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Kingdom: Animalia
Phylum: Onychophora
Class: Udeonychophora
Order: Euonychophora
Family: Peripatidae
Genus: Epiperipatus
Species: Biolleyi
Introduction: Epiperipatus biolleyi is a species of velvet worm of the phylum Onychophora. They are endemic to Costa Rica and can be found throughout the rest of central and south America. They are most at home in the rainforest, particularly dark and humid areas. Since they tend to avoid light, they usually prefer to spend most of their time in holes, burrows, logs, caves, or any other natural crevice they can find. Their diet consists of insects, spiders, and other arthropods, as well as gastropods and terrestrial annelids. They are slow movers and can only walk up to 10 mm/s, even with all those extra legs.
Morphology: These little guys are instantly recognizable from their skin covered in scales and papillae giving it the look and feel of a velvety texture, this is in fact where it gets the name "velvet worm". These papillae are responsible for all water regulation within the worm, since they do not drink water they must be constantly absorbing water which is why they require a very humid environment. Velvet worms can range in color from greenish-blue to pinkish-orange, which most members of this species display. Their cylindrical bodies are segmented and lined with 26-30 pairs of legs depending on sex, with the females having more. Each leg has no joints or internal structures, instead the shape is maintained by the internal fluids hydrostatic pressure. This hydrostatic pressure also acts as a sort of "pseudoskeleton" for the rest of the body, since they do not have an exoskeleton like their relatives the arthropods. On the ends of each leg you will find chitinous and retractable claws, the greek word for claws being onyches to which the phylum owes its name to.
Slime: Velvet worms have one of the most unique weapons in the entire animal kingdom. There are two modified limbs with oral papillae lateral to the mouth that contain slime glands that can secrete projectile streams of milky-white slime. This slime is stored throughout a branched system of reservoirs strewn within the body segments. When threatened or hunting prey, the velvet worm will activate its slime glands, which act similar to a syringe and push out the slime in a stream going as fast as 5 m/s up to a distance of one foot. While ejecting its stream of slime, they will move their glands in an oscillatory pattern to create a sort of net to ensnare their prey. Once it covers the prey, the slime starts to harden and traps the prey where it stands, the velvet worm will then proceed to stab it with its single razor sharp tooth injecting it with saliva full of digestive enzymes that turn their tissue into a nutritious soup. They now will suck up their prey and eat any remaining slime as well. The reason why they eat their slime is because it is incredibly costly to produce. It can take up to 24 days to fully replenish their slime reserves, which when full account for about a tenth of their weight. The slime itself is ninety percent water by weight, with the rest being partly various proteins which give it its stickiness and tensile strength, as well as sugars, lipids, and the surfactant nonylphenol, which keeps it from sticking to the worm or from drying before reaching its target, it is the only organism known to produce this substance.
Reproduction: Velvet worms of this family are some of the rare cases of a viviparous invertebrate. The males insert their spermatophore into the females genitals very similar to how it is done in salamanders. Females possess a special reservoir for male sperm to keep until the perfect time for fertilization. Once the egg is internally fertilized, the embryos of viviparous species will develop in the uterus, while oviparous embryos are laid in a small clutch of eggs. All species but one reproduce sexually, Epiperipatus imthurni has no males and the females reproduce through parthenogenesis. The females of all velvet worms are on average larger than the males and possess more legs, the females are also dominant in social hierachies, which are very common within this phyla. The groups consists of about 15 related individuals who usually live in the same log, or burrow and will hunt together. After a successful hunt, the dominant female eats first, with the other females following, then males, and lastly offspring. To determine if others are part of their group, they will run their antennae down the length of eachother, usually reacting violently to members of other groups. They also choose their mates this way, which are almost always within the same group. Their brain is surprisingly complex which allows for this degree of social and predatory behaviour.
Phylogeny: There are two families in this phylum; Peripatidae and Peripatopsidae, with this species being from Peripatidae. This family is mostly found in central and south America. Peripatopsidae is mostly found in Australia and its surrounding islands. These two families differ in several characteristics, with the most obvious being the amount of legs each one has, Peripatidae can have 22 up to 43 pairs of legs while Peripatopsidae can have 15 up to 24 pairs of legs. Another key difference is their reproductive methods, Peripatidae are almost always viviparous and provide their embryos with nutrients via a placenta, while Peripatopsidae has no placenta, and its members are usually oviparous or ovoviviparous but there are some viviparous species.
Evolutionary significance: The evolution of Onychophora is of particular interest because of their relationship with perhaps the most successful of all animal phyla, the Arthropods. These two phyla along with Tardigrada make up the proposed clade Panarthropoda, although this is still debated. However, this placement has gone over many changes throughout the years. They were first thought to be sister taxa to annelids, based off obvious morphological similarities. However further research done into their anatomy and molecular data placed them in the clade Ecdysozoa as a sister taxa to Nematoda, which is still a theory some believe in today. Since their appearance has not changed much since their divergence from these groups about 500 mya in the cambrian, onychophora is considered a living fossil and was one of the first terrestrial animals.
Bibliography:
Monge-Nájera, J., and F. Aguilar. “Behavior of <i>Epiperipatus Biolleyi</I> (Onychophora: Peripatidae) under Laboratory Conditions”. Revista De Biología Tropical, Vol. 41, no. 3 A, May 2016, pp. 689-96, https://revistas.ucr.ac.cr/index.php/rbt/article/view/24081
Read, J., & Hughes, R. N. (1987). Feeding behaviour and prey choice in macroperipatus torquatus (Onychophora). Proceedings of the Royal Society of London. Series B. Biological Sciences, 230(1261), 483–506. https://doi.org/10.1098/rspb.1987.0030
Baer, A., & Mayer, G. (2012). Comparative anatomy of slime glands in Onychophora (velvet worms). Journal of Morphology, 273(10), 1079–1088. https://doi.org/10.1002/jmor.20044
Monge-Nájera, Julian. (1995). Phylogeny, biogeography and reproductive trends in the Onychophora. Zoological Journal of the Linnean Society, 114(1), 21–60. https://doi.org/10.1111/j.1096-3642.1995.tb00111.x
Podsiadlowski, L., Braband, A., & Mayer, G. (2007). The complete mitochondrial genome of the Onychophoran epiperipatus biolleyi reveals a unique transfer RNA set and provides further support for the ECDYSOZOA hypothesis. Molecular Biology and Evolution, 25(1), 42–51. https://doi.org/10.1093/molbev/msm223
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