Galápagos green turtle
|Galápagos green turtle|
Not evaluated (IUCN 2.3)
Latreille in Sonnini & Latreille, 1802
(Baillie and Groombridge, 1996)
The Galápagos green turtle (Chelonia agassizii) used to be classified as a subspecies of the green sea turtle (Chelonia mydas) but was changed for a few reasons. One, caudal dimorphism (vertebrae of the same species changing), means that Chelonia agassizii (or "Chelonia agassizi") has a more domed shell. Another is the colour of the shell – Galápagos green turtles have a darker shell colour. It is endemic to the tropical and subtropical waters of the Pacific Ocean. They are often categorized as one population of the east Pacific green turtle. This title is shared with the other green sea turtle nesting populations inhabiting the Pacific Ocean. More specifically they are referred to as the black sea turtle due to their unique dark pigmentation. The Galápagos green turtle is the only population of green sea turtle to nest on the beaches of the Galápagos Islands and this fact is the derivative of their name.
There have been various debates over the binomial nomenclature of this population due to the distinct morphological characteristics that set them apart from other populations of green sea turtle. It has been difficult for researchers to obtain valid information on the lifestyles of the Galápagos green turtle due to their continuous migrations and submergence in the ocean; most information has been obtained through tagging experimentations. The Galápagos green turtle, along with all other population of green sea turtle, is listed as endangered on the IUCN Red List of threatened species. All populations are still suffering reductions in numbers despite the many conservation efforts being practiced.
The mature adult Galápagos green turtle is much smaller than those of other Green sea turtle populations; this may be why they are more genetically isolated than other populations. The carapace is dark in color, usually black to dark olive-brown, is oval in shape and tapers toward the tail. The carapace has a distinctive formation that is more sloped or domed than individuals of other populations. There are five vertebral scutes all alike in size with a hexagonal shape and flat edges. The lateral scutes are similar in size but only four run along each side. The carapace is tougher than the plastron, or the underside of the shell, and is also darker in pigmentation. They have been recorded up to 84 cm in length, while other Green sea turtles have been recorded up to 99 cm in length. Their legs are shaped like flippers to aide in swimming; they are broad and generally flattened. The head is rounded and lizard-like with no teeth and does not have the predominantly hooked beak like many other green sea turtles. The sexes are similar in most aspects except size, the females are slightly larger, and the males have a longer tail.
Galápagos green turtles get their name from their specific nesting habitat, the Galápagos Islands. They are the only subpopulation of sea turtle to nest in these islands. The Galápagos straddle the equator off the coast of Ecuador and consist of 17 islands that are volcanic in origin. This is not their only habitat as they are a highly migratory species and spend much of their time cruising the warm waters of the Pacific Ocean. Colonies of green sea turtles have been observed nesting in 80 countries around the globe and they forage along the coasts of approximately 140 countries. There have been recordings the Galápagos green turtle or the black sea turtle from the Baja California peninsula to the Galápagos Islands and Peru and as far west as the Hawaiian and Marshall Islands. Much of their time near shore is spent foraging and resting. Black sea turtles have been recorded to spend much of their time in the bays and lagoons of the Baja California peninsula foraging and resting during the period of maturation. Distances of migration are recorded to range between 1233 and 2143–km and are performed over various time periods. They come to the Galápagos primarily to nest and only the females come ashore and lay eggs, the males stay submerged for most of their lives.
Female Galápagos green turtles most often only lay eggs every 2–3 years and spend the time between resting and foraging. Fertilization occurs underwater with only females emerging for nesting. Mating has not been witnessed outside of the nesting season and usually occurs off shore near the nesting sites. Nesting space can be somewhat limited due to the consistency of the beaches in the Galápagos, being of volcanic origin. There are many rocky areas and cliffs with limited space on sandy beaches. Also, the sandy areas tend to be quite dry and this causes many cave-ins of the nests. Females in the Galápagos have modified their nesting habits due to this factor and usually keep one hind flipper in the nest while depositing the eggs to prevent the unwanted cave-in. Clutch size varies from 50 to 200 eggs per nest and can take up to three hours to accomplish. However, Galápagos sea turtles are known to have smaller clutch sizes than other populations. After the eggs are laid the female will cover them with sand and press down with her plastron for compaction. They have also been noted to make a false nest next to their primary nest to try and fool off predators. When leaving the female turtle will attempt to cover any trace of her presence by flinging sand around in the area of the nest.
