Amphipods are small, sideways-compressed crustaceans belonging to the order Amphipoda, often called scuds, sideswimmers, or sand fleas. These tiny, shrimp-like organisms inhabit environments from the deepest marine trenches to freshwater lakes and moist terrestrial soils. With over 10,000 described species, amphipods are one of the most diverse groups of invertebrates, colonizing nearly every aquatic and some terrestrial habitats globally. This massive ecological diversity is reflected in their varied and opportunistic dietary habits, which are fundamental to their widespread role in various ecosystems.
Feeding on Dead Organic Matter
The majority of amphipod species function primarily as scavengers and detritivores, consuming non-living organic material. This detritus includes decaying plant and animal tissues, shed exoskeletons, and fecal matter. This feeding strategy is widespread, especially among benthic species living on the seafloor or lake bottom. This consumption aids in nutrient recycling, converting waste and debris into accessible forms for other organisms.
Amphipods are often described as nature’s recyclers because they shred and process coarse organic material, such as fallen leaves or dead seaweed. This shredding activity accelerates the breakdown of large particles, making the material available for microbial decomposition in aquatic environments. Examples of these common detrital feeders include terrestrial sand hoppers scavenging along shorelines and many freshwater scud species. Some deep-sea amphipods are specialized carrion-feeders, rapidly consuming the remains of larger dead animals that sink from the surface.
Amphipods as Primary Consumers
While many amphipods consume dead material, a significant number of species act as herbivores or primary consumers, feeding directly on living plant matter and algae. This dietary role is prominent in shallow marine and freshwater environments where vegetation is abundant. Herbivorous amphipods often graze on microalgae and diatoms that grow on the surfaces of rocks, sediment particles, or larger aquatic plants.
Some species are specialized mesograzers, consuming the epiphytes—tiny organisms and algae—that grow on the leaves of macroalgae or aquatic flowering plants like seagrass. This grazing activity influences the health and structure of underwater meadows and kelp forests by controlling the growth of competing algae. For instance, certain amphipods graze on fast-growing erect algae, which can indirectly benefit the seagrass by relieving it from competition for light and space.
Active Hunting and Specialized Feeding Methods
Beyond scavenging and grazing, amphipods display a wide array of specialized feeding methods, including active predation and suspension feeding. Certain species, often categorized as omnivores, actively hunt smaller invertebrates, such as meiofauna or insect larvae. Prey consumption sometimes makes up over half of their diet. The invasive species Dikerogammarus villosus, for example, is an aggressive omnivore that preys on many smaller invertebrates, including other native amphipod species.
Filter-feeding, or suspension feeding, is a specialized method where amphipods use modified appendages to capture tiny food particles, such as phytoplankton, suspended in the water column. This feeding is common in species that live in burrows or tubes, allowing them to draw in water and filter out their food. Hyperiid amphipods, often found in the open ocean, may live in a parasitic or commensal relationship with gelatinous organisms like comb jellies or salps, consuming the host’s tissue or stealing food.
The Ecological Impact of Amphipod Diets
The diverse diets of amphipods make them indispensable components of both aquatic and terrestrial food webs. Their feeding strategies, encompassing detritus, algae, and live prey, establish them as a fundamental trophic link, transferring energy from lower to higher levels. By consuming primary producers and decaying matter, amphipods become a substantial food source for a wide variety of larger animals, including fish, shorebirds, and marine mammals.
Amphipod feeding also has a significant physical impact on their environment, particularly in soft-sediment habitats. As they burrow and move through the substrate, they engage in bioturbation, which mixes and aerates the seafloor. This activity is important for biogeochemical cycling, influencing the distribution of organic matter and oxygen within the sediment.