A Ray of Hope

Courtesy kathleenreed via Flickr

It's a story that lends itself to hyperbole. An unknown disease causing the literal disintegration of a large group of animals across a wide geographic area. Sea star wasting disease has been on the minds of every marine science professional for the last two years. And amazingly...wonderfully, this issue has captured the attention of the general public as well. Through traditional media, the internet, and personal experience, sea star wasting has become, I think, the most visible issue facing the oceans today.

In case you haven't heard, throughout the North East Pacific, from Alaska to California sea stars have been dying off in large numbers. The animals begin to show signs of distress by curling their rays in unusual ways. Then white lesions (any kind of damage to tissue) appear on the outer skin. These lesions then disintegrate further until holes appear. Eventually the holes grow so large that limbs separate from their bodies, and the animals crumble into piles of skeletal plates. Some reports have stated that the disease causes the arms to walk away from the body, but that is a blatant exaggeration. Partly because stars don't have brains, their limbs can survive for a surprisingly long time after they've been separated from the central disk. So while the disease does cause the arms to come off, it's not what's causing them to keep moving. Sea stars can even deliberately drop off limbs in an attempt to protect the rest of their body from disease and predators. Then they grow a new ray in it's place.

 "We can rebuild him, we have th...." "No that's okay he'll do it himself"

Courtesy Jill Siegrist via Flickr

So why can't the stars regenerate from the damages of the disease, and what the heck is causing it in the first place? Well for the last two years the answer has been a big, fat, "I dunnuh", but that's because researchers have been furiously looking into it, and good experimentation takes time. There has been amazing collaboration between aquariums, research labs, and everyday folks to study the spread and cause of wasting. From this collaboration a new study has identified a virus that is associated with sick stars.

Last year a team of researchers discovered the first virus associated with echinoderms. They found the pathogen inside the tissues of sea urchins on Hawaiian coral reefs. This virus was a type of densovirus which are most commonly found infecting arthropods, like crabs, shrimps, and insects. In the urchins the virus wasn't causing any disease, but as the outbreak of sea star wasting became more severe the scientists wondered if something similar might be at fault. 

First they needed to see if there were any viruses in the stars at all, so the scientists separated virus sized particles from the tissues of sick stars and injected healthy ones with this material. They also boiled samples of those particles before injecting other healthy stars; doing so destroys the DNA that viruses could use infect the organisms. Sure enough the stars that received potentially active viruses became sick with wasting and the ones that received the boiled samples did not.

MMMM Nothing like a nice hard-boiled virus to start the day

Courtesy michelle@TNS via Flickr

From there the team ran a viral DNA analysis on the sick animals and found a densovirus that is unique to stars. They named the pathogen Sea Star associated Densovirus or SSaDV for short, and the more copies of the virus the stars were carrying the more likely they were to start wasting. Interestingly the team found that for most stars, the larger the animal, the greater the viral load, but the opposite was true for the sunflower star (Pycnopodia helianthoides). Sunflower stars were one of the first and most heavily impacted by the disease, so I'm curious if this association may have something to do with that. Through this, and some other lines of evidence, these researchers have found a compelling correlation between this virus and sea star wasting.

So is that it? Can we all wash our hands of this and get on with out lives? In short, no. The study confirms the existence of a virus associated with wasting, but it doesn't look at how the virus interacts with the sea stars' cells. It's extremely likely that the virus alone isn't what's causing the stars to die. Especially since the researchers looked at samples of stars collected as far back as 1943 and found the same viral DNA. And when you think about it that makes sense. When you contract a virus you don't get sick purely because the virus is in your body. You get sick because the virus combines with your stress from work, and the bacteria in your environment, and the fact that you stayed up late having drinks, to tax your immune system until it can't suppress the virus anymore and you get symptoms.

There has been extensive coverage of this study, but the problem is that many news outlets are claiming the answer has been found and they have ignored an important takeaway from the paper's conclusion. From the paper itself: "However it remains to be seen how infection with SSaDV kills asteroids, what the role is for other microbial agents associated with dying asteroids, what triggers outbreaks, and how asteroid mass mortalities will alter near-shore communities throughout the North American Pacific Coast." (Hewson et al. 2014). Essentially the author's are saying " this is a good start, but we have a lot to look into."

