Phylum: Chordata (cord)
Note: This article is the ninth in a series that elaborates upon the natural history of the marine animals that you and your diving pals are likely to encounter when you dive. The pieces are presented in order according to the taxonomic system used by scientists when they describe and classify plants and animals. An overview of the articles and the taxonomic system is provided in the February 2001 issue.
In addition to tunicates and salps — creatures covered in last month’s issue and described in the subphylum Urochordata (Euro-core-dot-uh) — the phylum Chordata (core-dot-uh) includes vertebrates. The vertebrates, animals that have a backbone, are described in the subphylum Vertebrata (vurh-ta-braaat-uh). This group includes sea snakes, turtles, crocodiles, alligators, caimans, fishes and mammals.
This month’s feature is all about one class of vertebrates, the reptiles. Sea turtles, sea snakes, marine iguanas, alligators, caimans and marine crocodiles are reptiles. However, since Dive Training is a magazine for divers, and not a science text, I am going to concentrate on sea turtles, sea snakes and marine iguanas because these animals are the marine reptiles commonly encountered by divers in various parts of the world.
Reptiles possess dry skin that is covered with scale-like coverings called scutes that efficiently slow, or retard, water loss. These animals are also characterized by the fact that they use lungs to breathe air (meaning they lack gills) and their three-chambered heart. The majority of reptiles lay eggs but in some species the eggs hatch internally.
The first reptiles appeared on earth about 300 million years ago and they became abundant during the 50 million years that followed. Shortly after that, at least in terms of evolutionary time, the earth experienced a period of severe drought and glaciation. The special adaptations of reptiles, which included water-resistant skin and comparatively efficient lungs, gave them considerable advantages over other life forms. The most highly publicized reptiles, the dinosaurs, appeared at least 225 million years ago.
Worldwide, there are roughly 6,000 species of living reptiles described in four orders. Members of the order Crocodilia (crock-oh-dill-ee-uh) which includes the crocodiles, alligators and caimans live in and around water as do the turtles and tortoises which are described in the order Chelonia (chuh-lone-ee-uh). The largest order, Squamata, (sqwuh-mot-uh) includes lizards and snakes, the vast majority of which live on land. Sea snakes are an obvious exception. The fourth order, Rhynchocephalia, (rin-co-sea-fail-ee-uh) includes some primitive reptiles that can be found on a handful of islands off the coast of New Zealand.
Most evolutionary specialists believe that turtles first appeared on earth about 200 million years ago. Somewhere between 50 million and 100 million years later some large marsh-dwelling species are believed to have gradually expanded their realm by venturing into the sea. Given evidence from fossil remains, paleontologists suspect that one species of ancient sea turtles likely attained a length of at least 10 feet.
The living seven (a debated number) species of sea turtles occur worldwide throughout tropical, subtropical and temperate seas. While they hatch on land, sea turtles spend the vast majority of their lives in the sea. Females return to shore to lay their eggs, and males only leave the water on rare occasions to rest or escape predators.
Sea turtles are described in two families with the main distinguishing characteristic being the possession or lack of scutes. The species known as the leatherback (Dermochelys coriacea) lacks scutes, and vital body organs are protected by a leather-like carapace instead. Leatherbacks are described in the family Dermochelyidae (derm-o-ka-lia-day) while all other sea turtles are members of the family Cheloniidae (chu-lon-ee-ii-day).
Attaining proportions of 6 feet (1.8 m) long and 1,300 pounds (585 kg), leatherbacks are the largest of the modern sea turtles. The two smallest species are the Kemp’s ridley (Lepidochelys kempi) and the olive ridley (Lepidochelys olivacea). As adults both attain a length of just over 2 feet (.61 m) and typically weigh between 80 and 100 pounds (36 and 45 kg). The other four species are the green turtle (Chelonia mydas), hawksbill (Eretmochelys imbricata), loggerhead (Caretta caretta) and the flatback (Natator depressus) which is encountered primarily in Australian waters.
