Loricifera

Loricifera, or corset animals, are a phylum of tiny marine creatures that live in sediment. Their name comes from Latin words meaning "to bear a corselet," referring to their protective outer shell, the lorica. They measure between about 100 micrometers and 1 millimeter. These animals were first discovered in 1983 by R.M. Kristensen near Roscoff, France, though the first specimen was collected in the 1970s. They attach so firmly to gravel particles that they stayed hidden for so long. Loriciferans are found at all depths, in various sediment types, and across all latitudes. There are 44 described species, with roughly 100 more collected but not yet formally described.

Morphologically, Loricifera have a head (containing the mouth and brain), a neck region, and a trunk surrounded by six plates forming the lorica. Their brain is well-developed, and each scalid (a sensory structure) connects individually to it via nerves. They lack circulatory and endocrine systems. Many larvae are acoelomate, while some adults are pseudocoelomate; others remain acoelomate. Development is usually direct, though some species have Higgins-larvae with toes and two or three pairs of hair-like structures on their loricas. Most adults are gonochoric (separate sexes), but a few species are hermaphroditic. Pliciloricids have complex life cycles that include paedogenetic stages and parthenogenetic reproduction. Fossils of Loricifera date to the late Cambrian.

Taxonomically, morphological studies traditionally group Loricifera with Priapulida in the Vinctiplicata, and together with Kinorhyncha they form Scalidophora, sharing a chitinous cuticle, rings of scalids on the introvert, flosculi, and two rings of introvert retractors. However, the monophyly of Scalidophora is debated. A 2015 study using 18S and 28S rRNA sequences suggested Loricifera is related to Nematoida and Panarthropoda, while a 2019 genome-wide study provisionally placed them as sister to Nematoda. A 2022 study again recovered a monophyletic Scalidophora but noted that non-monophyly remains possible.

Evolutionarily, Loricifera are thought to be miniaturized descendants of a larger ancestor, possibly resembling the Cambrian fossil *Sirilorica*. Their microscopic, non-mineralized bodies leave a scarce fossil record, making detailed evolutionary history difficult to trace. The 2017 discovery of *Eolorica deadwoodensis* from the Cambrian may provide new insights.

Three Loricifera species have been found in completely anoxic sediments of the L'Atalante basin in the Mediterranean Sea, over 3,000 meters deep. These are the first multicellular organisms known to spend their entire lives without oxygen. Initially, their mitochondria were thought to function like hydrogenosomes for anaerobic respiration, but by 2021 questions arose about whether they even have mitochondria. These animals complete their life cycle in total darkness and absence of oxygen, inhabiting a dense, salt-saturated brine that does not mix with overlying water. The brine contains sulfide at 2.9 mM due to sulfate-reducing microbes, yet supports a rich microbial community of chemosynthetic prokaryotes and eukaryotic heterotrophs.

Taxa within the phylum include Nanaloricidae (Kristensen, 1983), Pliciloricidae (Higgins & Kristensen, 1986), Urnaloricidae (Heiner & Møbjerg Kristensen, 2009), and extinct taxa of uncertain placement.