Amphipods are a highly diverse group of small crustaceans that populate virtually every aquatic environment on Earth. These invertebrates, often nicknamed “scuds” or “sideswimmers,” are recognizable by their shrimp-like appearance and their tendency to swim on their sides. With over 10,700 species currently recognized, they represent a significant portion of the planet’s invertebrate biodiversity. Amphipods are globally distributed, occupying a wide range of ecological niches and sustaining numerous ecosystems worldwide.
Classification and Habitat Diversity
Amphipods are classified within the Phylum Arthropoda, Subphylum Crustacea, and belong to the Order Amphipoda. They are a major group within the Class Malacostraca, which also includes familiar organisms such as crabs, lobsters, and true shrimp. The species within this order are grouped into six suborders, reflecting their immense biological variation.
The majority of amphipod species are found in marine environments, inhabiting areas from the intertidal zone down to the deepest oceanic trenches. Their presence is nearly ubiquitous in the oceans, where they thrive in habitats ranging from rocky coasts to soft sediments.
While marine species dominate, roughly 2,000 to 2,250 species live in freshwater systems, representing about twenty percent of their total diversity. These freshwater forms are numerous in cool and temperate regions, often found in rivers, lakes, and subterranean waters. Ancient bodies of water, such as Lake Baikal, host remarkable endemism, including over 290 unique amphipod species.
A small number of amphipods have successfully colonized terrestrial or semi-terrestrial habitats, known commonly as landhoppers or sandhoppers. These species live in moist environments like leaf litter or along shorelines. Their ability to adapt to such varied conditions underscores the order’s evolutionary flexibility and global reach.
Distinctive Physical Traits
The defining physical trait of the Order Amphipoda is their laterally compressed body shape, meaning they are flattened from side to side. This compression distinguishes them from other crustaceans, such as isopods, which are typically flattened from top to bottom. Their body is divided into three main sections: the head, the thorax (pereon), and the abdomen.
Amphipods are characterized by the absence of a carapace. The head region is fused to the first segment of the thorax and bears two pairs of antennae used for sensing the environment. Their eyes are sessile, meaning they are fixed directly onto the head rather than mounted on stalks.
The thoracic and abdominal regions feature numerous, specialized appendages, giving rise to the name Amphipoda, which translates to “different feet.” The first two pairs of thoracic appendages are modified into grasping limbs called gnathopods. These gnathopods are used for manipulating food items or for reproductive behaviors like clasping a mate.
The remaining five pairs of thoracic limbs are called pereopods, which function as walking legs. These pereopods are often arranged in two functional groups, aiding in locomotion. The abdomen bears three pairs of swimming appendages called pleopods and three pairs of tail appendages called uropods, which assist in various movements.
Roles in Ecosystems
Amphipods perform foundational functions that support the health and structure of aquatic and semi-terrestrial ecosystems. They are primarily known as detritivores and scavengers, playing a significant role in breaking down organic debris. They consume decaying plant material and scavenge carrion, converting this dead material into biomass.
This shredding activity is important for nutrient cycling, as it facilitates the breakdown of large organic particles into smaller pieces available to microorganisms and other invertebrates. Some species also act as grazers, consuming micro-algae and other epiphytes. Their diverse feeding habits ensure that energy is efficiently transferred throughout the food web.
The abundance of amphipods makes them a major food source, forming a significant prey base for organisms at higher trophic levels. They are a standard component of the diet for various predators, including fish, shorebirds, penguins, and small cetaceans. In coastal regions, their population densities can reach high concentrations, providing a reliable source of nutrition.
Amphipods also serve a practical function as bioindicators, providing measurable insight into the environmental quality of their habitat. Because they are sensitive to pollutants and other stressors, their presence, absence, or overall health can signal changes in water quality. The effects of parasites, which frequently use amphipods as hosts, can also be monitored to understand broader impacts on the ecosystem’s food web.