A Cooler Ocean Predator Than Sharks? Consider the Mantis Shrimps
By Thomas Cronin
When you think about fearsome predators in the ocean, the first thing that pops into your mind is probably a shark. Sure, sharks are ok, with their sleek, menacing shape and their gaping jaws with rows of jagged teeth. But if you were a fish living on a coral reef or cruising along the shore over the sands of a tropical island, you would fear a far more terrifying predator.
Consider an armored, tank-like creature looking something like a lobster. Most are quite small, often tinier than your little finger, though some can be as long as your forearm. This animal doesn't swim around like a shark; instead, it hides in the sand or in rocky holes in coral, searching the water above with constantly roving eyes. It can snatch prey right out of the water in a tiny fraction of a second.
And it accomplishes this feat without claws. Instead, it's armed with a powerful pair of what scientists call "raptorial appendages" that end in a brutal hammer or a series of vicious, pointed spines. These prey-catching arms look somewhat like the front legs of a praying mantis, which gives these creatures their name—mantis shrimps.
They're crustaceans—the group of hard-shelled animals that includes crabs, lobsters and shrimps. The strength of the mantis shrimps' raptorial arms together with their amazing eyes make them perfect predators.
Massively Powerful Predators
Mantis shrimps' raptorial appendages contain massive muscles that can extend them to their full length in hundredths of a second, producing strike forces that in some species can smash through the glass wall of an aquarium or instantly dismember a crab. These smashing attacks are so forceful they produce tiny bubbles in the water. When these cavitation bubbles collapse in a flash of light, they release additional energy onto the target. Boat propellers and turbine blades are often ruined by cavitation forces; mantis shrimps use them to crack the hard shells of their victims.
Other species, with spiny raptorial appendages, impale fish or shrimp with a vice-like grip that allows the mantis shrimp to drag them down into its burrow—often, in the blink of an eye.
Mantis shrimps—properly called stomatopod crustaceans—first appeared in the oceans about 400 million years ago, and have been evolving on their own route to perfection ever since. By now, they are only distantly related to any other living animal, including ones that arose from their crustacean ancestors. They're so unusual that they seem to have arrived from another planet—in fact, vision scientist Mike Land jokingly calls them "shrimps from Mars."
There are almost 500 known species of mantis shrimp. However, they stay well concealed in their rocky and sandy burrows, and only a few scientists study them, so there are probably many new mantis shrimps yet to be discovered. Almost all live in shallow, marine waters and most inhabit the tropics.
Remarkable Eyes of the Mantis Shrimp
Like all crustaceans (insects, too), mantis shrimps have compound eyes—think of the eyes of crabs, bees or butterflies. Each eye has hundreds of separate facets, each of which is a single unit of the entire compound eye. But mantis shrimp eyes are far more specialized than all other compound eyes, in some ways more than any other eyes biologists have ever discovered.
For one thing, each eye is like three eyes squeezed into one. The three parts all look at the same point in space, much as our two separate eyes focus on the same scene. We use our two eyes to locate an object in space. Mantis shrimps can work out the distance to objects they're looking at using a single eye.
Two eye parts, at the top and bottom of the eye, are probably involved in this distance vision. The third part is built from parallel rows of facets that run around the middle of the eye like a belt. Usually there are six rows, though a few species have only two. This part of the eye is called the "midband," and it supports many special abilities.
Further, most mantis shrimps see ultraviolet light – part of the electromagnetic spectrum that causes sunburn in you or me and that is invisible to our eyes. Mantis shrimps not only sense this light, but with their specialized midbands they even see separate colors of it.
This feature is on top of another set of color detectors that see the same visible light we're used to—but in eight color channels as opposed to the three primary colors we see. Imagine trying to build a TV that looks right to a mantis shrimp. Besides the red, green, and blue colors that your TV uses to create a vivid picture, it would require pixels for violet, indigo, blue-green, orange and a deeper red than we can see.
And the midband can do even more. It can detect the polarization of light—where all the waves vibrate in the same plane. Our eyes cannot see this property of light. Mantis shrimps image things using it.
So putting together all its visual talents, when a mantis shrimp sees a fish, it's in patterns of ultraviolet colors, eight primary regular colors and polarized light. Their eyes gather all this information and pass it on to the animal's brain, so it can decide what to attack, when to attack it, how far away it is, and what it looks like in a dozen different ways. It's hard for a human to even imagine the visual world of a mantis shrimp.
Letting Down Its Defenses
With superpower vision coupled to explosive predatory arms, it seems like mantis shrimps would be invincible. But even these animals have their worries. Mantis shrimps can not only kill other animals, like fish, octopus or crabs. They can also kill each other. This raises a serious problem. Eventually, it's time to reproduce—but how does a mantis shrimp know when another one it meets wants to mate rather than make a murderous assault?
