Sea animals that can survive on land challenge conventional understanding of marine life. These creatures periodically or routinely leave water for terrestrial environments. Their ability to transition involves specialized features, allowing them to endure conditions different from the ocean. Exploring these capabilities reveals diverse evolutionary paths.
Adaptations for Terrestrial Survival
Survival on land for marine animals require significant modifications. Respiratory systems adapt from water-based gas exchange to air breathing. Most marine mammals, like seals and sea lions, have lungs specialized for efficient oxygen uptake and management during dives, allowing rapid air exchange at the surface.
Some fish, like mudskippers, supplement gill respiration with cutaneous breathing, absorbing oxygen through their moist skin. Sea snakes also use cutaneous respiration, with some species absorbing up to 33% of their oxygen through their skin for extended underwater periods. Land locomotion requires specialized structures, as aquatic fins or tails are often ineffective. Pinnipeds, including seals and sea lions, use powerful flippers for land movement; seals “galumph,” while sea lions walk more coordinately. Mudskippers use pectoral fins to “walk” or crawl, and some marine invertebrates, like crabs, have sturdy exoskeletons and jointed legs for terrestrial mobility.
Preventing desiccation is another challenge. Many maintain skin moisture through mucus secretions or by staying near damp environments. Mudskippers must keep their skin wet for breathing and often reside in muddy burrows for moisture. Some species, like sea turtles, have thick, scaly skin that reduces water loss.
Diverse Examples of Land-Venturing Marine Animals
Many marine animals have evolved the ability to spend time on land, showcasing diverse forms and behaviors. Pinnipeds, including seals, sea lions, and walruses, are prominent examples. Seals often haul out on beaches and ice floes to rest, give birth, and molt, using strong flippers to propel themselves. Sea lions are more agile on land than seals, rotating hind flippers forward to walk with greater ease.
Reptiles like sea turtles and some sea snakes also make land excursions. Female sea turtles return to sandy beaches to lay eggs, essential for reproduction. Sea kraits, a type of venomous sea snake, are semi-aquatic with wide ventral scales, enabling effective land movement, often coming ashore to digest food or lay eggs. The marine iguana of the Galapagos Islands, primarily a land animal, forages for algae underwater, showcasing a reversal of typical land-to-sea adaptation.
Several fish species defy the perception of fish as exclusively aquatic. Mudskippers are well-known, spending much of their lives out of water in mangrove swamps and tidal flats, using strong pectoral fins to navigate muddy terrain and climb roots. Walking catfish can move across land, typically to find new water sources if theirs dries up. Eels are another example, known to traverse damp ground to migrate between aquatic habitats.
Invertebrates also contribute to this group. Various crab species are adept at living both in and out of water, with some intertidal crabs spending most time on rocky shores or sandy beaches. These crustaceans have hard exoskeletons that reduce water loss and strong legs for terrestrial movement. Octopuses have also been observed briefly leaving water to hunt prey in tide pools or move between pools.
Motivations for Terrestrial Excursions
Marine animals venture onto land for various reasons, driven by biological needs and environmental pressures. Reproduction is a primary motivation for many species. Female sea turtles must return to sandy beaches to dig nests and lay eggs, as eggs require a terrestrial environment for incubation. Similarly, pinnipeds gather in large colonies on beaches or rocky outcrops during breeding seasons to mate and give birth.
Foraging for food also draws marine animals onto land. Some crabs, particularly those in intertidal zones, scavenge for organic matter or hunt small invertebrates exposed by the receding tide. Octopuses have been observed briefly emerging from tide pools to snatch crabs. These excursions are often opportunistic, taking advantage of food sources unavailable in water.
Escaping aquatic predators is another driver for terrestrial movements. Seals and sea lions, vulnerable to predators like orcas and large sharks, frequently haul out onto land to rest and find refuge. This behavior provides a temporary sanctuary from threats in their primary habitat.
Thermoregulation, maintaining an optimal body temperature, prompts some species to bask on land. Marine iguanas, after diving into cold ocean waters to feed, often warm themselves by sunning on volcanic rocks. Resting is also a common motivation, with many marine animals seeking stable, predator-free land to conserve energy.
Risks and Environmental Challenges on Land
While terrestrial excursions are necessary for certain behaviors, they also expose marine animals to significant risks and challenges. One immediate danger is desiccation, the loss of body water to dry air. Marine animals are adapted to a watery environment; prolonged air exposure can lead to severe dehydration and physiological stress if they cannot maintain internal moisture.
Vulnerability to land predators increases when these animals are out of their aquatic element. Their specialized aquatic locomotion is often cumbersome on land, making them slower and less agile, thus easier targets for land carnivores. Young or injured individuals are particularly susceptible. Temperature fluctuations on land also pose a challenge, as air temperature can vary more widely than the stable ocean temperature. Animals must regulate body temperature by seeking shade, basking in the sun, or returning to water.
The physical strain of gravity is another challenge. Large marine animals, evolved for buoyancy in water, can experience crushing pressure on internal organs and skeletal structures when unsupported by water. This is why large whales, if stranded, quickly suffer severe internal damage. These challenges significantly influence the duration and frequency of terrestrial excursions, limiting them to periods necessary for specific activities like breeding or resting, often near the water’s edge.