Introduction to small crustaceans
Small crustaceans are not a systematic group and thus they only share being crustaceans and being small, i.e. usually less than 25mm in length. For this web-guide we have decided to separate the Decapoda (crabs, shrimps, lobsters) and sessile Cirripedia (barnacles, goose-barnacles) from the remaining crustaceans, which are then lumped in the "Small Crustacea" with a common identification key. The systematic groups included in "Small Crustacea" are Copepoda, Cladocera and Peracaridea (Amphipoda, Isopoda, Mysidacea, Tanaidea). All these groups are difficult to identify to species level, and usually microscopic characters are necessary. Even these microscopic characters are not always unequivocal and often complexes of several "sibling species" are recognized within one genus.
Drawing with anatomy. From Stephensen 1928.
Fortunately the higher taxonomic level groups are rather easy to distinguish, and this is what this introduction will do. Obviously larval stages of decapods and cirripeds may be seen as part of the group of small crustaceans. All groups of crustaceans go through a nauplius stage. Cirripeds then go through a "cypris-stage", similar to ostracods. Fortunately, to this day no ostracod species have been introduced by human activities to marine or brackish waters of northern Europe (Wolff, 2005; Gollasch & Nehring, 2006), though one species, Eusarsiella zostericola (Cushman, 1906) has been reported from southern UK (Eno et al., 1997). Decapods pass through one or several "zoea-stage(s)" following the nauplius-stage, and these are rather easily identified as being decapods. Larval stages will not be included in this key.
Some small crustaceans are parasitic. These will be dealt with in a separate section also encompassing helminth and nematode parasites.