There are four species of horseshoe crabs in the world, with the most familiar to Americans being Limulus polyphemus, the Atlantic horseshoe crab. Here’s a group of them mating:
They are arthopods, but not “true” crabs, which are in the subphylum Crustacea. Horseshoe crabs are in a different subphylum, the Chelicerata, more closely related to spiders and scorpions than to the crabs we know.
A good website on the group describes its morphology, behavior, and medical uses (yes, they’re useful in making drugs; see here). Plus they’re really cute. Here’s a description of the legs of modern animals:
The horseshoe crab has 6 pairs of appendages on the posterior side of the prosoma. Five pairs of walking legs or pedipalps enable the horseshoe crab to easily move along benthic sediments. Each has a small claw at the tip except the last pair. The last pair of legs has a leaflike structure at the terminal end that is used for pushing and clearing away sediments as the crab burrows into marine bottom. The base of each leg is covered with inward pointing spines called gnathobases that move food towards the mouth located between the legs. As the legs are moving, food is crushed and macerated. There are also 2 small chelicera appendages that help guide food into the mouth.
And this is what they look like upside down. Note the five pairs of walking legs, which are “uniramous,” that is, each leg comprises a series of segments attached end to end:
Horseshoe crabs are famous for having changed little in morphology since they first show up in the fossil record over 400 million years ago (!): they are thus called “living fossils”. (In fact, the title of Richard Fortey’s new book on living fossils is Horseshoe Crabs and Velvet Worms. Well, living species are remarkably similar in general external features to their long-dead (and extinct) relatives, but there are distinct differences, and of course we know nothing about the difference in their internal features, nor in the structure of the DNA of ancient species (go here for more criticisms of the notion that horseshoe crabs are living fossils). Nevertheless, there is surprising “stasis” of morphology over a very long period of time, and we’re not sure why that is.
A new paper in the Proceedings of the National Academy of Sciences by Briggs et al. (reference below, the “Briggs” is Derek Briggs of Burgess Shale fossil fame) casts further doubt on the “living fossil” status of horseshoe crabs, for not even their external morphology has remained reasonably similar. In particular, their legs have changed drastically.
The authors examined a fossil horseshoe crab found in Herefordshire (UK) dated at about 425 million years old. This is the specimen, which doesn’t look promising, but they’re able to make out telling details of the appendages:
And what they find is that the limbs, unlike the uniramous limbs of modern horseshoe crabs, are biramous: each limb branches into two parts, with each branch comprising a series of segments attached end to end. As the paper describes,
Appendages 2 to 6 project just beyond the head shield (Fig. 1 F and H). Appendages 2 to 5 are biramous (Fig. 1 F and I). The inner rami (endopods) insert in a series posterior of the chelicera, surrounding a raised central area occupied by the mouth. The outer rami insert along the outer margin of the ventral body wall. There is no evidence that these two rami were connected by an elongate limb base like the coxa in Limulus. Nonetheless the rami clearly represent elements of the same limb rather than successive limbs alternating in morphology.
Here’s a reconstruction showing first the group of biramous limbs (each bifurcated limb has a green section and a blue section) and then two pairs of limbs, with one part attached, as noted above, to the outer margin of the body wall, and the other part attached further inside around a raised ventral area (the “inside limbs,” as you can see from the above diagram of a modern crab, are the ones that remain). Phys.Org describes how these reconstructions were made:
The name of the new fossil, Dibasterium durgae, refers to the double limbs and to Durga, the Hindu goddess with many arms. It was reconstructed in three dimensions by stacking digital images of physical surfaces exposed by grinding away layers in tiny increments.
Two pair of limbs. Modern crabs are missing the blue member of each pair.
What has happened, evolution-wise? Obviously, an entire set of limbs, the outer member of each biramous pair, has disappeared somewhere during the evolution of modern horseshoe crabs. We don’t know exactly why this happened, or, if it was due to selection, what form of selection would favor such loss. But the authors do speculate about the genetic basis of the loss.
In particular, in modern crabs the areas where the outer limbs would have been express the gene Distal-less (Dll) only in a transitory fashion, while the inner limbs—the ones that are present—express more marked expression of Dll. The gene is known to be required for limb formation in other arthropods, like the fruit fly Drosophila, so it’s possible that the loss of the outer set of limbs somehow occurred by loss of Dll expression at some time in the distant past. The authors emphasize that this is a hypothesis that is untested, for of course there’s no way we know of to see what gene expression was like in the ancient biramous fossils.
I suppose along with that theory goes a “macromutational” corollary that perhaps a single mutation in the Dll gene changed its expression so that the outer set of limbs was eliminated in one fell swoop. If Dll is indeed involved, I would find the single-mutation scenario unlikely. When we see a change in animals that looks like it could be caused by a single mutation that has similar effects in modern species (like the Bithorax mutation in Drosophila, which causes the appearance of four wings instead of the usual two), it’s dangerous to say that, for example, the gain or loss of wings in insect evolution was due to single mutations at a gene like Bithorax. Such changes could well be due to the accumulation of several to many genes of smaller effect.
Regardless of its cause, the difference in limb number between ancient and modern horseshoe crabs shows that what are regarded as “living fossils” may well be at one extreme of “changeability” among long-lived groups, but they have by no means remained completely unchanged.
Briggs, D. E. G., D. J. Siveter, D. J. Siveter, M. D. Sutton, R. J. Garwood, and D. Legg. 2012. Silurian horseshoe crab illuminates the evolution of arthropod limbs. Proceedings of the National Academy of Sciences, online publication, 10.1073/pnas.1205875109.