Amphionids have been studied very little, and as a result, the amount of information about them is very limited. No adult males or brooding females have yet been found.
Until the 1970s, the amphionidacea were classified as a family among the 30-odd families of caridean shrimps of the infraorder Caridea, because of similarities between the early larval stages of amphionidacea and those found among caridean shrimp species. Larval amphionids at various stages were sometimes classed as distinct species. In 1973, Williamson proposed reclassifying the amphionidacea as a separate, single-species order within the superorder Eucarida. He based his proposal on characteristics unique to the amphionids, including the brood chamber of the female and the structure of the first pair of pleopods in the female.
For the purposes of this chapter, the taxonomic breakdown for the Amphionidacea is: phylum Arthropoda, subphylum Crustacea, class Malacostraca, and order Amphionidacea. There is one family, Amphionididae, one genus, Amphionides, and one species, Amphionides reynaudii (H. Milne Edwards, 1832).
Amphionides reynaudii is the single extant species in its order that would have once contained others all descended from an ancestor in which females shed eggs directly into the water before evolving the distinctive brood chamber.
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