Physalia Physalis: A Mysterious Balloon Animal

For anyone who reads this blog, my obsession with jellyfish is well known.  They are such fascinating creatures and are the subject of my current (and next) dissertation chapter. One of the most feared jellyfish is the Portuguese Man O'War.  Its sting is debilitating, and can be deadly if not treated. Stories of swimmers becoming entangled in the tentacles of the Man O'War abound in my native Florida and in my immediate family. Concerns about their sting is not overblown: a Man O'War's sting is 75% as powerful as the cobra (Naja). In a word: Intense. 

The natural history of this organism is amazing.  The Man O'War is not a jellyfish, but is instead a siphonophore.  What's the difference? Siphonophorae, unlike medusae, are actually a collection of separate organisms called zooids. They co-exist as a colony, and cannot live separately, but they are not a single organismal system. How great is that?  The tentacles of the man o'war are separate organisms from the float. It's a complicated creature, the physalia. But what makes it so interesting to me is all that scientists don't know. Which is quite a bit.  

Biologists don't know how these animals breed, how they avert prey, their life cycle, or their general habits.  Why? 

The Portuguese Man O'War has never been kept successfully in captivity.   I've previously posted on the difficulties involved in maintaining jellyfish in captivity. They require very specific water movement, food, and temperatures.The Man O'War is a super special case. 

Unlike most other jellies, the Man O'war has a "float". This part of the organism is just as it sounds- part of the structure is filled with air, and floats along the ocean surface.  In fact, Man O'Wars are not individually mobile- they go where the current takes them (they don't attack people, but accidentally float into them).  In captivity, if they bump up against the side of a tank, their skin forms hard spots. These spots keep the organism from rolling; a behavior investigators believe is performed to keep the float wet. A tank to hold specimens would need to be roomy, so that the organisms could float without touching the side of their enclosure, as well as roomy enough to allow their particular rolling behavior. 

Besides being very big in diameter to allow room for maneuvering  a tank to maintain captive man o'wars needs to be very deep. The organism has super long tentacles (which are highly dangerous to other organisms because of the stinging nematocysts contained therein). When completely relaxed, some of the longer tentacles can reach over 9 meters. The organism can retract these tentacles, but it seems they are almost always extended in the natural environment. So, a tank would have to be very wide and very deep. And, it would need some type of water movement. 

Physalia cannot control their mobility. They belong to a family of pelagic organisms that are free floating (including the Velella Velella- commonly known as a "By-The-Wind-Sailor"- my favorite Cnidarian). Unlike jellies like Aurelia Aurita (moon jellies) that perform the characteristic contraction/pumping motion associated with jellyfish (although even Aurelia need some current or they get tired and die quickly in captivity), Siphonophorae like Physalia and Velella travel by currents.  Recreating this in a tank environment is difficult.  The continuous circular current of kreisel tank (utilized when keeping Aurelia in captivity) is perfect for organisms that live a submerged life; but it does little for an organism that lives on the surface. Investigators have tried many forms of tide in captivity:  utilizing a spray mechanism to create current and push the organism away from the side (this disoriented and tangled the jellies), keeping the jellies in cages in a moat (it mangled the tentacles), and utilizing structures that blocked the sides of the tank (but prevented the somersaulting motion needed to keep the float wet). The most advanced system has only succeeded in maintaining the organisms in captivity for about 6 months. 

Keeping marine organisms in captivity is only the first step to studying them. You must also keep them healthy in captivity and learn to mimic their natural environment. Once you master this, the organism might grow, produce sexual materials, and perhaps if you are lucky reproduce viable offspring.  If one organism does all of this unlikely stuff, you could possibly watch a new generation grow in captivity.  And this is really when you learn something. 

To date, investigators in scientific laboratories at universities and aquariums have struck out. And so, scientists are still guessing about the life cycle of these amazingly enigmatic creatures. In 2006, J. Pierce quoted the studies of Ernst Haeckel (1888), Robert Bigelow (1891), and George Parker(1932) regarding the physiology and supposed behavior of these organisms. That's a very long time for scientific theories about a non-extinct or even non-threatened organism to go unconfirmed or challenged.  Until the physalia can be kept in captivity, we may not know if the float can be deflated, or how these organisms reproduce. 

But don't despair: A succession of tanks, initiated by a theory by Robert Browne in 1901, has lead to a century of tanks built specially for rearing captive jellyfish and plankton. Yesterday, I visited the Birch Aquarium at the Scripps Institution of Oceanography. There, I saw both Aurelia Aurita and Cassiopea Xamachana reared in that facility.  More on that adventure in my next post... 

Some awesome sources:

Pierce, J. (2006) "Aquarium Design for Portuguese Man-Of-War" International Zoo Yearbook 

Parker, G.H. (1932) "Neuromuscular Activities of the Fishing Filaments of Physalia" Journal of Cellular and Comparative Physiology

Bigelow, Robert (1891) "Notes on the physiology of Caravella maxima, Haeckel (Physalia caravella,

Eschscholtz)" Johns Hopkins University Circular

Haeckel, Ernst (1888) Report on the siphonophorae collected by H.M.S. Challenger during the years

1873–1896. Report on the Scientific Results of theVoyage of the H.M.S. Challenger, Zoology