sea turtles They have been traversing the oceans for millions of years, and yet they continue to surprise us today with their biology, their impossible migrations, and their delicate role in the balance of ecosystems. Alongside them, the one considered the largest fish in the worldThe whale shark shares the spotlight as a peaceful giant of tropical and subtropical seas. Together, they represent two fascinating stories of adaptation, fragility, and wonder in marine life.
In this article we will review in detail features, curiosities and threats that affect sea turtles, as well as key facts about the largest fish on the planet. We'll talk about their diet, their habitat, how climate and human activity influence their survival, and why their conservation is much more important than it might first appear.
World Sea Turtle Day and its ecological importance
Each year, the 16 JuneJust a few days after World Oceans Day (June 8), World Turtle Day is commemorated, a date intended to remind us that these marine reptiles are among the oldest and most threatened species on the planet. It is not merely a symbolic event: it serves to focus attention on the serious conservation problems they face and the need to protect their coastal and marine habitats.
Sea turtles belong to two distinct families: Cheloniidae and DermochelyidaeThe first group includes most species with hard shells, while the second is represented by the iconic leatherback turtle, whose shell is more of a flexible carapace covered in skin. This morphological diversity translates into very different adaptations for swimming, feeding, and moving through the oceans.
From an ecological point of view, turtles are essential for the ocean healthWhen grazing on seagrass Seaweed beds and coral reefs help maintain these ecosystems in balance. Their activity indirectly benefits economically important species, such as tuna, lobster, and shrimp, which depend on these well-preserved habitats to thrive.
The disappearance of sea turtles would cause a notable imbalance in the food chains of many seas. Fewer herbivorous turtles would mean more uncontrolled algae blooms, which can smother seagrass meadows and reefs. This, in turn, impacts commercially important fish and the entire fishing economy. Therefore, talking about sea turtles is also talking about food security and human well-being in numerous coastal communities.
They are also considered one of the oldest reptile lineages They are still alive, with origins dating back tens of millions of years. They have survived brutal climate changes and continental drift, but now they face a new challenge: direct and indirect human pressure on their nesting beaches and feeding grounds.
Species of sea turtles and their dimensions
Currently, the following are recognized seven species of sea turtleseach with particularities in size, diet, distribution and conservation status. Ordered from largest to smallest, they are: the leatherback turtle (Dermochelys coriacea), The loggerhead (Caretta caretta), the green turtle (Chelonia mydas), the eastern right turtle (Natator depressus), the hawksbill turtle (Eretmochelys imbricata), the bastard turtle or parrot turtle (Lepidochelys kempii) and the olive ridley turtle (Lepidochelys olivacea).
La leatherback sea turtle It is the true giant of the group. It can exceed 1,80 meters in length and, in the most extreme cases, approach 900 kilos in weight. Its shell is not rigid like that of the other species, but rather a bony structure covered with thick skin with longitudinal ridges, which makes it very hydrodynamic for crossing entire oceans.
At the opposite extreme, the olive tortoiseThe smallest of the marine mammals, it rarely exceeds 70 centimeters in length. Even so, it is a robust animal, perfectly adapted to ocean life, with powerful fins capable of enduring long journeys between feeding grounds and spawning beaches.
If we consider the entire group, most sea turtles fall within a range of 60 centimeters to 1,8 meters They are up to 10 meters long, with a highly variable weight ranging from tens of kilograms to several hundred. This size, combined with a shell that is an integral part of their skeleton, provides them with protection against many predators, although it is not an infallible barrier against large sharks, orcas, or human activity.
The shell deserves a separate mention: it is not an added “armor”, but a integrated bone structure The spine and ribs are covered with horny plates (in the Cheloniidae family). This anatomical feature conditions their entire biology: their muscles, their swimming style, and even the way they breathe when they surface.
In Spanish waters, the most frequently appearing species are the loggerhead turtle and green turtleOf these, only the bobwhite has occasionally nested on beaches of the peninsula and islands, which makes these events exceptional and of great scientific and conservation interest.
