In lizards and turtles, these glands are above the eyes and can secrete a concentrated salt solution which end up bathing the eyes. This removes excess body salts, the tears cleanse the eyes. The ability to pump out the excess salts varies and determines which environment the organism can live.

 

Marine Lizards

Only one marine lizard exists and is the Marine iguana (Amblyrhynchus subcristatus) which lives in large colonies on the Galapagos. They have become secondarily adapted to marine life. They spend most of their time basking in the sun on warm rocks along the shore, warming up after swimming in the cold water to eat seaweed.

Flattened tail for swimming (like snakes)/ webbing on all four feet /

powerful claws to anchor in heavy seas / salt glands to get rid of salt taken in while eating macrophytic marine algae (seaweed) / can regulate buoyancy by expelling air and / develops BRADYCARDIA, a marked decrease of heartbeats per minute in response to diving (from normal 43/min to 7-9/min).

 

The marine iguana derives nourishment exclusively from sea flora and subjects itself to considerable exertion during its feeding dives, and though they can stay under for a considerable amount of time, they usually spend most of the day on the lava boulders along the shore. This probably has to do with the animal being cold blooded and loosing its heat to the surrounding water quickly. It props itself up on the rocks to absorb heat. (temp. tolerance from 21°C (70°F) to 49°C(120°F).

 

The marine iguana populations appear to be flourishing unlike the land iguanas which are under considerable stress from introduced animals. Where these lizards came from is still a question and whether they migrated from the coast or are leftover from a sinking land mass is still a mystery.

Bioscience Aug 1978 p 512-515

 

Saltwater Crocodile (Crocodylus porosus) inhabits mangrove areas and estuaries in the eastern Indian Ocean, Australia and some western Pacific Islands, and while most live on the coast, they are known to venture into the open sea. The largest is 30' but usually are 20'. They are very aggressive, eat people etc.

 

SEA SNAKES

There are about fifty kinds of sea snakes with most occurring and probably evolved in warm south Asian and Australian coastal waters. A few species are found as far away as Japan and the Persian Gulf. One, the yellow-bellied Pelamis, has crossed to the tropical Americas west coast and lives by the millions in the bay of Panama.

 

While sea snakes differ in form and behavior, there are a few features in common. They have flattened, rudder like tails, nostrils set on the top of the snout instead of on each side and equipped with flaps to keep them closed underwater, and smaller size of the broad belly plates that land snakes use in crawling. They have a gland in the mouth that helps get rid of salt from the seawater they drink. They have only one lung like most snakes but it is very long, lined with blood vessels to aid in O2 absorption and at the end is a simple sac to store air. Controlled heartbeats, BRADYCARDIA, reduces pulse rates by 50% when under water. Diving time varies with species, activities and water temp. The more active the less time under and the warmer water decreases diving times.

Reproduction: Sperm is implanted into oviducts of the female and she can store these for weeks or years until the eggs are ripe, fertilization is in the oviduct and many species have the embryo develop there and are born alive. Most are ovoviviparous and have no need to come ashore but a few do to lay eggs.

Olive sea snake...unmarked mustard yellow skin Aipysurus laevis up to 6', large mouth and blunt cobra head and dark cobra eyes. Its a fish eater and with cannot catch fish unless it corners them in a crevice...would die in a fish tank with free swimming fish.

Yellow bellied -Palamis platurus -gulf of California to Ecuador is equipped with fangs in the front of their upper jaws which to inject their potent venom.. They float on the surface attracting small fish to it like a piece of wood drifting in the water.

Sea snakes are related to the cobras, and are the most venomous of all snakes though they are not very aggressive. A large number of humans ..Asian fisherman mainly, have died from sea snake bites..some of which possess venom many times more powerful than any land snake. They do have short fangs and small heads and some won't bite even when provoked, but bites and fatalities do occur. The bite is painless but after several hours the legs of the victim become paralyzed, his eyes close and jaws lock. He may live for several days before convolutions and respiratory failure bring death.

Apart from Asians who eat sea snakes, sea eagles are known to feed regularly on the reptiles, seizing them when they come up for air, grab them and drop them on the rocks.