It's even possible that disease is a normal means for the

environment to handle overpopulation of echinoderms

Courtesy US Fish and Wildlife via Flicker

It all seems a bit bleak, but like I said before there has been an incredible amount of collaboration, and unprecedented visibility to the plight of West coast stars. Knowing the densovirus is associated with the disease won't stop it, but now we have jumping off point to further our understanding. This is a unique opportunity for you, as an interested person, to participate and keep this research alive.

So if I could ask one thing of you all it's this: Keep paying attention. Stay up to date, visit your local aquarium and ask questions, follow researchers on twitter. You can even go out and survey beaches for wasting stars yourself and scientists will use your data. Together we can develop a strong understanding, citizen and scientist alike, of what this disease is and does. So if you make statement about stars, or upload some pictures to social media I encourage you to attach the hashtag #RayOfHope, and we'll see if we can keep the momentum going.

For more information on the study that identified the virus check out this great summary from Ed Yong with National Geographic. Or read the paper yourself for free on the National Academy of Sciences website. 

References:

Hewson et al., "Densovirus associated with sea-star wasting disease and mass mortality", Proceedings of the Natural Academy of Sciences, Oct 2014, DOI 10.1073/pnas.1416625111  

Underwater Basket Weaving

Hey everyone, this week we're diving into a bit of a mystery. There's a pretty good chance that by now this little gem has come across your social media feed!

The original of that video had 8 million views on facebook alone last I checked. I've gotta say it's really cool that people are so curious about ocean animals. But the question on everyone's mind seems to be, as my mom succinctly put it: "What the heck is it?" Despite its facehuggerly appearance this is a native of the earth, or should I say the sea? What you're looking is a basket star. I should also mention that I'm not the first to identify this guy/gal. Both the Echinoblog, and IFLScience have tackled this mystery.

First off basket stars aren't actually a true sea star. You may remember from the post on catch connective tissue that sea stars are members of the echinoderm phylum. More specifically the sea stars we're most familiar with make up the asteroidea class (a class is one grouping more specific than a phylum) So if you're feeling pedantic and mischievous you can tell people you found tons of asteroids on the beach and not be lying. However the basket star is not an asteroid! Basket stars are part of a class of animals called ophiuroids (pronounced "off-yer-roids), and are more commonly called brittle stars.

 Jazz Hands!

Courtesy  Paul Thompson via Flickr

Even though most brittle stars look quite a bit like traditional sea stars, being in a separate class means they are as different from a true sea star as a sea urchin is. One of the most notable differences between sea stars and brittle stars is in how they get around. Sea stars use their hundreds of suction cup tube feet to grip tightly to the bottom and cruise along. Their rays (also referred to as arms) act as more of a platform for those strong tube feet to operate from. Brittle stars don't use their tube feet to walk. Instead they pick themselves up on their rays and stroll or slither like something out of the Nightmare before Christmas. Their tube feet lack suction cups and are used to grab food and help move it towards their mouth.

Basket stars are a really cool specialized group of brittle stars. They are well adapted for collecting plankton out of the water with their arms. In the above video you can only catch the view for a second, but at one point the basket star opens all its arms, and you can see the central disk. The disk is pentagonal and one trunk-like arm grows out of each side. Each of those five arms then branches dozens of times to create a wide net. The arms of the basket star are covered in microscopic hooks, a nice coating of mucus, and are capable of coiling around themselves to form traps that hold onto their planktonic prey. Below you can watch as some euphasiid shrimp are added to a basket star's tank at the Seattle Aquarium.

That video is a little sped up, but you can see how those branches form a wide net and are waved back and forth to sweep for more food. Grabbing food out of the water like this is called suspension feeding. Sometimes you'll hear it called filter feeding, but that's a bit different. When there isn't an obvious load of plankton around them, basket stars usually cling to a hard surface or the branches of corals. They curl their rays up above their bodies into the current forming a basket shape. Hence the name.

What a basket ca...I'm not even gonna let myself finish that joke

By Peter Southwood (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Once a basket star has enough food trapped on one of their rays they'll slowly move it towards their star shaped mouth. Incidentally brittle stars don't have an anus, so they excrete their waste through the same hole they consume food. Anyway inside the mouth are five sets of comb-like teeth. The star slides its arms over the teeth and the prey are scraped off like frosting from a fork. Am I the only one who does that? I can't be the only one who does that.

