Fish Feels Its Way With Fins


Cartoon animators have got it right — fish really do use their fins for more than just swimming. Or at least bluegill sunfish (Lepomis machrochirus) do. They use their pectoral fins — those are the ones on the sides — to reach out and touch obstacles, helping them to navigate. That’s the finding from a new study from two Harvard biologists published in the Journal of Experimental Biology.

Bluegill sunfish may be familiar because they’re the a popular sportfish in the eastern United States. Throw out a hook with a worm into the local lake, and you might just catch yourself a bluegill supper.

These fish live in the littoral zone, close to shore, an area that can be full of vegetation that the fish have to navigate through. The fish have decent vision, and they also have help from their lateral line, a special organ that helps detects movement and vibration in the water.

But maybe the fish have another way to help them spot obstacles in the water, the Harvard team thought. The researchers began by creating an obstacle course for the fish, with evenly spaced acrylic tubes in a big tank. Then they filmed fish swimming through the tank. To cut off the fish’s vision, they turned out the lights. To disable its lateral line, they treated the fish with cobalt chloride.

The scientists had hypothesized that only when its others senses were disabled would the fish reach out with their fins. To their surprise, however, the fish used their fins to touch the acrylic tubes even when they could see and/or use their lateral line. The bluegill sunfish would use their pectoral fins to propel themselves forward between posts, but they would also wrap those same fins around the obstacles.

“Fish did not appear to push off of the posts to change heading or move forward. Forward motion did not initiate until the beat following the tapping contact with the obstacle posts,” the researchers write.

The pectoral fins act a bit like our arms while we’re swimming, propelling and steering a fish through the water and helping it to feel its way through the weeds.

Image from U.S. Fish and Wildlife Service, via Wikimedia

How Silky Sharks Lose Out In Our Quest For Tuna

silkysharkSharks are pretty amazing creatures, as I pointed out yesterday. But since humans tend not to respect these animals — whether out of fear or other feelings — we kill a lot of them. Just take a look at the infographic below. The image, though, which has been making the rounds on the internet lately, might be off, at least a little. That’s because a team led by the Institut de recherche pour le développement in France just estimated the number of silky sharks (above) killed by fish aggregating devices set to catch tuna, and the numbers are shocking. Their study appears in Frontiers in Ecology and the Environment.

Fish aggregating devices (FADs) are huge structures of bamboo and netting that are set adrift in the open water. Tuna have a natural tendency to aggregate among floating objects, so fishermen can use satellite tracking to monitor the devices and just motor up to them and catch a whole bunch of fish.

But a lot of other critters can get tangled up in FADs, including sea turtles, marine mammals and silky sharks, a pelagic species that got its name from its silky smooth skin. In the new study, the researchers tagged and tracked 29 silky sharks off of the Seychelles and in the northern part of the Mozambique Channel. Over the next 94 days, four of the sharks became entangled in FADs.

Extrapolating from their tagged sharks, and assuming that there are some 3,750 to 7,500 active FADs in the Indian Ocean, the researchers calculate that 480,000 to 960,000 silky sharks are killed by the devices in the Indian Ocean each year, and perhaps as many as 2 million worldwide.

“FADs have been used with increasing frequency worldwide for the past 20 to 30 years,” the researchers write, “but it is only now that the unexpected impact on silky sharks in the Indian Ocean has been detected.”


Top image courtesy of flickr user Syn

Shark’s Tail Is A Deadly Weapon (If You’re A Sardine)


As if sharks weren’t scary enough, now there’s definitive proof that pelagic thresher sharks (Alopias pelagicus, above) can hunt with their tails, not just their teeth. Even worse, they’ve got two ways of thrashing their tail around, so you’d never know from where death was coming.

But don’t worry. These shark tails are only dangers to sardines.

