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The Fault in Our Stars Might be a Virus

In June 2013, starfish on the western coast of North America started wasting away. At first, their arms curled from the tips, and they tied themselves into pretzel-like knots. Their bodies deflated. White festering sores appeared on their flesh. As the lesions spread, their flesh rotted away and their arms fell off. Within days, healthy animals had disintegrated into mush.

This condition, known as sea star wasting syndrome (SSWS), was recorded as far back as the 1970s, but the scale of this recent event is unprecedented. It has hit at least 20 species all along the Pacific, from Alaska to California. In less than a year, huge, thriving populations have completely wasted away.

As the stars blinked out, scientists compiled a list of possible causes that included storms, rising temperatures, and pollutants. But an infection always seemed likely. The disease seemed to move from place to place with the character of a spreading epidemic. Most tellingly, starfish in aquariums started dying too. These animals were housed in controlled captive environments but they were immersed in water pumped in from the surrounding ocean—and that was enough to kill them. Filtering the water through sand didn’t help. The only measure that spared the stars was sterilising the water with ultraviolet light. Whatever was killing the animals was microscopic and biological.

Now, a team of scientists led by Ian Hewson from Cornell University have identified the most likely culprit behind the grisly outbreak—a new virus that they call sea star-associated densovirus, or SSaDV.

A sunflower starfish distingegrates due to SSWS. Credit: Hewson et al, 2014.
A sunflower starfish distingegrates due to SSWS. Credit: Hewson et al, 2014.

Densoviruses are best known for infecting insects and crustaceans, but last year, Hewson’s team discovered them in a group of Hawaiian sea urchins. “We didn’t associate those viruses with any disease,” he says. “I jokingly told a colleague: Boy, what if there was some kind of mass mortality?” When the SSWS outbreak hit, the team leapt at the chance to study it.

First, they blended tissues from wasting starfish and passed them through filters with extremely small pores—small enough to exclude bacteria but big enough to let viruses through. They inoculated healthy starfish with these extracts, and the animals started wasting within a couple of weeks. If they boiled the extracts first, the animals were unharmed. This confirmed that the disease was transmissible and was caused by something the size of a virus. By sequencing the killer extract, the team showed that it contained the genome of a new densovirus—SSaDV.

They then collected tissue samples from 465 wild starfish, belonging to three species. The symptomatic animals were more likely to carry SSaDV than their healthy peers, and in higher numbers. And the more viruses they had, the worse their symptoms were.

The association wasn’t perfect: some diseased starfish showed no signs of the virus, while some healthy ones did. But Hewson thinks that there are easy explanations for these patterns. Diseases animals don’t have the virus everywhere. “If you take a sea star and divide it into pieces, you can detect virus in 80 percent of them. So 20 percent will be a false negative,” he says. There’s also a lag between infection and symptoms, so “healthy” animals might be exposed without having fallen sick yet. “Some of the ‘healthy’ samples came from an aquarium where all the animals later died,” Hewson adds.

“The authors present persuasive evidence that they have identified an agent associated with SSWS,” says Ian Lipkin, a virus hunter from the Mailman School of Public Health. “Nonetheless, as they note themselves, there  is much more work to be done before we will know whether the densovirus they describe is necessary and sufficient to cause disease.”

Vincent Racaniello, a virologist from Columbia University, agrees that the evidence is strong. “The crucial experiment that remains to be done is to isolate infectious virus in cell culture, inoculate it into sea stars, and show that it causes wasting disease,” he says.

Hewson agrees that this step is crucial, but he also thinks that it will be very difficult. Ideally, he’d like to infect starfish in the absence of all other microbes to show that the virus is truly responsible for the disease. But these animals pump seawater through their bodies and they are naturally riddled with microbes. The alternative is to grow starfish tissues in the lab, but no such cultures exist for marine invertebrates. “That’s a real stumbling block,” says Hewson. “We’re trying to isolate a cell culture of a sea star but that’ll be a long and tedious process.

He also suspects that the virus may not actually cause the symptoms of SSWS directly. It could, for example, disrupt the sea stars’ ability to control the bacteria that they normally co-exist with. “The lesions are probably just the native bacteria taking advantage of an immunocompromised host,” he speculates. “So, associating the virus with those lesions may be a challenge.”

There’s also another mystery: why is the current outbreak of SSWS so dramatic when the newly discovered virus is actually an old presence? The team found its DNA within starfish that had been collected from the Pacific coast as far back as 1942, and that have been sitting in museum jars ever since. So why weren’t North America’s starfish melting away while World War II was raging?

“Viruses do smoulder in populations,” says Hewson. Familiar names like HIV and Ebola were affecting humans at small scales long before they triggered huge scary epidemics. The same may apply to SSaDV. In recent years, booming sea star populations in the Pacific Northwest may have given the virus newfound impetus. “There were mountains of sea stars underwater, tens of metres high,” says Hewson. “If you talk to crab-fishers in the region, their crab pots were full of these stars and they were getting annoyed.” The virus could have more easily jumped from host to host, or developed mutations that made it more transmissible or virulent.

