Sea turtles occupy the tropical and subtropical seas all over the world. They have populated the oceans for over 100 million years and are supposed to have evolved from an ancient family of terrestrial reptiles that developed paddle-like limbs as they adapted to life in the sea (Ministry of Natural Resources, 2005). They are very much migratory and travel huge stretches of ocean connecting feeding and breeding grounds (Ministry of Natural Resources, 2005). Sea turtles are characterized by a long, streamlined shell. Depending on the species, sea turtles colour range can be olive-green, yellow, greenish-brown, reddish-brown, or black. Limbs and flippers are tailored for swimming (Sea World, 2005). Most sea turtles are 53-114 cm (21-45 in.). The largest species, the leatherback, can reach 1.2-1.9 m (3.9-6.2 ft.). The biggest on record is 2.9 m (9.5 ft.). Major differences in size are not seen between the sexes. They can weigh between 27 to 186 kg (60-410 lb.) in smaller species. Leatherbacks can weigh 200 to 660 kg (441-1,454 lb.), with reported weights up to 870 kg (1,918 lb.). Sea turtles may be herbivorous (plant eating), carnivorous (meat eating) or omnivorous (eating both meat and plants). They have a life span of up to 80 years (Sea World, 2005).
1.1. Scientific Classification
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Sea turtles belong to the Kingdom - Animalia, Phylum - Chordata, Class - Reptilia, Order - Testudines, Suborder - Cryptodira, Family - Cheloniidae or Dermochelyidae. Sea turtles fall into one of two families. Family Cheloniidae includes sea turtles which have shells covered with scutes (horny plates). Family Dermochelyidae includes only one modern species of sea turtle, the leatherback turtle. Rather than a shell covered with scutes, leatherbacks have leathery skin (Sea Turtle Conservancy, 1996). Genus and species - There are 7 known marine turtle species and 1 subspecies worldwide. These are the Hawksbill turtle (Eretmochelys imbricata), Green turtle (Chelonia mydas), Kemp's Ridley turtle (Lepidochelys kempi), Olive Ridley turtle (Lepidochelys olivacea), Loggerhead turtle (Caretta caretta), Flatback turtle (Chelonia depressa), Leatherback turtle (Dermochelys coriacea) and the subspecies the Black turtle (Chelonia mydas agassizii), (Ministry of Natural Resources, 2005).
Link to Species Id key: http://www.seaturtlefoundation.org/biology/sea-turtle-identification/
1.2. Population Status
Total population figures are not known because male and juvenile sea turtles don't come onto land and are therefore hard to count. Population data are typically based on the numbers of adult females that come onto land to nest (Sea World, 2005). Researchers depend on changing numbers of nesting females from year to year to decide population trends (Sea World, 2005). All sea turtle species are now considered 'Critically Endangered', 'Endangered' or 'Vulnerable on the IUCN's Red List of threatened species. Critically endangered turtles are the hawksbill turtle, Kemp's Ridley and Leatherback. The Loggerhead, Olive Ridley and Green turtle are listed as Endangered. Also, all sea turtle species are listed in Appendix 1 of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). They are all also protected under the Convention on Migratory Species (CMS), (Ministry of Natural Resources, 2005).
Sea turtles were common 130 million years ago during the Cretaceous period and fossil records date back as far as 200 million years (EuroTurtle, 2006). The present day genera and species originate from the last 60-10 million years. As with modern sea turtles, older sea turtles could not withdraw their head, tail and limbs into their shells. Older sea turtles also had spines for protection. The aquatic chelonians showed a variety of adaptations and there were a number of lines of evolution (EuroTurtle, 2006). There was a lessening of the bony shell, most likely to save on weight. Archelon of the cretaceous period (3.6 metre shell) was very similar to the modern Cheloniidae. Recently some bones of a giant sea turtle were found in South Dakota, USA (EuroTurtle, 2006). The fossilized remains of this turtle represent the largest sea turtle ever found - probably 20 feet (six metres) wide. It has been dated at around 70 million years old, when much of the area was covered by water (Encyclopaedia Britannica, 2011).
2. Life Cycle and Seasonality
Fig. 1: Generalised diagrammatic lifecycle of the sea turtle (Lanyon et al., 1989).