Pacific fish eating fish don't normally eat sea snakes. This does not apply to Atlantic fish but then again there are no sea snakes in the Atlantic...yet. Basically in the Pacific, the fish know that if you eat a sea snake, you die. Sharks, swallowing sea snakes with other food have often spit it out. There is thought that the snakes bite the captor as it is swallowed thereby killing the captor being regurgitated alive before the captor dies-suggesting biting inside is a method of survival.  Any exceptions to the rule that did feed on the snakes, died out and not too many fish are bred to enjoy these snakes.

Sea snakes are becoming victims of over exploitation and are hunted now for their skins. Some species are becoming rare.

SEA TURTLES

Class: Reptilia Order: Chelonia Family: Cheloniidae

Sea turtles have been hunted for meat, eggs, shells, leather, and decorative objects and are now becoming victims of pollution. While they are relics of the past, they aren't obsolete nor predestined for extinction. They are beautifully adapted for life at sea and are found in most ocean habitats. They are strong swimmers and protected from predators by size and shell. They can stay under for a long time and some can live in salt water without ever having to drink fresh water. These adaptations make them slow and vulnerable when they leave the sea to lay eggs on land, the only time they have to leave the sea.

 

Characteristics: Non-retractable heads and limbs. They have powerful paddle-shaped front flippers to swim and hind fins are used for stabilizing and steering, shells are streamlined and flattened top to bottom to decrease water resistance, fatty deposits and very light spongy bones increase buoyancy enabling them to float easily. (green fat in green turtle.) There is little or no competition between the species.

Feeding: Most turtles feed in shallow coastal waters (food extremely abundant) . The green turtle feeds on meadows of turtle grass (thalassia) throughout the tropics.. the carnivorous turtles, loggerheads, feed on crabs, shellfish, sponges, fish, and horseshoe crabs. The hawksbill can pry mussels off rocks and also eats clams, jellyfish and algae. The Pacific Ridley feeds on sea urchins and other inverts. living in eel grass beds. The leatherback feeds on jellyfish far from shore and is equipped with a mouth lined with sharp spines to hold its prey and a digestive system adapted to withstand the stings. Its also the largest of the sea turtles and are the only turtle whose shell is covered by a leathery skin and whose backbone is NOT fused to the carapace.

 

 

 

 

 

 

Migration: Without any landmarks to guide them, some sea turtles swim a thousand miles or more across open oceans to lay their eggs on tiny island beaches they have not been to since they were hatched 10-50 years before. Their powers of navigation bring them back to their ancestral nesting areas every 2-4 years. Migratory routes have been studied by tagging adults at nesting site. When they approach their nesting sites, usually small desolate beaches with few terrestrial predators, they remain offshore and mate. Its possible that the developed eggs in the female have already been fertilized and that this mating is for future eggs. The sperm can be stored from 2-4 years.

Nesting

While the males wait offshore, the females swim through the surf and crawl up on the beach and begin to dig their nests with their front and hind flippers, often flinging sand into their eyes. This is washed out by their salty tears. Using the hind flippers, a cylindrical egg chamber is scooped out and about 100 eggs are deposited. They are covered with sand to protect them 1. from land crabs gulls and rats and 2. from drying out.and 3. keeps them at the right temperature. Females may repeat this up to five times during the summer breeding season (500 eggs total) before returning to the feeding grounds.

Hatchlings

After 60 days the baby turtles (2oz) scamper towards the sea equipped with about a weeks supply of yolk. They head toward the faint glow of the rising sun to locate the ocean, hatching at night. Instincts drive these hatchlings towards the lighter horizon so if they emerge during the day, all types of problems follow ,not to mention the frigate birds and buzzards feeding on them.

 

For years the most popular explanation has been that turtles smell their way back to their natal beaches. It is believed that hatchlings can imprint on distinct chemical characteristics of the beach and years later, when they reach breeding age, remember and retrace trails of these chemicals carried by ocean currents.

The population of green turtles was studied in this effect. They migrate between their feeding grounds off the coast of Brazil and nesting grounds on Ascension Island, midway between south America and Africa...a 2800 mile round trip.

 

Many years of tagging show that adults of some species go back to the same beach and sometimes almost the same spot on that beach to lay eggs. Establishing that those adults (or any adults) were hatched there, will require tagging of a hatching and seeing that the same turtle comes back when it is sexually mature...10-50 years later.

