Tantulocarida

Tantulocarida is a highly specialised group of parasitic crustaceans that consists of about 33 species, treated as a class in superclass Multicrustacea. They are typically ectoparasites that infest copepods, isopods, tanaids, amphipods and ostracods.

Description
Eyes are completely absent.

The tantulus larvae has a head with a ventral oral disc but no appendages, a six-segmented thorax with six pairs of legs, and a limbless abdomen consisting of one to six segments in addition to a telson.

Body length
Members of this subclass are minute – less than 0.3 mm in length and have a dramatic reduction in body form compared to other crustaceans, with an unsegmented, sac-like thorax and a much reduced abdomen. One tantulocarid species, Tantulacus dieteri, is the world's smallest arthropod, with a total body length of only 85 um.

Life cycle
The tantulocarid life cycle is unique among crustaceans. The tantulus larva transforms directly from a non-feeding (lecithotrophic) and free-swimming organism into a parasite without any instars. When entering the parasitic stage much of the body, such as the muscles, degenerates, even if the body itself becomes bigger. As a parasite it is permanently attached to its host, and after piercing its host's cuticle with an unpaired stylet, a rootlet system used to absorb nutrients enters through the hole and grow into the host’s tissue. The adult form develops inside the larva, and can become either a sac-like parthenogenetic female, or a fully developed free-living, non-feeding and sexually-reproducing male or female. The eggs inside the parthenogenetic female are eventually released as fully developed tantulus larvae. The finding of what appears to be a benthic non-feeding nauplius larva suggests that eggs produced by sexual females hatch as nauplii instead of tantulus larvae. Both the parthenogenetic and sexual females are semelparous.

Classification
Five families are recognised:

Basipodellidae Boxshall & Lincoln, 1983 :
 * Basipodella Becker, 1975
 * Hypertantulus Ohtsuka & Boxshall, 1998
 * Nipponotantulus Huys, Ohtsuka & Boxshall, 1994
 * Polynyapodella Huys, Møberg & Kristensen, 1997
 * Rimitantulus Huys & Conroy-Dalton, 1997
 * Serratotantulus Savchenko & Kolbasov, 2009
 * Stygotantulus Boxshall & Huys, 1989

Doryphallophoridae Huys, 1991 :
 * Doryphallophora Huys, 1990
 * Paradoryphallophora Ohtsuka & Boxshall, 1998

Microdajidae Boxshall & Lincoln, 1987 :
 * Microdajus Greve, 1965
 * Xenalytus Huys, 1991

Cumoniscidae Nierstrasz & Brender à Brandis, 1923 (formerly family Deoterthridae: Onceroxenidae Huys, 1991 :
 * Amphitantulus Boxshall & Vader, 1993
 * Aphotocentor Huys, 1991
 * Arcticotantulus Kornev, Tchesunov & Rybnikov, 2004
 * Boreotantulus Huys & Boxshall, 1988
 * Campyloxiphos Huys, 1991
 * Coralliotantulus Huys, 1991
 * Cumoniscus Bonnier, 1903
 * Deoterthron Bradford & Hewitt, 1980
 * Dicrotrichura Huys, 1989
 * Itoitantulus Huys, Ohtsuka Boxshall & Itô, 1992
 * Tantulacus Huys, Andersen & Kristensen, 1992
 * Onceroxenus Boxshall & Lincoln, 1987