Remipedia

Remipedes are elongate, slender and blind crustaceans ranging in length from approximately in adults of smaller species such as Godzilliognomus frondosus to in larger species such as Godzillius robustus. Similarly to many troglofauna (animals which live underground), nectiopodans have no pigmentation and are eyeless. The body is divided into two main tagmata (grouping of segments into a morphological unit): the cephalon and the trunk. They do not have the carapace of some other crustaceans. Head The head is quite short, and is covered by a larger head shield. The shield is generally a rounded oblong, however, the shape and features of that shield can be different between the species, for example, Pleomothra fragilis has two curved, elongate, rather long and thin spine-like posterolateral processes. However, in small species such as Godzilliognomus frondosus, the fewer number of somites (16) is fixed. Although the trunk is not divided into tagmata, the somites are variable throughout the body: the first trunk somite, which is more or less covered by the head shield depending on the species, is noticeably shorter and narrower than the following somites. Starting from the largest somites in the middle of the trunk, they gradually decrease in size towards the end of the body. The body ends by a terminal anal somite, that possesses no limb and bears a pair of cylindrical caudal rami that can reach more than twice the length of the anal somite. Venom Nectiopodan remipedes possess a highly developed venom apparatus. Two equally sized venom glandss are situated in the anterior trunk, which are linked to reservoirs in the brachia via ducts. Around each reservoir are two large muscles used to expel the venom. Four adductor muscles help the stabbing motion of the maxillules and prevent the backflow of the venom. The major components of this venom are the peptidase S1 (PS1), enzymes which may play a role in immobilization and digestion of the prey. Chitinasess are the second most abundant toxin type, and are used to break down the chitinous exoskeletons of their prey. This toxin is present in a particularly important quantity in nectiopodans. The venom also possesses neurotoxins, which are likely used to paralyze their prey. == Ecology ==

Most remipedes live in dark, poorly oxygenated saltwater. The only exception is Speleonectes epilimnius that lives in highly oxygenated surface water. Nectiopodan remipedes tend to live in anchialine systems. Most species are found in the Caribbean region (Turks and Caicos Islands, the Bahamas and Yucatán for example). The isolated species Lasionectes exleyi lives in one cave in western Australia. The only species known not to live exclusively in an anchialine system is Speleonectes kakuki which was found in a fully marine sub-seafloor cave in the Bahamas. Distribution Extant remipedes (Nectiopoda) are found in the following regions: • – Andros, Sweetings Cay, Grand Bahama, Great Exuma, Great Guana Cay (Exuma Cays), Cat Island, Abaco Islands, San Salvador Island • – North Caicos, Providenciales • – North West Cape (Western Australia) • – Matanzas Province • – Lanzarote (Canary Islands) • – Quintana Roo • - Caye Chapel • – Distrito Nacional Cueva Taína, Santo Domingo Este. Several hypotheses try to explain the very disjunct distribution of remipedes: the vicariance hypothesis suggests that the current remipede populations represent relicts of a global Tethyan distribution during the Mesozoic that was scattered by tectonic movements; the regression hypothesis suggests that global marine populations were scattered by sea regressions, and that they adapted to the remaining environments where they could survive, in this case, anchialine systems; the deep-sea hypothesis considers the possibility that hypogean populations come from deep-sea populations that arrived in cave systems through underwater crevices and fissures. Active or passive migrations of opportunistic organisms is also possible. Reproduction and life cycle Remipedes are hermaphrodites; female gonopores are on the protopods of the seventh trunk limbs, while the male gonopores are on the protopods of the fourteenth trunk limbs. Mating behaviour has never been observed. The male system is organized as follows: the two testes begin in the seventh trunk somite and extend to the tenth trunk somite, they contain various developmental stages of spermatocytes and spermatids. Thin seminal ducts transport spermatids from the tenth trunk somite to the fourteenth trunk somite and the gonopores. Together, few spermatids form larger spermatophoress (38 μm in Speleonectes benjamini). Posterior to the gonopore is a genital plate covered with glandular pores that produces a glue-like secretion that might help to fix the spermatophores to external surfaces such as the substrate or other individuals. Sternal bars between the gonopores may help transfer spermatophores to those surfaces. == Systematics ==

