Friday, October 30, 2015

Friday Fluff #3: For When Enough is Enough

Thing the first: Thar Be Snail Ogres!

Does this look like a beautiful lady to you?
What is the DEAL with sailors?!
Image by Toriyama Sekien via Wikipedia
I'm sharing a cool Echinoblog post I found in the only way anyone finds anything these days - Twitter. In time for halloween, Christopher Mah describes the mythology of and actual inspiration for several Japanese monsters. This being a Mollusc-centric blog, I will feature the Snail Ogre, Sazae-oni, here, which somehow transfigures into a drowning woman, or something convincingly enough like it to attract sailors, who become the snail's next meal. The real snail this legend is based off of is the horned turban snail (Turbo cornutus1). This image also reminds me of Aku.

There are a number of other monsters on there that are worth checking out and attempting at Halloween costumes, such as the Samurai crab, and/or Stephen Hawking, and the internet darling of the year, the dumbo squid.

Thing the second: Killer Whale Espionage

Because you know you want a giant orca photo...L94 and her baby nursing in BROAD DAYLIGHT! She's not even using a blanket, how indecent! Photo credit: NOAA Fisheries, Vancouver Aquarium
The world is all atwitter, quite literally, about the use of drones in research of the Southern Resident Killer Whales. I've blabbed about some of the recent population growth of this group (here), and there have been even more calves born this year. NOAA just released a video podcast show-and-tell of some recent photos obtained via hexacopter. The contraption, which is strapped to two pool noodles for flotation, flies above the whales to get good pictures of them without disturbing them. They can ID and measure the whales to track growth rates.

Surveillance photos indicate that the babies keep on coming (welcome, Baby L-122!), and the hexacopter provides insight into family life of the growing pods. This year has been a good year for the whales, lots of babies and seemingly well-fed whales. Hopefully that trend lasts.

Thing the third: See Slug Day!

Yesterday was Sea Slug Day! I don't really even have that much to say about it, except that it's worth trolling #SeaSlugDay for the photos, but don't say I didn't warn you that it was a black hole of a time suck! You sort of start to wonder if this group isn't really just an alien race living among us. Or we are a prison planet for all of some other planet's criminals: what with kleptoplasty, and kleptocnidae - they're evidently really just into stealing things, evidently.

Super cutie: O. muricata on E. pilosa

As an undergraduate, I did research on what is the least colorful species in the most colorful group of organisms in the world - Onchidoris muricata. That doesn't mean it wasn't charismatic! I was researching whether this tiny white blob, which you would be well within your rights to assume was some previous beach walker's snot rocker, was interested in eating an invasive bryozoan(2), in the Gulf of Maine. The bryozoan, Membranipora membranacea, was super abundant there, growing all over rocks and kelps. We gave the snot rocket nudibranch a choice between it's presumed preferred food (a native bryo Electra pilosa) and the new bryozoan (which they are probably serving at that gastro street-food from around the world fad place down the street from me), and found that the slug preferred to eat the fancy new food, and could eat it faster.

This was back in 2004, when we wondered, what could this mean for future slugs? Just this year, people are starting to think about whether invasive species are good food for native species - they could be right? Well it turns out that in a lot of cases, they are often beneficial as a supplement to a balanced diet, so they don't harm native species. But on their own, they aren't as good as the diet the predator evolved to eat. So, no you can't have Doritos for breakfast, lunch, AND dinner.

Phew, for Friday fluff, that really was some academic non-sense I just prattled on about(3)! Here's a palate-cleansing video... 

Check out this video I found in my archives. Marney Pratt, my Advisor on the project, grew some bryozoans onto a microscopes slide, and fed them a red algae so that they would show up clearly against the background: the red dots you see are individual bryozoans, called zooids. The big white blob is a single Onchidoris, and you're looking at it from beneath, through the side of a tank and the glass slide. Marney filmed this time lapse of the slug eating a single zooid. If you watch closely, there is one zooid that gets slurped out of its little house toward the bottom right. This is sort of like how an oyster drill (predatory snail) consuming an oyster in that "sucks-his-guts-out" is still an applicable description.

SEE! I told you they were charismatic!

References and Miscellany

1. For Turbo Corn Nuts see disambiguation.
2. Bryo (= moss) zoan (= animal). These are colonial organisms that do really cool defensive stuff...maybe some other day though...
3. But if you weren't completely deterred, you could read the paper we (/Marney) wrote here.