Habitat, distribution and migrations of sea turtles
Sea turtles are distributed throughout tropical, subtropical and temperate oceans They are cosmopolitan animals, but each species has its own preferences for temperature, coastline, and depth. Five of the seven species have wide ranges as long as they find warm or moderately warm waters; the other two, the bastard and the eastern right whale, have more restricted areas.
La bastard turtle It appears predominantly in the Gulf of Mexico and nearby areas, while the eastern right turtle It is concentrated mainly in northern Australia and southern Papua New Guinea. The other species, to a greater or lesser extent, can cross entire oceans, taking advantage of currents and productive feeding areas.
Throughout the year, and depending on their reproductive cycleTurtles change their environment: they move between feeding areas (often on the continental shelf) and nesting beaches, which are usually sandy and well-preserved. This means that, depending on the time of year, they can be found closer to the coast or further out to sea, and even move towards somewhat cooler waters if food conditions are favorable.
Brianda hawksbill turtlesIts distribution extends across the Atlantic, Pacific, and Indian Oceans, with a particularly notable presence in the Caribbean Sea and other tropical and subtropical regions. Individuals have been recorded in more than 100 countries, although documented nesting is limited to about 70. They prefer coral reefs, rocky areas, and shallow coastal zones with sponge-rich bottoms and relatively easy access to sandy beaches where they can spawn.
It is not common to see hawksbill turtles over 65 depth meterssince their lives revolve primarily around reefs and shallow platforms. In the western Atlantic, the hawksbill turtle population extends from southeastern Florida to southern Brazil, passing through the Bahamas, the Antilles, and, notably, the Mexican Caribbean. On the Yucatán Peninsula, especially in Quintana Roo, some of the main nesting sites throughout the northern hemisphere, thanks to the combination of good beaches and very productive reefs.
Reproduction, nests and survival of offspring
One of the most striking aspects of the biology of sea turtles is their reproductive strategyMales spend their entire lives at sea and never set foot on land again after leaving the beach where they were born. Females, on the other hand, only leave the water to dig their nests and lay their eggs in the sand during the breeding season.
Many females show impressive loyalty to their place of origin: after years of oceanic life, they return to the same beach where they were bornThis behavior, known as philopatry, relies on a highly sophisticated orientation system in which the Earth's magnetic field plays a key role. Turtles are able to use the planet's geomagnetic variations as an internal map that tells them where to head.
Nests usually contain around 150 eggs Although the exact number can vary depending on the species and the size of the female, a turtle may lay several clutches in a single season to compensate for the extremely high mortality rate of hatchlings. This enormous egg production is an evolutionary gamble: many hatchlings are produced knowing that only a tiny fraction will survive to adulthood.
The temperature of the sand is a critical factor, as it determines the sex of the baby turtlesGenerally speaking, warmer temperatures favor the production of females, while cooler temperatures result in a higher proportion of males. With current global warming, this is becoming a serious problem, because many beaches are producing almost exclusively females, which compromises the future balance of the populations.
The most dramatic phase of the life cycle occurs in the first few minutes of life. After hatching, the hatchlings emerge at night and rush en masse towards the sea, guided by the glow of the horizon. During this brief journey, they are extremely vulnerable: only one in a thousand It eventually reaches adulthood. Crabs, birds, foxes, or other predators take advantage of this explosion of small turtles to feed, and any alteration in the environment (artificial lights, pets, vehicles on the sand) can disorient them or further increase mortality.
Feeding and adaptation to marine life
The diet of sea turtles is very varied and changes according to the species and stage of life. Some are predominantly herbivorousSome, like the adult green turtle, feed mainly on seagrass and algae, helping to keep the seagrass meadows healthy. Others are more carnivorous or omnivorous, like the hawksbill turtle, which consumes a curious combination of sponges and small invertebrates.
La hawksbill turtle It is the only known reptile that specializes in eating sea sponges (spongivore). Many sponges contain toxic chemical compounds, which accumulate in the turtle's tissues. This makes its meat potentially dangerous for humans, causing severe poisoning in those who consume it. In addition to sponges, it also eats mollusks, algae, crustaceans, sea urchins, fish, and jellyfish, so it is considered omnivorous.