Environmental changes may be important. Carol Blanchette from the University of Santa Barbara says, “We have been sampling sea stars across southern California throughout this epidemic and it is likely that in our region, as well as in others, environmental causes like increased temperatures have played an important role.” She finds the virus evidence convincing, but thinks it “may only be one part of the story”.

SSaDV may just be part of a natural cycle that controls sea star populations if they grow too big. Then again, some starfish act as keystone species—they wield disproportionate influence over their habitats by controlling populations of mussels and barnacles that would otherwise takeover. Their loss could dramatically reshape the coastlines of the Pacific Northwest, from thriving communities into black mussel monocultures.

SSaDV is also found in other related animals like sea urchins and brittle stars. No one knows if it causes disease in these groups, but the worry is that the virus could use its sea star hosts as a platform for building in numbers and launching outbreaks in other species.

Even if that was true, Hewson doubts that anything can be done. Even if scientists could develop a cure or vaccine, it would be impractical to inoculate the animals on a wide scale. “We’re just trying to understand this as a natural phenomenon,” he says.

Reference: Hewson, Button, Gudenkauf, Miner, Newton, Gaydos, Wynne, Groves, Hendler, Murray, Fradkin, Breitbart, Fahsbender, Lafferty, Kilpatrick, Miner, Raimondi, Lahner, Friedman, Daniels, Haulena, Marliave, Burgem, Eisenlord & Harvell. 2014. Densovirus associated with sea-star wasting disease and mass mortality. http://dx.doi.org/10.1073/pnas.1416625111

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Starfish Spot The Way Home With Eyes On Their Arms

Most starfish have eyes on the tips of their arms. They’re hard to see and even if you spot them, you might not recognise them as eyes. But they can see you (as long as you’re not moving too fast).

The starfish in the top image is an Indo-Pacific species called the blue star (Linckia laevigata). Here’s a close-up of one of its arms. That groove runs up the entire underside of the arm and contains thousands of tube feet, which the animal uses to crawl about. The eye sits at the end of the groove, where the white arrow is pointing.

Credit: Garm & Nilsson, 2013. Royal Society.
Credit: Garm & Nilsson, 2013. Royal Society.

It’s a tiny red nub, barely half a millimetre wide. It’s usually exposed, but the starfish can retract it into the arm if danger threatens.

Credit: Garm & Nilsson, 2103. Royal Society.

Scientists have known about starfish eyes for ages, and they’ve assumed (reasonably) that the animals use these organs to see. But no one has properly tested this, or probed the limits of their vision. Anders Garm from the University of Copenhagen and Dan-Eric Nilsson from Lund University are the first.

They started with a detailed study of the blue star’s eyes. Each one sits on the end of a modified tube foot, and contains 150 to 200 separate light-collecting units called ommatidia. You can see them below, stained with a red dye. The set-up is similar to an insect’s compound eye, but with one big difference: insect ommatidia have lenses to focus light onto the underlying cells, while starfish lack lenses of any sort.

Credit: Garm & Nilsson, 2013. Royal Society.
Credit: Garm & Nilsson, 2013. Royal Society.

Each eye has a fairly large visual field that extends over 210 degrees horizontally and 170 degrees vertically—a slightly wider range than what your eyes can cover.  And since the starfish has five of these eyes at the end of its flexible arms, it can probably see in every direction at once. Alternatively, it might be able to narrow its view to certain directions by moving the flexible black tube feet that surround each eye, and using them as blinders.

In the wild, blue stars live on coral reefs. When Garm and Nilsson placed them in front of a real reef, the starfish walked around randomly at first. But when they got within 2 metres of coral, they made a bee-line for it.

They needed their eyes for that. If Garm and Nilsson dissected out the eyes (don’t worry—they grow back after a few weeks), the blinded starfish couldn’t navigate home. They walked at the same speed, but they couldn’t find the coral.  This is clear evidence that the blue star sees with its eyes, and uses them to navigate.

However, its vision is rather poor. It’s colour-blind, and sees the world only in shades of light and dark. Its light-detecting cells work very slowly, so fast-moving objects are invisible to it. And it has poor spatial resolution, so it can’t see fine detail.

But none of this matters. Garm and Nilsson suspect that the starfish doesn’t need to see colours, details, or speedy objects, because it mostly uses its eyes to detect coral reefs. Which are large environmental shapes that don’t move. The reefs would also appear as dark splotches against a bright ocean, in the starfish’s monochrome view of the world.

“When vision first evolved, it must have supported some tasks,” says Garm. “What behaviours could you control with a very crude image? Detecting large predators? They move fast and you’d need fast vision.” And to process images quickly, you’d need a decent amount of brainpower—something that the owners of the first eyes probably lacked.

An alternative, and one that Nilsson has championed, is that the first eyes evolved to detect large, stationary objects, so that animals could recognise their habitats and find their way home. “To do that, you only need poor eyesight,” says Garm. A starfish, guided back to a reef by its five arm-tip eyes, could be giving us an example of what the first visual systems were used for.

