and seemingly synthetic hue
of burgundy or murex or midnight blue,
it drifts in temperate seas
as in irons, the ruffled float
riffling in the breeze
(First verse of a poem by George Bradley, entitled, 'Its
Bladderlike Sail', Grand Street, Vol. 4, No. 4 (Summer,
1985), pp. 109-110).
These creatures belong to the Coelenterates and are commonly referred to as jellyfish, along with the scyphozoa, or jellyfish proper.
(Zoologists frequently only consider the medusae of the scyphozoa as 'jellyfish' though the term is commonly used in a broader sense, and why
not? 'Jellyfish' is after all a descriptive term and not a precise scientific term, much as the word 'worm' can be applied to a wide range of
creatures, including certain vertebrates). The group of coelenterates to which Physalis belongs is a remarkable group called the
siphonophores. Like scyphozoa (e.g. the Moon jellyfish) the siphonophores originally had a life-cycle involving an alternation of generations
between sessile non-sexual polyps and free-swimming sexual medusae. However, the life-cycle in siphonophores is radically altered.
Siphonophores grow from single floating polyps which multiply asexually by budding, but with the progeny remaining physically and intimately
connected to one-another to form a single composite or colonial organism. In the Portuguese Man-of-War the float is a single individual, called
the pneumatophore, and each tentacle is an individual - the long fishing tentacles are called dactylozoids and function only to capture food
and defend the colony - being incapable of feeding themselves, as is the float. The tentacles hang down from half of the float, along with the
shorter feeding polyps called gastrozoids. Each gastrozoid has a mouth and a short tentacle and feeds the colony (some later lose their
mouth and develop into long dactylozoids). Also in the colony are short gonozoids which bear numerous spheroidal female gonophores and
male gonophores - the gonophores are the sexual 'organs' and are really modified medusae that remain attached to the colony and are
incapable of swimming. Odd gelatinous zooids may also be present, which resemble simple gelatinous projections.
The various zooids originated from a single individual and continue to bud or branch from zooids growing beneath the float, with the zooids
grouped in bunched called cormidia (singular cormidium). The tentacles cluster on one-side of the float, either the left side or the right side.
Thus, some individuals have their tentacles on the left and sail on the right (so-called right-handed individuals) and these sail on the port tack.
Other individuals have their tentacles on the right and sail on the left (left-handed individuals) and these sail on the starboard tack (1). The
gastrovascular systems of all the individuals are interconnected, so that food consumed by the gastrozoids may be shared by all. (Remember
that in coelenterates the circulatory system is part of the gut or stomach and forms the gastrovascular cavity).
What we see here is a division of labour - the pneumatophore gives the colony buoyancy, the dactyloxoids catch food and defend the colony
and the gastrozoids ingest food and the gonozoids function in reproduction.
The sail can be collapsed and erected as required and the float deflated or inflated. The float may deflate, for example, in stormy weather,
whilst the tentacles retract and then spread out near the surface to increase stability. The float may also deflate in excess sunshine, to prevent
it from drying up. Dipping (2) is a behaviour frequently seen, in which the sail collapses and the float is turned to one side and dipped in the
water to wet it. This behaviour increases with wind speed and in a sense prevents the float from drying up. However, strictly speaking the floats
do not dry as such, but as water evaporates from the windward side, salt from the water is deposited on the surface and this salt will draw out
water by osmosis. To prevent this, the sail dips in order to wash off the salt (indeed, spraying on salty water triggers the response so the
deposition of salt on the float/sail is the critical signal). The float then rights itself and the sail re-inflates. The sail has partial chitinous partitions
within it, to give it rigidity when inflated.
The float is usually said to contain a mixture of gases similar in composition to air, though with less oxygen, however, studies have shown that a
variable amount of the toxic gas carbon monoxide is also present, sometimes accounting for 35% of the gas (3). This carbon monoxide is
secreted by a gland (the pneumadena), comprising a single layer of cells, in the base of the float. The float is a double-walled chamber. The
outer wall is called the pneumatocodon and the inner wall the pneumatosaccus and in between the two is the gastrovascular cavity of the float.
