I am not a big fan of reality TV, but I will confess that I am a loyal viewer of the Discovery Channel’s ‘Deadliest Catch’ series. The show chronicles the adventures of the crews of several crabbing boats of the Alaskan fleet as they pursue red king crabs on the Bering Sea. What fascinates me, and I suspect other viewers, is the vicarious experience of watching the crews working for long stretches in unimaginable conditions.
I know that this landlubber would not last an hour on any boat as it heaved in 30-foot seas, let alone while hauling 800-pound crab pots on an ice-covered deck, in 60-mile-an-hour winds, for 20 to 30 hours straight. That’s definitely not for me. My crab-catching is limited to plucking hermit crabs the size of golf balls off the sands of some quiet Florida beach in 80-degree weather.One might think that not only is there no comparison between my beachcombing and the dangerous business of Alaska crab fishing but that the two kinds of crabs involved have very little in common.
The typically diminutive hermit crabs have to contort their bodies into abandoned snail shells, while the four- to nine-pound red king crabs, the largest of the more than 100 species of king crabs, freely prowl the ocean bottom in search of worms, clams, mussels, starfish and other prey.
Looking at those monsters of the deep, safely steamed on your plate at Red Lobster, one might think that such tasty beauties would be more closely related to other crabs on the menu, like stone crabs, than to the largely inedible hermit crabs. But think again. DNA evidence links king crabs to hermit crabs more closely than to any other crabs.
In fact, it is now very clear that the majestic king crabs evolved from a hermit crab ancestor.The genetic link between the two crabs was first detected by Cliff Cunningham, now at Duke University, and colleagues almost two decades ago.
In a new article in the journal Systematic Biology, K H Chu and colleagues from the Chinese University of Hong Kong, National Taiwan Ocean University and the Australian Museum in Sydney, have strengthened that genetic link and shown that hermit crab ancestors also gave rise to so-called squat lobsters, free-living crabs with a lobsterlike form that are not directly related to lobsters.
The surprising stories of king crab and squat lobster evolution from hermit crab ancestors are examples of a frequent lesson in the era of DNA-based genealogy – looks can be deceiving. Very dissimilar forms can evolve from a given type of ancestor, and similar-appearing forms can evolve entirely independently from unrelated ancestors.
On how the hermit became king
As different as the hermit and king crab body forms appear at first glance, when one looks more closely at some details of their anatomy and behaviour, some features can be seen that reflect their relatedness and help us to imagine how a small, contorted shell-toting hermit evolved to be king.
For example, many hermit crabs have a soft, asymmetrical coiled hindbody that allows them to fit into the right-handed twisted shells of their mobile homes. In the evolution of the king crab, this hindbody has become shortened and folded flat under the front part of the body. Nevertheless, traces of this asymmetry can be seen in the king crab hindbody. Furthermore, asymmetric hermit crabs usually have a larger right claw, as do king crabs.
We can also better picture how a shell-toting hermit ancestor eventually gave rise to large free-living king crab by considering the lifestyle of the coconut crab. This species can reach up to 9 pounds and is the largest land-dwelling arthropod. In fact, it is a member of the hermit crab superfamily. While the adult crabs are free-living and hard-bodied, juvenile coconut crabs have a soft hindbody and live within the snail shells.
The two life stages of this one species demonstrate that the shift from shell-toting hermit to large king would not be as complicated as it might first appear. Another well-established example of deceptive looks in the animal kingdom involves the rock hyrax, elephant and manatee. Glancing at the modern forms of these mammals, one would hardly suspect that they are very closely related.
The hyrax, weighing 8 to 10 pounds and resembling a guinea pig, lives among rock crevices in Africa and parts of the Middle East. It hardly seems possible that is could be close kin to the 10,000-pound, trunk-swinging African elephant or the aquatic manatee. But again, DNA evidence makes it crystal clear that these species, which belong to three different orders, all belong to one larger group known as the Paenungulata that arose from a common ancestor that lived in Africa roughly 60 million years ago.
Kinship
Each group – the hyraxes, the sirenians (manatees and dugongs) and the elephants, is the other groups’ closest living relatives.Those close relationships are also reflected in specific features of the animals’ anatomies and behaviour. For example, if one looked closely at males for their testicles (not that I recommend doing so, particularly for elephants), one would not find them.
The testes are inside the abdomen in each group. The animals also share common details in their tongues, hooves and some skeletal features. In addition, all have very long gestation periods; the hyrax’s is seven to eight months, which is exceptional for a relatively small mammal.
While the evolutionary relationships here are surprising, it should be no surprise that the first naturalist to appreciate how looks could trick us was Charles Darwin. One stroke of his genius in “On the Origin of Species” was to begin the book with a discussion of domesticated animals, like the pigeons he had bred for many years.
Darwin noted the great disparities among the English carrier, short-faced tumbler, pouter, fantail and other breeds. He suggested that these many birds, “if shown to an ornithologist, and he were told they were wild birds, would certainly, I think, be ranked by him as well-defined species.”
Yet, Darwin pointed out, they were all descended from the rock pigeon.This observation then laid the foundation for Darwin’s explanations of the connections between the great diversity of forms on the planet – connections that continue to surprise us 150 years later as biologists now mine the DNA record of life.
New York Times News Service
