New findings about the ancient origins of sea spider appendages have shaken up the world of evolutionary biology
Sea spiders have claws on the front of their bodies, and that’s news. Amy Maxmen, of Harvard University, and William Browne, of the University of Hawai‘i, have proved that at least one species of sea spider has a set of claws connected to the first segment of their bodies. The two graduate researchers were working in the laboratory of Mark Martindale at the University of Hawai‘i’s Kewalo Marine Laboratory. They recently discovered these frontal claws, which may be unique among arthropods, a large species group that includes lobsters, spiders, and all insects. Arthropods have eyes connected to their frontal lobes but no claws. Maxmen and Browne’s sea spiders have both.
The two researchers published an article outlining their findings in the scientific journal Nature in October 2005. The article has caused a furor among arthropod experts, and raised the intriguing possibility that living sea spiders share features with arthropod species that lived in the oceans hundreds of millions of years ago, and had large claws attached to the first section of their bodies. The finding should spur further research on the evolution of sea spiders, also called pycnogonids (Latin for “knobby knees”), and how they relate to other animals in the arthropod phylum.
Fossil records dating back 500 million years show ocean-dwelling arthropods had prominent pincers attached to their first body segment. Scientists had long believed that these claws disappeared among all arthropods. However, research on the evolution and biology of sea spiders has been scant, and few fossil records of these creatures exist. These spindly, eight-legged animals resemble land spiders in appearance. But sea spiders have long snouts (proboscises) used to suck out the juices of sea anemones and hydroids. Sea spiders also have frontal claws rather than fangs seen in land spiders. The creatures live in both cold and warm waters, and range in size from a few millimeters in length to a full foot across. Most, including the ones that Browne and Maxmen described, measure less than three millimeters in diameter.
Maxmen became interested in sea spiders after identifying them as bycatch on nearby fishing docks frequented by Honolulu-based fishing vessels close to the Kewalo Lab, where she and Browne work with Martindale, their advisor at UH. (Maxmen and Browne also received assistance from Gonzalo Giribet of Harvard University, who, along with Martindale, was cited as a co-author on the Nature article.) Maxmen and Browne harvested sea spider eggs from the leg of a male specimen and hatched them in the lab. They tracked the nervous- system development of the larvae using a powerful optical microscope and dyes to tag tiny nerve pathways.
The researchers first located two nerves within the pair of stubby claws attached to the very front of the sea spiders, which, like all arthropods, have highly segmented bodies. Then, Maxmen and Browne tracked the nerves’ pathways to the first segment of the sea spiders’ bodies, where the very front part of the animal’s brain is located, thus cementing their hypothesis. They plan to continue research into the evolution of sea spiders. Their future insights could help biologists better understand evolution in arthropods in general, an extremely diverse and biologically significant group of organisms.
William Browne, Ph.D., is a National Science Foundation post-doctoral fellow working in the laboratory of Mark Martindale, Ph.D., at the oceanfront Kewalo Marine Laboratory. Browne studies head and nervous system development of crustaceans, including sea scuds. Amy Maxmen is studying sea spiders for her Ph.D. thesis at Harvard University, where she is a doctoral researcher in the Department of Organismic and Evolutionary Biology. The Kewalo facility is one of the only sophisticated research labs in the world with direct seawater supply from a tropical ocean.
Photos: Amy Maxmen - Amy Maxmen Photo: Jon Chase/Harvard News Office