Baby hawksbill turtles, and other species, usually spend their first years in high seasThey hide among masses of floating algae (such as sargassum). There they find refuge and food in the form of small crustaceans, bryozoans, tunicates, and annelids that live in association with these algae. As they grow, they move to more coastal environments, where their diet diversifies or specializes, depending on the species.
Other sea turtles feed on anemones, jellyfish, mollusks, crabs, and various crustaceans. Their fondness for jellyfish has unintentionally become a Mortal trapMany fish mistake plastic bags or other floating debris for their usual prey. Ingesting these plastics can cause intestinal blockages, perforations, or false feelings of fullness, which can lead to death.
Turtles have also developed adaptations to be able to drink salt water without dehydrating. They have specialized glands near their eyes that expel excess salt, which gives the impression of "crying" when they surface to breathe. Furthermore, their physiology allows them to remain submerged for long periods, slowing their metabolism and using oxygen very efficiently.
Long-distance migration and orientation
Sea turtles are tireless travelers. Some populations complete migrations of thousands of kilometers between their feeding grounds and nesting beaches, repeating every few years routes that their ancestors traveled long before maps or satellites existed.
The hawksbill turtle, for example, can travel more than 2.000 kilometers between widely separated habitats in the Pacific, as has been documented between Australia and the Solomon Islands. In the Atlantic, journeys of up to 3.000 kilometers have been recorded between feeding areas in Mexico and nesting beaches in the Dominican Republic, which gives an idea of their physical endurance and spatial memory.
This migratory capacity relies on various orientation mechanisms, but one of the most surprising is its use of terrestrial magnetic fieldTurtles detect slight variations in the intensity and direction of the magnetic field and use them as a kind of internal navigation system. Combined with visual, chemical, and ocean current signals, this allows them to locate specific beaches decades after leaving them as hatchlings.
During their migrations, turtles cross areas of intensive fishing, areas with heavy maritime traffic, or regions affected by pollution. Each leg of the journey involves facing added threatsFrom collisions with vessels to the possibility of becoming trapped in nets or drift fishing gear, these migrations are becoming increasingly risky.
Generally speaking, they spend most of their lives in continental shelf waterswhere productivity is high and they find food relatively easily. However, some species, such as the leatherback, venture more frequently into the open sea and deep ocean zones in search of jellyfish swarms or other gelatinous prey.
Threats: plastic, fishing, illegal trade, and climate change
Despite having survived major extinctions and environmental changes throughout geological history, sea turtles today face a set of human-caused threats that seriously compromise their survival. long-term survivalSix of the seven species are included on the IUCN Red List (International Union for Conservation of Nature) at varying levels of threat.
In terms of conservation categories, leatherback, loggerhead, and olive ridley turtles are considered "Vulnerables"It is estimated that there are approximately 60.000 breeding female loggerhead turtles and around 800.000 female olive ridley turtles, although these figures vary depending on the source and the study area. The green turtle is listed as an "Endangered" species, with an estimated 203.000 individuals worldwide.
The most critical situations correspond to the hawksbill turtle and the Kemp's ridley turtle, classified as “Critically Endangered”Estimates suggest there are around 8.000 hawksbill turtles and only about 1.000 Kemp's ridley turtles. Data is not as comprehensive for the eastern right turtle, but it is estimated that there may be around 10.000 individuals remaining, which is also not a reassuring figure.
Among the main threats is the marine plasticTurtles ingest bags, fragments, and other debris, mistaking them for jellyfish or other organisms. This not only causes digestive blockages but can also lead to entanglement, hindering their swimming and breathing. Furthermore, plastic carries chemical pollutants that are absorbed into the turtles' bodies and accumulate in their tissues.
La bycatch This is another major problem. Turtles need to surface periodically to breathe, but if they become entangled in nets, longlines, or trawls, they can drown. These catches are unintentional—fishermen don't typically target turtles—but they represent one of the biggest causes of mortality. Introducing turtle excluder devices and other responsible fishing practices can significantly reduce this impact.
El illegal trade And direct consumption hasn't disappeared entirely either. In some places, turtles are still hunted for their shells (especially the hawksbill, used in crafts), their skin, their eggs, or their meat. For years, turtle soup was considered a delicacy, and although it's now banned in many countries, the black market persists and worsens the situation for the most vulnerable species.
