The Bluebottle jellyfish, Physalia utriculus, is a common, if unwelcome, summer visitor to Sydney beaches. At the mercy of the wind, they are sometimes blown into shallow waters, and often wash up onto the beach.
On the eastern coast of Australia, it is the NE winds and warmer currents that bring them and other organisms that make up the armada or fleets of blue coloured floating colonial cnidarians and their predators, to beaches on the incoming tides.
The Bluebottle or Pacific Man-of-War is not a single animal but a colony of four kinds of highly modified individuals (zooids). The zooids are dependent on one another for survival.
The float (pneumatophore) is a single individual and supports the rest of the colony. The tentacles (dactylozooids) are polyps concerned with the detection and capture of food and convey their prey to the digestive polyps (gastrozooids). Reproduction is carried out by the gonozooids, another type of polyp.
The float is a bottle or pear-shaped sac that can exceed 15 cm. It is mainly blue, though its upper margin may show delicate shades of green or pink. It is a living, muscular bag that secretes its own gas, which is similar to air. The float has aerodynamic properties and it seems likely that sailing characteristics may be modified by muscular contraction of the crest. Physalia sails at a slight angle downwind and the course is determined by the curvature of the float and the underwater resistance of the rest of the colony. The float may project either to the left or to the right; the left-handed forms sail to the right of the wind and vice versa. Thus, if the sailing angle of one form leads to its stranding on the shore, the others sailing to the opposite side of the wind may escape.
The Bluebottle belongs to the phylum Cnidaria, which includes corals and sea anemones. Two other floating colonial cnidarians which may be found with Bluebottles are the By-the-wind sailor (Velella) and the blue-green Porpita pacifica. The float of Velella is a flat, oval disc with many gas-filled tubes. It is about 5 cm across with a slender diagonal sail, allowing the animal to sail at an angle to the wind. The float of Porpita is a flat, circular disc up to 2.5 cm across with many gas-filled tubes, but no sail. Both of these species possess fishing tentacles with stinging capsules that have no effect on humans.
Physalia utriculus is commonly encountered in the summer months on the eastern coast of Australia, and during Autumn and winter in southern Western Australia
The Bluebottle, Pacific man-o-war, is found in marine waters in the Indian and Pacific Oceans. The only other species, Physalia physalis , the Portuguese man-o-war is found in the Atlantic ocean.
Throughout Australia, bluebottles are more common on exposed ocean beaches after strong onshore north easterly winds wash them ashore and are rarely found in sheltered waters.
Commonly encountered in the summer months on the eastern coast of Australia, and during Autumn and winter in southern Western Australia.
Feeding and diet
Bluebottles feed mostly on larval fish, molluscs and small crustaceans such as copepods and amphipods.
The digestive polyps are the 'stomachs' of the colony and respond quickly to the presence of food, wriggling and twisting until they fasten their flexible mouths to it. Once attached they become all mouth, spreading out over the surface of the morsel. The resting polyp measures only 1-2 mm in diameter but the mouth may expand to more than 20 mm. They digest the food by secreting a full range of enzymes that variously break down proteins, carbohydrates and fats.
The most impressive members of the colony are the tentacles. As Physalia drifts downwind, the long tentacle fishes continuously through the water. Muscles in the tentacle contract and drag prey into range of the digestive polyps. The prey consists mostly of small crustaceans and other members of the surface plankton which it ensnares in a tangle of nematocyst threads.
Nematocysts are among the most complex intracellular structures known and may be only 0.001 mm in diameter. Each is a hollow sphere with its external wall turned in at one point as a long, hollow, coiled thread or tube turned outside in. The opening left in the surface of the capsule is covered by a hinged lid held down by a hairlike trigger. When the stinging capsule is stimulated the tube shoots outward turning itself right side out. The tube is usually armed with spines or barbs that aid in the penetration of, and anchorage in, the victim's flesh. Stinging capsules contain a toxic mixture of phenols and proteins that is injected into the victim through a terminal pore in the thread.
Other behaviours and adaptations
Bluebottles differ from true jellyfishes in several ways. The gas-filled float supports a number of specialised tentacles, which are actually members of a complicated colony. The individual members, or 'zooids', cooperate to form what looks to us like one animal-a jellyfish. Some zooids are specialised for stinging and capturing fishes and other marine animals, some are specialised for eating prey, and some are the reproductive members of the colony. Even the gas float itself is a modified colony member. The floats are of two sorts-ones that face left and others that are angled toward the right. This means that the same wind will push the two variations in different directions, avoiding all the colonies becoming washed up on the beach and dying.
Life history cycle
Bluebottles are hermaphrodites, so each individual gonozooid consists of male and female parts. The fertilised egg develops into a planktonic larval form which produces the large Physalia colony by asexual budding.
Danger to humans
About 10-30,000 stings each year are reported along the east coast of Australia from Physalia with about 500 reported from Western Australia and South Australia. No fatalities have been confirmed from these animals in the Southern Hemisphere but there have been several fatalities from the related species, P. in the Northern Hemisphere.
Most beachgoers in Australia swim at non-tropical beaches, and so are most likely to come across the more harmless, non-tropical stinger varieties such as the common Bluebottle. For the average person, getting stung by one will present no harmful danger, however, for the very young, elderly, people allergic to them or in extreme cases, they can present further complications.
How to treat a Bluebottle sting:
- Find a place to rest with someone who can watch over you.
- Don’t rub the stung area.
- Wash off any remaining tentacles with saltwater. Rinsing the stung area well with seawater will remove any invisible stinging cells.
- Immerse the stung area in hot water at a temperature you can comfortably tolerate. Studies have shown that 40 degrees Celsius will produce relief after 10 minutes. The heat is important as it kills the protein in the venom.
- If the symptoms persist or for stings that cover a particularly large area, or across the throat & face call triple zero (in Australia).
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