University of Queensland
This hybrid black tip shark contains both Common and Australian black tip DNA.
Credit: AnnaMargittai, Flickr
Sharks and humans have a tortured relationship. But underneath the fearsome killer instinct and razor-sharp teeth is an ultra-sensitive machine that nature has tuned to perfection. A shark's body is bristling with sensory organs, listening, feeling and responding to an underwater world humming with activity. Their famed ability to detect even a whiff of blood is just the beginning. Sharks have ears and eyes similar to humans, as well as electrical receptors that can detect a struggling fish or Earth's magnetic field. A line of pressure-sensitive cells runs down each side of their body as well, letting them know if anything stirs in the waters nearby.
Sharks' electric sense perception deserves its spooky abbreviation, “ESP.” Sharks' ampullae of Lorenzini (pictured) are little tubes that are extremely sensitive to the electrical signals emitted by struggling, wounded fish – or even from a single muscle twitch. This allows them to hone in on struggling fish, or prey that may stir while buried in the sand. Electroreceptors first appear early on in the development of shark embryos (inset), recent research found, developing out of the same stem cells that shape humans' head and facial features. The finding suggests that electro-sensing was probably common in our ancient ancestors until they came ashore and lost the ability.
“And, you know, the thing about a shark... he's got lifeless eyes. Black eyes. Like a doll's eyes. When he comes at ya, doesn't seem to be living...” -- Jaws, 1975 Like most pop culture references to sharks, this famous quote from Steven Spielberg's Jaws is overstated. The shark is famous for its empty stare, but erroneously so -- sharks have pupils just like humans that can expand and contract to control the amount of light they let in. Their eyes even have color (pictured). Sharks can see very well in low-light conditions common underwater, but their eyes don't have lenses and so can't reolve shapes.
Nostrils of Death
Just like a human's, the shark's nose picks up chemicals mixed into the surrounding environment. But since this all occurs underwater, the nose needs to be able to channel water across its feathered-shaped olfactory organs - called lamellae - and scan for a hint of something good to eat.
The Sounds of Supper
Sharks' sense of hearing and taste are both linked to touch – they literally feel their way to their next meal. While sharks are able to detect sound through an organ in the utriculus portion of the inner ear, they primarily rely on small hairs in the ears to feel changes in the water. Taste is similar -- sharks often bite their prey once before deciding whether or not to dive in for a meal. This behavior is mostly based on how the quarry feels in the shark's mouth; sharks don't exactly taste their "snack," so much as feel how soft, hard and potentially nutritious it will be.
Feeling the Pressure
A shark's lateral line is essentially a pressure meter: as any solid body moves through the water, it sends out pressure waves. Sharks both detect that movement and sense direction, feeling from which direction the pressure waves came as if they were touching the object itself. Humans have no sense comparable to the lateral line.
Like most animals, sharks are eating, mating machines. And as the dominant predators of the seas, sharks have the ability to inspire fear and encourage us to keep our distance. But they are also marvels of natural engineering, able to function effectively in the often dark, murky and confusing underwater environment. They are streamlined, beautiful, nearly perfect; as the dominant species on the planet, we still have much to learn from them.
- The first-ever observed shark hybrid was found in Australian waters.
- The shark is a combination of Australian black-tip shark and its global counterpart, the common black-tip.
- The hybrid appears to be robust, with a number of generations discovered.
Scientists said on Tuesday that they had discovered the world's first hybrid sharks in Australian waters, a potential sign the predators were adapting to cope with climate change.
The mating of the local Australian black-tip shark with its global counterpart, the common black-tip, was an unprecedented discovery with implications for the entire shark world, said lead researcher Jess Morgan.
"It's very surprising because no one's ever seen shark hybrids before, this is not a common occurrence by any stretch of the imagination," Morgan, from the University of Queensland, said.
"This is evolution in action."
Colin Simpfendorfer, a partner in Morgan's research from James Cook University, said initial studies suggested the hybrid species was relatively robust, with a number of generations discovered across 57 specimens.
The find was made during cataloging work off Australia's east coast when Morgan said genetic testing showed certain sharks to be one species when physically they looked to be another.
The Australian black-tip is slightly smaller than its common cousin and can only live in tropical waters, but its hybrid offspring have been found 2,000 kilometers (1,243 miles) down the coast, in cooler seas.
It means the Australian black-tip could be adapting to ensure its survival as sea temperatures change because of global warming.
"If it hybridizes with the common species it can effectively shift its range further south into cooler waters, so the effect of this hybridizing is a range expansion," Morgan said.
"It's enabled a species restricted to the tropics to move into temperate waters."
Climate change and human fishing are some of the potential triggers being investigated by the team, with further genetic mapping also planned to examine whether it was an ancient process just discovered or a more recent phenomenon.
If the hybrid was found to be stronger than its parent species -- a literal survival of the fittest -- Simpfendorfer said it may eventually outlast its so-called pure-bred predecessors.
"We don't know whether that's the case here, but certainly we know that they are viable, they reproduce and that there are multiple generations of hybrids now that we can see from the genetic road map that we've generated from these animals," he said.
"Certainly it appears that they are fairly fit individuals."
The hybrids were extraordinarily abundant, accounting for up to 20 percent of black-tip populations in some areas, but Morgan said that didn't appear to be at the expense of their single-breed parents, adding to the mystery.
Simpfendorfer said the study, published late last month in Conservation Genetics, could challenge traditional ideas of how sharks had and were continuing to evolve.
"We thought we understood how species of sharks have separated, but what this is telling us is that in reality we probably don't fully understand the mechanisms that keep species of shark separate," he said.
"And in fact, this may be happening in more species than these two."