Here’s something cool that I’ve meaning to write about for a long time. A native Powelliphanta land snail with an apparently pigment-less foot and head:
That snail (a close relative of speedy carnivore featured here) popped up in Kahurangi National Park at the end of last year. Apart from just being kind of cool, the un-pigmented individual is interesting for a geneticist that studies land snails. For the most part, dark pigmentation in snails results form melanin (which is perhaps the most common pigment in the animal world). That’s true for pigmentation of the shell as well as the animal that caries it around. As you can see, this snail has normal pigmentation on its shell, so clearly its still able to make melanin. The genetic mutation (or developmental defect) that has left this snail white hasn’t broken the genes for pigmentation, just the mechanism that moves that pigment around the body wall of the snail.
The ghost Powelliphanta is a pretty cool snail, but there’s actually an albino snail that’s even more interesting. Every now and again a truly albino individual of the freshwater snail Biomphalaria glabrata pops up. Looking at these mutants we can learn something about the evolutionary history of the these snails:
Photo is CC 2.5 and comes from Lewis FA, Liang Y-s, Raghavan N, Knight M et al in PLoS Tropical Diseases
Free from the pigments that would usually make shell opaque we can see the feature that sets Biomphalaria and other species form the family Planorbidae (ramshorn snails) apart from every other snail. The planorbids are the only red-blooded snails on earth. So why are these snails so different?
As we all know, in order to live animals need to get oxygen from their environment into their bodies. For small animals this doesn’t represent a huge problem. Oxygen will flow form areas of high partial pressure (a concept analgous to concentration, but accounting for some of the weird ways gasses behave) to areas in which Oxygen is being used up. So, for instance, most insects pull air directly into their bodies with a set of open tubes (called tracheae). Once the air makes it into those tubes oxygen will passively diffuse into the insect’s tissues.
Big animals have a much bigger problem*. Not only do larger animals need much more oxygen to fuel their bodies, they also have to actively transport that Oxygen because the distances it is required to travel can’t be achieved by passive diffusion. Lungs and gills are both organs dedicated to pumping more oxygen into animal bodies, and many animals use blood, and special proteins dissolved in blood, to move oxygen about.
In vertebrates the oxygen-carrying protein is called hemoglobin. Very simply, a hemoglobin molecule is a cage used to hold iron atoms in such a way that they will bind to an oxygen atom. The iron containing group in the hemoglobin protein (called heme) gives our blood its red colour and its hemoglobin circulating through that snail’s body that makes it red.
As with every problem life faces, invertebrates have come up with many more interesting ways to move oxygen around than their spined relatives. Annelids (earthworms and their kin), brachiapods and spoon worms have a whole set of iron-containing proteins to do the job. Even more interestingly, molluscs and some arthropods have a protein that uses Copper rather than Iron atoms to co-ordinate an oxygen molecule. This molecule, called hemocyanin, takes on a green-ish blue hue when oxygen binds to it and changes its conformation.
Most snails get through life fine with hemocyanin as the only oxygen-carrying molecule in their blood, so why have Biomphalaria and their cousins become red-blooded? Part of the reasons lies in their lifestyle. Planorbid snails breath with lungs (which only work in air) but live underwater. If you make your living by holding your breath while diving then you really want to have some way of holding on to as much of the oxygen you get form each breath for as long as possible. It seems that Biomphalaria hemoglobin is more efficient at using the oxygen stored in lungs while diving than any hemocyanin could be.
It’s all well talking about why an animal might have evolved a particular trait. But in evolutionary biology it’s generally much more intresting to try and work out how. How does an air-breathing snail make its own hemoglobin from scratch? A team lead by Bernhard Lieb asked just that question a few years ago, and found the answer: Biomphalaria hemoglobin was made by cobbling together parts of existing proteins. When Lieb et al (2006, doi: 10.1073/pnas.0601861103) isolated hemoglobin from red-blooded snails they found it was made up of two different components (called peptides), each of which has 13 different sub-components (called domains). When the team compared the sequence of those peptides and their domains to other molluscan proteins they found similarties between the hemoglobin sequences and another iron-containing protein called myoglobin.
Myoglobin is a small molecule that is usually restricted to muscles where is acts as a store of Oxygen (in snails, myoglobin is most commonly found in the muscles that drive the radula, the rasp like organ used to break down food). The Biomphalaria hemoglobin sequences are more closely related to each other than they are to myoglobins from any other species. This pattern suggests the sequences that make up the snail hemoglobin descend from a single common ancestor. Subsequent changes to each of these descendants have allowed the descendants proteins to group together and become “super myoglobins” capable of transporting oxygen through the body.
*The huge number of ways size matters in biology were wonderfully explained by JBS Haldane. I’d reproduce the most famous passage here, but it’s probably even better if you discover it by yourself.