There had been rumors for years that pelagic threshers could kill small fish with their tails (even the flickr page of the image above mentions it). So a team from the Thresher Shark Research and Conservation Project in the Philippines and Bangor University in Wales set out, armed with underwater video cameras and SCUBA gear, to get the action on film and find out how the sharks accomplished their kills. The results of their study appear in PLOS One.

The researchers dove into the waters off the small coral island of Pescador in the Phillippines many times from June to October 2010, eventually capturing 25 shark hunting events, 22 in which the shark swung its tail overhead and three sideways.

Here’s how an overhead strike works: A shark lunges toward a school of sardines then draws its pectoral fins down, which changes the shark’s pitch and stalls its movement forward. The tail whips overhead, all the way to the shark’s snout, striking at the sardines. Finally, the shark turns a full 180 degrees and chomps up the stunned fish. All this happens in just a few seconds, and the shark’s tail reaches speeds as fast as 48 miles per hour.

The sideways tail-slap takes a little longer, but it’s more of a follow-up weapon. The sharks observed carrying out this move had already completed a successful overhead strike.

The tail-slap worked about a third of the time, but it had the added advantage of being able to catch a shark more than one fish in just one move. That’s a definite advantage; the researchers note that carnivorous ocean-dwelling sharks usually only pursue one piece of prey at a time (perhaps this should make swimmers feel slightly safer after a shark has gone after one of their fellow beachgoers).

Bigger sharks are faster tail-slappers, the scientists found. That’s likely because their longer tails make better whips.

Image courtesy of flickr user Rafn Ingi Finnsson

Aww…Kissing Fish Win Fan Favorite In Underwater Photo Contest


Several years ago, when I went snorkeling on the Great Barrier Reef, I didn’t bother buying or renting an underwater camera. While my photo skills on dry land are pretty decent, I knew that beneath the ocean’s surface I was pretty clueless and my time would be better spent just admiring all the amazing creatures in my sight rather than fiddling with a box. And now, having viewed the winners of the 2013 Annual Underwater Photography Contest hosted by the University of Miami Rosenstiel School of Marine & Atmospheric Science, I’m glad that I didn’t even try my hand at this. The winners are masters at this art. Just take the fan favorite (above). The pair, a species called Mandarinfish or Mandarin dragonets (Synchiropus splendidus), were caught on camera by Italian amateur photographer Pietro Cremone in Puerto Galera, Philippines. These vividly colored fish can be found throughout the western Pacific, from Hong Kong to Australia. And while this pair may look like they’re kissing, mating is actually a bit different. Small groups of males and females gather at night on the reef. A pair will get in alignment, rise about a meter from the reef itself, then release their eggs and sperm. And that’s where parenthood (and romance) ends for these fish.

Petrel Bones Show How Humans Changed The Open Ocean Food Web

Pterodroma sanchwichensisIn one marine biology class I took long ago, the way we study the open ocean was compared to trying to study a forest by throwing a bucket out of a helicopter as you fly over and making conclusions based on whatever you happen to drag back up. Ocean exploration these days is a bit more sophisticated, but it’s still not easy, especially if someone wants to see how things have changed over the last several thousand years or so. But scientists figure out elegant solutions to these problems.

In a study published this week in PNAS, a research team led by the Smithsonian’s National Museum of Natural History investigated how the food web of the North Pacific Ocean has changed since humans began fishing there on a large scale by looking at the chemistry of petrel bones. Hawaiian petrels turn out to be a good proxy for what’s going on in the oceanic food web for several reasons: The birds breed only on the Hawaiian Islands, and their remains, dating back thousands of years, can be found in many archaeological and paleontological sites there. That makes the petrels special because most creatures that play a role in the oceanic food web die at sea. Even better, these bones contain a record, in the form of carbon and nitrogen isotopes, of where they foraged and what they ate when they were alive.

