Stung by a Moon – Celebration of the Aurelia aurita genome
On 3 December 2018, the first jellyfish genome was published online at Nature Ecology and Evolution. To be clear, the is the first genome of a cnidarian with a jellyfish stage (i.e. medusa). What was the lucky species? Aurelia aurita (species 1 complex), the Moon jelly. Moon jellies are a commonly encountered species around the world, and probably one of the first jellies most people can picture in their heads; a stout, translucent jelly with short tentacles and the charismatic clover-leaf pattern on the top of their bell. They are ubiquitous in aquariums that care for jellies, and are the most common jellies kept as pets (not including corals or anemones). Moon jellies are also common in biological research, with several laboratory strains (including Roscoff and Eilat) used for studying nervous system evolution and cellular regeneration. If you have taken an introduction to zoology course, you have probably seen theAurelia life-cycle in your textbook.
To celebrate the release of this genome, I thought I would take a closer look at what we understand about the venoms of these common jellies. Since these species are found across the globe, they are a common jelly for beachgoers to “interact” with, not to mention researchers and aquarists that work with these animals on a near-daily basis. Needless to say, Moon jellies have likely stung their fair share of humanity.
But as you can imagine, these animals would not have become so common in aquaria or labs if they were truly dangerous. In fact, I could only find two reports of these jellies causing major health issues. One is a case report in 1988 by Burnett et al where a 30-year old marine biologist experienced local skin irrational and pain for over a week after being stung by “an unusually large” A. aurita. The second case is also a sting by a “quite large” A. aurita on a 25 year old man diving in Florida, which cleared up in 8 days after treatment with medication (Simmons et al 2014). While generally considered harmless, there have been incidences of massive jellyfish blooms around the global that have caused problems: issues with jellyfish stings by Aurelia have been reported in the Caribbean, Spain and Israel (Mariottini and Pane 2010). Just this year over 2,000 people were stung by Moon jellies in Florida, though no major injuries were reported. Even if a single sting from one individual is relatively benign, in a large number of jellies (called a bloom), these animals likely pack a more powerful punch. And even if most of the time these animals only cause some itchiness, moon jellyfish stings appear to be highly variable, and we have no idea why that variability occurs.
So, what do we know about moon jelly venoms? Though the studies are sparse, crude venom from Aurelia has shown to be dermonecrotic and display vasopermeability and hemolytic properties (Mariottini and Pane 2010). Basically, their venom attacks skin and blood. Crude venom also displays phospholipase A2 activity (Radwan et al 2001), which is a common toxic component of animal venoms (and typically cnidarians) that can cause a range of myotoxic and neurotoxic effects (i.e. they burst open cells and fry your neurons). Phospholipase A2 transcripts (evidence of gene expression) has also been found in the nematocysts content of Aurelia, as well as with evidence of highly potent pore-forming toxins from a nematocyst proteome (Rachamin et al 2014). Other proteolytic (protein busting) properties have been found in crude venoms as well (Pong Prayoon et al 1991;Lee et al 2011) and recently neurotoxic activity has been characterized in vivo using crab models (Ponce et al 2013). There is even a study from Japan showing that venom from Aurelia collected in the Truk Islands of the Federated States of Microneasia completely and irreversibly blocked muscle contraction in frogs (Kihara et al 1988).
All of these toxins and toxic properties are common bioactive properties of venoms, but what about specific effects on humans? Work on Mexican Caribbean A. aurita showed that extracted venom from tentacles was equally hemolytic to human, sheep, and bovine erythrocytes at low doses but was more toxic to human cells at higher doses (Segura-Puertas et al 2002). In fact, these hemolysins appear just as potent as hemolysins isolated from Sea Nettles, which are generally thought to be more dangerous to people. Strangely, hemolysins from sea nettles are equally potent to human erythrocytes as sheep (Segura-Puertas et al 2002), suggesting the mechanism is somewhat different from the Aurelia hemolysin.
So, what is the deal? Is the mechanism of these hemolysins really that different from sea nettles, or is there perhaps simply a lower dose of hemolysin in Aurelia, by virtue of having smaller tentacles than the sea nettle, thus lending to a smaller effect? And are these toxins specifically targeting humans? Shouldn’t that this evidence indicate that all moon jellies are dangerous?