2.1. Adult Reproductive Cycle
Males and females begin the reproductive cycle by migrating from their feeding grounds to their breeding grounds. Feeding and breeding grounds may be separated by some thousand kilometres (Berlie et al., 2004). Many sea turtles exhibit seasonal reproductive cycles with a distinct spring mating period (Amoss et al., 1998). Usually, mating will start in March and happens in waters nearby to the nesting beaches where the females will ultimately lay eggs (James et al., 2008). Males will make this journey every year, but females only migrate every 2-4 years. Once near these grounds fertilization will happen internally (Meyer, 2010). During mating the male mounts the female, holding her with claws in his fore flipper (Berlie et al., 2004). Females will mate with numerous males before the start of a nesting season (SEE Turtles, 2007), storing sperm to fertilize multiple clutches of eggs that will be laid over the course of a the nesting season (Broderick et al., 2002).
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Several weeks after mating (between late April and September (James et al., 2008)), the females come onto land to nest (Berlie et al., 2004), generally near the area where they themselves hatched and emerged (SEE Turtles, 2007). Nesting occurs year round with a distinct dry season peak but non-stationary seasonality is also evident (Chaloupka, 2010). Nesting normally takes place at night, when females will find an appropriate nesting site, clear away the surface sand (making a body pit), and dig out a flask shaped nest with their hind flippers. This may be 2-3 ft. deep. They lay between about 70-130 eggs (Table 1.) in the nest and fill it with sand (Berlie et al., 2004). Once laying begins, the female will go into a 'nesting trance' and are less easily distressed during this point. They then throw sand around the nest for camouflage and go back to the sea. Most turtles nest more than once during a season (Table 1.), with about two weeks (Fig. 1) separating each nesting occasion (Berlie et al., 2004). After a female nests, she returns to the feeding grounds until the next breeding migration (SEE Turtles, 2007). During this time they will continue to mature and forage, replenishing the stores needed to undergo the next reproductive season (SEE Turtles, 2007).
Table 1: Sea Turtle Comparison - Reproduction (nesting, eggs and hatchlings).
no. of eggs
2.3. Development of hatchlings
The development time differs among each species (7 to 10 weeks (Berlie et al., 2004)) and is subjective to environmental conditions like the temperature of the sand (SEE Turtles, 2007). The developing hatchlings don't have sex chromosomes so their gender is determined by the temperature of the nest. There is a "pivotal" temperature, ranging between 28-29 degrees Celsius, at which embryos within a nest develop into a mix of males and females. Temperatures above this range produce females and temperatures below produce males (SEE Turtles, 2007). The hatchlings then start to break out of their eggs, using a small temporary tooth located on their snout called a caruncle. They hatch over a period of a few days (Berlie et al., 2004) and once out of their eggs, they will stay in the nest for a number of days (SEE Turtles, 2007). During this time they will absorb their yolk, which is attached by an umbilical to their abdomen. Once in the sea, the hatchlings spend the first couple of days of their lives in a "swimming frenzy" (Berlie et al., 2004), where they will use this yolk to provide them the much-needed energy to get from the nest to the open sea. The "swimming frenzy" may last for several days and varies in intensity and length among species, it gets the hatchlings away from dangerous near shore waters where predation is high (SEE Turtles, 2007). Less than one in a thousand hatchlings is believed to survive to adulthood (Berlie et al., 2004).
2.4. Juvenile Development
After hatching the "lost years" begin. This is a time when they spend many years in a number of juvenile oceanic habitats until they join other adults at feeding areas (Berlie et al., 2004). They are thought to drift passively in ocean current systems (Fulgêncio de Moura et al., 2010). After their open ocean phase, the juveniles will go back to coastal areas where they will forage and continue to grow until they reach sexual maturity (SEE Turtles, 2007). During this coastal phase, juvenile and sub-adult turtles forage over a huge area. The time to sexual maturity differs among species but ranges between 10-50 years (SEE Turtles, 2007). The smaller Kemp's and olive ridleys mature around the age of 10 years. Loggerheads, hawksbills, flatbacks, and greens mature between 20 and 50 years (fig. 1), and the enormous leatherback around 15 years. Once sea turtles reach sexual maturity, males and females will leave coastal areas and migrate, often thousands of miles, to breeding areas where they will mate and start their adult, reproductive stage (SEE Turtles, 2007).
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At varying stages of their life cycle, many sea turtles can be found in three different ecosystems. These three ecosystems are known as the terrestrial zone, the oceanic zone and the neritic zone (Bolten, 2003). The neritic zone is the inshore environment from the sea floor to the surface (water depth < 200m) and includes the continental shelf. The oceanic zone is the open ocean environment from sea floor to surface (water depth > 200m) (Bolten, 2003). First ovipostion, embryonic development and hatching occur in the terrestrial zone. Second post-hatching and early juvenile development and foraging occur in the oceanic zone. Next a later juvenile development and foraging stage occurs in the neritic zone (Crowder et al., 2010). Later, migration takes place in the oceanic zone to explore new environments and access seasonal foraging grounds. Lastly sea turtles return to the neritic zone for breeding (Crowder et al., 2010).