Reference: Garm & Nilsson. 2013. Visual navigation in starfish: first evidence for the use of vision and eyes in starfish. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2013.3011

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Starfish go five ways, but two ways when stressed

A typical starfish has five-sided symmetry. With no clear head, the starfish can move in any direction, led by any one of its five arms. If you were feeling particularly cruel, you could fold one up in five different ways, so each half fitted exactly on top of the other. We humans, like many other animals, have only two-sided symmetry. We’re ‘bilateral’ – our right half mirrors our left, and we have an obvious head.

These two body plans might look radically different, but looks can be deceiving. Chengcheng Ji and Liang Wu from the China Agricultural University have found that starfish have hidden bilateral tendencies, which reveal themselves under times of stress.


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Sea urchins use their entire body as an eye

Purple sea urchins look like beautiful pincushions. They have no obvious eyes among their purple spines, but they can still respond to light. If you shine a spotlight on one, it will sidle off to somewhere darker. Clearly, the purple sea urchin can see, and over the past few years, scientists have worked out how: its entire body is an eye.

For decades, scientists knew that sea urchins can respond to light, even though they don’t have anything that looks remotely like an eye. The mystery deepened in 2006, when the full genome of the purple sea urchin was published. To everyone’s surprise, its 23,000 genes included several that are associated with eyes. The urchin has its own version of the master gene Pax6, which governs the development of animal eyes from humans to flies. It also has six genes for light-sensitive proteins called opsins.

While these genes are usually switched on in the developing eye, Maria Arnone found that the sea urchin’s versions are strongly activated in its feet. Sea urchins have hundreds of “tube feet”, small cylinders that sway around amid the spines. They can use the feet to move around, to manipulate food, and apparently to see.


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Pocket Science: Stealth mode in the sea

The oceans are full of animals that seek safety in numbers, gathering together to confuse predators. But some opt for the opposite strategy.

Alexandrium is part of the sea’s collection of plankton. It’s a single-celled creature but it can create colonies by amassing together in long chains. At their most extreme, these colonies can form large swarms to produce harmful red tides.

As chains, Alexandrium swims and grows faster, but it is vulnerable to predators such as copepods – small relatives of crabs or shrimp. Erik Selander from theTechnical University of Denmark found that when the chains detect the chemical traces of copepods, they break apart. By turning back into single cells, they make themselves harder to find. They also swim at a slower pace to avoid creating telltale movements in the water. When threatened by predators, these plankton enter stealth mode.


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Fishing bans protect coral reefs from devastating predatory starfish

CrownofThornsStarfish.jpgBlogging on Peer-Reviewed ResearchA complete ban on fishing can save coral reef communities in more ways than one. A few weeks ago, I blogged about a study which found that the coral trout, a victim of severe overfishing, was bouncing back in the small regions of the Great Barrier Reef where fishing has been totally forbidden. It certainly makes sense that fish will rebound when fishing ceases, but a new study reveals that the bans have had more indirect benefits – they have protected the corals from a predatory starfish.

The crown-of-thorns starfish (Acanthaster planci) is a voracious hunter of corals and a massive problem for reef conservationists. It’s bad practice for any science writer to anthropomorphise an animal, but the crown-of-thorns really does look incredibly, well, evil. Its arms (and it can have as many as 20) are covered in sharp, venomous spines. As it crawls over the reef, it digests the underlying coral by extruding its stomach out through its underside.

From time to time, their numbers swell into plagues of thousands that leave behind the dead, white skeletons of corals in their wake. These outbreaks eventually die off as the starfish eat themselves out of food supplies, but not before seeding downstream reefs with their tiny larvae that drift along the southern currents. During their peak, they destroy far more coral than other disturbances such as bleaching events or hurricanes.

Now, Hugh Sweatman at the Australian Institute of Marine Science has found that these outbreaks are much less frequent in the “no-take marine reserves”, where fishing is absolutely forbidden. Every year between 1994 and 2004, Sweatman carried out a census of starfish numbers in up to 137 areas across the Great Barrier Reef’s massive length.


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Sand dollars avoid predators by cloning themselves

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Many animals have cunning ways of hiding from predators. But the larva of the sand dollar takes that to an extreme – it avoids being spotted by splitting itself into two identical clones.


Sand dollars are members of a group of animals called echinoderms, that include sea urchins and starfish. An adult sand dollar (Dendraster excentricus) is a flat, round disc that lives a sedate life on the sea floor. Its larva, also known as a pluteus, is very different, a small, six-armed creature that floats freely among the ocean’s plankton.

A pluteus can’t swim quickly, so there is no escape for one if it is attacked by a hungry fish. Instead, Dawn Vaughan and Richard Strathmann from the University of Washington discovered that the pluteus relies on not being spotted in the first place.

They exposed 4-day-old larvae to water which contained mucus from the skin of a potential predator – the Dover sole. Within 24 hours, every single larva that was exposed to the mucus has grown a small bud that eventually detached and developed into a second larva, genetically identical to its parent and smaller in size. In contrast, larvae that were exposed to untouched seawater stayed undivided.