Gammarus lacustris

G.O. Sars, 1863

lacustrine scud

Gammarus lacustris is a freshwater amphipod crustacean widely distributed across the Holarctic region, including northwestern Europe, Russia, and North America. This exhibits remarkable phenotypic plasticity in reproductive traits, with females at higher elevations producing fewer but larger compared to lowland . It serves as a key intermediate for numerous helminth and is an important food source for fish and other . The species is increasingly threatened by amphipods such as Gmelinoides fasciatus, which can outcompete it through superior predator avoidance .

Gammarus lacustris by (c) Matt Muir, some rights reserved (CC BY), uploaded by Matt Muir. Used under a CC-BY license.

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Pronunciation

How to pronounce Gammarus lacustris: //ˈɡæmərəs ləˈkʌstrɪs//

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Habitat

Gammarus lacustris inhabits freshwater lakes and slow-moving rivers across a broad range of environmental conditions. It has been documented in shallow and deep lakes, from low-altitude calcium-rich waters to high-altitude, cold, calcium-poor lakes. In Norway, it occurs at elevations from 832 to 1396 m above sea level, with occurrence increasing significantly with lake size. The shows to dilute water, having been recorded in lakes with calcium concentrations below 1.0 mg L⁻¹ and pH below 6.0, with a lower lethal threshold for calcium near 0.5 mg L⁻¹. It is more abundant in fishless lakes than in lakes with fish .

Distribution

Documented from northwestern Europe, Russia, and North America. Its precise range has yet to be fully defined. In Norway, it shows a marked distribution pattern on the Hardangervidda mountain plateau, being present in 79% of lakes in central/eastern areas but nearly absent from western areas, likely due to colder water temperatures and lower ionic strength. The follows the thermocline in water bodies.

Seasonality

is triggered by decreased day length and light intensity. Laboratory studies demonstrate that reproduction occurs only in maintained at short (12 hours light) or in dim light conditions for several weeks. This photoperiodic response ensures regular reproduction despite variable climatic conditions.

Diet

that consumes decaying organic matter. Also feeds on , primarily diatoms.

Life Cycle

Females carry in a pouch on the side. Egg number, size, and development time vary with environmental conditions: alpine produce fewer but larger, heavier eggs than prairie populations. Development time decreases with increasing incubation temperature, with large eggs requiring longer incubation than small eggs. Eggs incubated at 4°C fail to produce young. Juveniles resemble and undergo several . Time to sexual maturity and number of per year vary with environmental factors. Under fish pressure, females may begin at smaller body sizes (7 mm) compared to -free conditions (9 mm).

Behavior

Typically exhibits photophobic . When parasitized by Pomphorhynchus laevis, behavior can shift to photophilic, increasing visibility to and likely aiding transmission. Shows digging behavior to avoid , though less effectively than the Gmelinoides fasciatus. Reduces activity in the presence of predatory fish . In laboratory conditions, releases chemicals that affect the depth distribution of other zooplankton .

Ecological Role

Serves as a and algivore in freshwater . Functions as an important food source for fish, birds, and some insects. Acts as an intermediate for at least 12 of helminth , including cestodes, , and acanthocephalans. Considered an for ecosystem health and stability. Its abundance and reproductive patterns respond to fish pressure, with structure shifting toward smaller, earlier-maturing individuals under exploitation.

Human Relevance

Used as a bioindicator for freshwater health. Serves as a model organism in ecological and parasitological research. Increasingly important in studies of dynamics, particularly regarding its displacement by Gmelinoides fasciatus. Vulnerable to water quality changes and climate warming, which may alter its reproductive strategies and geographic range.

Similar Taxa

Gammarus pulexCo-occurring native European gammarid; distinguished by preferences and morphological details, though reliable separation requires examination of uropod structure and other fine characters.
Gmelinoides fasciatus amphipod that has displaced G. lacustris in many Eurasian lakes; distinguished by superior digging , more substantial activity reduction in response to , and different ecological tolerances.
Gammarus duebeniNative European gammarid with overlapping distribution; co-exists with some where G. lacustris is excluded due to competitive inferiority.