Axiidea is an infraorder of decapod crustaceans. They are colloquially known as mud shrimp, ghost shrimp, or burrowing shrimp; however, these decapods are only distantly related to true shrimp. Axiidea and Gebiidea are divergent infraoders of the former infraorder Thalassinidea. These infraorders have converged ecologically and morphologically as burrowing forms. Based on molecular evidence as of 2009, it is now widely believed that these two infraorders represent two distinct lineages separate from one another. Since this is a recent change, much of the literature and research surrounding these infraorders still refers to the Axiidea and Gebiidea in combination as "thalassinidean" for the sake of clarity and reference. This division based on molecular evidence is consistent with the groupings proposed by Robert Gurney in 1938 based on larval developmental stages. Axiidea are noted for the burrows with complex architecture that they make in the ocean floor sediment. These burrows can be classified based on their external characteristics in the sediment as well as the trophic group that the species falls into. The population density of most species of Axiidea tends to be high, so these organisms play an important role in the biogeochemical processes of the ocean floor sediments, and in the creation of habitats that favor various marine benthic communities. Classification The infraorder Axiidea belongs to the group Reptantia, which consists of the walking/crawling decapods (lobsters and crabs). The cladogram below shows Axiidea as more basal than Gebiidea within the larger order Decapoda, from analysis by Wolfe et al., 2019. The infraorder Axiidea comprises the following families: Axiidae Huxley, 1879 Callianassidae Dana, 1852 Callianideidae Kossmann, 1880 Ctenochelidae Manning and Felder, 1991 Gourretiidae Micheleidae Sakai, 1992 Strahlaxiidae Poore, 1994 A few subfamilies of Axiidea have been proposed to become families, but have not for a variety of reasons. Examples of these subfamilies include the subfamily Gourretiidae, discovered by Sakai in 1999. Gourretiidae is a subfamily of the Ctenochelidae, and has been proposed to become a family instead, but phylogenetic analyses do not yet support that proposal. Similarly, molecular studies do not support the subfamily Eiconaxiidae being separate from family Axiidae. There is also no molecular evidence to separate the subfamily Calocardidae from Axiidae. The cladogram below shows Axiidea's internal family relationships from analysis by Wolfe et al., 2019. Description The length of an adult Axiidea can range from about in some species, to over in other species. The color of the