The float also possesses muscle fibres to control its volume and shape and to perform behaviours such as dipping.
Most siphonophores have a different arrangement of polyps. The polyps bud from a long stalk that hangs in the water (this stalk is essentially
compressed to a disc in Physalia) and there may or may not be a float and instead swimming medusae, still attached to the colony, provide
propulsion. Some forms have both a float and swimming medusae. The epidermis of siphonophores, which connects all the individuals, can
conduct electrical signals (in the absence of nerves) and acts as a neuroid system.
Physalia, the Portuguese Man-of-War (Portuguese
Man o' War or bluebottle), is found in all oceans, but prefers
the warmer waters of the Indian and Pacific oceans. It has a
remarkable float, usually bluish (sometimes violet) in colour
and typically up to 30 cm long or more. The float and its crest
or sail can be seen above the water with the long tentacles
trailing beneath the water's surface. Two species are actually
recognised, Physalis physalis or Portuguese Man o' War and
the smaller Physalis utriculus or Pacific Man o' War (though
probably both are commonly referred to as Portuguese Man
These creatures float on the waters, sailing by means of the
collapsible crest or sail (which seems very variable in size
judging from various photographs) dragging their fishing
tentacles through the water. The tentacles are armed with
stinging nematocysts and the venom of the larger species,
Physalia physalis, can be lethal to humans. The tentacles
typically reach up to 10 m to 30 m in length, though there are
reports of tentacles growing up to 50 m.
- Hydrodynamics of sailing of the Portuguese man-of-war Physalia physalis; G. Iosilevskii and D. Weihs; J. R. Soc. Interface, 2008.
- Note concerning Physalia behaviour at sea; A.H. Woodcock; J. R. Soc. Interface, 2008.
- Fine Structures of the Carbon Monoxide Secreting Tissue in the Float of Portuguese Man-of-War (Physalia physalis L.); D.E. Copeland;
Biological Bulletin, Vol. 135, No. 3 (Dec., 1968), pp. 486-500 .
- Colonies of colonies in Physalia; P.F.S. Cornelius; In Biology and systematics of Colonial organisms, G. Larwood and B.R. rosen (eds)
1979; Academic Press.
Above: Physalia floating on the water's surface.
Physalia have proven difficult to keep in aquaria as they typically degenerate
after a day or two in captivity. It has been suggested that the long tentacles
touching the bottom of the tank may trigger this degeneration. It is suggested
that tanks should be at least 50 feet deep and have fans positioned so as to
blow the Physalia and keep it in the centre of the tank.
|Jelly Creatures - Siphonophores
The gonophores shed their egg and sperm into the sea where fertilisation takes place. The resultant larva develops into a juvenile colony with
a single gastrozoid and one tentacle. More zooids are added by budding. At all stages of the life cycle the organism is pelagic and never
settles on the bottom to live a sessile existence.
Physalia is at the mercy of the winds and sometimes entire swarms are blown onto shore. The fact that some have their sails arrange to tack
to the left and some to tack to the right, means that prevailing winds will be unable to drive the whole population ashore! Winds can also
funnel Physalia into tight-packed swarms on the ocean's surface that sometimes extend for hundreds of kilometres. Such aggregation might
facilitate fertilisation of the gametes shed into the water, but must also create tight competition for food and have significant effects on the
local populations of fish and crustaceans in the surface waters. Do Physalia attack and attempt to sting each other in these aggregates or are
they peaceful affairs?
A jellyfish-like creature generated from computer code in Pov-Ray. (Click link to learn how to make models
Velella (By-the-wind Sailor)
This creature was once classed as a siphonophore, but is now usually placed in a separate group. It resembles a siphonophore with a
disc-shaped coenosarc up to 7 or 8 cm across. The coenosarc is filled with branching channels (presumably filled with water below and air
above) that are lined by photosynthetic zooxanthellae (microscopic algae) which tap the energy of sunlight to make food for themselves and
their coelenterate host. Above this structure is the float, with chitin reinforced walls, which contains concentric air-filled rings called air tubes.
