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Biology

Elastic nerves make rorqual whales the Mister Fantastic of the ocean

Article: Vogl, A.W., Lillie, M. A., Piscitelli, M. A., Goldbogen, J. A., Pyenson, N. D., Shadwick, R. E. 2015. Stretchy nerves are an essential component of the extreme feeding mechanism of roqual whales. Current Biology. 25, R345-R361. doi:10.1016/j.cub.2015.03.007

For those of you less familiar with Marvel comics canon, Mister Fantastic (a.k.a. Dr. Reed Richards) was a scientist that became a superhero when he and his merry band of explorers were bombarded by cosmic radiation while cruising above Earth in a space ship he created. As a result of this cosmic radiation, he gained the super human ability to stretch his body into any shape he desires. While rorqual whales cannot tie themselves into literal pretzels, they do exhibit a very similar, stretchy ability.

Whales in the Balaenopteridae family, including the ever popular humpback whale or the enormous blue whale, are unique among whales in that they obtain food through a means called lunge feeding (Fig 1).  These whales are easy to identify by their rorquals – long ridges of blubber running from the tip of their mouth down to about midway on the whales’ stomachs. Rorqual whales open their immense mouths and capture huge amounts of water containing their prey (like krill or small fish like herring) and then slowly concentrate the prey by squeezing water out of their mouths, catching prey in their filamentous baleen. This process requires rorquals to engulf vast amounts of water – sometimes more water than there is whale!

Fig 1

Figure 1. Rorqual whales, like humpbacks, fins, and blue whales, use lunge feeding to obtain their food. They open their mouths and swallow great volumes of water containing their prey and slowly push water out of their closed mouths, leaving just the food behind. In order to accomplish this, whales have highly stretchy tissue on the bottom of their oral cavity, called ventral grooved blubber (or VGB).

 

This type of feeding creates a huge amount of strain for the whale’s body. This is where the rorquals come in (that’s right, they’re not just decorative!). The rorquals act like the pleats of a skirt and fold excess skin, blubber, and tissue to keep the whales streamlined when not feeding, and expand during lunge feeding to comfortably engulf huge amounts of prey-filled water.

It turns out, though, there is more to this story. Recently, a team of researchers led by Dr. Andrew Vogl from the University of British Columbia were dissecting a washed up fin whale, when they discovered a very strange, long, cord-like tissue in the whale’s mouth. What was particularly interesting about this tissue was its ability to stretch and bounce back when placed under tension, much like a bungee cord. Close examination revealed the stretchy cord to be a nerve. Dr. Vogl and his team are the first to realize that rorqual whales have stretchy nerves in their mouths.

This discovery doesn’t seem very exciting – we already know that they have stretchy patches of skin and tissue under their mouths so shouldn’t it make sense that the nerves in the mouth are just as stretchy? But many studies of humans and other vertebrates have found that nerves aren’t stretchy at all. Some of the most common sports injuries we experience relate to nerve damage from overextension, or stretching, including muscles aches, pains, spasms, and in serious cases, paralysis.

Studies have shown that stretching a typical nerve found in most members of the animal kingdom more than 10% starts to cause changes in nerve performance. Stretching it to 30% longer makes the nerve completely lose function. But whale nerves? They can stretch up to anywhere from 75 to 115% of the original nerve length (Fig 2). So how is it that a human’s nerve can barely stretch at all without causing some sort of athletic injury, but a whale’s can extend almost twice its original length without causing any problems whatsoever?

Figure 2. The smaller image is a segment of a nerve taken from a dissected fin whale tongue in its resting (un-stretched) state. The larger image shows the extent that the nerve can stretch while still quickly returning to its original length without any observable damage. Nerves in the tongue and mouth of rorqual whales can extend from 75 to 115% longer than their resting state. In contrast to this, a 10% extension in a human nerve reduces nerve performance and a 30% extension would cause nerve failure and permanent damage.

Figure 2. The smaller image is a segment of a nerve taken from a dissected fin whale tongue in its resting (un-stretched) state. The larger image shows the extent that the nerve can stretch while still quickly returning to its original length without any observable damage. Nerves in the tongue and mouth of rorqual whales can extend from 75 to 115% longer than their resting state. In contrast to this, a 10% extension in a human nerve reduces nerve performance and a 30% extension would cause nerve failure and permanent damage.

The answer, they found, was in how the nerves are built. A typical (non-rorqual whale) nerve, the core is composed of densely packed nerve fibers which conduct the electrical impulses that direct your muscles to move. Surrounding these nerve fibers is a sheath of collagen – a tough, but not all that flexible, structural protein. However, the nerve fibers in the mouths of rorqual whales are much longer and folded, much like an accordion (Fig 3). This compressed bundle of nerve fibers are then wrapped in a wall that’s made up of collagen and another protein – elastin.

Figure 3. Nerves (center image) running through the VGB and tongue in rorqual whales are made up of nerve fiber bundles (right image) in the inner core, surrounded by an inner wall of collagen (top left image) and an outer wall of elastin (black spots with the arrow pointing to them, lower left image).

Figure 3. Nerves (center image) running through the VGB and tongue in rorqual whales are made up of nerve fiber bundles (right image) in the inner core, surrounded by an inner wall of collagen (top left image) and an outer wall of elastin (black spots with the arrow pointing to them, lower left image).

When the whales are filling their oral cavities with water, the elastin lets the nerve stretch, and the compressed nerve fibers within are allowed to elongate. After reaching some critical stretch length, the collagen in the nerve wall tenses up, forcing the nerve to stop stretching before injury can occur (Fig 4). Then, the whale is able to slowly contract the jaw muscles and push out the water through their baleen. So while the nerve itself is stretching quite a bit, the actual nerve fibers, the sensitive conductors of nerve impulses, are not actually under any tension, but rather are just stretching to their full length.

Figure 4. When in the resting state, the nerves’ wall and core are highly bundled and compressed. When extended, both the nerve wall and nerve core unfold but do not undergo over extension, which causes nerve damage.

Figure 4. When in the resting state, the nerves’ wall and core are highly bundled and compressed. When extended, both the nerve wall and nerve core unfold but do not undergo over extension, which causes nerve damage.

This finding adds another piece to the rich mosaic of whale biology and evolution. Discoveries like these, based on simple exploration, are sometimes the findings that shift how we think about entire fields of science. Discoveries like these, happy accidents, happen fairly often in science and are a surprising source of some very important discoveries. Do you know of any other examples of accidental discoveries of important scientific findings? Can you imagine where discoveries like these can lead humans?

Irvin Huang
A recent convert to oceanography, I’m studying under Dr. Anne McElroy at Stony Brook University’s School of Marine and Atmospheric Sciences. My research uses biochemical and genomic methods to investigate how coastal organisms respond to environmental stress.

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