Scientists Trace Origin Of Shark’s Electric Sense

The dark markings indicate gene expression in the electrosensory organs in the head of an shark.

Gainsville, Florida (Feb 6 2006 18:53 EST) Sharks are known for their almost uncanny ability to detect electrical signals while hunting and navigating.

Now researchers have traced the origin of those electrosensory powers to the same type of embryonic cells that gives rise to many head and facial features in humans.

The discovery, reported by University of Florida scientists in the current edition of Evolution & Development, identifies neural crest cells, which are common in vertebrate development, as a source of sharks’ electrical ESP.

It also fortifies the idea that before our early ancestors emerged from the sea, they too had the ability to detect electric fields.

“Sharks have a network of electrosensory cells that allows them to hunt by detecting electrical signals generated by prey,” said Martin Cohn, a developmental biologist with the departments of zoology and anatomy and cell biology, and the UF Genetics Institute. “That doesn’t mean they can only detect electric fish. They can sense electricity generated by a muscle twitch, even if it’s the weak signal of a flounder buried under sand.”

Likewise, sharks are widely thought to use the Earth’s magnetic field for navigation, enabling them to swim in precise paths across large expanses of featureless ocean, Cohn said.

“If you think of this in the big picture of evolution of sensory systems, such as olfaction, hearing, vision and touch, this shows sharks took a pre-existing genetic program and used it to build yet another type of sensory system,” Cohn said.

UF and University of Louisiana researchers analyzed electroreceptor development in the embryos of the lesser spotted catshark, an animal that is largely motionless during the day and hunts at night, mainly in the seagrass beds of the eastern Atlantic Ocean.

Using molecular tests, scientists found two independent genetic markers of neural crest cells in the animal’s electricity-sensing organs. Analysis shows these cells migrate from the brain and travel into the developing shark’s head, creating the framework for the electrosensory system – a previously unknown function of a much-studied group of cells, according to Renata Freitas, a doctoral candidate in UF’s zoology department and first author of the paper.

The process mirrors the development of the lateral line that allows fish to mechanically sense their environment, and organs of the inner ear that enable people to keep their balance. But scientists suspect as human ancestors emerged from the sea, they discarded their lateral lines as well as their ability to sense electrical fields.

“Our fishy ancestors had the anatomy for it,” said James Albert, a former UF biologist who is now at the University of Louisiana. “You can imagine how valuable this system would be if you were aquatic, because water is so conductive. But it doesn’t work on land – air doesn’t conduct electricity as well. When it happens, it’s called a lightning bolt and you don’t need special receptors to sense it.”

All primitive animals with backbones could sense electricity, according to Michael Coates, an associate professor of organismal biology and anatomy at the University of Chicago. Mammals, reptiles and birds lost the sense over time, as did most fish alive today.

But in sharks and a few other species, such as sturgeons and lampreys, electrosensory capability endured.

“Most fish you see today have large eyes,” Coates said. “But sharks are predators that do not particularly rely on vision. If you see a hammerhead shark searching for flatfish, it moves its head back and forth, almost as if it were using a metal detector. Knowing that the electrosensory system may have developed with involvement of neural crest cells is valuable for people trying to reconstruct vertebrate evolution. It gives us further indication of how all of the various sensory systems come on line.”

But the idea that the neural crest truly is the source of the electrosensory system will raise eyebrows, scientists say.

“It’s a very interesting paper for two reasons,” said Glenn Northcutt, a distinguished professor of neuroscience at the University of California, San Diego, and a leading expert in vertebrate neurobiology. “For the first time, someone has shown which molecules may be responsible for guiding the development of the receptors of the lateral line system. I think this will hold true and is a very important finding. But I’m skeptical about the claim the neural crest gives rise to electroreceptors. It still requires a definitive experiment, where the developing neural crest cells are marked with dye, the embryo develops and the dye clearly shows up in the electroreceptors.”

Dye tests are a classical way of mapping cell movements during development, and have been used to explore the origins of limbs and brain cells. In the current research, scientists used genetic markers to trace neural crest cells.

A STING IN THE TALE FOR SHARKS AND RAYS

Expert findings show sharks and rays are now amongst Europe’s most threatened animals as more are added to the IUCN Red List of Threatened Species

Gland, Switzerland, 20 February 2006 (IUCN)  The number of species of sharks and rays on the IUCN Red List of Threatened Species will increase based on the findings of a three-day expert workshop, hosted by the Joint Nature Conservation Committee (JNCC), that examined the conservation status of the species in the Northeast Atlantic and Mediterranean waters.

