If you think you know how to categorize animals as aquatic or terrestrial, think again. Are frogs aquatic or terrestrial? What about sea otters? And what about a human embryo floating in its amniotic fluid and breathing it into its lungs?
Strict definitions are not reliable, because there are always exceptions. Nevertheless, consider the following criteria, the best I can come up with so far, for aquatic and terrestrial animals.
A1. An aquatic animal spends its entire life cycle in a liquid medium that is mostly water.
A2. The volume of water making up the animal’s habitat is significantly larger than the body volume of the animal.
T1. A terrestrial animal spends its entire life cycle outside of water. If it enters the water to feed or for other activities, the total time spent in water is insignificant relative to the animal's life span.
T2. If there is an aqueous envelope around the animal, the envelope’s volume is significantly less than the body volume of the animal.
The problem is that if we go by these definitions, then there is no such thing as a terrestrial animal, because all animal embryos are surrounded by a watery medium and the volume of the medium is significantly larger than the body volume of the embryo at least during the early stages of its development. We need to modify the definition for terrestrial animals by adding a 3rd criterion.
T3. During the early stages of the animal's embryonic development, there may be an aqueous envelope around the embryo with a volume significantly larger than the body volume of the embryo if the egg or the parent’s body within which the embryo and the aqueous envelope are located are not themselves surrounded by a liquid medium.
Now the definitions are beginning to get cumbersome. It gets worse when we start considering those animals that fit into one set of criteria during one part of their lives and into the other one during the rest. For example, there are many snail species that live where the sea and the land meet. How do we decide if they are terrestrial snails or aquatic snails? Traditionally, those species that live outside the water and enter it only to reproduce have been classified as aquatic snails* (for example, Melampus bullaoides and Cerithidea scalariformis), while those that live and reproduce outside the water have been classified as land snails regardless of how close their habitats may be to the sea (for example, Truncatella species).
I have so far found only 3 relevant, but brief literature discussions of what it means to be terrestrial. The 1st one is by Labandeira & Beall (1990):
...we consider a "terrestrial organism" to be one that is obligately adapted to land; facultative behavioral adaptation of marine arthropods to a land-based existence is technically excluded...When both groups are discussed, we will refer to the first group as terrestrial and the last group as semiterrestrial. True terrestriality requires an obligate physiological commitment to life on land.I don't find this a useful criterion, because it requires further explanation. What do they mean by "an obligate physiological commitment to life on land"?
The criterion used by Martens et al. (2004) to decide that the ostracods from forest leaf litter were terrestrial, however, agrees with mine. In response to another researcher, who did not consider similar ostracods to be terrestrial, this is what they said:
...did not consider his ostracods from terrestrial habitats themselves to be terrestrial, because these animals gather a film of moisture around and within their valves, so that they still breath through water. We reject this point of view, as nearly all forms of animal respiration, even that of humans, requires a degree of moisture, either externally or internally (within lungs). When animals live in terrestrial conditions, i.e. outside of free standing or flowing water, we consider them terrestrial.Finally, here is Little's (1990) opinion:
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 just that they are bigger, as earthworms are usually bigger than soil-dwelling nematodes, then they can be said to be truly terrestrial.This criterion seem to introduce not a relative but an undefined absolute size criterion; in other words, very small animals can't be terrestrial. I don't find it convincing because of that reason.
A note of caution is necessary here. If we spend too much time thinking about such matters, we will be missing the point. Exercise is good for one's body and mind, an exercise in futility, on the other hand, is a waste of time. What matters is the characterization of the lifestyles of the animals of interest and the understanding of their evolutionary origins.
*If we applied the same reasoning to turtles, then there would be no such thing as a sea turtle, because they all lay their eggs on land.
Koen Martens, Patrick De Deckker & Giampaolo Rossetti. (2004). On a new terrestrial genus and species of Scottiinae (Crustacea, Ostracoda) from Australia, with a discussion on the phylogeny and the zoogeography of the subfamily, Zoologischer Anzeiger, 243:21-36.
Labandeira, C. & Beall, B. S. (1990). Arthropod Terrestrialization. In Arthropod Paleobiology, Short Courses in Paleontology No. 3: 214-232. Mikulic, D. & Culver, S. J. (Eds).
Little, C. (1990). The Terrestrial Invasion. Cambridge University Press.