Thirty extant species are recognized as of early 2022, divided among eight families and twelve genera. All are placed in the order Nectiopoda. The second order, Enantiopoda, comprises the fossil species Tesnusocaris goldichi and Cryptocaris hootchi. • Cryptocorynetes haptodiscus Yager 1987 • Cryptocorynetes longulus Wollermann, Koenemann & Iliffe 2007 • Family Morlockiidae García-Valdecasas 1984 • Genus Morlockia García-Valdecasas 1984 • Morlockia williamsi (Hartke, Koenemann & Yager 2011) [Speleonectes williamsi Hartke, Koenemann & Yager 2011] • Morlockia emersoni (Lorentzen, Koenemann & Iliffe 2007) [Speleonectes emersoni Lorentzen, Koenemann & Iliffe 2007] • Morlockia atlantida (Koenemann et al. 2009) Hoenemann et al. 2012 [Speleonectes atlantidus Koenemann et al. 2009] • Morlockia ondinae García-Valdecasas 1984 [Speleonectes ondinae (Garcia-Valdecasas 1984)] • Family Speleonectidae Yager 1981 • Genus Lasionectes Yager & Schram, 1986 • Lasionectes entrichoma Yager & Schram, 1986 • Genus Speleonectes Yager 1981 • Speleonectes epilimnius Yager & Carpenter, 1999 • Speleonectes gironensis Yager, 1994 • Speleonectes kakuki Daenekas et al., 2009 • Speleonectes lucayensis Yager, 1981 • Speleonectes minnsi Koenemann, Iliffe & van der Ham, 2003 • Speleonectes tanumekes Koenemann, Iliffe & van der Ham, 2003 • Family Xibalbanidae Olesen et al. 2017 • Genus Xibalbanus Hoenemann et al. 2013 • Xibalbanus cokei (Yager, 2013) Olesen et al. 2017 [Speleonectes cokei Yager, 2013] • Xibalbanus cozumelensis Olesen, Meland, Glenner, van Hengstum & Iliffe, 2017 • Xibalbanus fuchscockburni (Neiber et al. 2012) Hoenemann et al. 2013 [Speleonectes fuchscockburni Neiber et al. 2012] • Xibalbanus tulumensis (Yager 1987) Hoenemann et al. 2013 [Speleonectes tulumensis Yager 1987] • Family Pleomothridae Hoenemann et al. 2013 • Genus Pleomothra Yager 1989 • Pleomothra apletocheles Yager 1989 • Pleomothra fragilis Koenemann, Ziegler & Iliffe 2008 Fossil record The remipede fossil record is extremely poor: only two species are currently known: Tesnusocaris goldichi from the Tesnus Formation and Cryptocaris hootchi from the Mazon Creek fossil beds, both dating back to the Carboniferous and classified within the family Tesnusocarididae and the order Enantiopoda. They are both very different from the nectiopodans, which has led some authors to question their placement within Remipedia. Among other differences, enantiopodans possess compound eyes and the trunk somites bear two pairs of uniramous appendages (duplopody). No fossil member of Nectiopoda are known. Extant species The first extant species to be described was Speleonectes lucayensis, discovered by Jill Yager while cave diving in Lucayan Caverns on the Grand Bahama Island in 1979 and described in a paper in the Journal of Crustacean Biology in 1981. The novel nature of this species was recognized and the class Remipedia was erected in the same paper. The name "Remipedia" is from the Latin '''', meaning "oar-footed". Recent molecular studies have grouped Remipedia with Cephalocarida, Branchiopoda, and Hexapoda in a clade named Allotriocarida. and combined morphological and transcriptome studies. In other studies Remipedia and Cephalocarida are grouped together form the clade Xenocarida, which in turn was sister to Hexapoda in a clade named Anartiopoda or Miracrustacea ('surprising crustaceans'). The relationship of Remipedia and other crustacean classes and insects is shown in the following phylogenetic tree, which shows Allotriocarida, along with Oligostraca and Multicrustacea, as the three main divisions of subphylum Pancrustacea, embracing the traditional crustaceans and the hexapods (including insects). }} ==References==