Basket stars are found throughout the world from shallow water to the abyssal plane. The one from the original video is probably Euryale aspera which is a shallow living basket star found throughout the Indian ocean and tropical western Pacific. One of the things I think is coolest about basket stars is that they seem to have a strong association with a variety of coral species. Not only do coral branches make a good holding place for adult basket stars, they may even be an important nursery for juveniles. Young of the species most commonly found around N. America, Gorgonocephalus eucnemis, are usually found living just inside the polyps of the sea strawberry coral (Gersemia spp.). While this seems to be some type of symbiotic relationship, it isn't entirely clear if the little basket stars are stealing food from the polyp they're living on, or just using their mouth as a platform to feed from.

References:

Stöhr, S., O’hara, T., & Thuy, B. (March 2nd 2012) “Global Diversity of Brittle Stars (Echinodermata: Ophiuroidea)” PLOS ONE DOI: 10.1371/journal.pone.0031940,
Accessed via http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0031940

"What is that weird thing on facebook???" The Echinoblog
http://echinoblog.blogspot.com/2014/10/what-is-that-weird-thing-on-facebook.html

“Gorgonocephalus eucnemis” Encyclopedia of Life, 

http://eol.org/pages/599654/details

"Gersemia” Encyclopedia of Life, 

http://eol.org/pages/41391/details

Skin Deep

A calm ocean can seem uninteresting from above. At first glance it's this big expanse of basically nothing, but just beneath the surface lie wonders. The same can be said of sea stars. When we find them at low tide sea stars barely move. Many people aren't even sure that they're animals, and we regularly had people at the touch-pools of the Seattle Aquarium ask if they were fake.

 Well he does exaggerate how much he can bench, but I wouldn't call him fake

Courtesy Jerry Kirkhart via Flickr

  Right under the surface of sea stars, and the other echinoderms like sea cucumbers and urchins, lies what I think is one of the coolest adaptations of any animal on earth: catch connective tissue. This material is the source of echinoderms' amazing ability to become completely rigid, or jello soft.

Before we get into the meat of how this stuff works let's think about how it might be useful. At any one time different sections of a star's body can be rock hard, near liquid, and everything in between. This is an amazingly good strategy for an animal that moves over uneven terrain. Imagine you're a sea star and you're trying to find a tasty mussel to eat. As you crawl along your leading arm comes into contact with a big boulder. Well no big deal, you can make that arm go soft and bend to any angle you might need to climb onto it. As you reach the top of the stone you notice a strong current trying to blow you away. Again no problem, you can make a couple of arms go rigid to add strength to your grip which keeps you from waving around. Once you're held on nice and tight one of the arms that you're not using to cling for dear life can go soft and tap around the rock's surface in search of prey. Below you can see sped up video of a blue linckia star (Linckia laevigata) moving, and you can get a good idea of what I'm talking about. Check out how flexible the sections bending around the edges of the coral are, and how stiff the parts on the flat.

 Clearly this catch connective tissue is some useful stuff, but how does it work? In humans, our skin and connective tissues are made up of fibrils (small bundles of strands that make up a fiber) of collagen held together by connecting proteins.

Collagen fibrils (the big strands going up) and the proteins holding them together

Courtesy Zeiss Microscopy via Flickr

This is true for catch connective tissue as well.  The difference is that we have a more or less set amount of proteins holding the fibrils together whereas echinoderms can change the number of links. The more proteins holding the fibrils together, the less they can slide around on one another, and the more rigid the whole tissue. The fewer the proteins, the more the fibrils can slide around, and the softer the tissue. It's almost like bundling sticks with rubber bands. In what might be the greatest naming ever, the molecules that trigger the hardening and softening of the tissue are called tensilin and softenin. Many echinoderms can even reduce the number of proteins so low that they can literally walk away from sections of their body. This is called autotomy (dibs on the band name) and it's a great way to escape predators that might do much more damage by ripping off one of their appendages. Thankfully echinoderms have incredible regenerative abilities, so they can regrow parts of their bodies. There's a wonderfully quirky, yet easy to understand, video explanation of this incredible tissue at creaturecast.org.