Sea turtles are very much migratory. Juveniles make transoceanic migrations, and they occupy a range of different habitats over the course of their lives. Juvenile loggerhead sea turtles undergo at least one main ontogenetic habitat shift, the change from an oceanic habitat to a neritic habitat (Crowder et al., 2010). Sea turtles span a large range of body sizes during their ontogeny; therefore they will often alter their habitat and diet as they grow in order to obtain optimal growth rates while minimizing predation mortality (Crowder et al., 2010). Sea turtles are solitary migraters and it is therefore difficult to examine each individual's oceanic path as it's never identical to any other's (Takayoyoshi, 2008). However, with the ocean as their main habitat, sea turtles have the ability to move to other regions allowing them to explore new environments and foraging grounds or interact with other individuals (Takayoyoshi, 2008). Migration between breeding and foraging sites of adult animals often arises as a response to environmental heterogeneity (Avens et al., 2004), such as seasonal cycles in resource availability and lethal water temperatures that occur during winter months (James et al., 2008). Early tracking studies revealed that foraging sites are widespread in many populations. For example, leatherback turtles may disperse widely from nesting areas to scattered oceanic foraging sites, while green turtles may similarly disperse after breeding to widely separate coastal sites (Fossette et al., 2010). More recently, studies have started to reveal a foraging dichotomy between oceanic and coastal sites, even for individuals from the same breeding population (Fossette et al., 2010).
3.3. Species Distribution & Habitat
Green sea turtles (Chelonia mydas mydas) are found in the Atlantic Ocean, Gulf of Mexico, along the Argentine coast, the Mediterranean Sea, and Indo-Pacific (Fig. 2). Their habitats include tropical and subtropical areas near continental coasts and around islands (Sea World, 2005).
Fig. 2: Green Sea turtle (Chelonia mydas mydas) distribution.
Black sea turtles (Chelonia mydas agassizii) are found along the west coasts of North and South America, from Baja California to Peru (Fig. 3). They inhabit bays and protected shores. They aren't normally seen in the open ocean (Sea World, 2005).
Fig. 3: Black sea turtle (Chelonia mydas agassizii) distribution.
Loggerhead sea turtles (Caretta caretta) are found worldwide in coastal tropical and subtropical areas (fig. 4). They often venture into temperate waters and prefer coastal bays, but their habitats can include coastal streams, creeks, brackish water, bays, lagoons and the open ocean (Sea World, 2005).
Fig. 4: Loggerhead Sea turtle (Caretta caretta) distribution.
Kemp's Ridley sea turtles (Lepidochelys kempii) are found in the Gulf of Mexico as adults (fig. 5). Juveniles are found between temperate and tropical coastal areas of the north-western Atlantic Ocean but young turtles can travel as far as northern European waters and as far south as the Moroccan coast. They inhabit shallow areas with sandy or muddy bottoms rich in crustaceans (Sea World, 2005).
Fig.5: Kemp's ridley sea turtle (Lepidochelys kempii) distribution.
Olive ridley sea turtles (Lepidochelys olivacea) are found in the tropical regions of the Atlantic, Indian, and Pacific Oceans (fig. 6). Their habitat is mostly coastal; travelling or resting in surface waters (Sea World, 2005).
Fig. 6: Olive ridley sea turtle (Lepidochelys olivacea) distribution.
Hawksbill sea turtles (Eretmochelys imbricata) are found throughout central Atlantic and Indo-Pacific regions (fig.7), near coral reefs and rocky outcroppings in shallow coastal areas (Sea World, 2005).
Fig. 7: Hawksbill sea turtle (Eretmochelys imbricata) distribution.
Flatback sea turtles (Natator depressus) are found in the northern, north-western, and north-eastern regions of Australia (fig.8) and they are completely coastal (Sea World, 2005).
Fig. 8: Flatback sea turtle (Natator depressus) distribution.
Leatherback sea turtle (Dermochelys coriacea) are found in the north-eastern and south-eastern Pacific, in the North and South Atlantic, and throughout the Indian Ocean (fig. 9). They are the most widely distributed of all sea turtles and is found farther north than any other reptile, marine or terrestrial. They are highly oceanic and only approach coastal waters during the breeding season (Sea World, 2005).