Amphipods are small crustaceans, a diverse group of over 10,700 recognized species. These animals typically have a laterally compressed body and lack a hard outer shell, distinguishing them from crabs or shrimp. They are found across the globe, inhabiting nearly every aquatic environment, and even some terrestrial ones.
Ocean Habitats
Marine environments host the largest number of amphipod species, from shallow coastal areas to the deepest ocean trenches. In intertidal zones, species like sandhoppers, which belong to the family Talitridae, commonly inhabit beaches and are often found under seaweed or within rock pools. These areas provide shelter and a source of detritus as food.
In shallow coastal waters, amphipods thrive in diverse habitats such as vibrant seagrass beds, dense kelp forests, and soft sandy bottoms. Many species are epibenthic, meaning they live on or near the seafloor. Some amphipods also engage in symbiotic relationships; for instance, whale lice are parasitic amphipods that live on whales.
The deep sea, including the abyssal plains and even the Challenger Deep, harbors a unique array of amphipods, some of which can grow to considerable sizes. Deep benthic species are particularly prevalent in the Arctic and Norwegian Seas. These creatures adapt to extreme pressures and cold temperatures, playing a role in nutrient cycling in these remote environments.
Freshwater Environments
Freshwater bodies worldwide support a substantial number of amphipod species, with approximately 2,250 species identified. These crustaceans inhabit various freshwater systems, including lakes, rivers, and streams, often found beneath rocks or within aquatic vegetation where they feed on organic debris. Notable concentrations of diverse freshwater amphipods are found in ancient lakes like Lake Baikal, which hosts over 350 unique species, and in the Caspian Sea basin.
Beyond surface waters, many amphipods have adapted to subterranean environments, with about 750 species living in caves and groundwater systems. These subterranean amphipods are particularly diverse in regions such as the Mediterranean Basin, southeastern North America, and the Caribbean. Approximately 45% of all freshwater amphipod species are subterranean, thriving in isolated underground habitats.
Terrestrial Dwellings
While less common than their aquatic counterparts, some amphipod species have successfully colonized terrestrial environments. These land-dwelling amphipods, often referred to as “landhoppers” or “sandhoppers,” prefer damp, humid conditions away from direct sunlight. They typically reside in leaf litter, under decaying logs, within soil, or in garden compost piles.
Specific examples include species like Talitrus saltator and Arcitalitrus sylvaticus. These terrestrial amphipods are globally distributed, with significant populations found in areas that were historically part of the ancient supercontinent Gondwana. They have also expanded their range into parts of Europe and North America.
Why Amphipods Live Where They Do
The distribution of amphipods across marine, freshwater, and terrestrial habitats is influenced by several environmental factors. Salinity is a primary determinant, as amphipods exhibit a wide tolerance, living in everything from freshwater to water with double the salinity of typical seawater. Temperature also plays a significant role, with most amphipod species preferring cooler and temperate climates, making them less common in tropical regions.
Food availability is another factor; most amphipods are detritivores or scavengers, relying on decaying organic matter. Some species also graze on algae, while others are omnivores or predators of smaller organisms. The type of substrate, whether sandy, muddy, rocky, or vegetated, dictates where certain species can establish themselves, with many being epibenthic or subterranean.
Oxygen levels in the water or moisture content on land are also influential. Areas with decreasing dissolved oxygen, often linked to rising temperatures, tend to support less diverse amphipod communities. Their reproductive strategy, involving brooding young without a free-swimming larval stage, limits dispersal. This contributes to unique distribution patterns and speciation across geographical barriers.