The internet's favourite law of evolution is that everything turns into a crab. It is not a law, it is not everything, and the real story is funnier than the meme.
The word is carcinisation, coined by L. A. Borradaile in 1916 to describe crustaceans independently converging on a flat, compact, tucked-tail crab shape. The number people usually cite is five. The number nobody cites, and which is the actual joke, is seven.
Below are the resources worth linking instead of the encyclopedia entry, ordered by how much use they are.
The source of every number anyone quotes. Establishes at least five independent origins of the crab form and at least seven losses. Also does the useful work of separating what actually converges (carapace flattening, abdomen tucking) from what does not, which is the part the meme flattens entirely.
A free preprint version is available at preprints.org if the Wiley paywall is in the way.
Roughly 6,000 words that take the joke seriously and turn it into an argument about humans encasing themselves in technological shells, trading individual resilience for collective efficiency. This is the one that has actual thematic weight rather than just fun facts.
Usable lines: "The crab is a deep symbol of our devil's bargain with technology." And, on the shape of the landscape rather than a destination, "There is no final form, merely several very deep holes in possibility space."
Joanna Wolfe, an author on the BioEssays paper, patiently explaining that the meme smuggles in teleology. Evolution has no destination and no hierarchy of better species. Short, authoritative, and the right thing to link if you want to make the point without lecturing.
Where the good quote lives. Gerhard Scholtz, deadpan: "I would say not every crab is a crab." Also notes that nobody actually knows how many times it has happened, which is a more honest framing than the confident number.
The press release version of the BioEssays paper. Useful mainly because it states the five and seven figures in plain language with direct quotes, which saves you a paywall trip if you just need the numbers verified.
Fine for the Borradaile etymology and the list of lineages. Not what you want to send anyone, because it has none of the comedy and none of the argument.
Convergence is real and the meme is wrong about why. Evolution is not aiming at a crab. A crab is just a low spot in the landscape that a lot of paths happen to roll into, and plenty of lineages have rolled back out.
If you use one fact, use decarcinisation. Five arrivals and seven departures is the whole argument in two numbers, and it is much better than the version everyone already knows.
Sources
All resources listed above with direct links. Primary citation: Wolfe JM, Luque J, Bracken-Grissom HD. "How to become a crab: Phenotypic constraints on a recurring body plan." BioEssays 43, no. 5 (2021): 2100020.