Mantis shrimps have been forced to evolve ways to recognize when it's safe to get intimate and to signal their own nonlethal intent. They use their special vision for this too. Mantis shrimps are often brightly colored, and they display patterns—invisible to us—in ultraviolet and polarized light. The complicated displays inform other members of their species, or of different ones, what they plan to do. If their plans include reproduction, and the viewer is of similar mind, then they can safely mate and initiate a new generation of their species.
So, yes—sharks are all right. But do they have bullet-like strikes? Do they have super-vision? Can they take down prey in milliseconds? It's mantis shrimps that have these abilities, and they use them to become some of the world's most impressive predators.
Reposted with permission from our media associate The Conversation.
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By Tara Lohan
The conclusion to decades of work to remove a dam on the Middle Fork Nooksack River east of Bellingham, Washington began with a bang yesterday as crews breached the dam with a carefully planned detonation. This explosive denouement is also a beginning.
The History<p>The Middle Fork Nooksack drains glacier-fed headwater streams that run off the icy summit of 10,778-foot Mt. Baker. The Middle Fork joins the North Fork and then the mainstem of the Nooksack River, which travels to Bellingham Bay and Puget Sound. The entire Nooksack watershed stretches 830 square miles across Washington and into British Columbia.</p>
A Plan Comes Together<p>The Middle Fork dam is not a pool dam built for water storage. Much of the time, water flows over the top until dam operators drop a floodgate to divert water to new locations. That water travels about 14 miles through tunnel and pipeline to Mirror Lake, then Anderson Creek, and to Lake Whatcom before finally being delivered to residents' taps.</p><p>Before removing the dam, engineers had to move the water intake 700 feet upstream and situate it at an elevation that still enabled city water withdrawals throughout the year, regardless of flow conditions.</p><p>They also needed to make sure that the rushing water didn't sweep up fish and accidentally send them through the water-supply system.</p><p>"The solution required a fairly complex design in the intake structure, including a fish exit pipe out of that structure to put fish back into the river in a way that meets current environmental permit standards," explains LaCroix.</p>
Project layout for the removal of the Middle Fork Nooksack diversion dam and rebuilding of water intake. City of Bellingham<p>Despite the cost and the work, she says, being able to continue to meet their municipal water obligations while opening up habitat for threatened species has been a win-win.</p><p>"I think there's a lot of benefits to having a dam removal versus fish passage — the main one being that you get a free-flowing river that can be a dynamic ecosystem and change over time," she says. "A static fish ladder just can't provide that same level of ecosystem benefit."</p>
Restoration Success<p>Despite local authorities' championing dam removal on the Middle Fork, the project has largely flown under the radar, overshadowed in the Pacific Northwest by heated discussions about a much larger potential project — removing <a href="https://www.seattletimes.com/seattle-news/feds-reject-removal-of-4-snake-river-dams-in-key-report/" target="_blank">four federal hydroelectric dams on the lower Snake River</a>, a major tributary of the Columbia River.</p><p>Proponents of dam removal there see it as the best chance for recovering threatened salmon populations, including Chinook, which could help starving Southern Resident killer whales. Those dams also provide irrigation water, barge navigation and hydropower, so there's been more pushback against removal efforts.</p><p>Previous dam removals around the country, however, have proved successful at aiding fish recovery and river restoration.</p><p>Most notably the 1999 demolition of <a href="https://therevelator.org/edwards-dam-removal/" target="_blank">Edwards Dam on Maine's Kennebec River</a> restored the annual run of alewives, a type of herring essential to the food web. The fish run has gone from zero to 5 million in the two decades since dam removal. Blueback herring, striped bass, sturgeon and shad have also extended their reach. And the resurgence has brought back osprey, bald eagles and other wildlife, too.</p><p>The overwhelming success of river restoration on the Kennebec helped to spur a nationwide dam removal movement that's now seen 1,200 dams come down since 1999. Last year a record <a href="https://www.americanrivers.org/conservation-resource/a-record-26-states-removed-dams-in-2019/" target="_blank">90 dams</a> were removed in 26 states, including <a href="https://therevelator.org/cleveland-forest-dam-removal/" target="_blank">20 dams in California's Cleveland National Forest</a>.</p>
Spider excavators remove on dam on San Juan Creek in California's Cleveland National Forest. Julie Donnell, USFS<p>The results have been seen in the Pacific Northwest, as well, which boasts the largest dam removal thus far in the country. In 2011 and 2014, the demolition of <a href="https://therevelator.org/elwha-dam-removal/" target="_blank">two dams</a> on Elwha River, which runs through Washington's Olympic National Park, opened up 70 miles of habitat that had been blocked for a century. Scientists have started seeing all five species of salmon native to the river coming back, particularly Chinook and coho. Bull trout, they've observed, have increased in size since the dams were removal.</p>
Benefits on the Middle Fork Nooksack<p>McEwan hopes to see a similar outcome on the Middle Fork.</p><p>Like the Elwha the Middle Fork Nooksack is a relatively pristine river with little development, and dam removal is expected to provide a big boost to fish. The additional miles of spawning habitat are important, but so is the temperature of that water.</p><p>The dam removal will open access to cold upstream waters, which are ideal for salmon and getting harder to come by as climate change warms waters and reduces mountain runoff.</p><p>"This is really great for the climate change resiliency for these species," says McEwan.</p><p>Steelhead will get back 45% of their historic habitat in the river, and scientists expect Chinook populations to increase in abundance by 31%.</p><p>That <em>could</em> help Southern Resident killer whales.</p><p>"When you get to the ocean, it's a little bit of a black box in terms of what you can model and say definitively is going to help, but more fish is better for orcas," McEwan says.</p><p>Upstream habitat will see benefits, too.</p><p>Oceangoing fish like salmon enrich their bodies with carbon and nitrogen while at sea. When they return to their natal rivers to spawn and die, the marine-derived nutrients they carry back upriver become important food and fertilizer for both riverine and terrestrial ecosystems — aiding everything from trees to birds to bears.</p><p>"Once the fish start making their way back, it will start changing the whole ecological system," says Delgado.</p><p><span></span>But any ecological benefit from salmon restoration, either in the ocean or the upper watershed, won't be immediate.<br></p><p>"The population of salmon on the Middle Fork is so low that we expect it's going to take quite a while to rebound," she says. "But the big picture is that what's good for salmon is good for the region — our history and our destiny are intricately intertwined."</p><p>After decades of work, that process of restoration has finally begun.</p>
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It can grow to a maximum of six inches (16 centimeters), change color depending on mood and habitat, and, like all seahorses, the White's seahorse male gestates its young. But this tiny snouted fish is under threat.
