Few people ever observe the world's most prevalent animal, but scrutiny by University of Hawaii researchers may change the way scientists look at the little insect-like ocean-dwelling invertebrates known as calanoid copepods. The UH researchers have discovered that nerve cells of the species Undinula vulgaris are coated with myelin -- a substance generally thought to be limited, with very few exceptions, to humans and other vertebrates. If upheld, their findings, which are reported in the April 15 issue of the journal Nature, will require revision of basic biology textbook discussions of the difference between vertebrates and invertebrates.
Researchers at the University of Hawaii at Manoa's Pacific Biomedical Research Center, observed myelin in transmission electron microscope images of the calanoid copepods collected in Oahu's Kaneohe Bay. The authors had previously observed that Undinula vulgaris are unusually fast in sprinting from danger -- responding to stimuli about a hundred times faster than humans. Myelin is a white fatty sheath that coats parts of the vertebrate nervous system, including the long axons of nerve cells. Like insulation on an electric wire, myelin protects and speeds the electrical signal, providing a competitive advantage for organisms that must respond quickly to capture food or escape predators. Such an advantage is particularly important for large animals, in which nerve signals must travel long distances from sensory cells to brain and brain to muscles, and scientists had assumed myelin was a feature nearly exclusive to vertebrates.
Most calanoid copepods are only 3 millimeters long or less and eat phytoplankton (plant plankton), other zooplankton or detritus. The most prevalent of the zooplankton, they constitute the biggest source of protein in the ocean and form a critical link in the marine food chain between the phytoplankton on which they feed and the krill, fish and whales that feed on them. In essence, they are the insects of the sea -- more prevalent but harder to study than their land-based counterparts.
The Pacific Biomedical Research Center team studies the behavior and morphology of copepods, particularly how anatomy and structure relate to sensory perception and reaction. The copepods that have myelin surrounding their axons exhibit consistently and significantly faster response time to stimulus in laboratory tests. (Deeper dwelling copepods luminesce in response to danger and other copepods move away from stimuli, though more slowly.)
Team members include researchers Petra Lenz and Daniel Hartline, researcher and electron microscope supervisor Tina (Weatherby) Carvalho and graduate student April Davis. It was Davis who first noticed the characteristic onion-ring-like myelin circles around axons in copepod cross sections. Myelin-looking artifacts can occur as chemical fixation anomalies, so she and Carvalho confirmed the results by preparing samples using an ultra-rapid freezing technique.
Because myelin does not occur in less-evolved copepods, it appears to have developed separately from that of vertebrates and other invertebrates, notes Lenz. The researchers' work has implications for the study of evolution as well as the understanding of the biology of the oceans.
The copepod research is funded by a grant from the National Science Foundation Division of Integrative Biology and neuroscience. UH's electron microscope facility is supported by the National Institute of Health's Research Centers in Minority Institutions Program.
Related Web Material:
copepod research, http://www.pbrc.hawaii.edu/~petra/copepod.html
copepod image, http://www.pbrc.hawaii.edu/bemf/microangela/pleuro.htm
copepod myelin image, http://www.pbrc.hawaii.edu/~petra/myelin.html
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