How Many Crabs are There?

Recently, the internet has become enamored with carcinization. Apparently, a research article exploring carcinization was published in 2017ⁱ, but a few popular articles ran about it in late 2020.ⁱⁱ Carcinization has become so popular that “everything becomes a crab” has become its own meme.

What exactly is carcinization? It is the development of a crab-like body. Take, for example, the two creatures illustrated below.

They both look like crabs, right? They both have short, flat bodies with a hard shell and pincers, so they are both crabs, right? Actually, the creature on the left is a true crab and the one on the right is not. The one on the left is Pachygrapsus marmoratus, the marbled crab, and it belongs to the family Grapsidae in the infraorder Brachyura. The infraorder Brachyura is the group that is considered to be the true crabs. The crab on the right is Paralithodes camtschaticus, the red king crab, and it belongs to the family Lithodidae in the infraorder Anomura. Thus, since it is not in the infraorder Brachyura, it is not a true crab.

The thing about carcinization that seems to have amazed people it that it has supposedly happened at least five times. There is the infraorder Brachyura, the true crabs, already mentioned, and then there are four families within the infraorder Anomura that also look very crab-like. These are Porcellanidae (the porcelain crabs), Lithodidae (king crabs), Hapalogastridae (another family of king crabs), and Lomisidae (the hairy stone crab). To put some of these things in perspective, the king crabs are a common source of crab meat. Moreover, the world’s largest crab, the Japanese spider crab, is a member of Lithodidae, so it is actually a king crab and not a true crab at all. Another interesting tidbit of information is that while the families Porcellanidae, Lithodidae, and Hapalogastridae all have several species, the family Lomisidae has only a single species, Lomis hirta, which lives exclusively around Australia. Australia always seems gets to have all of the weird stuff.

It is interesting to note that the infraorder Anomura, the group that contains all of these not-true-crabs, is a very diverse group. Some of the other types of crustaceans found in that infraorder are squat lobsters, hermit crabs, and coconut crabs. In case you are wondering, hermit crabs and coconut crabs were not included in the previous list of examples of carcinization because, despite being called crabs, they are actually not crab-like enough: they still differ in a few important ways. Which raises the question, what exactly makes something crab-like? Here is a list of the three important features:

1. Wide, flat carapace. The carapace is the shell of the crab. Note that many other crustaceans, such as a lobster or crayfish, also have a carapace. In a crayfish, the carapace is the shell in front of the tail. Unlike a crab, its carapace is long and deep.
2. The plastron is wide and flat. The plastron is basically the underpart of the shell. No surprise that if the carapace is wide and flat, the plastron will also be wide and flat.
3. The pleon is flat, short, and bent underneath the plastron. The pleon is the abdomen, so think of the tail of a crayfish or a lobster. It may be surprising to learn, but crabs have an abdomen, too, but it is so flat and usually held tightly to the underside of its shell, that it is easily overlooked.

Now, you may be wondering, “If all of these different creatures look like crabs and have these three features in common, why are they not all classified as crabs?” To make it real simple, is that they have different internal anatomies. Sure, there are some parts of their internal anatomies that are similar. It is thought that the similarities in their internal anatomy are driven by having similar external anatomies. Basically, in order for their outward shapes to be the same, some parts of their insides have to be the same also. For example, it should be no surprise that the circulatory system (blood vessels) that run into the legs will be similar among all of the crab-like animals. After all, they will have to follow the pattern of the body in order to supply nutrients to the body. However, this does not hold for everything, and so some organs, such as the ovaries and the hepatopancreas, differ among the crab-like creatures.

Now it is time to address the elephant in the room. The very term of “carcinization” implies a process by which an animal becomes crab-like. What does “process” and “becomes” imply? Evolution. The idea of carcinization implies an evolutionary process. Hopefully, it is evident based on everything that has been said so far that the concept of “crab-likeness” is a very real thing, and that creatures that differ from one another may externally all look crab-like. However, the idea that these things all became crab-like implies that they all began with different appearances and over time developed crab-like features.