Building an Ocean Seahorse Destination<p>Seahorses are found in tropical and temperate coastal water worldwide, but are most abundant around Australia, China and the Philippines. </p><p>Trade in the tiny creatures is strictly regulated because of their use in traditional medicine, aquariums and their sale as dried curios. But because they are poor swimmers and cannot easily move elsewhere, habitat loss is a particular threat for these curious animals. </p><p>Seahorses wrap their tails around seagrass and corals to avoid being carried away on currents. They use the habitat to spawn and hide from predators such as crabs, while also feeding on riches of plankton and small crustaceans living in the reef.</p><p><span></span>Where corals aren't available, <a href="https://onlinelibrary.wiley.com/doi/abs/10.1002/aqc.1217" target="_blank">scientists</a> found seahorses taking up residence in fishing nets and old crab traps abandoned at the bottom of the ocean. </p>
Mixing With the Locals<p>Baby seahorse mortality is high in the wild because they are easily caught, so those bred in the protected environment of the aquarium weren't ready to be released into the wild until early May.</p><p>The team released 90 new arrivals into Sydney Harbor, placing some directly into the purpose-built hotels, and others onto a net that wild seahorses had already settled on.</p><p>Before setting them free, the researchers marked each young seahorse with a fluorescent tag with unique IDs inserted just beneath the skin to track how they get on in the different environments. </p><p>"The most exciting part was being able to put these animals into the wild and then go back a month later and still see them surviving and growing," said McCracken. </p><p>The seahorses will be old enough to mate and reproduce around October or November 2020. And researchers hope that by then, they will be able to breed with the wild population. </p>
Building a Global Seahorse Hotel Chain<p>With seahorses everywhere facing the loss of their coral reef homes, similar projects have sprung up in places like Greece and South Africa, home to the world's most endangered seahorse, the Knysna seahorse. </p><p>"The endangered South African seahorse is benefiting from something quite similar, even though it wasn't intentional," said Peter Teske, professor at the Department of Zoology, University of Johannesburg.</p><p>In the South African <a href="https://www.researchgate.net/publication/322649251_An_endangered_seahorse_selectively_chooses_an_artificial_structure" target="_blank">case</a>, seahorses have bedded down in "Reno mattresses" — wire cages filled with rocks — that were used to build a new marina. Researchers from NGO Knysna Basin Project found the structures acted as a refuge for the animals.<span></span></p><p><span></span>While Teske describes the seahorse hotels as "a positive news story" and a great way to create public awareness of conservation, he added that establishing artificial habitats in some areas will only prevent the extinction of local populations.</p><p>"For a complete recovery, it is necessary to give the natural habitat a chance to regenerate," said the seahorse expert. </p>
Underwater Mascot<p>In Australia, the researchers hope the project could provide an opportunity to raise awareness not only of the plight of the Sydney seahorses but the other animals with which it shares its ocean habitat.</p><p>The waters around Sydney and the east coast are rich in biodiversity and include several threatened species like the weedy seadragon — a relative of the seahorse — and the grey nurse shark. Like the seahorse, they're also under pressure from pollution, ocean traffic and habitat loss through storms and coastal construction. </p><p>"It's a good thing to get people's support and interest. The seahorses are a useful vehicle to get people concerned if the harbor is in trouble," said David Booth, professor of marine ecology at the University of Technology Sydney who is also working on the project. </p><p>The hotels have become an attraction for divers hoping to catch a glimpse of these small but near mythical creatures. </p><p>"Everyone loves seahorses," added Booth, "they are so popular." </p>
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