Phylum Onychophora

Posted on January 30, 2011 by Tim 3 Comments

It’s has been many years since I studied the Tree of Life. Like much of evolutionary science it is near constantly in a state of flux as understandings between organisms is gained. Organisms are separated and classified by their origins and relatedness. Not too long ago all organisms were separated into 2 or 3 Kingdoms. Now there are Domains (3?), which contain the Kingdoms. Last I learned there were 6 Kingdoms but this is probably outdated. The Kingdom Animalia contains 32 give-or-take Phyla. Most people learned that this was divided into the vertebrates and invertebrates. Technically, one shouldn’t classify organisms by what they don’t have, especially at the higher levels of classification. However, this is a simple notion to grasp since the Phylum Vertebrata contains us (and most of the animals we learned as kids) and the invertebrates contain everything else. Examples of invertebarate Phyla that most people have heard of include sponges, jellyfish, flatworms, segmented worms, arthropods, molluscs, and echinoderms. Today I want to show you something from an enigmatic Phyla that most people have not heard of – the Onychophora or velvet worms.

A close up view of an onychophoran showing its rudimentary antennae and lobopod feet.

One of the reasons scientists classify organisms is to determine their evolutionary sequence and which characters were new as they evolved into other creatures. An example would be that amphibians evolved from fish-like animals that developed limbs from fins, started breathing air, and moved on land. Or that birds evolved from reptiles by developing specialized scales that became feathers. This of course is an over simplification but you get the idea. Onychophora are enigmatic because scientists don’t know where to definitively place them in this scheme of A –> Z. Onychophora look a little like worms with legs, or like squishy centipedes. They have soft bodies and lobopod legs. That is the legs move by waves of hydrostatic pressure changes in the body. These legs are unique in the animal kingdom. Some of their biology resembles that of the segmented worms (simple gut, soft body, simple sense organs, unjointed appendages) but some features resemble arthropods (growth by shedding, respiration, ‘large’ brain, and basal antennae). For this reason onychophorans are frequently simplified as evolutionary links between the segmented worms (Phylum Annelida) and the arthropods (Phylum Arthropoda).

Oddly, there are very few onychophora species. There are more than 15,000 annelids and the number of arthropod species is in the tens of millions. However, there are less than 100 known species of onychophorans. Another ‘cool’ feature of onychophora is their means of defense and hunting. When encountering potential food or when molested onychophora can spray jets of glue from their mouths. This liquid quickly becomes stickier on contact with air, trapping prey or deterring, if not gluing shut predators’ mouths. During this trip to Sani I encountered 3 onychophorans – all the same species I believe. I have only seen one other onychophoran and that was in Costa Rica. These things were not easy to depict photographically but their uniqueness in the animal kingdom warrants this little write up.

An overhead view of an onychophoran. The liquid to the right is a stick glue sprayed from its mouth when I poked it.

Web maker meets web sprayer. A spider became an experimental victim when I placed it in front of an onychophoran.