Occasionally, we come across an image or a video that compels us to ask, “What is that?” Such was the case when I saw the mesmerizing work of Aaron Ansarov and his Man-of-War project.
The Portuguese man-of-war, Physalia physalis, is a marine animal that drifts on the ocean surface preying mostly on fish.
Unlike many other marine animals, it has no head, brain, gills or skeleton. Instead, it has a float, and thousands of appendages. Although it looks like a jellyfish and is often mistaken for one, P. physalis is actually a siphonophore, a colony of highly specialised hydrozoan animals called zooids, which forms its long appendages.
The float is essentially two nested containers – an outer muscular case with a sail-like crest and an inner air bladder. Physalia travels by drifting, its direction determined by the wind and the water. The float is able raise and lower its crest by contracting and relaxing the muscles of the outer case. And it can adjust how low it sits in the water by controlling the gas density in the air bladder, using a gas gland that is located on the inner layer of the float.
Underneath the float are 12 stalks (cormidia) from which all the zooids bud out. The budding pattern is very specific, and zooids mostly appear in groups of three called “tripartite groups”. Each group contains a dactylyzooid for catching prey, a gastrozooid for feeding and digestion, and a gonodendron for reproduction. When fully grown, Physalia can have hundreds of tripartite groups organized in branches and sub-branches.
The longest zooid of the tripartite group is the mature dactylzooid. It has a blue ampulla and a ribbon-like tentacle (10-12 m) covered with nematocysts. Nematocysts are tiny capsules with miniature harpoons that pierce into the fish and discharge a paralyzing venom. Once the fish is attached to the tentacles, the muscles of the tentacle contract and draw the paralyzed prey towards the hundreds of gastrozooids waiting to digest the food
The gastrozooids are responsible for providing nutrients to the entire organism. They have a long tubular shape with a mouth on the free end. When stimulated by the presence of food, a gastrozooid will spread open its mouth and release digestive enzymes. The enzymes break off small pieces of the prey which is then engulfed and digested. The nutrients are then channeled to the float and other zooids. Gastrozooids have a reddish-brown appearance partly due to the accumulation of haemoglobin from the blood of their prey.
The third element of the tripartite group is the gonodendra which is responsible for reproduction. Unlike dactylzooids and gastrozooids, the gonodendron is composed of several specialized zooids arranged in branches and sub-branches creating a delicate globular mass.
It is worth noting that Physalia physalis is often described as a colony of animals, like coral, because of the way it is assembled. However, whereas the zooids in coral are all identical and complete individuals, Physalia’s zooids have become specialized and are no longer able to survive independently. Evolutionary biologists propose that the “zooids” in Physalia are more accurately described as a unique arrangement of organs that is part of a single animal.
Thanks to Catriona Munro for her correspondence. Ms. Munro is a graduate student at The Dunn Lab, Brown University, specializing in the evolution of siphonophores.
Excellent video of Portuguese Man-of-war (Physalia physalis) in open waters by National Geographic Video.
General information the taxonomy, habitat, and ecological impact. “Marine Invertebrates of Bermuda, Portuguese Man-of-war (Physalia physalis)”
In-depth reading. “Studies of Physalia physalis (L.)” by A.K. Totton and G.O. Mackie. National Oceanography Centre, University of Southampton.
Cormidium: (plural: cormidia) Cluster of zooids.
Nematocyst: A capsule within specialized cells in the tentacles of cnidarians, such as jellyfish and corals, containing a barbed, threadlike tube that delivers a toxic sting to predators and prey.
Pneumatophore: A float with a sail-like crest and an inner air-sac.
Zooid: The units that make up a colonial animal. Each zooid is structurally similar to a free living animal.
Photo credit: Video and photos by Aaron Ansarov, used with permission. Schematic drawings by Emei Ma, adapted from Totton(1960), reproduced under Creative Commons. Source: http://www.biodiversitylibrary.org/item/28335#page/345/mode/1up.