''Gammarus lacustris'' is an aquatic

amphipod Amphipoda () is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods () range in size from and are mostly detritivores or scavengers. There are more than 10,700 amphipod species cur ...

Description

''Gammarus lacustris'' is semi-transparent and lacks a webbed tail. It may be colorless, brown, reddish or bluish in color, depending on the local environment. It has seven abdominal segments, a fused cephalothorax, and two pairs of antennae. Unlike other

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s, amphipods lack

carapace A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...

s and have laterally compressed bodies. Gammarids are referred to as scuds or sideswimmers. ''G. lacustris'' resembles a freshwater

shrimp A shrimp (: shrimp (American English, US) or shrimps (British English, UK)) is a crustacean with an elongated body and a primarily Aquatic locomotion, swimming mode of locomotion – typically Decapods belonging to the Caridea or Dendrobranchi ...

Life cycle

The female carries eggs in a brood pouch on its ventral side. ''G. lacustris'' in higher elevations were more likely to have fewer but larger eggs than those living at lower elevations. ''G. lacustris'' undergoes several molts and juveniles resemble the adult.

Ecology

''Gammarus lacustris'' plays an important role in many of the

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

ecosystem An ecosystem (or ecological system) is a system formed by Organism, organisms in interaction with their Biophysical environment, environment. The Biotic material, biotic and abiotic components are linked together through nutrient cycles and en ...

s that it inhabits. It is a

detritivore Detritivores (also known as detrivores, detritophages, detritus feeders or detritus eaters) are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces). There are many kinds of invertebrates, ...

and may also consume

algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

, mainly

diatom A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...

s. It is considered an

indicator species A bioindicator is any species (an indicator species) or group of species whose function, population, or status can reveal the qualitative status of the environment. The most common indicator species are animals. For example, copepods and other sma ...

for the overall health and stability of the ecosystem. ''G. lacustris'' can also inhabit a wide array of environments, ranging from low altitude calcium-rich lakes to high altitude, cold, and calcium-poor lakes. This influences its biology: at low altitudes, it is known to die after first reproduction, but in colder waters, it lives to reproduce repeatedly.

Temperature

In many species feeding behaviour is affected by temperature, i.e. the amount of food consumed often increases at warmer temperatures. However, when ''G. lacustris'' feeds on three different parasite species ( ''Diplostomum'' spp., ''Apatemon'' spp. and ''Trichobilharzia'' spp.) no such effect of temperature on feeding is seen.

Parasites

As a small aquatic invertebrate ''G. lacustris'' is an important food source for many organisms. Birds, fishes, and some insects are known to prey upon ''G. lacustris''. As many other species of small

invertebrates Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordate subphylum ...

, it serves as an

intermediate host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include ...

for several

parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

species (e.g. '' Pomphorhynchus laevis''). ''G. lacustris'' typically shows photophobic behaviour, but when parasitized this can be altered to photophilic behaviour when infected by ''Pomphorhynchus laevis,'' which is a host manipulation parasite, like many others. Parasites can affect the diel migration of ''G. lacustris'', making it more visible and susceptible to predation most likely aiding parasite transmission. More mature parasites have greater effects on hosts.

Distribution

''Gammarus lacustris'' has been noted in northwestern Europe, Russia, and North America. Its precise range has yet to be defined. It can be found in shallow or deep lakes and in slow-moving rivers. It is more abundant in fishless lakes than in those with fish. Its distribution follows the

thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a distinct layer based on temperature within a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) with a high gradient of distinct te ...

in the water.