With the exception of leatherbacks, which are almost always encountered in the open sea except during their nesting season, all sea turtles inhabit shallow coastal waters, lagoons, bays and estuaries as well as the open sea. Exactly when and where members of a given species are likely to be found depends upon the species, time of year, the animal’s sex and stage of sexual maturity. Leatherbacks and several other species migrate great distances, often traveling as far as 3,000 miles (4,800 km) between their feeding and nesting grounds, and some individuals have been documented to travel twice that far in a given year. On the other hand, hawksbills do not migrate at all.
Perhaps even more surprising is that not only do migration patterns differ markedly between the species, they also differ significantly between populations of the same species. For example, some well-studied local populations of green sea turtles do not migrate at all while members of other populations commonly travel as far as 1,500 miles (2,400 km) in the annual trek between their primary feeding and nesting areas. Green turtles are the species of sea turtle most often seen by snorkelers and divers in the Caribbean and associated waters. They are named for the greenish hue of their body fat, not their shell color.
Not surprisingly, sea turtles are superb swimmers and they are well-adapted for long-distance travel as well as sudden bursts of speed. Over the eons their flippers have been modified to help them swim rather than walk or crawl on land. Their long, paddle-like fore flippers provide thrust while their rear flippers serve as a rudder, providing directional control and enhancing stability.
Sea turtles often appear lazy. When not resting on the sea floor, they often cruise at a speed that is slow enough that divers can catch up with them. But make no mistake, all species are capable of rapid bursts of speed and several have been clocked in excess of 20 miles per hour (32 kph) over short distances.
Not only are sea turtles excellent swimmers, they are also superb divers. Leatherbacks have been documented to dive as deep as 1,000 feet (303 m) and some scientists are convinced that they commonly go as deep as 3,900 feet (1,182 m) on feeding excursions. Like all of their cold-blooded reptilian cousins, sea turtles have relatively slow metabolic rates, a characteristic that enables them to make feeding dives that often last as long as 45 minutes. When resting on the sea floor instead of swimming and feeding, sea turtles have been known to remain submerged for as long as five hours without coming to the surface for air.
While not breathing for as long as five hours is an amazing feat, green turtles and loggerheads hibernate and are known to spend several months buried in mud on the sea floor.
The preferred diet of sea turtles differs significantly from species to species, and within the same species diet varies with aging. Some turtles are herbivores (plant eaters); others are carnivores (prey on other animals); and still others are omnivores (eat plants and animals). The jaws of various species are well-designed to help them take advantage of their preferred food sources. As examples, the head and jaws of hawksbills are narrow and pointed, a design that helps them capture a variety of crustaceans, mollusks, sponges, tunicates and other animals that commonly inhabit crevices in the latticework of a reef. The jaws of Kemp’s ridleys and loggerheads are designed for crushing and grinding. These species have a diverse diet that varies from seagrasses and jellyfishes to hard-shelled crustaceans and mollusks.
Leatherbacks have delicate jaws that are built to cut like scissors. Their primary prey consists of jellyfishes, salps and other soft-bodied organisms. Their mouths are lined with numerous small spines that angle back toward the stomach, an adaptation that helps them swallow their prey. During the first year or so of their lives green sea turtles prey chiefly upon a variety of crustaceans, mollusks, echinoderms, some cnidarians and some fishes. But as they age their jaws become more serrated and their dietary preference shifts to various alga and sea grasses.
Reproduction in sea turtles
Reproduction in sea turtles is a fascinating phenomenon. People often encounter the females when they leave the water and crawl up on sandy beaches to dig nests and deposit their eggs. It appears to be a grueling ordeal for the turtles.
The age of sexual maturity varies greatly. Studies have demonstrated that in hawksbills and green turtles sexual maturity is sometimes reached in only three years, while it takes as long as 13 years for some other healthy individuals.
