The Portuguese war (Physalia physalis), also known as the man-of-war or bluebottle,[1] is a marine hydrozoan found in the Atlantic Ocean and the Indian Ocean. It is considered to be the same species as the Pacific man o' war or bluebottle, which is found mainly in the Pacific Ocean.[2] The Portuguese man o' war is the only species in the genus Physalia, which in turn is the only genus in the family Physaliidae.[3]
The Portuguese man o' war is a conspicuous member of the neuston, the community of organisms that live at the surface of the ocean. It has numerous microscopic venomous cnidocytes which deliver a painful sting powerful enough to kill fish, and even, in some cases, humans. Although it superficially resembles a jellyfish, the Portuguese man o' war is in fact a siphonophore. Like all siphonophores, it is a colonial organism, made up of many smaller units called zooids. Although they are morphologically quite different, all of the zooids in a single specimen are genetically identical. These different types of zooids fulfill specialized functions, such as hunting, digestion and reproduction, and together they allow the colony to operate as a single individual.
The name man o’ war comes from the man-of-war, a sailing warship,[4] and the animal's resemblance to the Portuguese version (the caravel) at full sail.[5]
The bluebottle, Pacific man o' war or Indo-Pacific Portuguese man o' war, distinguished by a smaller float and a single long fishing tentacle, was originally considered a separate species in the same genus (P. utriculus). The name was synonymized with P. physalis in 2007, and it is now considered a regional form of the same species.[6] [7]
The man o' war is described as a colonial organism because the individual zooids in a colony are evolutionarily derived from either polyps or medusae,[8] i.e. the two basic body plans of cnidarians.[9] Both of these body plans comprise entire individuals in non-colonial cnidarians (for example, a jellyfish is a medusa, while a sea anemone is a polyp). All zooids in a man o' war develop from the same single fertilized egg and are therefore genetically identical. They remain physiologically connected throughout life, and essentially function as organs in a shared body. Hence, a Portuguese man o' war constitutes a single organism from an ecological perspective, but is made up of many individuals from an embryological perspective.[8]
Most species of siphonophores are fragile and difficult to collect intact.[10] [11] However, P. physalis is the most accessible, conspicuous, and robust of the siphonophores, and much has been written about this species.[12] [13] [14] [15] [16] [17] [18] [19] [20] The development, morphology, and colony organization of P. physalis is very different from that of other siphonophores. Its structure, embryological development, and histology have been examined by several authors.[6] [21] [22] [23] These studies provide an important foundation for understanding the morphology, cellular anatomy, and development of this species.
Like all siphonophores, P. physalis is a colonial organism: each animal is composed of many smaller units (zooids) that hang in clusters from under a large, gas-filled structure called the pneumatophore.[24]
Seven different types of zooids have been described in the man o' war, and all of these are interdependent on each other for survival and performing different functions, such as digestion (gastrozooids), reproduction (gonozooids) and hunting (dactylozooids). A fourth type of zooid is the pneumatophore. Three of these types of zooids are of the medusoid type (gonophores, nectophores, and vestigial nectophores), while the remaining four are of the polypoid type (free gastrozooids, tentacle-bearing zooids, gonozooids and gonopalpons).[6] However, naming and categorization of zooids varies between authors, and much of the embryonic and evolutionary relationships of zooids remains unclear.
The pneumatophore or bladder is the most conspicuous part of the man o' war. This large, gas-filled, translucent structure is pink, purple or blue in color; it is 9to long and rises as much as 15frac=2NaNfrac=2 above the water. The pneumatophore functions as both a flotation device and a sail, allowing the animal to move with the prevailing wind. The gas in the pneumatophore is mostly air which diffuses in from the surrounding atmosphere, but it also contains as much as 13% carbon monoxide, which is actively produced by the animal.[25] [26] In the event of a surface attack, the pneumatophore can be deflated, allowing the animal to temporarily submerge.[27]
New zooids are added by budding as the colony grows. Long tentacles hang below the float as the animal drifts, fishing for prey to sting and drag up to its digestive zooids.[28]
The colony hunts and feeds through the cooperation of two types of zooids: tentacle-bearing zooids known as dactylozooids (or palpons), and gastrozooids. The palpons are equipped with tentacles, which are typically about 10sigfig=1NaNsigfig=1 in length but can reach over 30sigfig=1NaNsigfig=1.[29] Each tentacle bears tiny, coiled, thread-like structures called nematocysts. Nematocysts trigger and inject venom on contact, stinging, paralyzing, and killing molluscs and fishes. Large groups of Portuguese man o' war, sometimes over 1,000 individuals, may deplete fisheries.[6] Contraction of tentacles drags the prey upward and into range of the gastrozooids. The gastrozooids surround and digest the food by secreting digestive enzymes. P. physalis typically has multiple stinging tentacles, but a regional form (previously known as a separate species, P. utriculus) has only a single stinging tentacle.
