Last Friday you guys voted overwhelmingly for Hermit Crab Basics as this week’s MLS topic. Unfortunately I’ve been dealing with the flu all week so I haven’t been up for doing the research. I’ll tackle that next week. Tonight I’d like to show you my newest marine acquisition, which was brought in just a few hours ago by a friend of mine who has been fishing rock crabs for about thirty years. This is the first albino he’s ever seen.
(Last week’s diary was called Cassiopeia, about the upside-down jellyfish, and is here if you missed it.)
In vertebrates albinism is usually caused by inheritance of two recessive genes which prevents the body from producing melanin, resulting in white skin and hair, feathers or scales covering the body. These genes come from parents who may not necessarily be albinos themselves, but are carriers of this trait. The chances of an albino being born to two carrier parents is one in four.
When albinism occurs in invertebrates it is usually due to a genetic mutation rather than inheritance. The color of a crab’s exoskeleton is produced by the combination of many different color pigments. For different species different colors tend to dominate. Occasionally an error occurs which causes one or more of the pigments to mask the others. These individuals are known as color morphs. As you can see below, normally a rock crab’s shell is mostly red with some white and orange pigmentation blended in.
photo by NOAA
I’m using the term "rock crab" here because that is the generic name given to any of the number of species found in the genus Cancer. This particular one is called a Jonah Crab (Cancer borealis).
I’ll be doing a diary at some point on the color morphs of lobsters, so I won’t go into a lot of detail about them here, but the new albino crab now shares a large aquarium with two of my lobster color morphs. Lobsters and rock crabs are both cold-water species that will not survive at room temperature. So I have their tank equipped with a chiller to keep the temperature in the fifties. Here she is with my blue lobster. The divider keeps them from killing each other.
Also housed in this aquarium is a large calico lobster. Calicos, which are much less common than the more spectacular-looking blues, have an exoskeleton that has black and yellow pigments dominating different parts of the body. Here she is below.
I’m often asked what the odds of a color morph occurring are. Different sources have different numbers, and I’m not sure if anyone even knows. My best guess is 1 in 5 million for blue lobsters and 1 in 25 million for calicos. Albinos are by far the rarest color morph, occurring in roughly 1 in every 100 million lobsters. Considering that about 3000 tons of Jonah crabs are landed each year, and none of the fishermen I’ve talked to so far has ever seen another albino before, I think this figure probably is pretty close to the odds of this white crab also occurring.
One thing to keep in mind is that like most marine invertebrates, crustaceans produce hundreds of thousands to millions of eggs per female to offset predation of the larval stages. Very few of these will survive to adulthood. It is likely that color morphs suffer higher mortality than normal-colored ones, which means the birth of an albino may be fairly common, but their low survival rate is actually what makes them so rare.
Unless otherwise noted, all photos were taken by Rebecca Bray.
Other diaries in this series can be found here.