Why adult turtles would return to their natal beach is another story. They presumably make long-distance journeys to get to habitats they need during only part of their life cycle. Herbivorous green turtles, for example, feed in calm, shallow coastal waters but need steep, sandy beaches to get above the high tide line when it is time to lay their eggs. Because these habitats are often nowhere near one another, migration during the breeding season makes sense. HOWEVER, some green turtle populations appear to swim by and ignore perfect nesting beaches while making their long migration. Its possible that geological and biological factors we can't see are important. Example, a wide current moving away from Ascension Island probably carries the vulnerable, planktonic (drifting) hatchlings quickly away from the coastal predators. This little extra in terms of survival and reproductive success could be worth the extra thousand(s) miles of migration. ALTERNATIVELY, some migratory routes may be vestiges of ancient behavior no longer adaptive. Perhaps small distant islands were once larger or closer together than they are today and with the spreading sea floor, turtles nesting on islands once only a few miles from shore...attractive because of the lack of egg-eating predators...were gradually forced to swim farther and farther out to sea.

 

Research has shown in the case of the green turtles, that the current going past Ascension Island, sweeps right into the faces of those turtles grazing in the feeding grounds off Brazil. Also its been established that turtles have a well developed nasal epithelium and excellent olfactory acuity. Even if this olfactory imprinting does turn out to be what governs the sea turtle migration, the mechanism is an imperfect one because turtles often colonize new nesting habitats, which would be essential for the species to survive because beaches only last for 100, 1000, or 10,000 years but the turtles have been around for 100million years!

 

Problems

In 1947, more than 40,000 Kemps Ridley sea turtles came ashore in a single day to lay their eggs on the species main nesting beach in Mexico. Thirty years later, the wholesale collection of eggs and slaughter of nesting females has so decimated the species that no more than 250 animals have nested in a single day since 1978 and is down now to less than 117 during their largest nesting.

Conservation efforts have reduced the egg poachers and the hunting of females but still a new threat which could wipe out the remaining turtles that are the only hope for their future. Every year thousands of endangered and threatened turtles drown in the nets of shrimp fisherman who deploy their nets off our coast in the same waters used by sea turtles. When the turtle finds itself in the way of a net, it usually swims faster but soon it tires and is caught in the net and drowns. 96,000 since 1981 (reported).

Turtle Excluder Device force large fish/turtles out the side through trap doors allowing them to go free but the shrimp go in the net. However, it is only voluntary to use it and less than one percent of the US shrimp boats are using them!

 

 

Artificial Imprinting

David Owens and Mark Grassman of Texas A&M U. measured responses of turtles to sand, seawater mixtures containing chemicals that were presumably imprinted as hatchlings. This carried over to try to artificially imprint hatchlings of the endangered Kemps ridley turtle (Lepidochelys kempi) to a new nesting beach.

 

Because todays only nesting beach is near Rancho Nuevo, Mexico on a 15 mile stretch of beach, they are trying to reestablish a second nesting beach at Padre Island TX which was at one time used for nesting. Each year, researchers collect between 2000 and 3000 eggs (caught before they touch the sand) from Rancho Nuevo, place them in styrofoam boxes filled with Padre Island sand, and send them by air to Texas. After hatching, the baby turtles are allowed to run down the beach at Padre Island in case imprinting occurs at this time and then are collected again and raised in tanks for about a year before they are released. The olfactory imprinting has not been proven yet and this is still an experiment but with the turtles so close to extinction it is worth the gamble. Because the Kemp ridley takes about 10 yrs to mature sexually, the success of this project won't be known for a few more years. Some turtles have returned according to reports in the last two years. If a large population of these turtles start to show up on Padre Island, it will be good evidence to support olfactory imprinting.

 

Tiny magnetic particles, called magnetite, have been found in sea turtles as will as other migratory animals indicating a possible magnetic compass sense. Navigation by stars however, is considered unlikely because most marine turtles can barely see above water..in fact a blind olive ridley, nested normally with its species on a beach in Costa Rica.

Laura Tangley, June 1984 BioScience Vol 34 No. 6 353-356.

 

 

 

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