Friday, October 9, 2015

Carcinus, and the Amazing Technicolor Crab Shell OR Video Games for SCIENCE!

I think most people assume that reading scientific papers is totally boring, and that only someone who is really fascinated in some completely arcane microdiscipline could possibly get anything out of it. I wish I could disabuse you of that assumption, but, by and large, yeah, they're pretty boring unless you really get your jollies reading about "A framework to assess evolutionary responses to anthropogenic light and sound"(1). There are certainly some notable exceptions, mostly when scientists, some more successfully than others, try to demonstrate they have a sense of humor(2). But what I like is really digging to find the hidden gems in papers.

Today, for instance, I learned that, "Humans adopt a similar search pattern to birds…when looking for prey on a computer screen". Huh! Did you know that? I did not know that! And, naturally, there are citations!(3)

Sooooo, why do I need to know that? (4) Because it turns out it's easier to get humans to play a video game than to get birds to do what you want - such is the way science actually gets done.

OK, but that's really a methods question, let's get to the point of the paper:
Carcinus maenas just can't decide what it wants to wear. Images from Stevens et al. 2014
The above photos are ALL ONE SPECIES (SRSLY), the European green crab, Carcinus maenas. Notably, this species is regularly listed as one of the worst invasive species on the planet, but that really doesn't have anything to do with this variation in color(5). And it turns out this is not the only species to have this insane catalogue of costumes that undoubtedly drives undergraduate zoology students insane: Also sporting coats of many colors:
Then what is the deal with all these fashion-forward crabs? Well some intrepid volunteers in Singapore attempted to test the hypothesis that the variety of colors helps camouflage crabs when they are young, to keep them from getting eaten by their main predator - you guessed it, shorebirds(9). So this is where it's important to know that humans and shorebirds look at a computer screen similarly, because these volunteers got to play video games - FOR SCIENCE! 

Before we get to the shoot-em-up part (pew, pew!), let's dive into the question a bit more. First off, a quick straw poll, when you hear the word camouflage, what image pops into your head? I'm going to go out on a limb and guess it was something like this:

Out on a limb FTW! Their frowns are the best.
The camouflage that we often think of, that is literally embodied by the chameleon, is called background matching. Which sounds pretty straightforward - trying to hide by pretending you aren't there. But there is another type of camouflage that you are probably familiar with, and it looks like this:

Yes, this is also camouflage, it turns out.
This is called disruptive coloration. This type of camouflage isn't about avoiding detection, it's about making it hard for someone who wants to eat you to figure out just where you start and where you end. You can see how it works really well when you get a dazzle(10) of zebras together and your eyes start to cross, but it also works when a zebra is by itself, at least theoretically. By creating false edges and boundaries, this coloration makes it more difficult for the edge-detection software in your brain to identify and track the animal as an individual. Not convinced? Try this thought experiment. In disruptive coloration, high contrast is an asset because the strong difference in brightness between the white and black stripes overpowers or distracts the brain from detecting the weaker differences in color with the animal an its background(11)

So let's bring it all together:

Video Games + Camouflage + Green Crab

This paper just came out in Current Zoology (by Peter Todd and colleagues) using video games to test how these white spots might contribute to camouflage in green crabs. They did this by creating fake crab shells and putting them on fake backgrounds, and having students come play their video game, which I'm calling "Nab the Crab", timing how quickly they could find the fake crab shell on the computer screen.

The researchers varied the shells and the back ground in different ways - testing several aspects of camouflage. Here is some really compelling evidence that you can learn something from video games:

Figure 6. from Todd et al.: This is how long it took game players to find the simulated crab shells (examples on the bottom).

On the far right, notice that the game players found the plain shell in no time flat (about  1 second!) - the plain shell is clearly no good at hiding this fake crab from shorebirds (because remember the thing from the beginning about visual search patterns...). Next look at the left two bars, those two shells had large spots on them, but one (LE) had large spots overlapping the edge of the shell, while the other (LI) had the spots all inside the shell. Game players were MUCH faster finding the crab when the spots were all on the inside of the shell (15 versus 33 seconds). The same was true for middle and small sized spots, but the difference in speed was less.  This is all great support for the fact that spots at the end of the shell could act as disruptive coloration!