When sea turtles mate it is quite common for more than one male to pursue a female. Telling males from females in the field can be tricky, but in all sea turtles the tail of the male is considerably longer. So if you see a male and female close to each other you have a good chance of determining who is who. Fertilization is internal and the animals copulate in the water. During the mating season males can become aggressive toward one another and toward intruding divers. On rare occasions when divers get too close or break up an amorous encounter, male turtles have been known to bump the divers in an effort to encourage them to leave the area. These are big, strong, hard-shelled animals, equipped with powerful jaws and I trust no more need be said.
Some time, from a few days to a few weeks after mating, the females head to shore to dig a nest and lay their eggs. In all species, nesting and egg laying tends to occur at night during the warmer times of the year. The females crawl high up on the beach well beyond the high tide line and dig a “body pit,” or hole, in the sand with their flippers. Digging a nest above the high tide line ensures that the nest will not be eroded by a rising tide before the hatchlings are ready to emerge from the eggs. The entire process can take several hours.
Toward the back of the hole, usually on the end of the nest that is closest to the water, the female excavates a second, smaller, deep hole known as the egg cavity. Not long afterwards she will begin to deposit as many as 100 eggs into the egg cavity. The soft-shelled eggs drop into the egg cavity where they harden over time.
After depositing the last egg, the female quickly begins to cover her nest with sand and then she crawls back into the sea. A female can lay as many as nine clutches of eggs during a single breeding season, and she is likely to mate every two to three years.
The incubation period ranges from as few as 50 to as many as 70 days. Turtles usually hatch at night, and upon emerging from their eggs, hatchlings head right for the water. Life is not kind to newborn sea turtles as they are heavily preyed upon by sharks, other fishes, crabs, birds as well as cats, dogs, pigs and goats. Specialists estimate that only one in 10 live as long as one year. However, those that do manage to live through the trials and tribulations of their first year sometimes manage to live to reach the age of 50.
Sometimes great numbers of nesting sea turtles gather during their mating and nesting season. On the Caribbean coast of Costa Rica at a beach named Tortuguera (Place of the Turtle) thousands of green, hawksbill and leatherback turtles nest each year from May through November. Almost 60 years ago in 1942 as many as 50,000 Kemp’s ridley turtles gathered along the coast of Mexico’s Yucatan peninsula near the shores of Ano Nuevo to mate and nest. However, today Kemp’s ridleys are the world’s most highly endangered species of sea turtle and by the summer of 1995 only 800 nesting females were counted at Ano Nuevo, the only nesting site in the world used by Kemp’s ridleys.
Sadly, Kemp’s ridleys have been pushed to the brink of extinction by pressures from shrimp fisheries in the Gulf of Mexico. Unfortunately, the story doesn’t end here. Nearly all of the world’s living sea turtles are listed as threatened or endangered species, and their future remains uncertain. In many Third World countries, sea turtles are still hunted, and even in areas where taking sea turtles is illegal there is little to no enforcement. And in many areas, nesting sites that have been used for eons have been commercially developed in recent years. Artificial lighting from street lights, sidewalks and the glow of city lights is believed to confuse some hatchlings who use an instinctive orientation to moon light to head for the water immediately upon hatching.
However, there is some good news. Strong conservation efforts and restocking programs such as one operated in Grand Cayman are in place in some parts of the world.
Sea snake is the name given to several venomous, air-breathing marine reptiles. Sea snakes are believed to have descended from a family of Australian snakes that lived on land, and it is not surprising to evolutionary specialists that 32 of the world’s 50-plus species inhabit the waters of Australia’s Great Barrier Reef. As is the case with many animals that have not been studied as thoroughly as many specialists would like, there is some debate about the classification of sea snakes. Most specialists accept that within the class reptilia there are two subfamilies of sea snakes, Hydrophiinae (hydro-fi-uh-nay) and Laticaudinae (lat-ee-caw-di-nay), both of which are part of the family Elapidae (ee-lap-uh-day). Other specialists consider the two just-mentioned subfamilies to be families in their own right, not part of the family Elapidae.
