Onychophora: An Arthropod RelativeHonor Project Fall 2017Sarah Pooch

Fossils from the CambrianThere are many different species from the Cambrian that are marine relatives of Onychophora, who lived in the seas around 520 million years ago (existing fossils are around 300 million years old), however there are two we will focus on:

Halucigenia and Aysheaia

Images courtesy of the Burgess Shale.

HallucigeniaSo named after the Latin word hallucinatio, which means ‘wandering of the mind’, these creatures had a very bizarre body plan, with spindly legs on the bottom and great spikes off the top.  They are a member of the “lobopodans” specifically, due to the

presence of leg-like extensions from their body. https://www.youtube.com/watch?v=9-Z9Ssgb0Kg

Image courtesy of the Burgess Shale.

AysheaiaNoted by the Burgess Shale as “a very primitive arthropod-like animal with spiked legs”, Aysheaia are also members of the “lobopodans”.  It was first described by anthropologists as being an ancestor of the annelids, before later being redescribed as related to the modern day Onychophorans, such as the Velvet Worm.                                        

https://www.youtube.com/watch?v=6hE-Snc1qy8

Image courtesy of the Burgess Shale.

Modern Onychophorans: Body PlanBody contains a hemocoel, that has circular and longitudinal muscles that line the body wall.  They have non-jointed appendages, paired antennae, jaws, and oral papillae to make up their three head segments.  The body is covered with a thin layer of chitin and the skin is covered in tiny papillae (overlapping ‘scales’ that makes the skin appear ‘velvety’).  They are unsegmented, with the long, smooth body almost appearing more like an unsegmented annelid with legs.

Velvet Worm - a modern Onychophoran

MotionOnychophorans move in a unique fashion that is still not fully understood.  They use their hemocoel as a hydrostatic skeleton, because they shed their cuticle in patches when they molt.  This allows for them to change the length of their body, aiding in forward

motion (this is not fully understood).  Because their legs are not jointed, they are extended by hemocoelic pressure.  They then carry their body forward past the stationary point in the extended limb tip.  Throughout this motion, the body itself remains rigid, despite the fact that it is going through continual changes in length and width.

In short, locomotion is thought to be the result of a complex interaction between the muscles in the limbs and the pressure generated by the hemocoel.

https://www.youtube.com/watch?v=PNw5RZ5R0U8

FeedingWhile some Onychophorans are herbivores or omnivores, most are carnivores that actively hunt down their food, with the help of a proteinaceous glue that they shoot out of large “slime glands”.  These glands are located at the tips of the oral papillae.  This glue is targeted at their prey (isopods, termites, and small molluscs generally), which when it hits them, begins to harden immediately, holding the prey still until the Onychophoran gets there to eat it. There are some species that use special digestive enzymes (salivary secretions) to help soften up the prey, which they then suck up the semi-liquid tissues into its mouth.  In the case of the herbivorous and omnivorous Onychophorans, they can use the ‘glue’ to act as a self-defense mechanism.

https://www.youtube.com/watch?v=LY8TgD6-7kg

https://www.youtube.com/watch?v=do9YivjrAFk

Internal AnatomyThey have a brain, which lies above the esophagus and connects to a ladder-like nervous system that runs down the length of the body.  There is a single pair of eyes connected to the protocerebrum in the brain.  They use a system of trachea and spiracles (near the base of the legs) to bring oxygen into their body, with a highly permeable cuticle that restricts them to moist environments.  Their excretory system is similar to that of a crustaceans, but they excrete uric acid, rather than urea or ammonia.  This means that they must eliminate water at a fairly rapid rate in order to prevent a toxic buildup of waste in their body. Their blood-filled hemocoel acts as an open circulatory system, that is reminiscent of those of arthropods.  They also have a tubular heart that helps pump blood.

ReproductionThe Onychophorans have separate sexes and utilize both internal and external fertilization (species dependent).  They also typically exhibit sexual dimorphism, primarily in that the males and females are of different sizes, with the female being larger than the male.

After fertilization, there are a variety of ways that Onychophorans ‘raise’ their eggs.  Some are oviparous (lay eggs outside of the body), many retain the eggs inside the uterus of the female, with there being differences here as well.  Most do not attach them to the mother’s body (inside the uterus), but there are those that maintain contact with the eggs through a sort of placenta, that provides the growing eggs with nutrients taken from the mother’s body.

Reproduction - Velvet WormsOne of the most well-known modern day Onychophorans, is the Velvet Worm (so named because of its velvety appearance due to the chitin ‘scales’ on its body surface).  These animals demonstrate all three types of reproduction: oviparous (egg-laying), ovoviviparous (egg-live bearing), and viviparous (live-bearing).  It is believed that both the oviparous and viviparous forms descended from the ovoviviparous form.  Both sexes have a pair of gonads, which are derived (along with the gonoduct)

from the true coelom tissue.  (The gonoduct differs between the types of reproduction.)  

Photo credit to Wikipedia

Arthropod RelationsThey share similar characteristics with the Arthropod line, such as: chitin cuticle, hemocoel, and arthropod-type hemocyanins.  Molecular evidence also exists that shows these animals are fairly closely related to arthropods, with 12S rRNA being the one mentioned.

Referenceshttp://burgess-shale.rom.on.ca/

http://lanwebs.lander.edu/faculty/rsfox/invertebrates/peripatus.html

https://en.wikipedia.org/wiki/Onychophora (photo of the eggs)