The workshop confirms the widely-accepted notion that slow-growing sharks and rays are exceptionally vulnerable to over-fishing, and that deep-water species are being depleted at an alarming rate. Some formerly important commercial species are now so rare that they are no longer being sought by fishermen, but their risk of extinction is still rising because of continued incidental capture in fisheries for more abundant species. This situation is exacerbated by the lack of shark fisheries management in European waters.

“Sharks and rays are amongst the most threatened animal groups in the UK today. I welcome the development of a Red List baseline, against which to monitor the hoped-for changes in their status that should arise from increased awareness of their plight,” said Dr Malcolm Vincent, JNCC’s Director of Science.

Nearly 100 species of sharks and rays were evaluated against the IUCN Red List Categories and Criteria. Categories range from Extinct to Least Concern and Data Deficient. Species deemed Vulnerable, Endangered or Critically Endangered are considered threatened with extinction and are added to the global IUCN Red List of Threatened Species. The IUCN Shark Specialist Group, which convened the meeting, will compile these assessments for a regional report that will include recommendations for conservation action.

Proposed additions to the Red List include three species of angel sharks, two species of skates, and several species of deep-water sharks, all of which are considered Critically Endangered in the region, as well as two species of coastal ray, now considered Endangered. The species found to be at lowest risk were generally small and fast-growing coastal species, like cuckoo ray and lesser-spotted catshark, and very deep ocean species that are still beyond the reach of today’s fishing fleets.

Angel sharks, formerly abundant large coastal sharks, were once a common sight in fish markets, but have largely vanished, almost unnoticed, from the European seas that are their world stronghold.

Now officially declared extinct in the North Sea by the International Council for the Exploration of the Sea (fisheries advisers to European countries), the angel shark was nominated in 2001 for strict legal protection in British waters, but we are still waiting for government action on this proposal, said Sarah Fowler, Co-Chair of the Shark Specialist Group. Workshop participants emphasised the urgency of protecting this, and many other imperilled species.

Three species of deep-water sharks, taken as incidental catch in fisheries and increasingly targeted for their meat and rich liver oil, were assessed as threatened. A population decline of 80-95% prompted a Critically Endangered classification for the region’s deep-water gulper shark.

These exceptionally slow-growing sharks are simply not biologically equipped to withstand such intense fishing pressure, said Tom Blasdale, Marine Species Adviser at the JNCC. We welcome recent European Union action to manage deep-water gillnet fisheries, but similar measures are still urgently needed to protect deep-water sharks taken by trawls and longlines.

The shortfin mako shark, a favourite target of commercial and recreational fishermen around the world, was proposed as Vulnerable in the Northeast Atlantic and Critically Endangered in the Mediterranean Sea.

This wide-ranging species is increasingly the target of fisheries and yet lacks any type of protective measures in this region, warned Alen Soldo of the Institute of Oceanography and Fisheries in Croatia. Of particular concern are mako sharks in the Mediterranean, where our findings revealed ongoing fishing pressure well beyond the reproductive capacity of the species.

In contrast to similar workshops held in North America, South Africa, and Australia, the workshop yielded little if any good news, due largely to the lack of shark and ray conservation measures in this region. Protection is granted by just a handful of European countries for the three largest species (basking shark, devil ray, and great white shark). The few European shark and ray quotas in place are routinely set far in excess of actual catches and therefore do not limit fishing pressure. They also cover only part of these stocks. Scientists advice for zero catch of many depleted shark and ray species has been ignored. There are no international limits on shark catch, even as fisheries for wide-ranging shark species (such as mako and blue sharks) expand and evidence of their declines mounts.

Scientists from government agencies, universities, and private institutions participated in the workshop including authors of published papers on shark and skate population status and experts who develop advice on shark quotas for European and international fisheries of the Northeast Atlantic. Experts from England, Scotland, Ireland, Italy, Spain, Portugal, Croatia, Russia, Sweden, Canada, and the USA took part.

The IUCN Red List of Threatened Species is the world’s most authoritative guide to the status of biological diversity. The workshop was the eighth in a global series to assess all of the world’s shark and ray species and develop regional conservation priorities. Resulting Red List proposals are preliminary until accepted by the global Shark Specialist Group network.