Sea snakes range throughout the entire Indo-Pacific region, but the vast majority inhabit waters close to continental coasts, islands, atolls and shallow reef systems that stretch from the Persian Gulf through Indonesia and Australia.
However, the most widely distributed species, the Pelagic Sea Snake (Pelamis platurus) occurs from the coastal areas of East Africa to the Pacific Coast of the subtropical Americas. The Pelagic Sea Snake inhabits the waters of the open sea and is seen hundreds of miles (several hundred kilometers) from the nearest point of land.
Many species of sea snakes display markings and color patterns that are truly stunning while others are solid and drab. Sea snakes can attain a length of as long as 6.5 feet (2 m), but most are only half this length or shorter. One group of sea snakes, which is sometimes referred to as the true sea snakes, bear live young. These snakes lack the enlarged scales that can easily be seen on most terrestrial snakes. The other group is known as kraits (or sea kraits). These animals leave the water to mate and lay their eggs. Sea kraits possess strap-like body scales that are missing in the true sea snakes.
When divers speak to one another in laymen’s terms, some will use the terms “sea kraits” and “kraits,” but most of us lump all of the animals into a single group of animals we call sea snakes. In this piece, unless I specifically use the terms “sea kraits” or “kraits,” it is correct to assume that I am grouping all of the animals when I use the term “sea snakes.”
With venom that is considerably more potent than that of cobras, sea snakes are among the most venomous animals on earth. Fortunately, however, sea snakes are rarely aggressive. But make no mistake about it, they are highly venomous, being equipped with extremely potent neurotoxins.
It is very rare for a diver to get bitten, and in fact it is undocumented for any divers or snorkelers that were not handling or that did not accidentally step on sea snakes to have ever been bitten. A persistent myth about sea snakes states that they can’t bite very well because their fangs are in the rear of mouths that are quite small. Not totally true. Being 2.5-4.5 mm in length, their fangs are indeed shorter than many terrestrial snakes, but their mouth and fangs are more than adequately developed to bite a person’s thigh, and they often swallow fish that are more than twice the diameter of their own necks.
Most bites and fatalities occur when fishermen try to remove a sea snake entangled in their nets. Most sea snakes are rather helpless when on land, but even so they should not be handled. One of the more poisonous species is the Beaked Sea Snake. An average-sized specimen can produce 10-15 milligrams of venom at any one time, and 1.5 milligrams is likely to prove fatal to humans. At times sea snakes demonstrate a sense of curiosity about divers, and it can be a little unnerving when you see a snake that is far more venomous than a cobra swimming right at you. Indeed, it is nice to know that these animals are not aggressive toward divers and snorkelers who don’t grab or step on them.
Sea snakes are very efficient swimmers, “S-ing” their way through the water in movements that appear similar to the movement of many terrestrial snakes. Their widened and flattened, paddle-like tail is used in an oar-like fashion providing them with plenty of thrust.
Lacking gills, sea snakes must rise to the surface to obtain air. However, they also acquire dissolved oxygen from the water they swallow and expel. When submerged, valves over their nostrils close to keep water out, and they often remain submerged for several hours at a time.
Sea snakes feed primarily on a variety of small, bottom-dwelling reef fishes. Many snakes are daytime hunters that actively probe holes and crevices in their search for prey. Lacking gills, sea snakes are air breathers and they must periodically return to the surface for air. While hunting, many species work as deep as 130 feet (39 m). However, most sea snakes tend to hunt along reefs that are in much shallower water.
Some species of eels sometimes get mistaken for sea snakes and that is easy to understand given their roughly similar body shape and some of the patterns found in common species of eels and sea snakes. However, it is easy to differentiate between eels and sea snakes. Sea snakes can be identified by their paddle-shaped tails and their large, distinctive scales. Eels, on the other hand, lack scales and many species have long fins that run along the length of their body.