The main reproductive zooids, the gonophores, are situated on branching structures called gonodendra. Gonophores produce sperm or eggs. Besides gonophores, each gonodendron also contains several other types of specialized zooids: gonozooids (which are accessory gastrozooids), nectophores (which have been speculated to allow detached gonodendra to swim), and vestigial nectophores (also called jelly polyps; the function of these is unclear).
Man o' war individuals are dioecious, meaning each colony is either male or female.[24] Gonophores producing either sperm or eggs (depending on the sex of the colony) sit on a tree-like structure called a gonodendron, which is believed to drop off from the colony during reproduction. Mating takes place primarily in the autumn, when eggs and sperm are shed from gonophores into the water.[24] As neither fertilization nor early development has been directly observed in the wild, it is not yet known at what depth these occur.
A fertilized man o' war egg develops into a planula that buds off new zooids as it grows, gradually forming a new colony. This development initially occurs under the water, and has been reconstructed by comparing different stages of planulae collected at sea. The first two structures to emerge are the pneumatophore (sail) and a single, early feeding zooid called a protozooid. Later, gastrozooids and tentacle-bearing zooids are added. Eventually, the growing pneumatophore becomes buoyant enough to carry the immature colony on the surface of the water.
See also: Ocean surface ecosystem.
The Portuguese man o' war is a carnivore. Using its venomous tentacles, it traps and paralyzes its prey while reeling it inwards to its digestive polyps. It typically feeds on small fish, molluscs, shrimp and other small crustaceans, and zooplankton.[30]
The organism has few predators; one example is the loggerhead sea turtle, which feeds on the Portuguese man o' war as a common part of its diet.[31] The turtle's skin, including that of its tongue and throat, is too thick for the stings to penetrate. Also, the blue sea slug specializes in feeding on the Portuguese man o' war,[32] as does the violet sea snail.[33] The ocean sunfish's diet, once thought to consist mainly of jellyfish, has been found to include many species, including the Portuguese man o' war.[34] [35]
The man-of-war fish, Nomeus gronovii, is a driftfish native to the Atlantic, Pacific and Indian Oceans. It is notable for its ability to live within the deadly tentacles of the Portuguese man o' war, upon whose tentacles and gonads it feeds. Rather than using mucus to prevent nematocysts from firing, as is seen in some of the clownfish sheltering among sea anemones, the man-of-war fish appears to use highly agile swimming to physically avoid tentacles.[36] [37] The fish has a very high number of vertebrae (41), which may add to its agility and primarily uses its pectoral fins for swimming—a feature of fish that specialize in maneuvering tight spaces. It also has a complex skin design and at least one antibody to the man o' war's toxins. Although the fish seems to be 10 times more resistant to the toxin than other fish, it can be stung by the dactylozooides (large tentacles), which it actively avoids.[36] The smaller gonozooids do not seem to sting the fish and the fish is reported to frequently nibble on these tentacles.[36]
The Portuguese man o' war is often found with a variety of other marine fish, including yellow jack. These fish benefit from the shelter from predators provided by the stinging tentacles, and for the Portuguese, the presence of these species may attract other fish to eat.[38]
The blanket octopus is immune to the venom of the Portuguese man o' war. Individuals have been observed to carry broken man o' war tentacles,[39] which males and immature females rip off and use for offensive and defensive purposes.[40]
The stinging, venom-filled nematocysts in the tentacles of the Portuguese man o' war can paralyze small fish and other prey.[20] Detached tentacles and dead specimens (including those that wash up on shore) can sting just as painfully as those of the live organism in the water and may remain potent for hours or even days after the death of the organism or the detachment of the tentacle.[41]
Stings usually cause severe pain to humans, lasting one to three hours. Red, whip-like welts appear on the skin that last two or three days after the sting. In some cases, the venom may travel to the lymph nodes and may cause symptoms that mimic an allergic reaction, including swelling of the larynx, airway blockage, cardiac distress and shortness of breath. Other symptoms may include fever, circulatory shock and in extreme cases, even death,[42] although this is extremely rare. Medical attention for those exposed to large numbers of tentacles may become necessary to relieve pain or open airways if the pain becomes excruciating or lasts for more than three hours, or if breathing becomes difficult. Instances in which the stings completely surround the trunk of a young child are among those that may be fatal.[43]
The species is responsible for up to 10,000 human stings in Australia each summer, particularly on the east coast, with some others occurring off the coast of South Australia and Western Australia.[44]
Stings from a Portuguese man o' war can result in severe dermatitis characterized by long, thin, open wounds that resemble those caused by a whip.[45] These are not caused by any impact or cutting action, but by irritating urticariogenic substances in the tentacles.[46] [47]
Treatment for sting pain is immersion in water for 20 minutes.[48] The cnidocyte found in the box jellyfish react differently than the nematocyst in the Portuguese man o' war; cnidocytes are inhibited by application of vinegar, but nematocysts can discharge more venom if vinegar is applied.[49]
The species is found throughout the world's oceans, mainly in tropical and subtropical regions, but occasionally also in temperate regions.