Figure 1 from Todd et al. High and low contrast spots.
The researchers also tested whether increased contrast would help, so they made crabs with bright white spots and crabs with grey spots, and in most cases, the higher contrast crab was harder to find.
Figure 2 from Todd et al. Background.
Of course all of this depends on your background, and some of these effects of camouflage depended on what kind of background the crab was on. If you imagine that these crabs like to live on beaches, you can imagine these backgrounds as small pebbles (on the left) and larger cobbles (on the right). In general, game players took longer to find the crab on the small/pebble background regardless of where the spots were on the shell or what color they were - this makes sense, a busy background makes it harder to detect pattern if the brain has to do a bunch of work interpreting the busyness.

Which is all to say that humans playing video games is only very slightly similar to the process by which natural selective forces of predation by shorebirds might have altered the frequency of shell coloration in Carcinus maenas. Nevertheless, I'm all for video games for science. I found Smorball this summer (Go Piggers!) - by playing you are helping digitize the library of biodiversity literature! I also went on a few virtual safaris, one in Gorongosa National Park, and one in Samburu.

So next time you're caught goofing off playing video games, you can just just say you're doing science like a shorebird. 


References and Miscellany
1. A title I literally just pulled out of the latest issue of Trends in Ecology and Evolution, THE most important ecology journal published, but it sounds kind of interesting, don't you think?
2. This is perhaps the most inscrutable example known to man.
3. But salient to this quoted sentence is Jackson et al. Behavioral Ecology 2005
4. Also, who is adopting who's search pattern here?!
5. You'll hear more about this later, after years of trying to work with this species, I have finally found my way to a project on European green crab.
6. Krause-Nehring et al. 2010 Zoology (link)
7. Rachel Folz (link)
8. Silva et al. 2014 Braz. J. Biol. (link)
9. There is a method to my madness.
10. A group of zebras can also be called a "zeal" but "dazzle seemed more to the purpose here. 
11. Now, I should point out, the story of zebra stripes is probably more complicated than disruptive coloration, and an apparent sudden urgency of solving the "how the zebra got its stripes" question has resulted in new theories, including insect avoidance and keeping cool. I'm going to suggest that stripes probably have multiple benefits for zebras, and might have all played a role in their evolution. I use them as an example because they do make my point about disruptive coloration.

Wednesday, June 17, 2015

Species species of the Week week #10 or EVEN MOAR Tautonomical predation!

What's better than a tautonomical(1) predation and better even than tautonomical IGP

Tautonomical predation by the most absurd looking creatures ever!

Mola mola

The ocean sunfish
Figure 1. Look at your ridiculous face and tiny mouth you amazing alien! That's right, it's Mola mola eating Velella velella! Photo: Jodi Frediani - obvi

I wrote a few weeks ago about how massive numbers of Velella^2 just won't stop washing up on the west coast, really taking all the cache out of these animals. They're just not a rare enough sighting any more to constitute a hipster-find. But the plus side is that we get to see things like this happening. Jodi Frediani captured several pictures of this goggly-eyed prehistoric force of nature in Monterey Bay.

It turns out that people just assume(2) Mola mola are supposed to eat larger jellies(3) but their diet is pretty poorly understood, there really isn't very much quantitative data. Other animals that preferentially eat large jellies have mouths that are much better suited for it (Figure 2) and there is increasing speculation that they are much less picky eaters(4). I love how difficult these pictures make it look to catch a barely mobile floating jelly blob. These Velella must be worth it somehow. Maybe the Mola likes the crunchy sail bit - they're like potato chips.

Figure 2. The last thing a jellyfish sees. Sea turtles are sooooo kewt! Now, THIS is a mouth for biting into and holding onto gooey slippery jellyfish. Leatherback sea turtle mouth (Photo Credit: Geographic Consuling).
PS Happy Turtle Week!
So, tiny mouth - check. What else? 

This species is so endearing because it's huge and clumsy at swimming. In fact, when I was an undergrad, I was taught this was the largest species of plankton(5), but again, we are learning more that they are actually pretty swell swimmers, thankyouverymuch. I recall being on a whale watch and seeing one breach (the sunfish - we didn't see any dang whales on the whale watch). The weird shape of the fish (Figure 3) is apparently the likely result of adapting to life in the open ocean when you start out looking something like a pufferfish (Figure 4, and reference 4). The dorsal and anal fins are used for propulsion, which you can see really well in this video, and the tail has receded into a mere rudder (called a clavus). 
...ocean sunfish stroke their dorsal and anal fins generate a lift-based thrust that was likened to the symmetrical flipper beats of penguins. Essentially, M. mola uses its dorsal and anal fins as a pair of wings. It is worth noting that this is the only animal known to use two fins for this purpose that are not originally bilaterally symmetrical. (5)
It is worth noting indeed - you go M. mola

Figure 3. Super weird skeleton of a super weird fish. Notice how the vertebrae are nearly degenerated, but some of the spinal processes are really well integrated with the bones stabilizing the dorsal fin.
Lastly: behavior, because what good is a weird-looking animal if it acts totally normal? 