References:

Ana R. Ribeiro, Alice Barbaglio, Cristiano D. Benedetto, Cristina C. Ribeiro, Iain C. Wilkie, Maria D. C. Carnevali, Mário A. Barbosa, (September 14th, 2011) "New Insights into Mutable Collagenous Tissue: Correlations between the Microstructure and Mechanical State of a Sea-Urchin Ligament" PLOS ONE, DOI: 10.1371/journal.pone.0024822 Accessed via
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0024822

Yasuhiro Takehana, Akira Yamada, Masaki Tamori, and Tatsuo Motokawa, (Jan 15, 2014) "Softenin, a Novel Protein That Softens the Connective Tissue of Sea Cucumbers through Inhibiting Interaction between Collagen Fibrils" PLOS ONE. 2014; 9(1): e85644.
Published online, DOI: 10.1371/journal.pone.0085644,
Accessed via http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3893245/

Dunn lab and Creature Cast
http://creaturecast.org/archives/2457-creaturecast-echinoderm-skin

A Worm as a Weapon

One of the things I love about the ocean is that every separate surface is its own ecosystem. If you look at the shell of a crab you're likely to find plants and animals growing there. And that's just the ones that don't deliberately decorate themselves. Many times the organisms living on and around each other form some kind of symbiotic relationship. As you probably remember from school, symbiosis is when two organisms live together and can affect each other's behavior. Often times we say that both benefit one another, but that's only one of three types. Mutually beneficial symbiosis is conveniently called mutualism. You've probably already heard of the symbiosis where one organism benefits and the other is harmed; it's called parasitism. Finally when one organism benefits and the other doesn't get anything but also isn't harmed it's called commensalism.

All three types are frequently observed in this environment 

Courtesy Canyon 289 via Flickr

Chris Mah, who is an awesome scientist and blogger over at the Echinoblog did a post in 2010 about a team of scientists who took a closer look at a cool symbiosis. You can read it here. In his post Dr. Mah talks about how we thought that the banded scale worm (Arctonoe vittata), which lives on a number of invertebrates in the Pacific northwest, probably didn't benefit its host organisms. Consequently they are generally considered commensals. However a team of scientists decided to see if one of the worms' hosts, the leather star (Dermasterias imbricata), preferred to have a worm over not. If they did prefer to have the worms it would suggest the worm benefited the star and would actually be a case of mutualism. Well it turns out the stars chose worms over nothing, other stars, and even their favorite foods! The scientists who wrote the paper offer suggestions for how the worm benefits the star, but also recommend looking into it more. I'd like to offer the video below as evidence of the worm's benefit to the star.

That is the same banded scale worm biting the hell out of a sunflower star's (Pycnopodia helianthoides) rays. Seriously the worm looks like the love child of a graboid and a xenomorph when it chomps down. In this case the worm is protecting a keyhole limpet (Diodora aspera). As far as I'm aware sunflower stars don't eat leather stars, but the morning sun star (Solaster dawsoni) consumes pretty much everything; especially other stars. Solaster dawsoni is such an invertebrate killer that it's also commonly called the vampire or death star.

Oof!  The leather star is probably not gonna regenerate from that one.

Courtesy Brooke Reiswig at:

 http://www.wallawalla.edu/academics/departments/biology/rosario/inverts/Echinodermata/Class%20Asteroidea/Solaster_dawsoni.html

Clearly the leather star could potentially benefit from the predator warding prowess of the scale worm. Wouldn't it be cool if the scale worm also occurred on morning sun stars so they could be protected from one another, because they're totally cannibalistic!? Oh wait they do!

Courtesy same as above

 That specific worm could be jumping ship from the star being eaten, but banded scale worms have been documented living on morning sun stars. Obviously no scientific rigor has been applied to this hypothesis yet, so it may turn out to be a load of hooey, but it's exciting to look at evidence and begin to form questions. I hope we'll see a study or some video evidence of stars also being protected in the future.

References:

http://www.wallawalla.edu/academics/departments/biology/rosario/inverts/

http://www.sciencedirect.com/science/article/pii/0022098179901266

http://echinoblog.blogspot.com/2010/03/is-it-love-mutualism-in-leather-stars.html