“Hawaiian petrels spend the majority of their lives foraging over vast expanses of open ocean,” lead author Anne Wiley of the Smithsonian Institution said in a statement. “In their search for food, they’ve done what scientists can only dream of. For thousands of years, they’ve captured a variety of fish, squid and crustaceans from a large portion of the North Pacific Ocean, and a record of their diet is preserved in their bones.”

Three decades of collecting 17,000 petrel bones have given these researchers a pretty good dataset to work with. And when they looked at the isotope information from the bones, the scientists found that from about 4,000 to 100 years ago, petrels were munching on bigger prey. But when humans intruded on the food web, snapping up the bigger fish, petrels changed their diet and gobbled up smaller creatures.

“Our bone record is alarming because it suggests that open-ocean food webs are changing on a large scale due to human influence,” coauthor Peggy Ostrom, a zoologist at Michigan State University, said in a statement.

It’s yet another case showing how humans are influencing the planet, and not necessarily for the better.

Image courtesy of Brittany Hance, Imaging Lab, Smithsonian Institution

Maybe Your Pet Fish Wasn’t Suicidal

guppyWhen I was growing up, my brother had a black goldfish that I swear was suicidal — it kept jumping out of its bowl. If someone saw it happen, they’d just scoop the fish off the floor and toss him back into the water. But of course, the day came when the fish made a leap when no one was watching, and it didn’t survive.

Several of my friends have similar stories. And so does Daphne Soares, a biology professor at the University of Maryland College Park. She was studying the brains of Poecilia reticulata, a species of guppy, when one of her research subjects in the lab jumped out of its tank and into her cup of chai.

“Fortunately it was iced chai and it had a lid on, so he stayed alive,” Soares said in a statement.

Soon Soares and colleague Hilary S. Bierman were recording jumping guppies with a hi-speed camera to find out what was going on. Their study appears in PLOS ONE.

As with the fish that jumped into the chai, the guppies didn’t need any prompting to make a leap out of the water. They didn’t have to be chasing after food, racing to escape a predator, or swimming upstream to migrate like a salmon in order to jump. And unlike other fish that are known jumpers, P. reticulata prepared for their leap by swimming slowly backwards. Then they would start their jumping cycle with fast body thrusts that propelled them out of the water at speeds of more than four feet per second, allowing them to leap distances up to eight times their body length.

Aquarium owners may be familiar with P. reticulata because these fish are one of the most popular species sold as pets. Soares and Bierman, though, studied guppies taken directly from their native habitat in the Guanapo River in the mountains of Trinidad. And the researchers suspect that it’s this habitat that may be key for understanding why guppies jump.

In the mountainous region where the guppies live, the fish might encounter small waterfalls or other barriers as they try to move into new territory. Soares and Bierman hypothesize that the guppies evolved the ability to jump spontaneously as a method to aid this dispersal, which would help them to avoid competition with kin, prevent interbreeding, escape predators, or find new sources of food.

So perhaps my brother’s fish wasn’t suicidal after it — maybe it just didn’t like the bowl it lived in and was seeking a new home.

Image courtesy of flickr user Wolfgang_44

Great White Sharks Are Scavengers – Will You Think Less Of Them Now?

greatwhiteGreat white sharks have a well-deserved reputation for being fearsome predators. But a new study in PLOS One shows that they’re also crafty scavengers.

Researchers from the University of Miami in Florida, led by Captain Chris Fallows of Apex Expeditions in South Africa, documented four scavenging events over 10 years in False Bay, South Africa. False Bay is home to a breeding site of cape fur seals, and those seals attract lots of great whites — seals are a favorite great white food. But, the scientists found, when when there’s a dead whale nearby, the great whites will abandon the hunt and instead munch on a meal of whale.

“Although rarely seen, we suspect that as white sharks mature, scavenging on whales becomes more prevalent and significant to these species than previously thought,” coauthor Neil Hammerschlag, director of UM’s R.J. Dunlap Marine Conservation Program, said in a statement.