Nesting most often occurs at night so there is protection from predators. The prime season for nesting is from December to June, however peak months are January through March. Females are very uneasy when coming up to shore. If they feel threatened they will return to the water and wait until the shore is safer. Approximately two months after nesting the hatchlings emerge. They are approximately 46 mm in length when hatched, smaller than other Chelonian hatchlings.
Green sea turtles have been recorded and observed in the Galápagos over many centuries as far back as the 17th century by William Dampier. There has not been much attention brought to them due to the overwhelming research done on the Galápagos Giant Tortoises. Only over the last 30 years have extensive studies been performed covering the behaviors of the Galápagos green turtles. Much of the debate that has surrounded them recently is over the binomial classification of the species. It is the only subpopulation of green sea turtle to be given premise for separate species delineation, proposing Chelonia agassizii as a separate species. At this time no distinctions have been made. This is mainly due to analysis of mitochondrial and nuclear DNA of 15 nesting beaches. This analysis has showed little distinction between the populations of the East Pacific waters and those of other nesting areas.
It is the unique morphological distinctions of the Galápagos green turtle or the black sea turtle that have been giving rise to the debate. The two most notable distinctions are the considerably smaller adult size and the much darker pigmentation of the carapace, plastron, and the extremities. Other distinctions are the curving of the carapace above each hind flipper, the more dome-shaped carapace, and the very long tail of adult males. The main argument that separates agassizii from other Chelonians is that with a smaller adult size it would be very difficult for a male of another subpopulation to mate with a female black sea turtle. A male sea turtle must have a fairly strong hold of the females’ carapace during mating in order to successfully copulate; with the female agassizii being much smaller this feat would be impossible. This in turn creates an isolation of agassizii from the genetic variations shared between other populations of green sea turtle in the Pacific.
Three possibilities have arisen from their unique characteristics; agassizii is a separate species from mydas, the Galápagos green turtle is a subspecies of green sea turtle, or it is simply a color mutation. These facts have led to the debate over binomial separation however due to the significance of the DNA testing results there have been no distinctions made at this time.
Galápagos green turtles’ lifestyle is similar to other populations of Chelonians. The behavior of all sea turtles is difficult to track but many tagging experiments have been performed to assess migration patterns as well as feeding habits of Chelonians in the eastern Pacific. The results of this research indicates that the green sea turtles of the eastern Pacific including the Galápagos green turtles are highly migratory and ingest many different variations of forage including some animal matter. The female green turtles are known to nest colonially and to return to the same nesting beach every time they reproduce. This may be a result of comfort from past nesting success or it could be a result of returning to the origin of their own hatching beach. Populations in the eastern Pacific have also shown to return to the same foraging habitat after nesting and these areas are shared with other nesting colonies. Analysis of genetic data shows that populations in the eastern Pacific share the same life history but have developed variations of many details such as size and color. The populations in the Pacific and Indian Oceans are shown to have complete genetic isolation from the populations in the Atlantic and Mediterranean Sea. Black turtles are recorded to spend much of their maturation stage in the coastal areas off the Baja California peninsula in particular in the Bahia Magdalena lagoon and connecting channels. This particular area is rich in mangrove trees that provide forage and protection during this crucial stage of their lives. Another food source for the mature green turtles in the Pacific is marine algae, red alga is common along with green alga and also eelgrasses. This has been observed off the Bahia de los Angeles. In this area they have also been noted to consume many forms of animal matter as well. This may be incidental or intentional in origin, results are unclear. If incidental it may be a repercussion of consuming marine algae near the invertebrates or if intentional it may be to provide important nutrients, minerals, or proteins not obtained from vegetation.
