Sea Turtles

Sea turtles, along with other turtles and tortoises, are part of the order Testudines. There are seven species of sea turtles where five of these species are found around our area at Guinjata Bay up to Tofo.

Green Sea Turtle seen on local reef

Green Sea Turtle seen on local reef

The Green sea turtle the Hawksbill sea turtle the Olive ridley sea turtle the Leatherback sea turtle and the loggerhead sea turtle. (Kemp’s ridley and Flatback sea turtles are not found in our region)

The species are primarily distinguished by their anatomy: for instance, the prefrontal scales on the head, the number of and shape of scutes on the carapace, and the type of inframarginal scutes on the plastron. The leatherback is the only sea turtle that does not have a hard shell; instead, it bears a mosaic of bony plates beneath its leathery skin.

It is the largest sea turtle, measuring up to 2 meters in length at maturity, and 1.5 meters in width, weighing up to 590 kg. Other species are smaller, being mostly 1.2 meters and proportionally narrower.

Sea turtles are generally found in the waters over continental shelves. After taking to the water for the first time, males will not return to shore again. During the first three to five years of life, sea turtles spend most time in the pelagic zone floating in seaweed beds.

Green sea turtles in particular are often found in sargassum beds, brown seaweed in which they find shelter and food, and mostly spotted at Turtle creek a reef just 800 meters from Guinjata Dive Center.

Once the sea turtle has reached adulthood it moves closer to the shore. Females will come ashore to lay their eggs on sandy beaches during the nesting season.

Sea turtles are almost always submerged, and, therefore, have developed an angerobic system of energy metabolism. Although all sea turtles breathe air, under dire circumstances they may divert to anaerobic metabolism for long periods of time. When surfacing to breathe, a sea turtle can quickly refill its lungs with a single explosive exhalation and rapid inhalation. Their large lungs have adapted to permit rapid exchange of oxygen and to avoid trapping gases during deep dives.

Loggerhead Sea Turtle taking a breath of air

Loggerhead Sea Turtle taking a breath of air

It takes decades for sea turtles to reach sexual maturity. After mating at sea, adult female sea turtles return to land to nest at night. Different species of sea turtles exhibit various levels of philoparty. In the extreme case, females return to the beach where they hatched. This can take place every two to four years in maturity. They make from one to eight nests per season. The mature nesting female hauls herself onto the beach, nearly always at night, and finds suitable sand on which to create a nest.

Using her hind flippers, she digs a circular hole 40 to 50 centimeters deep. After the hole is dug, the female then starts filling the nest with a clutch of soft-shelled eggs one by one until she has deposited around 50 to 200 eggs, depending on the species. Some species have been reported to lay 250 eggs, such as the hawksbill. After lying, she re-fills the nest with sand, re-sculpting and smoothing the surface until it is relatively undetectable visually. The whole process takes thirty to sixty minutes. She then returns to the ocean, leaving the eggs untended.

Turtle gender depends on sand temperature while the egg is incubating. The hatchling’s gender depends on the sand temperature. Lighter sands maintain higher temperatures, which decreases incubation time and results in more female hatchlings. It takes several decades for adult sea turtles to reach sexual maturity. The mature turtles migrate, sometimes for thousands of miles, to reach breeding sites. Male and female turtles mate in the water, and the males return to deep sea to feed.

Marine vertebrates maintain a balance of dissolved solutes and water in the body fluids by excreting excess salt ions. Like other marine reptiles, sea turtles rely on a specialized gland to rid the body of excess salt ions, because reptilian kidneys cannot produce urine with a higher ion concentration than sea water. All species of sea turtles have a lachrymal salt gland in the orbital cavity, capable of producing tears with a higher salt concentration than sea water.

Sea Turtles are believed to have a communal relationship with some barnacles, in which the barnacles benefit from growing on turtles without harming them. Barnacles are small, hard shelled crustaceans found attached to multiple different substrates below or just above the ocean. The adult barnacle is a sessile organism, however in its larval stage it is plank tonic and can move about the water column.

sea turtles mating in our crystal clear waters at Guinjata Mozambique

sea turtles mating in our crystal clear waters at Guinjata Mozambique

The larval stage chooses where to settle and ultimately the habitat for its full adult life, which is typically between 5 to 10 years. A favorite settlement for barnacle larvae is the shell or skin around the neck of sea turtles. The larvae glue themselves to the chosen spot, a thin layer of flesh is wrapped around them and a shell is secreted.

Many species of barnacles can settle on any substrate, however some species of barnacles have an obligatory communal relationship with specific animals, which makes finding a suitable location harder. Around 29 species of “turtle barnacles” have been recorded. However it is not solely on sea turtles that barnacles can be found; other organisms also serve as barnacle’s settlements. These organisms include mollusks, whales, decapod crustaceans, manatees and several other groups related to these species.

Sea turtle shells are an ideal habitat for adult barnacles for three reasons. Turtles tend to live long lives, around 50 years, so barnacles do not have to worry about host death. Secondly, barnacles are suspension feeders.

Sea turtles spend most of their lives swimming and following ocean currents and as water runs along the back of the turtle’s shell it passes over the barnacles, providing an almost constant water flow and influx of food particles. Lastly, the long distances and inter ocean travel these sea turtles swim throughout their lifetime, offers the perfect mechanism for dispersal of barnacle larvae. Allowing the barnacle species to distribute themselves throughout global waters is a high fitness advantage of this commensalism. The barnacles are not parasitic to their hosts but have been found to have negative effects on the turtles they choose to reside on.

These effects however seem to depend on the quantity of barnacles affixed to its back. The barnacles add extra weight to the sea turtle, potentially increasing the energy it needs for swimming and affecting its ability to capture prey.

Sea turtles play key roles in two ecosystem types that are critical to them as well as to human’s oceans and beaches/dunes. In the oceans, sea turtles, especially green sea turtles, are one of very few creatures (manatees are another) that eat the sea grass that grows on the sea floor. Sea grass needs to be constantly cut short to help it grow across the sea floor.

Sea turtles act as grazing animals that cut the grass short and help maintain the health of the sea grass beds. Sea grass beds provide breeding and developmental grounds for numerous species of fish, shellfish and crustaceans. Without sea grass beds, many marine species humans harvest would be lost, as would the lower levels of the food chain. The reactions could result in many more marine species eventually becoming endangered or extinct.

Beaches and dunes form a fragile ecosystem that depends on vegetation to protect against erosion. Eggs, hatched or unhatched, and hatchlings that fail to make it into the ocean are nutrient sources for dune vegetation. Sea turtles use beaches and the lower dunes to nest and lay their eggs. Dune vegetation is able to grow and become stronger with the presence of nutrients from sea turtle eggs, unhatched nests, eggs and trapped hatchlings.

As the dune vegetation grows stronger and healthier, the health of the entire beach/dune ecosystem becomes better. Stronger vegetation and root systems help to hold the sand in the dunes and help protect the beach from erosion.