Each air tube opens to the air via two air pores on top of the float. On top of the float is a chitinous triangular sail. In plan view the creature is
somewhat oval, but wider at one end and angular to form a rounded-rhomboid shape. The tentacles of the creature are vividly coloured blue
or purple and hang-down beneath the float. There is an outer-ring of fishing tentacles (dactylozooids) armed with nematocyst batteries and
several inner rings of gastrozooids and mouth-bearing gonozooids bearing gonophore buds. The medusa of Velella is about 3 mm in size and
is free-swimming and has four-radial symmetry. In the centre, hanging beneath the float is the large central gastrozooid, with its large mouth.
Eight radial canals connect the stomach of the gastrozooid with a ring canal in the periphery of the disc. The sail is angled with respects to the
long-axis of the elliptical/rhomboidal float. The sail is designed to impart stability, and has a low centre of wind-pressure, making it less likely to
topple the animal, and indeed at wind speeds up to 20 knots the animal is stable, but wind speeds of 40 knots can cause it to flip over
end-over-end. The animal cannot right itself, though it can tense its muscles, so as to facilitate righting by wave action. If trapped
upside-down, the animal starts to degenerate after a few hours.
Above: a computer model of a siphonophore, similar to Agalma. In this type of siphonophore the zooids bud from a long stalk or
coenosarc, which in some forms, like Physalia, is reduced to a flattened disc. There are various types of zooid, each specialised to
perform a particular function. At the top is the first zooid to form, which in this case acts as a float (and is probably a modified medusa).
Below this are swimming zooids or nectophores which pulse, jetting out water, in much the same way that free-living medusae do. There
are six nectophores in this model. Below this is a pair of narrow tubular tentaculozooids, each armed with a single tentacle and several
tubular feeding gastrozooids, each again armed by a tentacle and equipped with a mouth. These tentacles will be equipped with clusters or
batteries of cells called nematocytes. Each nematocyte contains an organelle called a nematocyst. The nematocyst is a vessel containing a
much coiled microscopic 'harpoon', generally poisonous (though in some coelenterates some nematocysts may simply be sticky) which
discharges under pressure. In those forms in which the nematocyst harpoons can pierce human skin, the injected venom will cause the
jelly's sting. leaf-like zooids called bracts are also present. A bunch of male gonophores can be seen toward the top, with their
spermatocyte cargo depicted in yellow, and a bunch of female gonophores lower down, with their eggs shown in red. The gonophores
are modified sexual medusae that remain attached in most forms, although some types have free-swimming medusae (sometimes only the
female medusae) which do not feed, but degenerate after shedding their gametes. The gametes are shed into the sea and after
fertilisation they give rise to ciliated planula larvae which develop into a new primary zooid which produces a new colony by asexual budding.
Siphonophores come in a fantastic diversity of forms and many are bioluminescent. They are extraordinary creatures in which incomplete
asexual reproduction by budding gives rise to an integrated colony in which all the zooids remain connected via gastrovascular canals that
ultimately connect to the stomachs of the feeding gastrozooids.
Velella was once regarded as a colony of zooids, but is now often seen as an individual with several mouths, on account that it seems to have
a single integrated nervous system. The question remains as to whether it is an individual that resembles a colony or a colony that has
involved into an individual. Evolution often produces more complex organisms by repeating modules. For example, plants are modular
organisms and some plants can reproduce in this way, such as the crack willow tree which sheds twigs and branches which can establish and
become new individuals. Some marine worms reproduce asexually by budding off new individuals at the rear, but in some forms the new
individuals do not readily separate and the chain of individuals function as an individual. Taken to its conclusion this process possibly gave
rise to the segments that make up the body of an earthworm, a fish and a mammal such as the human being. Each segment retains its
ancestral nervous system, albeit in modified form, with the ganglia alongside the spinal cord in humans being vestiges of each segments
original 'brain'. In Velella we see another example of how modules, perhaps originally individuals in their own right, can become so well
integrated as to form the organs of a new modular organism. This is a familiar pattern in evolution - the replication of units, followed by their
modification to produce new parts and new complexity.