HARVARD GAZETTE ARCHIVES
A tale of a venomous dispute
How the spider got a head
By William J. Cromie
Harvard News Office
Living creatures have been found with claws growing out of the front of their brains. They are sea spiders netted off Hawaii by researchers from Harvard and the University of Hawaii.
The discovery adds to the lore of this obscure but bizarre and fascinating group of buglike beasts. They creep around shallow and deep parts of the oceans from warm tropical waters to frigid polar seas. They have bodies so slender that their stomach, intestines, and gonads must be squeezed into their thin legs. Males brood the eggs of their young in specialized arms.
Sea spiders as large as a foot across have been seen crawling along the deep ocean floor from the windows of submersible research vessels. Most of them, however, including those in this study, are a scant millimeter (.04 inch) in size. But big or small, they boast long snouts, on either side of which grow pincerlike claws.
Zoologists classify them as arthropods, a group that includes all the insects, land-loving spiders, and crustaceans from flea-size shrimp to lobsters. Together they make up the largest class of animals on Earth.
"In all other arthropods, the front section of the brain bears only eyes," notes Amy Maxmen, 27, a Harvard graduate student who studies sea spiders for her Ph.D. thesis. "Our observation is the first ever of a clawlike appendage arising from that part of the brain. The finding supports assumptions by others that some ancestors of living arthropods once had a pair of pincers or antennae, along with their eyes, extending from the forward parts of their brains."
"At first sight, this is a rather esoteric finding," according to two scientists who commented on the discovery as it is reported in the Oct. 20 issue of the science journal Nature. "But if it is correct, it will shake up the field of arthropod evolution."
"The evolutionary status of the front part of the brain has been a point of contention for a long time," Maxmen notes. Prior to her study, no living arthropods had anything but eyes growing out of this part of the head. However, it appeared that some extinct arthropods did. Thus, sea spiders, rather than being odd, water-living relatives of ordinary house spiders, may be the only surviving relatives of arthropods who walked the oceans' floors some 500 million years ago with appendages attached to the front of their heads. Fossils of such beasts reveal that many of them carried large, grasping or branched pincers that have become known as the "great appendage."
According to the commentators, the exciting implication is that sea spiders "are extraordinary living fossils retaining an organization of their heads that all other living arthropods lost hundreds of millions of years ago."
Ordinary spiders, including those who decorate houses with their gossamer webs, also boast pincerlike fangs protruding from their heads, as do scorpions. Flies and other insects wave antennae from their heads. Crustaceans like horseshoe crabs and lobsters display a wide variety of clawed headdresses. But these arise from further back in the brain.
All arthropods are characterized by their segments, jointed parts extending up or down from tail to head. Lobsters, for example, boast segments with slender legs, powerful claws, and antennae. Even arthropod brains are segmented. All the dizzying variety of head antennae, claws, fangs, and pincers come from the second segment of the head. They are operated by nerves in that part of the brain. Only their eyes are innervated by the first segment. Sea spiders are the grand exception.
To zoologists, the segments and their attachments define the biology and evolution of this dominating group of creatures. "Scientists use information about the structure of segments and appendages, combined with an animal's relationship to its near relatives, to try to understand what went on during evolution," Maxmen explains. "We are confident that the antennae of flies, fangs of spiders, and claws of horseshoe crabs evolved from the same structure in a common ancestor. But sea spiders are unique; their closest relatives remain unknown. Are they primitive members of a group that includes horseshoe crabs and true spiders, or do they belong in a class of their own?"
That's the question Maxmen and her collaborators tried to answer by looking at the brains of sea spiders and comparing them with other arthropods. To do so, she worked with Gonzalo Giribet, an associate professor in Harvard's Department of Organismic and Evolutionary Biology. She did fieldwork at the University of Hawaii with William Browne and Mark Martindale. Browne says that he once saw large congregations of big, fleshy sea spiders, each nearly a foot long, crawling along the deep ocean floor.
Maxmen collected tiny male sea spiders, about one-twenty-fifth of an inch long, living in Kewalo Basin on Oahu. She took eggs from their arms and waited for them to hatch. Such larvae are a mere two thousandths of an inch in size.
After that, came the hard work. Maxmen had to find and trace the development of the nerves that operate their claws. With the help of molecule-size tags and a powerful laser microscope, she found that the nerves came from the first head segment, further forward than nerves that move the claws of true spiders and the appendages of their insect and crustacean cousins. This finding makes it doubtful that land and sea spiders are true sisters.
But the case is not closed. "Our research presents an important difference between sea spiders and land spiders," Maxmen points out. "Now, additional studies comparing sea spiders to other arthropods are needed before the debate can be put to rest."
The controversy is by no means a minor dispute waged in a gentle manner in the quiet halls of zoological museums. The two scientists mentioned earlier who commented on Maxmen's report in Nature, Graham Budd of the University of Uppsala in Sweden and Maximilian Telford of University College London, put it this way: "The conclusions of Maxmen (and her collaborators) overturn entrenched ideas about the body plan of sea spiders, and, furthermore, lend support to some controversial theories of arthropod evolution. Unlike [true spiders], sea spiders lack a poisonous bite, but this paper is bound to inject venom into what is already one of the most controversial of zoological topics."
Maxmen, who is bound to be at the center of the dispute, says, "Hopefully, criticism will be in the form of new research that our paper has helped to spark."