Gammarus spp. (Arthropoda, Crustacea, Amphipoda) contains more than 100 freshwater, brackish, and marine species in the Northern hemisphere. They represent important keystone species in aquatic ecosystems and are often present in high abundance. As shredders and detritus feeders they contribute to the detritus cycle and the microbial loop. Gammarids are also carnivorous, feeding on small invertebrates and carrion. Due to their widespread distribution, importance in the food web, and sensitivity to a wide range of pollutants, they are important bioindicators for water quality assessment.

Gammarus spp. are ecologically highly successful, due to the following characteristics: wide trophic repertoire and foraging plasticity, migration ability and tendency to drift, which allows them to easily invade and colonize ecosystems, high reproductive capacity with several broods per female per year, a high number of offspring, and relative longevity (1-2 yrs).

Gammarus spp. and its American relative Hyalella azteca are standard test species in ecotoxicity testing in the USA and Europe. A new test guideline is currently being prepared to enhance the utility of gammarids as environmentally relevant test species. A variety of ecotoxicological tests based on different measurement parameters have been developed, and some of these have successfully been applied for both in situ and ex situ applications. From an evolutionary perspective, gammarids are particularly interesting. Gammarids contain several highly invasive Ponto-Caspian and Atlantic species, which have spread across Europe. Some species are currently in the process of splitting into several geographical forms. Although the genome of Gammarus pulex has yet to be fully sequenced, over 12000 ESTs are listed in the PubMed database, which might be the basis for future innovative knowledge in taxonomy and toxicogenomics.

The aim is to summarize the current and rapidly expanding knowledge on Gammarus spp., which is an important taxon, highlighting different aspects in zoological (genetics, behaviour, ecophysiology, evolution, taxonomy) and ecological (cryptic species, invasions) science and application in water quality assessment, including effects of climate change on ecophysiology and ecotoxicology. We invite both review articles and innovative research papers on the following topics covering both marine and freshwater Gammarus species.

A special issue on this taxon is timely because within aquatic ecology and ecotoxicology there is increasing interest in this genus and due to the modern genetic methods, new groups of species and sometimes taxonomic revisions might be needed. Potential topics include, but are not limited to:

• Taxonomy
• Genetics
• Evolution
• Ecology
• Ecophysiology
• Behaviour
• Ecotoxicology
• Invasion and biodiversity
• Gammarus spp. in water quality assessment

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