Big sharks are disappearing

he rapid decline of great sharks in the world’s oceans is disrupting the marine ecosystem by allowing more lowly fish to thrive, scientists warn today.
Overfishing of the ancient predators has lead to a sudden uprising of species they prey on, causing an abundance of skates, rays and smaller sharks, which are steadily devastating populations of shellfish, including scallops, oysters and clams, the researchers claim.
The findings suggest that the demise of the great sharks, whose primitive ancestors cruised the seas long before the rise and fall of the dinosaurs, may have unforeseen knock-on effects on marine life lower down the food chain.
Records from fisheries and research vessels dating from the 1970s to 2005 have revealed a dramatic nosedive in great shark populations. Tiger sharks and scalloped hammerheads may have declined more than 97% since the mid-1980s, while numbers of smooth hammerheads and bull sharks are believed to have fallen by 99% off the east coast of the US.
Writing in the journal Science, a team of marine biologists led by Ran Myers at Dalhousie University in Halifax, Nova Scotia, analyse fish research surveys over the past 16 to 35 years. The records show that while the abundance of 11 great shark species fell dramatically over the past 35 years, 12 of the 14 fish species they prey on had increased sharply.
In the waters along the US Atlantic coast, numbers of cownose rays, a staple of the great shark diet that can grow to four feet across, jumped 8% a year to an estimated population of around 40 million.
The explosion of the cownose population coincides with an almost complete collapse of scallops in the waters, leaving only those protected behind marine fences for local fisheries to take.
Without sharks to keep their numbers in check, researchers fear the migrating rays will drive down shellfish populations as they swim through, to the point where they are unable to recover .
Julia Baum, a co-author of the paper, said: “With fewer sharks around, the species they prey upon, like cownose rays, have increased in numbers, and in turn, hordes of cownose rays dining on scallops, have wiped the scallops out.”
Sharks are targeted by fisheries for their fins and meat, but are also taken as by-catch by fleets fishing for tuna and swordfish. As many as 73 million sharks are killed each year around the globe for the finning trade.
Ellen Pikitch, executive director of the Pew Institute for Ocean Science in Miami, said: “This is the first published field experiment to demonstrate that the loss of sharks is cascading through ocean ecosystems and inflicting collateral damage on food fisheries such as scallops. These unforeseen and devastating impacts underscore the need to take a more holistic, ecosystem-based approach to fisheries management.”
Charles Peterson, a researcher on the paper and marine biologist at the University of North Carolina in Chapel Hill, said the study highlighted the importance of maintaining populations of the ocean’s top predators. “Despite the vastness of the oceans, its organisms are interconnected, meaning that changes at one level have implications several steps removed. Through our work, the ocean is not so unfathomable, and we know better now why sharks matter,” he added.
In British waters, historic overfishing has seen the common ray decline to the point that surveys in the western channel have failed to spot any since the 1930s. More recently, numbers of blue and porbeagle sharks are believed to have fallen. The porbeagles are believed to be taken by Danish and French fleets, while Spanish long-line vessels take blue sharks migrating into British waters.
Last year, a team lead by David Sims of the Marine Biological Association in Plymouth tagged six blue sharks off the coast of Portugal to investigate their fate. Two were landed by fisheries within three months. “The ones that get here may be the survivors,” he said.
Dr Sims said the lack of hard data makes it extremely difficult to produce reliable assessments of fish populations, adding that many predators have such varied diets that cascade effects through ecosystems are complex and often difficult to pinpoint.
“There’s no doubt the fisheries are having an impact on the big shark populations, but what we really don’t know is what the ecosystem effects of that will be. There could be other factors involved that haven’t been measured,” he added.

Shark’s teeth

There is something unique about sharks’ teeth! A shark without teeth could not survive; it would starve.  Therefore, unlike many other animals like dogs and canines, sharks continuously get new teeth to replace those that fall out.  A shark’s mouth generally contains five or more rows of teeth, one behind the other. All rows, with the exception of the first, lay flat in the animal’s mouth. The next row rises up to replace any teeth that have fallen out or were broken. Sharks always make new teeth and have always spare rows of teeth.  Sharks’ teeth are adapted to what they eat. Unlike humans, sharks do not chew. They are not omnivores, but carnivores. They use their teeth to grasp prey and, if needed, tear the prey into smaller chunks they can swallow.  Most shark teeth are very sharp. Sharks’ jaws are powerful and the sharp teeth are capable of cutting through bone and even thin steel chains.

Shark teeth vary from being ferocious-looking curved spikes to flat triangular points, to points that are so small that they are not used for anything at all.  The larger sharks, like the great white and the tiger shark, have triangular teeth with jagged edges. This helps to keep hold of large fish and animals so as to tear chunks of meat from their bodies or slice through a turtle’s shell. A sand tiger’s teeth, on the other hand, are long and narrow which make them look frightening, but in fact this type of shark is not very aggressive. The shape of its teeth is ideal for grabbing hold of slippery prey, like fish and squid. However, the whale shark, one of the biggest sharks on earth, has very small teeth. Whale sharks don’t use their teeth for biting because they simply filter their food.

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