In addition to swimming all funny, and breaching (possibly to dislodge parasites), these fish sunbathe (possibly(6) to warm up after deep dives). Other nutso facts:
  • The most fecund fish, no, the most fecund vertebrate on the planet! Releasing an estimated 300 MILLION eggs per season.
  • Their larvae retain spines that not only look super punk, but are shared with pufferfishes they are related to (Figure 4)
  • "Mola" is the latin word for "millstone
  • They apparently also recruit seabirds to lend a bill with the parasite problem (no scales!)
  • Surprisingly, no apparent relationship with Moola moola

Figure 4. Larval punkfish. If this isn't a Pokemon character by now, I don't know what to think any more.
Photo: G. David Johnson

The result of all of this weirdness is entirely hypnotic to watch. Hypnotic isn't even the right word, it's more like disorienting. If you ever get a chance to go see them at the Monterey Bay Aquarium, or, god forbid, in their actual habitat(7), you will truly feel like an alien on your own planet. As in, how could I be on this planet so long and not know anything about anything?

References and Miscellany

(1) I still can't resolve whether it should be tautonomical or tautonymical, it's driving me kind of nuts. Don't I wish I had majored in linguistics?
(2) And you know what they say about assuming...
(3) To play the pedant here, Velella are technically not jellies. True jellyfish are Scyphozoans while Velella are hydrozoans, but I don't want to steal the limelight from the vertebrate star here!
(4) Pope et al. 2010 Rev Fish Biol Fisheries
(5) Plankton in the strict sense, means "drifter", those at the mercy of the currents
(6) Noticing a theme here with all the "possibly"s? The field is wide open!
(7) If this is your jam, do it soon. These fish face substantial mortality as bycatch in gill nets etc. Though the population is thought to be relatively stable - I again refer you to the number of qualifying "possibly"s in this post, meaning we don't know as much as we think we do.

Friday, May 29, 2015

Friday Fluff #2: For When Enough is Enough

Thing the First: Blue Tide hits Washington Coasts

What a freaking mess! Someone should do something about this! Photo: Tiffany Boothe, Seaside Aquarium

This is old news now, but I was resisting putting it on the blog for a long time. Really strong westerly winds have pushed literal heaps of everyone's favorite blue tautonomical cnidarian onto coastal beaches in Washington and Oregon over the last month. Is it called a "blue tide"? I don't know. Should it be? Maybe, "blue tide" is kind of redundant. In any case, yes, Velella velella has become a victim of its own ingenious dispersal ability (1), and create just endless fields of rotting blue mesoglea (2). The most stunning statistic coming out of this whole ordeal is that only 50% of science journalists know how to capitalize latin names. Editorial Fails include: USA Today, The Weather Channel, KOMO news (local, and having the opposite problem as the others). CNN did get it right, but then lost points for referring to "something fishy going on in Washington" which is as original as it is semantically correct - not at all. Anyway, there's a lot of cool pictures. This isn't really something anyone is concerned about, they aren't a huge health hazard, or an endangered species, and this isn't some major environmental catastrophe. This happens from time to time to these guys, and could happen more frequently as climate change alters the frequency of certain weather patterns.

Thing the Second: Pretty Subtidal Seattle OR Pretty (Subtitle: Seattle)

If you think it's beautiful above the waterline in Seattle this time of year (and you're right, it's ridiculous), check out this video my very talented friend Eliza Heery made celebrating our green waters! Eliza does what she calls Urban Marine Ecology, exploring how marine ecosystems function so closely to an extremely dense urban environment.

