04 May 2006

Those who made it to the terra firma

Among the more than 30 extant animal phyla, only 7 have evolved truly terrestrial representatives. Little1 proposed the following definition for a "truly terrestrial" animal: “If animals are effectively covered by a layer of water, then they are living as aquatic animals. If they are not so covered, which often means that they are bigger, as earthworms are usually bigger than soil-dwelling nematodes, then they can be said to be a truly terrestrial.” Provisionally and solely for the purpose of this post, I am defining a "truly terrestrial" animal as an animal that spends its entire lifespan on land and whose viability doesn't require it to be in a volume of water much larger than its own volume. This definition includes certain animals, such as slugs, that are almost always covered by a film of water whose volume is much smaller than the animals’ body volume, but excludes unfairly many insects (for example, dragonflies), many amphibians and snails that have aquatic larvae and terrestrial adults, but the phyla they are in are already in the list.

What are those phyla?

1. Platyhelminthes (land planarians).
2. Nemertina (land nemertines).
3. Mollusca (land snails and slugs).
4. Annelida (terrestrial oligochetes, e.g., earthworms, terrestrial leeches and the much rarer terrestrial polychetes).
5. Onychophora.
6. Arthropoda (insects, spiders, isopods, millipedes, terrestrial crabs, etc.)
7. Chordata (vertebrates, including mammals, birds, reptiles, etc.)

I am leaving out tardigrades (Tardigrada) and bdelloid rotifers (Rotifera), because, although some of their species live in mosses, lichens and in the soil and can survive desiccation much better than almost all truly terrestrial animals, they require to be surrounded with relatively large volumes of water to be active. Some nematode species (Nematoda) may be active outside of water at very high humidities, and thus, may also be considered truly terrestrial. But, I don’t know enough about the natural history of such species to confidently include them in my list.

I have mentioned this list here before, but back then I had forgotten to include onychophorans. Onychophorans are large, carnivorous, multi-legged, worm-like creatures that live in the forests of South America, Africa, South East Asia, Australia and New Zealand. Some authors consider them to be a subphylum of the Arthropoda.


1. Colin Little. 1990. The terrestrial invasion. Cambridge University Press.

1 comment:

pascal said...

Then there are the groups that decided that being on land wasn't all that special and went back:

ichthyosaurs, plesiosaurs, cetaceans are the fully aquatic vertebrates I can think of off the top of my head. I would imagine invertebrates had several opportunities to go back and forth - terrestrial/aquatic vascilation?