The marine iguanas (Amblyrhynchus cristatus) found in Ecuador’s Galapagos Islands are the world’s only seagoing lizards. As such, marine iguanas occupy a niche in the animal kingdom. These animals are not able to compete with tortoises and land iguanas in inland areas. However, they are well-equipped for life in the coastal, intertidal and subtidal environment of the Galapagos Islands where cool water provides ample algae for food, land temperatures enable them to recover their body temperature after feeding sojourns into relatively cold water and no predatory land mammals naturally occur.
Highly gregarious animals, marine iguanas sometimes gather in densities as high as 3,000 animals per square kilometer (roughly 40 percent of a square mile). Being cold-blooded animals that are incapable of self-regulating their core body temperature, marine iguanas often rest while stacked on top of one another in groups of as many as 50 animals in order to conserve heat. Images of this activity often cause laymen to think that they always gather in such dense concentrations, but that is not the case.
Marine iguanas are well-known for their diving and swimming prowess, however, it is normally only the large males that feed offshore when diving as the big males are the only members of their species that are strong enough to swim through breaking surf. These males primarily eat a variety of fast-growing red and green marine algae that is abundant in shallow reef communities. Females and juveniles feed in the intertidal zone or on algae that grows on exposed reefs close to shore.
When diving for food, the males commonly remain submerged for five to 10 minutes. Usually they prefer to dive in rather shallow water, working at depths from 3 to 16 feet (1 to 5 m). However, big males have been documented to feed in water as deep as 40 feet (12 m) while remaining submerged for as long as one hour.
In addition to feeding on marine algae, marine iguanas sometimes feed on the fecal matter deposited by sea lions and red crabs. As a result of their diet, the iguanas must be able to excrete a lot of excess salt, and they have the most effective salt glands among the reptiles. The glands are positioned above the eyes and are connected to the nostrils via a duct system. The excess salt is forcibly expelled by the frequent “sneezing,” and as a result the heads of marine iguanas are often salt-encrusted.
Despite the fact that the Galapagos Islands are on the equator, the surrounding water is considerably colder than the water in most tropical regions. Water temperature in the Galapagos varies a surprising amount, but water temperatures can drop into the mid- to low-50s degrees Fahrenheit (low- to mid-10s degrees Celsius). Cold surroundings greatly slow down the activities of cold-blooded animals, and as a result, most marine iguanas feed only once a day and larger animals commonly feed only once every two or three days.
As is the case with all reptiles, marine iguanas are ectothermic animals, meaning that they must regulate their body temperature by behavioral means. In other words, they do not sweat or vasodilate to cool, nor do they vasoconstrict in order to conserve body heat. Instead, they depend upon external factors and their behavior to control their body temperature.
Their daily activity pattern is greatly influenced by the temperature and the height of the tide. Shortly after sunrise, most marine iguanas orient themselves on land in a manner that enables them to take maximum advantage of the sun’s warming rays. Once their body reaches the optimum temperature of 96 F (35.5 C), the animals change position to prevent overheating. This usually means the iguanas face the sun and elevate the front half of their body to allow whatever breeze is circulating to help cool them.
When marine iguanas are on land, mocking birds and finches are commonly seen cleaning ticks and mites from their skin. In addition, lava lizards often perch high atop the heads of resting marine iguanas to take advantage of a perch that provides them with a good opportunity to catch flies and other insects.
Females and juveniles typically feed in the period just slightly before the lowest tide of the day. The larger males, which take advantage of the more abundant alga on the nearby submerged reefs, typically wait until the middle of the day when the sun has heated their bodies. They cool down while feeding, but have enough afternoon sunlight and radiant heat from the rocky shoreline to rewarm their bodies to help them manage the cooler nights.
Most marine iguanas are colonial during most times of the year. During their nonbreeding season there is normally very little aggression between the animals. However, the game changes during the breeding season, which varies from island to island within the Galapagos archipelago. During their breeding season, adult males become territorial and more brightly colored with mottled red, green, orange and black markings replacing their nonbreeding gray-to-black skin color. Breeding males will try to claim a territory by placing themselves at the highest point within their territory and then posturing to make themselves appear as big as possible. Physical confrontations involving head butting, biting and pushing occur when competing males vie for breeding turf. Males do not control females, and must woo the females as they move about from one male’s territory to the next.