Thing the Third: Foaming with rage and bruised all over the chest

Just some old crummy boat...wait it looks familiar! Photo: Richard Rodriguez, Bitter End Blog

The most faithful molluscophiles among you will remember my reference to John Steinbeck's (and Ed Ricketts', cough cough) description of the infuriating intractability of the the Sally Lightfoot Crab (Grapsus grapsus, naturally). The vignette was from the Log from the Sea of Cortez, detailing a whimsical collecting trip the likes of which will never occur again. Some have tried, like really tried, down to the brand of beer, which was evidently a crucial aspect of the trip. But I suspect we won't get narrative like Steinbeck out of it(3). Anyway, the point is that the boat Steinbeck and Ricketts took, the Western Flyer, is still in existence, sort of, in Port Townsend, WA. It's sunk more than once, and certainly has some tragedy in its history, but the new owner hopes to restore it and potentially return it to Monterey - hopefully not as a restaurant...A book about the post-Steinbeck/Ricketts history of the Western Flyer recently came out - in time for father's day, score.

Happy weekend!

References and Miscellany:

1. If you don't recall the mechanics of the By-the-wind sailor, might I recommend this resource?
2. Word of the Day! There, go impress your friends and family and don't say I never gave you nothin'!
3. However, evidently there is currently a museum exhibit resulting from the repeat journey (inscrutably) at the AZ museum of natural history

Monday, April 13, 2015

Don't read this if you don't want nightmares about things that will never happen to you

You know what totally gives me the willies? Two words:

Transmissible cancer

Yikes. Yes, that is cancer that can be "contagious". This is different from oncoviruses, which are viruses that cause cancer (like some strains of HPV). But this is where a cancerous cells touching uninfected individuals can cause them to "catch" the cancer as well. Until recently, only three types were known (Spoiler alert/you can breath a sigh of relief: NONE in humans):
  • Facial tumors in tasmanian devils (DFTD), which are transmitted when these guys chomp each other in the face squabbling over food, mates, territory, and who's more swole.
  • Canine transmissible venereal tumor (yes, that is what it sounds like), which was recently sequenced. 
  • And [sort of] a sarcoma in Syrian hamsters, too(1).
I have spared you pictures of any of the above, but feel free to do a Google image search - NO! DO NOT DO A GOOGLE IMAGE SEARCH!

Line at the Mya arenaria clinic. Soft shell clams waiting for a blood/hemolymph draw to test for cancer, photo from Metzger via Cell Press. It really looks like proper science what with the lab diaper and flasks, beakers, and bunsen burner in the background.

Well, welcome to the club, Steamer! I agree that name is really inappropriately adorable for a leukemia, but it has been so dubbed because it infects soft-shell, or "steamer" clams (Mya arenaria). This leukemia have been observed for over 40 years, and causes death in the majority of individuals who become infected, but the cause of the leukemia was unknown until recently. The natural question here is: "Leukemia? Do clams even have blood?" And the answer is sort of - they have hemolymph, which is a fluid that carries oxygen (and other junx) around the tissues of the clam's open circulatory system.

Graphical abstract from Metzger et al. 2015 Cell. One of the key findings in this paper is that the tumors all had the same genetic signature in infected clams, suggesting that they all came from a single source, even though the clam populations examined were found hundreds of miles apart. 

What/Who is Steamer? And how do I keep it the *bleep* away from me? Never fear! You won't have to give up your chowdahSteamer is a "jumping gene", a retrotransposable element(2) that can get copied into the DNA of a host organism. If you can't imagine why mishmashing DNA could be problematic, everything you need to know on the topic can be found here. All kidding aside, let me be completely clear about this - this retrotransposon WILL NOT infect humans. But soft shell clams are indeed at risk. The most recent research shows that Steamer can travel hundreds of miles in the ocean and infect clams via these jumping genes. So, this is unusual because a virus is not spreading the disease but the tumor cells (hemocytes) themselves are spreading the disease. Hence: transmissible cancer.

It's certainly consoling that this newest discovery in the realm of terrifying things nature can kill you with is so distant from us in the tree of life that it's not a direct threat to us (lovers of clam chowder likely disagree, however. Disagreement noted.). Nonetheless, this is the first time a transmissible cancer has been discovered in an animal other than a mammal, which opens the possibility that there are many more of these out there infecting many more types of animals than we suspected. 

Now try to sleep at night...

References and Miscellany:

1. It seems like this isn't a "naturally-occurring" transmissible cancer, in that the cancer was initially chemically induced, and then transmitted from one individual to another by injecting them with cells from the original tumor. So this isn't really quite the same thing, nonetheless, I'm including it here, because you will see it referenced in Wikipedia, and I wouldn't want you to think I didn't do my research. Also, be warned, the article I linked is pretty clinical.
2. Cripes, I am having a hard time finding relatable resources on retrotransposons! Try this? Entry level online resources describing this genetic process is absolutely a growth market. You could make a billion bucks if you made one decent you tube video on this topic. You're welcome.