The truth is that many of these components are common in most venoms, but almost all are very specific for invertebrate and vertebrate prey. In the same study on the Mexican Caribbean, fractions of that venom were lethal to crabs in about three minutes; instar II and III Artemia nauplii were affected only after five hours, whereas instar I nauplii were hardly affected at all. The jellies that were most harmful to human cells were also those that were most lethal to crabs (and assumingly Artemia), but even that venom is not effective towards the earliest stages of Artemia. That suggests that some of these individuals might have just been more toxic by genetic chance. So while Moon jelly venom sounds scary and all individuals contain some dangerous compounds, that doesn’t mean that all moon jellies will hurt you. Venom evolves meticulously for exactly the prey and/or predators that jellies (or any venomous animal) interact with. And humans aren’t really either for jellyfish, at least not in their native marine environment.
There has also been some recent work that suggests it might not even be true envenomation from contact with tentacles that is causing problems. The mucus released by moon jellies has been shown to contain enzymatic compounds that can potentially cause a sting without having to touch the jellyfish at all (Lui et al 2018). This is likely a form of defense, since mucus production is usually induced by a physical disturbance or general stress. Fisherman that touched “residual water” on their nets after going through a bloom of jellies would get skin irritation and local swelling (Lui et al 2018). Cnidarian mucus is also an understudied field that could prove valuable for discovering novel bioactive compounds and understanding ecological roles, as well as aiding in public health efforts. Even more fascinating is the possibility that jellyfish mucus could help breakdown oil (see this article to learn more).
I owned large Aurelia from Florida (they are the logo for this blog!), and comfortably (if not awkwardly) moved them into new tanks with my hands. It is not that they are not stinging me, or that toxicity has been “bred out” of them because they were raised in tanks (though comparison of venoms from lab-reared and wildtype animals has yet to be tested), but usually these moon jellies are just not a problematic jellyfish to people.
But that still doesn’t answer why moon jelly venom is so variable in terms of human envenomation. I do have a couple of ideas based on other venomous animals. It could be due to the size of the jellies, which may represent a shift in diet from small zooplankton to larger, vertebrate prey. This has been shown to occur in other venomous animal, like snakes and spiders, which shift their venoms as they switch prey types. And it would make sense for the two medical cases explained above. In fact, Simmons et al (2014) suggests that larger jellies may have larger stinging cells, which may deliver most venoms. But another study on Old World (Red Sea) and New World (Chesapeake Bay) Aurelia found that the Old World species were more potent than their New World counterparts. So maybe it isn’t size, but specific species or subspecies in certain areas. Or there is something about the different locations that is shifting venoms to great potency. But what is it about their local environments that cause this difference, especially towards humans? Perhaps there is simply a difference in ecological and environmental conditions that cues for more potency. Maybe there is also a genetic component that makes these particular animals more toxic.
And what about polyps? How does their venom compare, both in composition but also in function? Does a polyp need hemolytic toxins if it isn’t trying to eat any vertebrates that have blood? What happens to venom of strobilating polyps or ephyrae, which don’t even eat? Studies utilizing the recently released genome could help answer some of these questions, and in fact what I am hoping to research during my graduate work.
Not only is the release of a jellyfish genome a huge advance for evolutionary genomics, but for better understanding a commonly problematic species in our oceans. A. aurita (and other Aurelia species) are often responsible for major infrastructure disruptions, including shutting down nuclear power plants (e.g. CBC News). A reference genome allows ecological researchers to do population and environmental studies, which can be complimented with genetic work that utilizes the genome. A genome basically acts like a huge platform on which to lay gene expression data (transcriptomes) on top. Plenty can be done without a genome (there is TONS of research on a multitude of animals that don’t have a reference genome), but it does make it easier and more reliable for some experiments, such as looking at how different environmental conditions affect jellyfish reproduction and blooming behavior. This type of work may help us understand the genetic mechanisms behind large bloom events, and thus lead to more robust ways to predict these behaviors.
We do not currently have good ways to manage moon jelly blooms, though some have suggested consumption may be a good option. If you can’t beat it, eat it! Researchers in Denmark have developed ways to prepare “jellyfish chips” as a more palatable way to consume jellies (see my article on the KU EEB Outreach blog).
This ends my ode to Moon jelly venoms, but I am hoping that in the years to come more research on the toxicity of these animals will be completed.
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