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Porcelain Crabs and Carcinization

Porcelain Crabs: an amazing example of Carcinization!

Written by Roy Kittrell 28 October 2022.

Before we get started, I’m sure you’re wondering, what is Carcinization? Carcinization is an example of convergent evolution in which a crustacean evolves into a crab-like form from a non-crab-like form. The term was introduced into evolutionary biology by L. A. Borradaile, who described it as “one of the many attempts of Nature to evolve a crab”.

So today, let’s look into one of my favorites (as well as many underwater macro photographers), the stunning Porcelain Crabs. They are a small red dotted crustacean that live alongside clownfish on anemones. The usual ones we see here in SE Asia and in the pictures below from Flow Dive
Center’s recent trip to Lembeh are the species Petrolisthes maculatus.

These small crabs (around 3-4cm wide) are filter feeders as can be seen in the second pic in this series. They extend their nets to catch detritus which they then scrape off with a smaller set of little crab mittens and eat. They have large claws which they use to fight with other crabs for territory. Pic for scale at the end of this series.

It is said that ‘Nature abhors a vacuum’, and there is also a joke that ‘Nature abhors a lack of crabs’. What that means is that there are several examples of other decapods (shrimp, lobsters, squat lobsters etc) turning into a ‘true crab’ via convergent evolution.

Porcelain crabs are one of these examples! They are in fact squat lobsters that evolved into ‘true’ crabs. If you look closely at it’s face, you will see that it has long antennae (true crabs do not have this), and in the fourth picture you can see the 4th pair of legs has become redundant and is folded up flat alongside it’s carapace as the squat lobster tail became redundant and their body became more crab-like.

There are many such examples of carcinization in the animal kingdom; such as the mighty Coconut Crab, the largest land invertebrate that exists today, whose ancestors were more hermit crab-like, and the King Crab who is equally as mighty but lives in the deep ocean.

Truly remarkable animals, one of my favorites to photograph underwater! Make sure you keep a close eye on these beauties the next time you come across them and notice the amazing signs of evolution by mothernature!

Should you be interested in joining our next macro trip, make sure you keep a close eye on our upcoming trips! We will be sharing 2023 trips soon!

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