Remipedia is a class of blind crustaceans, closely related to hexapods, found in coastal aquifers which contain saline groundwater, with populations identified in almost every ocean basin so far explored, including in Australia, the Caribbean Sea, and the Atlantic Ocean. The first described remipede was the fossil Tesnusocaris goldichi (Lower Pennsylvanian). Since 1979, at least seventeen living species have been identified in subtropical regions around the world.[1]
Description
Remipedes are 1–4 centimetres (0.4–1.6 in) long and comprise a head and an elongate trunk of up to thirty-two similar body segments.[2] Pigmentation and eyes are absent.[3] Biramous swimming appendages are laterally present on each segment. The animals swim on their backs and are generally slow-moving.[4] They are the only known venomous crustaceans, and have fangs connected to secretory glands, which inject a combination of digestive enzymes and venom into their prey,[5] but they also feed through filter feeding. Being hermaphrodites, the female pore is located on the seventh trunk segment and the male pore on the fourteenth.[6]
Remipedia have a generally primitive body plan compared to other extant crustaceans, and are the only extant pancrustaceans to lack significant postcephalic tagmosis.[4] External respiratory structures like gills are absent.[7] Previously regarded as 'primitive', Remipedia have since been shown to have enhanced olfactory nerve centers (a common feature for species that live in dark environments).[8]
Based on studies of the free-living larvae, they appear to be lecithotrophic (non-feeding). Mouths, guts, and anuses appear in the juvenile stage. Because of the energy and nutrients required for swimming, molting, and to grow in size and length, it has been speculated that the larvae may have other sources of growth than its yolk; possibly symbiotic bacteria.[9][10]
With the exception of Speleonectes kakuki, which inhabits a fully marine, sub-seafloor cave in the Bahamas, all known species of remipedians have been found exclusively in anchialine cave systems.[11]
Historical phylogeny based on morphology and physiology has placed Remipedia under Mandibulata, in the subphylum Crustacea, and distinct from Hexapoda.
New research in evolution and development reveals similarities between larvae and postembryonic development of remipedes and Malacostraca, singling Remipedia as a potential crustacean sister group of Hexapoda. Similarities in brain anatomy further support this affinity, and hexapod-type hemocyanins have been discovered in remipedes.[15]
Recent molecular studies have grouped Remipedia with Cephalocarida, Branchiopoda, and Hexapoda in a clade named Allotriocarida.[16][17] Remipedia was found as the sister group to Hexapoda both in phylogenomic[18][17] and combined morphological and transcriptome studies.[16] 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[19] or Miracrustacea ('surprising crustaceans').[4]
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).[17]
Thirty extant species are recognized as of early 2022, divided among eight families and twelve genera.[20][21] All are placed in the order Nectiopoda. The second order, Enantiopoda, comprises the fossil species Tesnusocaris goldichi and Cryptocaris hootchi.[1]
^Yager J (2013). "Speleonectes cokei, new species of Remipedia (Crustacea: Speleonectidae) from a submerged ocean cave near Caye Chapel, Belize". Zootaxa. 3710 (4): 354–362. doi:10.11646/zootaxa.3710.4.4. PMID26106696. S2CID10850210.
