Physalia: Classification, Characteristics and Life Cycle

Table of Contents

• Classification of Physalia
• Characteristics of Physalia
• Anatomy
• Ecology of Physalia physalis
• Life Cycle

Physalia, a genus of the order Siphonophorae, found on the surface of the Indian, Pacific and Atlantic oceans, are colonies of polyps and medusoids. They may appear like a single multicellular organism but are made up of minute colonies called zooids. Two species of the genus are known: Physalia physalis and Physalia utriculus. Let us now study the genus in detail.

Classification of Physalia

Characteristics of Physalia

There are two species found in the genus Physalia: Physalia physalis (Portuguese man o’ war) and Physalia utriculus (Pacific man o’ war or blue bottle). The Portuguese man o’ war thrives in the Indian and Atlantic oceans, whereas the blue bottle thrives in the Pacific ocean. Both species are considered to be the same, below is a detailed description of Physalia physalis.

• It is a marine hydrozoan that belongs to the Neuston community. Neuston is a community of floating ocean organisms that live at the interface between air and water.
• It is named Portuguese man o’ war because of its great resemblance to the sailing warship.
• The pneumatophore of the species lives on the surface while the remainder of the body is submerged under water.
• Physalia physalis is a conspicuous, accessible and robust siphonophore as compared to other members of the order that are fragile and difficult to collect.
• It consists of numerous nematocysts that sting painfully, killing fishes and even humans.
• It is found commonly on the coasts of Indonesia, Japan and Eastern Australia, stinging thousands of beachgoers.

Anatomy

• The species is composed of smaller units called zooids that hang in clusters from a large gas-filled structure called pneumatophores.
• Almost seven kinds of different zooids are described in the species, namely nectophores, vestigial nectophores and gonophores in the medusoid body and tentacle-bearing zooids, gonopalpons, gonozooids and free gastrozooids in the polyploid body.
• The pneumatophore, also called bladder, is a conspicuous, translucent and tinged (mauve, purple, pink or blue) part that rises as high as 15cm above the water.
• It is a floatation device that allows the colony to move in the direction of the wind.
• The pneumatophores are filled with carbon dioxide that is actively produced by the animal. This gas diffuses into the surrounding area in the event of an attack, thus allowing the colony to submerge in water for protection.
• The colony of Physalia hunts and feeds with the help of gastrozooids and tentacle-bearing zooids.
• The tentacle-bearing zooids, also known as dactylozooids or tentacular palpons, are equipped with tentacles of about 10 m lengths that can extend up to 30 metres.
• The tentacles have small, thread-like, coiled structures that are called nematocysts.
• The nematocysts have venom that is injected at the time of attack by squids and fishes. It paralyses and kills the organisms.
• For preying, the cnidarian contracts its tentacles to catch the prey upwards and bring it near the gastrozooids (digestive zooids). The gastrozooids surround the prey and secrete enzymes for digestion.
• Gonophores, the reproductive zooids, are situated in branches called gonodendra.
• The gonophores produce sperm and eggs, the gonozooids are accessory gastrozooids, and the nectophores allow the detached gonodendra to swim.

Ecology of Physalia physalis

• The species is a carnivore that preys on fishes (both young and adult), zooplanktons, shrimps and other small crustaceans.
• The species has many predators, such as loggerhead turtle, blue sea slug, violet snail and ocean sunfish.
• The blanket octopus is a species that is immune to the toxic venom of Physalia physalis.
• The man-of-war fish (Nomeus gronovii) lives and feeds on the tentacles and gonads of the Portuguese man o’ war.
• Yellow jack is another species of marine fish that gains shelter from the stinging tentacles of Physalia, and in return, the species attracts fish to eat.

Life Cycle

They are dioecious in nature, which means that each colony is either male or female. The gonophores, borne on branch-like structures called gonodendron, produce either egg or sperm. The eggs and sperm are released into the water, and mating takes place primarily in the autumn season.

A fertilised egg develops into a larva and starts budding zooids. The growth usually takes place underwater. Pneumatophores are grown first, followed by gastrozooids and tentacle-bearing zooids. The pneumatophore eventually becomes buoyant enough for the colony to float on the surface of the water.

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• Explore the Difference between Polyp and Medusa