Physalia

Not to be confused with Physalis.

Physalia
Physalia physalis
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Hydrozoa
Order:	Siphonophorae
Suborder:	Cystonectae
Family:	Physaliidae Brandt, 1835
Genus:	Physalia Lamarck, 1801
Species
Physalia physalis

Deceased Portuguese man-of-war specimen from the University of Alberta invertebrate zoology collection.

Physalia is a genus of the order Siphonophorae, colonies of four specialized polyps and medusoids that drift on the surface of the Atlantic, Indian and Pacific oceans. Although these organisms look like a single multicellular organism, each specimen is actually a colony of minute organisms called zooids that have to work together for survival. A gas-filled bladder resembling a blue bottle provides buoyancy, and long tentacles of venomous cnidocytes provide a means of capturing prey. A sail on the float, which may be left or right-handed, propels Physalia about the sea, often in groups. These siphonophores sometimes become stranded on beaches, where their toxic nematocysts can remain potent for weeks or months in moist conditions. Both species of this siphonophore resemble a jellyfish in appearance, with their gas-filled float and cluster of polyps beneath, which can hang up to 30 or 165 ft (9 to 50 m) below the surface of the sea. They reproduce sexually and asexually, and their large bursts of population growth are referred to as "jellyfish blooms".

Because it frequently washes up on beaches on the coast of the Atlantic, Indian and Pacific Oceans and is seen in the open ocean, Physalia is the best-known genus of siphonophore, as nearly all siphonophores live in the cold, dark ocean depths where they can only be observed in their natural habitat by a submersible or ROV.

Physalia belongs to the Physaliidae family, the sister group to Rhizophysalidae under the Cystonectae suborder.

The genus was first described by Jean-Baptiste Lamarck in 1801.

Diversity

Multiple Physalia washed up on Maroubra Beach, NSW, Australia.

The family Physaliidae is monotypic, consisting of only one genus, Physalia. The genus is also monotypic, containing only P. physalis. There is a long history of the genus being described with multiple species, such as the Pacific man o' war (P. utriculus), however most of these species are now considered synonyms for P. physalis.

Distribution

Physalia are found globally in a wide range of oceanic conditions due to their pleustonic nature, meaning that their involuntary locomotion is determined only by currents and wind.

According to data recorded from beachings, dispersal of Physalia is suggested to be highly influenced by wind over temperature. Although Australian beachings are recorded in summer at a higher rate than any other season, research reveals that this is likely due to the seasonality of high and low pressure winds acting on the organism's sail, since species density has been shown to be unaffected by colder temperatures.

Drifting

Physalia drifting is mainly determined by wind, similar to drift migration seen in migrating birds. Which direction they drift from wind influence is determined by their dimorphism. Physalia have two forms of their enlarged pneumatophores that affect drift; left-handed, which shifts to the right of the downwind direction and right-handed, which moves to the left of the downwind. In strong winds, Physalia drift with its float oriented in the same direction of the wind.

A closer look at a Physalia washed ashore on a beach in Boca Raton, Florida.

It is thought that this drifting, controlled by the direct influence of wind drag, is what leads mass amounts of Physalia to wash up on beaches. Moderately strong winds, around 5 m/s, produce drifting velocities significantly higher than the typical speed of ocean currents. When Physalia get closer to the coast, they start to experience the influence of the slope-shelf currents, which are strong even in light winds. Because of the Physalia's long tentacles, these currents can overtake the wind drift and wash them ashore.

Predation

Although Physalia have an incredible defence system in place, they are not invincible and thus fall prey to various different kinds of predators. Loggerhead sea turtles, ocean sunfish, and violet sea snails all feed on Physalia. These organisms all have adaptations that render the vemon of Physalia useless, making Physalia a perfect snack! Interestingly, the blue sea dragon ingests the nematocysts of the Physalia and uses them to their own advantage, employing them whenever they see fit.

Symbiotic Relationships

Depiction of the symbiotic relationship between Physalia and the "Man-of-war fish" Gobiomorus gronovii

The relationship between Physalia and various animals in the Nomeidae family has been explored concluding some behaviours associated with commensalism. These drift fish live in close proximity to the tentacles of Physalia and generally, if undetected, remain unharmed. Although they may not be fully immune to the venom of Physalia, they are incredible swimmers, swimming just fast enough not to be caught. These fish feed on the tentacles and gonads of Physalia.  It has been speculated that the fast movement of the Nomeidae may incentivize other fish to approach Physalia facilitating a quick and easy meal, making the relationship potentially mutualistic.

Reproduction

Physalia specimens have separate sexes, or dioecious, and all of the gonodendra (branch-like reproductive structures) ) in a single colony will have characteristics of a single sex, either male or female. There is no description of recorded larval development, methods of gametogenesis, or gametes in this genus. Physalia reproduce both sexually and asexually. Sexually, they engage in broadcast spawning, releasing their gametes into the water. Asexual reproduction for this genus is through budding, where the new clones become an entirely new colony .

Morphology

Physalia are commonly found in colonies of zooids. Each zooid has a specialized function which work together to allow the organism to thrive. Superior to the zooids is a pneumatophore, which functions like a sail to travel using wind, their tentacles - growing up to an average of 10 metres- following. Pneumatophores are typically blue, violet, or pink! These tentacles contain nematocysts, releasing venom whenever necessary.

Differentiation from Rhizophysidae

Physalia (family Physaliidae) shares the suborder Cystonectae with the family Rhizophysidae.

The differences between these families are apparent when comparing morphologies. Physalia possess larger pneumatophores and trailing tentacles instead of a stem. The stem of Rhizophysidae species and other Siphonophores contain segmented colonial polyps along its length which carry out specific functions such as feeding and reproduction. In Physalia, these structures, such as the gastrozooid, are concentrated on the ventral side of the pneumatophore.

These families also differ in the ocean water column zones they inhabit. Physalia exist strictly on the surface of the water while members of the Rhizophysidae family are found in the epipelagic and bathypelagic zones.

Jellyfish Blooms

Cnidarians (including jellyfish, and other organisms such as Physalia) go through periods of large bursts in population growth called jellyfish blooms. These bloom events cause adverse affects to the food web. Physalia (and other Cnidarians) in large populations can disturb the prey of fish, such as smaller larval fish, or compete for the same resources as them, resulting in the ripple effect disrupting the food web.

Coastal communities are affected by their large numbers because of their disruption to the fish, leading to the reduction in catches, as well as damage to fishing supplies. Future bloom events are expected to multiply as the climate changes, which will mark further effects on the oceanic wildlife in areas Physalia and other Cnidarians occupy.