Once the breeding season ends, the males lose their breeding colors and no longer display aggression toward one another, and it becomes the females’ turn to compete. Mated females move to sandy areas where they establish nesting sites. They vigilantly guard the sites to make sure that no other female digs up their eggs or steals the site where one to four leathery eggs are laid. After a three- to four-month incubation period, the hatchlings emerge. Left on their own from that point forward they must immediately begin to try to defend themselves against and evade predators such as snakes, gulls, herons, hawks and other feral animals.
Things Aren’t Always As We Expect
Suppose you see an 8-foot-long tiger shark and a medium-sized loggerhead turtle in the same area. I think most of us would think that the turtle might end up as shark lunch.
A few years ago I came across a tiger shark and a turtle swimming in a channel in the Bahamas. The shark appeared to be following the turtle so naturally, I followed the action. As the duo entered shallow water, the tiger shark continued to close the gap until the shark was only a few feet away from the turtle.
I was fully expecting to see the shark try to make a kill when the turtle suddenly turned and swiftly rushed the shark. The turtle bit the tiger shark, tearing a sizeable chunk of flesh out of the tissue around the shark’s gill slits.
Instantly, the shark retreated, but it returned within a minute or two. Again the turtle took to the offensive and attacked the shark. This scenario repeated itself a few more times before the shark, a bit wiser for wear, departed and the loggerhead crawled up onto the beach to recover. The turtle soon re-entered the water.
In nature things aren’t always as we might expect them to be.
Determining Sex in Sea Turtles
By the time we graduated from junior high school (maybe even from elementary school) most of us had a pretty good idea how babies are made. We were reasonably aware that sperm from males combined with eggs from females to begin the process. Take it one step further and I think most of us believe that the sex of the offspring is generally determined when sperm and eggs unite. Not so in turtles.
The sex of turtles is not determined at the time of fertilization, but by the temperature of the sand surrounding the eggs in the nest. As a rule, the deeper the egg in the nest, the cooler the sand. These eggs yield females. Eggs deposited in the warmer, shallower part of the nest produce males.
On The Hunt
We were working in Australia shooting a sequence about sea snakes in a film that dealt with marine life on Australia’s Great Barrier Reef when we began to follow a 4-foot-long olive sea snake as it worked its way along a ledge. Moving at a steady pace, the snake appeared to be hunting as it poked its head into hole after hole in the latticework of the reef in about 75 feet (23 m) of water. Every few minutes the snake would enter a crevice and remain there for 30 seconds or so as it wriggled its way in, but it would soon re-emerge with nothing to show for its efforts.
We watched this scenario repeat itself for 20 minutes or so when the snake poked its head in the hole and remained there. The snake wriggled its way farther into the hole and then backed out and immediately began making its way toward the surface. Sea snakes are air breathers, so it wasn’t a surprise that the snake would be going to the surface to breathe. But what did surprise us was that as soon as the snake reached the surface and got air the animal turned around and instantly headed right back to the hole it had been in.
The current was running rather hard so we guessed that the snake was at least 150 feet from the hole at the surface, but it swam in a near straight-line course right back to this small, seemingly nondescript hole in the reef. A few seconds later, the snake emerged from the hole with a small fish in its mouth.
At that point we realized how important it is for the venom of sea snakes to be so highly toxic and fast-acting. The neurotoxin stops the prey quickly, allowing the snake to leave the victim, get air and come back to devour its catch without the prey moving away.
We also marveled at the snake’s ability, especially in such a strong current, to immediately find the hole where it made the kill. Sea snakes are believed to have rather poor eyesight, but possess a keen ability to interpret their surroundings by using sophisticated sensory chemoreceptors. We might regard sea snakes as being simple animals, but certainly this experience taught me how capable and sophisticated they are.