The sharks mostly eat the whale’s blubber — the part with the biggest caloric punch — but they often eat the fluke first, the researchers observed. The biggest sharks get the best bites, with littler sharks having to settle for bits of blubber that float away.

In a way, this is not a surprising study. On land, many apex predators have been observed scavenging for meals, including bears, wolves, and lions. But what is notable, I think, is that we don’t think of any of those creatures as scavengers. And we probably wouldn’t think of great whites as scavengers either, even after reading this new study.

Animals have reputations, and it’s hard to convince people that they may be undeserved. Hyenas, for example, are supposed to be lowly scavengers, but in reality, they’re predators that kill 95 percent of what they eat. Disney’s Lion King did them an incredible disservice — hyenas are actually as effective as a predator as a lion or leopard.

Why do we look down on scavenging? While picking up roadkill from the side of the road or diving into a dumpster has — at least in my opinion — a level of danger involved that makes these actions seem ill-advised, we all scavenge at some level. We eat the leftovers of catered meetings that are left in the company break room, or pick up books left in the “free” pile outside someone’s home. In New York City, you can furnish all of your apartment just from scavenging other people’s discarded belongings.

Scavenging is an effective technique for an organism to supplement its diet (or its life in general, as seen with NYC apartments). Taking advantage of a free meal, one you don’t have to expend much energy to obtain, is a smart move. And we already know that great white sharks are smart, sophisticated animals. After all, it’s one of the things that makes them so scary.

Image of great white shark off South Africa courtesy of flickr user Bring on the Photog

This Two-Headed Bull Shark Is Real

wagner-sharkA couple years ago, scientists in the marine science department at Florida Keys Community College received a unique gift — a baby bull shark with two heads. The little fish had been cut from its mother’s uterus and, sadly, had died.

The animal was later sent to Michigan State University and given an MRI and X-ray:


The creature, say researchers in a study in the Journal of Fish Biology, is not a conjoined twin but a true two-headed shark. It has two heads, two hearts, and two stomachs, but the rest of its body is singular. It’s the first time that dicephalia, as the condition is called, has been seen in a bull shark.

Top image courtesy of Patrick Rice, Shark Defense/Florida Keys Community College; bottom image courtesy of Michael Wagner; both via MSU

How Scientists Figured Out Salmon Use Magnetism To Navigate

salmonToday I wrote about salmon navigation on NPR’s food blog The Salt:

After hatching in a freshwater stream, young salmon make a break for the ocean, where they hang out for years, covering thousands of miles before deciding its time to settle down and lay eggs in the same stream they were born.

So how do these fish find their way back to their home river?

According to one theory, it’s all about magnetism. When salmon are young, the theory goes, they imprint on the pattern of the Earth’s magnetic field at the mouth of their native river. Years later, when the salmon head back home to spawn, they home in on that pattern. In a study published Thursday in Current Biology, the scientists behind that theory now say they have evidence that’s exactly how the fish are navigating.

What got cut from the story is how the scientists made the connection:

The researchers determined that the fish must be using magnetism to navigate by studying sockeye salmon from the Fraser River in Canada. The mouth of that river is blocked from the ocean by Vancouver Island, meaning that the fish can’t directly swim back into the river from the Pacific; they have to choose a route either north or south of the island to get home.

That set up a natural experiment: If the salmon are navigating by the earth’s magnetic field, they should pick the route entrance that has a magnetic field pattern that most closely resembles the one at the mouth of the Fraser.

But it gets a little complicated because the earth’s magnetic field varies by strength and direction of the magnetic lines, and that field wiggles ever so slightly year to year — as evidenced by the changing location of the magnetic north pole.

So over time, the northern route is sometimes a closer match for the salmon, and vice versa. And it turns out that “fish migratory routes are following that wiggling” in the magnetic field, says lead author Nathan Putman of Oregon State University in Corvallis, which means they’re probably using it to find the exit off the Pacific highway and the way to their home river.

Image credit: Current Biology, Putman et al.