Crabs are decapod crustaceans typically having a very short projecting tail-like abdomen, usually hidden entirely under the thorax. They generally have five pairs of legs, with pincers/claws on the front pair. They feed on algae, detritus, and other invertebrates. Some crustaceans, such as king crabs and porcelain crabs, are not true crabs, but have evolved similar features through a process known as carcinization (different from carcinogenesis, which is a bad thing). True crabs have four pairs of legs used for locomotion, whereas carcinized crabs have only three sets of “walking legs.”
Male crabs have larger claws. The abdomen in male crabs is narrow and triangular, while females have a broader, rounded abdomen where they brood fertilized eggs. Crabs mate belly-to-belly, often just after the female has molted and is still soft. A female can store the sperm for a long time before fertilizing her eggs. The fertilized eggs are released onto her abdomen and secured with a sticky material. Initially, the eggs are orange, and they change color as they mature. After one to two weeks when development is complete, the larvae is released into the water where they float and go through molting stages until they can live on the bottom. Even most land crabs release their larvae in the ocean. Crabs have an average of 2 million eggs at a time, but can have up to 8 million. Only about one in a million eggs survives to adulthood.
Crabs molt many times. First, the old shell is softened, and the crab takes in a lot of water to expand and crack open the old shell. Then the crab extracts itself. This takes many hours. If the crab gets stuck, it will die. After freeing itself from the old shell, the crab is very soft and hides until the new shell has hardened, expanding the new shell to make room for future growth. This process takes about two days.
It appears there is a lot we don’t know about crabs, including how many species there are. One source says there are over 4,500 species; another says there are over 7,000 species of crabs, and counting. They all agree that the sizes range from a few millimeters across to Japanese Spider Crabs which can be up to 12 feet across and live up to 100 years. Of these, only a few species are commonly consumed by humans. Over 1.5 million tons of crabs are caught annually for human consumption. Once a crab dies, it gives off toxins that permeate their body and make the meat unfit for consumption. This may explain why crabs are often transported alive and then cleaned and boiled alive for eating, which makes me question whether I will ever eat crab again. I have the same problem with eating lobster. Some sources assure the reader that the crabs die instantly, but do they? I get that we have to eat, I’m just not a fan of torturing my food first. But I digress.
I had to look up all this because I saw so many crabs in Indonesia, and I wanted to learn more. For one thing, I saw many porcelain crabs, aka Neopetrolisthes maculatus, and now I know they are not really crabs. They were all living on anemones, and many of the ones I saw were eating what looked like algae. Crabs always look like they are eating something, but the food bits are so small that I can never see them. The algae here was surprisingly obvious. I had to blow up this photo a lot before I could see the little tiny crab next to it. Given all the phases that a crab has to go through, what are the chances that could be mother and baby?—slim to none, but a girl can dream.
Zebra Urchin Crab, Zebrida adamsii. These small crabs live between the spines of sea urchins, but they do not need the urchin to live. We found this crab and coleman shrimp living on the same fire urchin.
Soft coral crab, Hoplophrys oatesii. These tiny crabs live on soft corals. They are so so small and blend so so well with the coral, I doubt I would ever be able to find one without help from a dive guide.
Anemone hermit crab, Dardanus pedunculatus. It is small and looks very plain until you light it up and see that it is actually quite beautiful. it collects sea anemones to place on its shell for defense. Its eyestalks are white with red bands, and the left claw is much larger than the right.
But the best was this, rough box crab, Calappa gallus. This crab uses its claws to camouflage itself from predators.
When it moved its claws, I saw the orange belly and took lots of photos, trying to get the best view. The crab kept using its front legs so that the orange was sometimes more visible than other times. Back on the boat, my guide told me that the orange was eggs. I knew it was something, but had no idea what. Eggs! Millions of eggs!