Friday, April 3, 2015

Friday Fluff: For when enough is enough.

It's after 4 on Friday, and I just spent a few hours trying to teach myself sql. Soooo ... brain. Because. But three easy pieces did cross into my field of view recently, and so:

1. Even extinct cone snails get all the attention!

Cone snails get a disproportionate amount of attention, mostly, I suppose because they are so diverse. Admittedly, they have some cool ecology, but nonetheless, those of us who don't work on them (or maybe just me) are sometimes a bit jealous of all the hubbub about a single group. So it was unsurprising when I saw this post from the Smithsonian blog about how entrancing even extinct cone snails are. A researcher from San Jose State University used UV light to show the color patterns on fossilized cone snail shells. Some of the pigmentation was no longer visible to the naked eye, but using this technique, this guy identified more than 10 new species! I feel like there should be some comment here about inspiration coming from the strangest places - like raves and velvet posters. Here's the original paper, with more cool photos.

Figure 2 reproduced from Hendricks 2015 PLoSOne demonstrating
how UV light can be used to reveal historic coloration of fossil cone snail shells

2. Between a rock and a limpet radula

Well, Rah rah radula indeed! In fact, we had better hail the radula or it will come for us, and we will not come out on top. Engineers at the University of Portsmouth (also in Science Daily) have proclaimed that limpet teeth are made of the strongest naturally-occurring material known to man. Here, "strongest" refers to highest tensile strength and has nothing to do with towing a 767. For those of you who still think terrestrial systems have anything to offer ("But, but, what about spiders?") it turns out these fangs have toppled spider silk as the champion of natural material strength. 

No, this picture does need to be this big. If the last post on attack snails wasn't enough to give you nightmares, this should be. Image from the University of Portsmouth. 

All of that from this:
Patella vulgata, the common limpet. Just think about how
strong teeth would be from an UNcommon limpet.
Image from Wikimedia Commons.

3. Beauty abounds thanks to El Nino!

California faces drought unprecedented in recorded history and John Steinbeck wishes he were alive to document it. But Californians have something positive to look forward to - population booms of this beautiful slug, Okenia rosacea, which benefits from warmer temperatures. The Hopkins rose nudibranch is being spotted much further north than is usual. More from Science Daily and the primary literature.
Image by Jerry Kirkhart from Los Osos, CA via Wikimedia Commons
Have you seen it on your beach? Send me pictures!

Wednesday, January 21, 2015

Dangerous hormonal snails OR Vacuum tube of death

So, here's a question: 

Why is this fish so dumb?

Good God, man! It's a giant snail vacuum mouth of death, coming at you very, very slowly - just move! All you have to do is ... anything! Nope. OK, you did nothing. 

What looks like a case of natural selection weeding out those fish without any common sense or athletic ability is evidently a case of chemical warfare. This species of snail, Conus tulipa, belongs to a group of predatory snails(1) that uses neurotoxins to immobilize and then overtake prey. The problem is, most of them use a poison dart like this:

Clearly that's not what's going on with C. tulipa. It's just a giant, obvious, excruciatingly slow, net-mouth coming right at the slimy little bug-eyed fish face - and that video is sped up! Because we are busy people who don't have time to watch this sort of slow-motion train wreck at its true glacial speed. 

But, OK, it's not hard to imagine that instead of darting them with a neurotoxin, they are releasing it in the water nearby before they "spring the trap". And that's what people thought for a while. A recent paper(2) shows that the fish aren't the victims of a toxin, but  a hormone. Fish are sent into hypoglycemic shock as C. tulipa releases an insulin into the surrounding water. This, according to WebMD, is likely to cause confusion, dizziness, sweaty skin, and "feeling shaky" in the fish. No, I'm kidding. Fish can't sweat. This is what it does to fish:

You just witnessed actual science! So the idea is the insulin makes fish susceptible to the snails' otherwise unambitious attack strategy. One of the other cool things is that, because there are many different forms of insulin, snails had to evolve an insulin that would work on fish! 

If snails have taught me nothing, it's that there's more than one way to skin a fish. And that it's totally fine to be lazy as long as you have a sweet chemical arsenal.

References and Miscellany:

(1) Don't you dare act shocked that there is such a thing as predatory snails! Did you not read the very first thing I ever published on this blog?! Pay attention, people.
(2) Actually, as of the writing of this post, it's not even published - just a preview still.