Gammarus are small, widespread crustaceans found in various aquatic environments. Though often unnoticed due to their size, they play a substantial role in the health and functioning of freshwater and brackish ecosystems. Studying Gammarus reveals their intricate connections within aquatic food webs and their importance to habitat balance.
What Are Gammarus?
Gammarus are classified as amphipods, a group of crustaceans characterized by their laterally compressed bodies. They are often described as resembling tiny shrimp, ranging from 4 to 22 millimeters in length. These crustaceans usually exhibit colors such as brown or greenish-brown, sometimes with light banding, but can also appear white or clear depending on their diet, water temperature, or age.
The curved body of Gammarus is a defining feature, allowing them to move efficiently through their environment. They possess two pairs of antennae, with the first pair longer than the second, and large, kidney-shaped sessile compound eyes. Their bodies are segmented, with each segment bearing limbs: pereiopods on the thorax and pleopods and uropods on the abdomen, which aid in swimming and scuttling.
Their Aquatic Homes
Gammarus species inhabit a diverse range of aquatic environments across the Holarctic region, extending into the Northern Hemisphere tropics, particularly Southeast Asia. Many species thrive in freshwater habitats such as clear, cold streams, rivers, lakes, and ponds, often preferring shallow, still areas with abundant vegetation or debris for cover. They tend to avoid direct light and may cluster near the bottom edges of backwaters in rivers.
While many Gammarus species are strictly freshwater, others are found in brackish or marine coastal waters, including estuaries. They prefer conditions with dissolved oxygen and a narrow range of cold temperatures, though some species can tolerate warmer temperatures.
Role in the Ecosystem
Gammarus species serve a dual function in aquatic ecosystems as both detritivores and grazers, contributing to nutrient cycling. As detritivores, they consume decaying organic matter, such as leaf litter, breaking it down into smaller particles. This shredding activity processes stream detritus, making nutrients available to other organisms. They also graze on algae and biofilms, playing a role in controlling algal growth.
Gammarus are a primary food source for a wide array of aquatic and semi-aquatic animals. Fish, birds, and various aquatic insects rely on Gammarus as a key part of their diet. Their abundance and widespread distribution make them a consistent and accessible food source, supporting the trophic levels above them. Moreover, some Gammarus species are used as bioindicators of water quality due to their sensitivity to pollution. Their presence or absence, or changes in their population, can signal the health of an aquatic environment.
Life Cycle and Reproduction
The reproductive process in Gammarus involves distinct mating behaviors and direct development of their offspring. Males are often larger than females. During mating, the male carries the female in a “precopula” embrace for a period influenced by temperature. This guarding behavior ensures the male fertilizes the eggs immediately after the female molts.
Following fertilization, the female carries the eggs in a specialized ventral brood pouch called a marsupium, formed by overlapping plates. The eggs develop directly within this pouch, bypassing a free-swimming larval stage, and hatch as miniature versions of the adults. Females can produce multiple broods during their lifespan, which typically ranges from 1 to 2 years, though some may live longer in cooler conditions. The number of offspring per brood can vary. The incubation period depends on water temperature, with development faster at higher temperatures.