This nerve can stretch to twice its resting length. Did you just squirm? If you did, you probably know that the nerves of vertebrates are notoriously inelastic things, and that stretching them even a little bit can result in painful injury. But the nerve in this image is special. In fact, it’s evolved to stretch this way.
Above: A segment of fin whale nerve, at its initial and stretched lengths. The nerve has been manually stretched to a point, more than twice its initial length, where it abruptly stiffens and resists further extension | Credit: Vogl et al./Current Biology 2015
Researchers led by University of British Columbia zoologist Bob Shadwick describe the newly identified elasto-nerve in Monday’s issue of Current Biology. But to understand the discovery’s significance, you need to know a little about the animal it came from—a creature that study co-author Nick Pyenson once described to io9 as a “mammal from space.”
The nerve in question once belonged to a fin whale, one of several species of cetacean known collectively as rorquals. The rorqual family comprises the biggest of the baleen whales, including such titans as the humpback, the minke, and the blue whale, which, at roughly 100 feet (~30 meters) in length and and 180 tons (~163,000 kg) in weight, is the largest known animal ever to have lived.
Another thing rorquals share in common—apart from, but related to, their size—is feeding strategy. Big animals require big diets. To meet their gargantuan nutritional requirements, rorqual whales rely on their large mouths and a technique known as lunge feeding to scoop up colossal quantities of prey-laden water. For reference, a blue whale’s jaw can measure upwards of 18 feet (6 meters) in length, extend almost perpendicularly to its body, and swing open and shut in the span of about ten seconds. For a rorqual whale, to feed oneself is to deftly wield a biological ladle the size of a swimming pool.
An artist’s depiction of a rorqual whale engaged in lunge-feeding | Illustration by Carl Buell
It is perhaps unsurprising, then, that rorquals have been found to possess all manner of unusual features that facilitate this remarkable feeding strategy. These adaptations include their aforementioned, highly flexible jaws; a pleated throat that expands, balloon-like, to accommodate huge volumes of water; and an invertible tongue that makes way for expanding mouthfuls of briny prey. Among the more recently discovered adaptations is a grapefruit-sized mass of vessels and nervous tissue, located at the tips of rorquals’ lower jaws, that assists in the coordination of successful lunge-feeding. This sensory organ was first described by Pyenson, Shadwick, and their colleagues back in 2012. In Monday’s issue of Current Biology, the same team describes another strange evolutionary trait unique to these fascinating creatures: uncommonly elastic nerves, which the researchers describe as “an essential component of the extreme feeding mechanism of rorqual whales.”
“These large nerves actually stretch and recoil like bungee cords,” said A. Wayne Vogl of the University of British Columbia, first author of the study, in statement. According to Vogl, the discovery was made accidentally, when, during the dissection of a fin whale, a member of the team picked up a dull, white, cord-like structure and stretched it.
The team had originally thought it a blood vessel. Vertebrate vessels are usually stretchy. Vertebrate nerves are not; a thin sleeve of inflexible collagen limits them from elongating past the point of injury, which, for most nerves, isn’t far at all.
But the nerve recovered from the fin whale was different. If contained a bundle of densely folded fibers sheathed not just in collagen, but chains of a highly elastic protein called—aptly—elastin. The elastin allowed the nerve to stretch and the fibers at its core to unfold, while the collagen, as with other vertebrate nerves, prevented things from stretching too far. When the nerve was relaxed, the fibers re-packed into their folded configuration, ready to unfurl again at a moment’s notice.
Credit: Vogl et al./Current Biology 2015, Illustration by Carl Buell
The other notable thing about this nerve was its location. It was recovered from the tissues lining the floor of the whale’s oral cavity—a cavity that, in fin whales, can expand in circumference by upwards of 150% to accommodate its tremendous mouthfuls of water. Other nerves, like the one pictured at the top of this post, were recovered from the whale’s tongue. In this light, say the researchers, their discovery makes a lot of sense.
The bulk filter feeding mechanism employed by rorqual whales “required major changes in anatomy of the tongue and ventral blubber to allow large deformation,” says Vogl, “and now we recognize that this also required major modifications in the structure of nerves in these tissues so they could withstand the tissue deformation.”
“This discovery underscores how little we know about even the basic anatomy of the largest animals alive in the oceans today,” added Pyenson in a statement. “Our findings add to the growing list of evolutionary solutions that whales evolved in response to new challenges faced in marine environments over millions of years.”
Read the full scientific study in the latest issue of Current Biology.
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