Orange unicorns
Have you ever seen an orange unicorn swimming in the ocean? Probably not. To see one, you would need to travel to the Caribbean and bring a microscope. These unicorns are less than a millimeter long and almost invisible to the naked eye. To be honest, they aren't actually unicorns at all, but rather the tiny larval stage of a particular species of crab, Cyclograpsus integer.
The adults of this species are also small, usually around just a centimeter wide and they live in tiny burrows under rocks along the shoreline, far enough from the water that they only get submerged during very high tides or big storms. Interestingly, the adults are often found in groups of 2-8 individuals, often a mix of males and females. Like all crabs, females carry fertilized eggs under their abdomens as they develop and then release larvae (called zoea [singular] or zoeae [plural]) into the sea when the eggs are ready to hatch.
Cyclograpsus integer adult on a chunk of coral rubble
You might be wondering by now why I was talking about orange unicorns at the beginning. The reason is the unique and beautiful coloration of Cyclograpsus integer zoeae. Most crab zoeae are almost entirely transparent but this species is exceptionally pigmented. The unicorn horn is the white pigmented dorsal spine (the pointy thing coming off the larva on the left side of each image below) projecting from their orange-tinted body. Under the microscope it's possible to see all the different colors scattered across their body but if you look at these tiny zoeae with your naked eye they look like bright orange blobs with a white horn sticking out, hence the orange unicorn.
Cyclograpsus integer zoea larvae, photographed against white (left) and black (right) backgrounds under a dissecting microscope
Why these zoeae exhibit such bizarre coloration patterns is unknown, but in my research on the tolerance of crab larvae to ultraviolet radiation, this species was both the most pigmented zoea I studied as well as the most tolerant to ultraviolet radiation damage. Thus, the pigments may at least be shielding the zoea from UVR but I suspect something else is also driving this coloration since these larvae look so different from other crab larvae I've studied. I suspected it might be a warning coloration to warn predators they are chemically defended, like how monarch butterflies use bright colors to warn potential predators of the poisonous chemicals they store in their tissues. However, I fed some Cylcograpsus integer zoeae to the most common fish in their habitat (reef silverside: Hypoatherina harringtonensis) and they were readily gobbled up like any other food, with no adverse effects on the fish.
Cyclograpsus integer adult habitat on Galeta island, Panamá.
So I have no idea why these zoeae look like orange unicorns. If you have any ideas, I would love to hear them. As an interesting side note, the most common question I am asked when discussing my research on the coloration of larvae is whether the coloration of larval crabs is similar to the coloration of adults. My gut response is always no, since larval and adult crabs live in entirely different habitats so there would be no adaptive reason for their coloration to be similar, but in this case both larvae and adult have a striking orange appearance. This could be because many pigments are not produced by the animals themselves but most be sequestered from the foods they eat. Mothers must provision their offspring with these pigments so it is possible that the coloration of adults and larvae is simply related to the diet of the adults.