P. physalis is a member of the neuston (the floating community of organisms that live at the interface between water and air). This community is exposed to a unique set of environmental conditions including prolonged exposure to intense ultraviolet light, risk of desiccation, and rough sea conditions.[50] The gas-filled bladder, or pneumatophore, remains at the surface, while the remainder is submerged. The animal has no means of propulsion; it moves passively, driven by the winds, currents, and tides. Winds can drive them into bays or onto beaches. Often, finding a single Portuguese man o' war is followed by finding many others in the vicinity.[51] The Portuguese man o' war is well known to beachgoers for the painful stings delivered by its tentacles. Because they can sting while beached, the discovery of a man o' war washed up on a beach may lead to the closure of the beach.[52] [53]
P. physalis uses a float filled with carbon monoxide and air as a sail to travel by wind for thousands of miles, dragging behind long tentacles that deliver a deadly venomous sting to fish.[54] This sailing ability, combined with a painful sting and a life cycle with seasonal blooms, results in periodic mass beach strandings and occasional human envenomations, making P. physalis the most infamous of the siphonophores.[55] Despite being a common occurrence, the origin of the man o' war or bluebottle before reaching the coastline is not well understood, and neither is the way it drifts at the surface of the ocean.
The Portuguese man o' war is asymmetrically shaped: the zooids hang down from either the right or left side of the midline of the pneumatophore or bladder. The pneumatophore can be oriented towards the left or the right. This phenomenon may be an adaptation that prevents an entire population from being washed on shore to die. The "left-handed" animals sail to the right of the wind, while the "right-handed" animals sail to the left. The wind will always push the two types in opposite directions, so at most half the population will be pushed towards the coast.[56] [57] Regional populations can have substantial differences in float size and the number of tentacles used for hunting. The regional form previously known as P. utriculus has a bladder rarely exceeding 10cm (00inches) in length and has one long hunting tentacle that is less than 3m (10feet) long. In comparison, the typical man o' war has a float of around 15cmto30cmcm (06inchesto10inchescm), and several hunting tentacles that can reach 30m (100feet) in mature colonies when fully extended.[55] When combined with the trailing action of the tentacles, this left- or right-handedness makes the colony sail sideways relative to the wind, by about 45° in either direction.[58] [54] Colony handedness has therefore been theorized to influence man o' war migration, with left-handed or right-handed colonies potentially being more likely to drift down particular respective sea routes.[58] Handedness develops early in the colony's life, while it is still living below the surface of the sea.[55]
Since they have no propulsion system, the movement of the man o' war can be modelled mathematically by calculating the forces acting on it, or by advecting virtual particles in ocean and atmospheric circulation models. Earlier studies modelled the movement of the man o' war with Lagrangian particle tracking to explain major beaching events. In 2017, Ferrer and Pastor were able to estimate the region of origin of a significant beaching event on the southeastern Bay of Biscay. They ran a Lagrangian model backwards in time, using wind velocity and a wind drag coefficient as drivers of the man o' war motion. They found that the region of origin was the North Atlantic subtropical gyre.[59] In 2015 Prieto et al. included both the effect of the surface currents and wind to predict the initial colony position prior to major beaching events in the Mediterranean.[60] This model assumed the man o' war was advected by the surface currents, with the effect of the wind being added with a much higher wind drag coefficient of 10%. Similarly, in 2020 Headlam et al. used beaching and offshore observations to identify a region of origin, using the joint effects of surface currents and wind drag, for the largest mass man o' war beaching on the Irish coastline in over 150 years.[61] These earlier studies used numerical models in combination with simple assumptions to calculate the drift of this species, excluding complex drifting dynamics. In 2021, Lee et al. provide a parameterisation for Lagrangian modelling of the bluebottle by considering the similarities between the bluebottle and a sailboat. This allowed them to compute the hydrodynamic and aerodynamic forces acting on the bluebottle and use an equilibrium condition to create a generalised model for calculating the drifting speed and course of the bluebottle under any wind and ocean current conditions.