Behaviours of hatchlings differ form that of adults. After a hatchling emerges from the sand it immediately guides itself out to sea. Hatchlings use the light of the night sky to find their way into the ocean. However in many areas with coastal development the hatchlings become confused from the false light and are sometimes guided in the wrong direction to their death, this is not as much of a problem for Galápagos green turtles due to lack of development in the islands. It is important that the hatchlings emerge at night to be protected from predators. They quickly move out to sea and swim for a period of up to 24 hours, this way they are far removed from shore and the predators in that area. They spend many years in the open ocean before entering the coastal habitat for the maturation period as juveniles. This period of maturation last from 10 to 20 years and for populations of black sea turtle is usually spent along the coasts of the Baja California peninsula and the connecting bays, lagoons, and channels that provide excellent forage and protection.
Economic significance to humans
Now that the Galápagos green turtle is an endangered species it has little positive economic significance for humans. They have in the past been harvested from egg to adult for various human uses. The eggs and meat from all life stages was utilized in many areas as a food source for humans and the meat is still considered a delicacy in some countries of South America. They were also harvested for their hides and for the oils of their fat deposits used in cooking, however the Galápagos green turtle has less body fat than other populations so not as much oil was yielded from their bodies. Now the green turtles have more of a negative impact on human economics due to their status as endangered. They are often involved as bycatch in many ocean fisheries and are protected through many acts of legislation.
As previously noted, the Galápagos green turtle, grouped with all populations of Chelonia mydas, is listed on the IUCN Red List of threatened species. They were placed on the list in the mid 1980s and have remained under protection since. One of the major problems that have led to their decline is the slow growth rate and long period from juvenile to sexual maturity. Chelonians average a period of 26–40 years to maturity; the Galápagos green turtle averages 33 years. The green sea turtle has been shown to have the slowest growth rate of any species of sea turtle and is generally long-lived if left undisturbed. Over all populations of Chelonia mydas have seen a decrease of approximately 48–67% of nesting females, however the population that nests in the Galápagos has remained quite steady with little to no change. The population of nesting females recorded in the Galápagos from 1976–1982 was approximately 1400 individuals and after further evaluation in 1999–2001 they again found around 1400 nesting females.
Reductions in populations of Chelonia mydas are recorded in every ocean ecosystem that they inhabit and the contributing factors that have led to their decline are all anthropogenic. The main threats are harvesting of eggs and individuals, bycatch in marine fisheries, and degradation of the marine and coastal habitats. The most common accidental threat is the entanglement in fishing equipment with the most harmful methods being drift netting, shrimp trawling, long-lining, and dynamite fishing. There are now many pieces of legislation that ban these practices in protected areas.
For Chelonians inhabiting the eastern Pacific, including the Galápagos green turtle, the threats are the same. The main factor leading to decline in this area was the intense and unregulated fishing operations run off the coast of Mexico between 1950 and 1970. In many countries in Central and South America the meat of sea turtles is considered a delicacy and they are to this day poached and hunted directly. The mortality of the sea turtles near their foraging area of the Baja California peninsula is approximately 7800 deaths yearly and this has led many organizations to rally for governmental protection of this area for the species. It was noted that approximately 9 out of 100 individuals will survive a year in the waters of the Baja California peninsula and as previously stated this in an intense foraging ground for all population in the eastern Pacific. Even with the passing of new protective legislation and the many efforts of conservation organizations, populations are continuing to decline, and considering the long generation length of 42.8 years any hit on the population is difficult to recover from.
One of the many conservation efforts being implemented to increase populations at this time is the use of hatcheries for protected egg incubation. The goal of this type of project is to create a protected environment for the hatchlings where they can incubate and emerge from the sand without threat of predators and then be released out to sea safely. Unfortunately this is not the result that has been documented from the practice. Often when hatchlings emerge from the sand in the incubator they remain enclosed inside and expend much of their preserved energy trying to find a way out. They also lose the extremely important dark, night hours used to get far away from shore. When they are released out to sea it is often in the early morning hours of dusk when many predators are about and this limits their chance of survival. In addition to the threat of being released during dusk the hatchlings are also often too weak to gain a swift and long distance movement from shore due to the expended energy lost in the incubator after hatching. These results have led many to believe that hatcheries are ineffective and should only be used as a last resort. There is no extensive research showing the survival rate of the hatchlings after they are released to sea but one study proclaimed a possible 50% loss after release.
|Wikimedia Commons has media related to Chelonia mydas agassizi.|
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