How should a creationist, one who believes that God created creatures according to distinct kinds, look at carcinization? First of all, it should probably be noted that these various crab-like creatures probably do not belong to the same kind. When I refer to “kind,” I am referring to a distinct type of animal that God originally created to reproduce within its own kind and not with anything else. It is generally thought by creationists that the classification group Family is closest to a kind, so in the absence of any evidence to the contrary, it is best to assume that Porcellanidae, Lithodidae, Hapalogastridae, and Lomisidae all belong to separate kinds from each other and from the true crabs. However, this is where we encounter our first big difference with the evolutionary interpretation of carcinization. According to evolution, the infraorder Brachyura, the true crabs, evolved the crab-like body once and the other four families evolved the same body type independently.ⁱⁱⁱ Here is where creationists differ: the infraorder Brachyura already has several families within it, and thus the crab-like body was not inherited from a common ancestor. For example, the blue crab belongs to the family Portunidae, while the fiddler crabs belong to the family Ocypodidae. That means that, while both blue crabs and fiddler crabs are considered to be true crabs, they belong to separate kinds. Since kinds represent distinct mating groups, in the sense that members of two separate kinds are not capable of interbreeding with one another, blue crabs and fiddler crabs are as unrelated to each other as they are to king crabs or porcelain crabs. In other words, blue crabs and fiddler crabs did not inherit their crab-like forms from a common ancestor. Rather, they were given their crab-like forms by a Creator who decided to use the same body type across multiple, unrelated kinds. Thus, the whole idea that carcinization occurred five times is replaced by the idea that a similar body type was given by God to multiple, unrelated kinds.

While we call them all true crabs, the group Brachyura is a human-defined group that contains several God-created kinds that we happen to think look like each other. This leads to our second major difference with the evolutionary idea of carcinization: carcinization is more of an artifact of our classification system than it is of actual events in nature. If we wanted to, we could reclassify Brachyura to include only external features. Doing so would reclassify Porcellanidae, Lithodidae, Hapalogastridae, and Lomisidae within Brachyura, and then every crab would be a true crab. However, we have chosen to base our classification system on more than just external differences, and thus Porcellanidae, Lithodidae, Hapalogastridae, and Lomisidae have to be put into the infraorder Anomura rather than Brachyura.

That there are various families, four of which are distinct from Brachyura, that all have a very similar body type is still very interesting. However, rather than being an astounding coincidence, it can instead by thought of as an example of an awesome God repeating a common pattern in multiple kinds of creatures. If we think of God as an artist, then it should be no surprise that His creative body plans show up in multiple, different works, just as human artists use repeated styles and subjects.

Thoughts from Steven

i Keiler, Jonas; Christian S. Wirkner; and Stefan Richter (2017) “One hundred years of carcinization – the evolution of the crab-like habitus in Anomura (Arthropoda: Crustacea)” Biological Journal of the Linnean Society 121: 200-222

ii Dunn, Thom (2020) “Animals have evolved into a crab-like-shape at least 5 separate times” Boingboing.net, retrieved from https://boingboing.net/2020/10/15/animals-have-evolved-into-a-crab-like-shape-at-least-5-separate-times.html on January 4, 2021 and Delbert, Caroline (2020) “Animals Keep Evolving Into Crabs, Which is Somewhat Disturbing” PopularMechanics.com, retrieved from https://www.popularmechanics.com/science/a34389129/crab-evolution-carcinization/ on January 4, 2021

iii Technically, Lithodidae and Hapalogastridae are so closely related, evolutionists think that the crab-like body evolved once in their common ancestor and then it split into two families. Nevertheless, Porcellanidae and Lomisidae are thought to have nothing to do with the evolution of the crab-like body of Lithodidae, Hepalogastridae, and Brachyura.