As the only member of the order Siphonophora that spends its life at home in both the air and sea simultaneously, Physalia physalis is highly recognizable even from afar. When considering the easy target this presents for predation, how has this fragile species been able to prosper in its natural environment? Being a colonial organism made-up of highly specialized zooids performing their synchronous functions has provided it with the necessary tools to improvise, adapt, and overcome.
The morphology of these zooids has greatly assisted with the species’ long-term survival. Most other zooids on their own are planktonic and are found in the far depths of the ocean. On their own they are extremely fragile and difficult to collect. When the zooids work together forming the Physalia physalis, they become more robust and easier to isolate, which is why many zooid studies are performed on the Physalia physalis. The Portuegse Man o’ War is a highly modified polymorphism, which occurs when zooids are modified so much that they act like the organs of one body instead of many individuals in a colony. The zooids in the species Physalia physalis are composed of units called cormidia. The cormidia of the Physalia physalis include the gastrozooids, dactylozooids that have long and short tentacles, and gonozooids with gonophores. Zooids exhibit these polymorphisms in order to take on highly specialized functions, because when they work together they increase their chances of survival.
Looking at the specialized task of each zooid, it’s clear that they have similarities to a typical organism’s organ functions. In development, the first observable zooid is the pneumatophore. The pneumatophore is a multi-layered structure and contains a gas gland. The aeriform cells within the gas gland keep the structure full of carbon dioxide, allowing it to float on the surface of water. Underneath the surface of the water, there are tripartite groups consisting of the dactylozooids or tentacular palpons, gastrozooids, and the gonodendron. The tentacular palpon is the largest structure of the organism, which can extend up to 165 feet in length. When in development, the younger tentacles are a purple color and will eventually mature to a turquoise blue. The tentacles are equipped with the nematocysts to paralyze prey they come into contact with and drag them towards the gastrozooids. The gastrozooids are the only zooids that perform digestive functions, using their mouths to attach to prey and release proteolytic enzymes that cause extracellular digestion of their food. After the digestion, the nutrients are moved to the gastric cavity, which connects to the rest of the colony to provide sustenance.
The last zooid that can be observed is the gonodendron. The gonodendron is unique in that it consists of several different parts, which scientists assume allows it to be released as a whole structure after maturity to begin reproduction. The gonophore contains the gametes for reproduction, and each Physalia physalis is dioceious, meaning the colony only has one sex. The gonodendron also has its own secondary gastrozooids, which allows it to have digestive functions once it is detached from the colony. In addition to this, it has a locomotive structure, the nectophore, which is thought to propel it through the water. Lastly, the jelly polyp is an unknown functioning structure contained on the gonodendron. All of the zooids structuring the Physalia physalis work together to ensure its survival and perpetuation.
If these polymorphisms are beneficial, then what is the origin of them? There are several theories about the origin of polymorphisms. The first is the Polyorgan theory proposed by Huxley and others, which states the zooids of a colony are essentially organs of the medusoid organism.The zooids will then multiply into different types of zooids and migrate from their initial positions. Then a new polymorphic individual is formed when these new, different types of zooids come together. The second theory is the Polyperson theory from Vogt and Gegenbaur, which states that the colony is not just one organism, but that it is composed of specialized parts which are modified zooids that have changed their structures and functions to better suit the colony. The development of these polymorphisms is essential in the life history of a coelenterate animal.
For the Portugese Man o’ War we have a hybrid of these two theories. In many ways it appears as though the zooids are essentially the organs of the individual, however, they still have highly specialized zooids that take on a unique function essential for survival. Over the course of the evolution of the Portuegese Man o’ War the function of these zooids are extremely specific making it impossible to function on their own.
We hope you have enjoyed reading our blog! If you want to continue to learn about the Physalia physalis up next we will discuss some of the interesting ecological facts.
Sources:
https://www.nature.com/articles/s41598-019-51842-1
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