El climate change This adds a layer of cross-cutting threat. Rising sea levels and the increased frequency and intensity of storms damage the beaches where turtles nest, eroding the sand and destroying nests. Furthermore, the rising temperature of the sand in nesting areas alters the sex ratio, favoring the birth of females and reducing the number of males, which could completely disrupt populations within a few generations.
Added to all this is the destruction of coastal habitat Uncontrolled urbanization and tourism: construction of infrastructure on the beachfront, artificial lighting that disorients hatchlings, use of vehicles on the sand, etc. Diseases are also observed in sea turtle populations, some of which could be related to pollution, the presence of introduced predators, or the increase in invasive species that disrupt the delicate balance between predators and prey.
The largest fish in the world: the whale shark
When we talk about largest fish in the worldalmost all roads lead to the whale shark (Rhincodon typusDespite its name and colossal size, it is not a whale, but a filter-feeding shark, the largest known fish currently inhabiting the oceans. It can easily exceed 10 meters in length, and individuals approaching or even exceeding 15 meters have been documented.
Interestingly, their diet is based on tiny organismsThe whale shark opens its enormous mouth as it swims slowly, filtering large volumes of water in search of plankton, fish eggs, krill, and small fish. This diet makes it a peaceful giant that rarely poses a danger to humans, beyond the risk of accidental collision if one swims too close without caution.
The whale shark's habitat is linked to warm waters These sharks inhabit tropical and subtropical regions, many of which they share with various species of sea turtles. They are frequently seen in areas where plankton is abundant, such as upwelling zones or areas where nutrient-rich ocean currents converge. In some parts of the world, their appearances are so predictable that they have become a major tourist attraction.
Like turtles, whale sharks face significant threats: targeted fishing in some countries, collisions with ships, bycatch in nets, and degradation of their feeding grounds. While they don't share all the specific problems of turtles (such as dependence on nesting beaches), their decline reflects the same issues. global pressure on marine ecosystems and the need to better manage our activities at sea.
On a symbolic level, the whale shark and sea turtles embody the idea of “vulnerable giants” of the ocean: enormous and seemingly powerful animals, but very sensitive to changes in water quality, the presence of pollutants, underwater noise or overfishing in their key habitats.
Conservation, protected beaches and human responsibility
The key to ensuring the future of sea turtles and the world's largest fish lies in a combination of local conservation measures and international agreementsIn the case of turtles, protecting nesting beaches is essential: keeping the sand clean, limiting vehicle access, controlling artificial lighting, avoiding buildings too close together, and minimizing tourist pressure during nesting season.
Nest monitoring programs, relocation of at-risk clutches, and visitor education are tools that have proven effective in numerous regions, such as the turtle release in ValenciaSea turtles can lay more than 150 eggs per nest and several times per season, but that potential only translates into adults if the environment is respected. balance between predators and prey and the added human impact that tips the balance against their survival is minimized.
In the fisheries sector, the adoption of more selective techniques, the use of turtle excluder devices in trawls, and the restriction of certain fishing gear in migratory passage areas contribute to reducing the accidental catchesAt the legislative level, the prohibition of the trade in shells, meat and eggs, along with effective monitoring, helps to curb the illegal market that does so much harm to the most vulnerable populations.
The fight against plastic in the oceans Addressing pollution from other sources is equally essential. Reducing the use of single-use plastics, improving waste management, and promoting coastal clean-up campaigns not only benefits turtles but all marine life. Similarly, well-managed marine protected areas provide refuges where turtles, whale sharks, and other species can feed, reproduce, and migrate with less pressure.
Looking to the future, the conservation of sea turtles and whale sharks is not just about saving charismatic species. It involves preserving the functioning of the ocean ecosystems upon which millions of people depend. Every turtle that reaches adulthood and every giant filter feeder that continues to swim in tropical waters are, ultimately, indicators that we are doing something right. Although the situation is complex, the combination of science, legislation, education, and changes in our consumption habits can make all the difference for these species that have been on the planet for millions of years and now depend, to a large extent, on our decisions.
