Is the Portuguese man o war a jellyfish?
Jellyfish
encompasses a broad group of animals under the group Cnidaria, which include
the true jellyfish (medusozoans) that we are familiar with. However, there are
many that are closely related to the jellyfish. These include hard and soft corals,
sea anemones and hydrozoans (such as the Portuguese man o war), and although
they are not true jellyfish, they share very similar ecological functions and
lifestyles. The life cycle of Cnidaria includes a drifting phase in the open
ocean, as well as a sedentary phase connected to the seafloor. Within the
different groups in the Cnidaria phylum, the time spent between the different
phases can vary considerably, and some phases are even lost. Adult jellyfish
release sperm and eggs into the water column to form fertilized eggs or larvae.
The larvae seek to settle on a surface or the seafloor as polyps and then
continue to grow and develop. While a coral
will live in one spot for the rest of its life growing and forming a skeleton,
true jellyfish will attach temporarily before budding off into their adult medusa
phase, being released into the water column as a young adult jellyfish.
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| Jellyfish silhouette in one of the jellyfish lakes of Palau. Photo by Dr Nick Evenson |
Jellyfish
are most famous for their sting. Jellyfish house stinging cells called
nematocysts that contain a venom-bearing harpoon within the cell triggered by touch
or pressure. The nematocysts are used to capture prey - juvenile fish and zooplankton - as well as
to defend themselves. Brushing against a jellyfish can be an awful experience –
imagine being hit by a thousand microscopic venomous harpoons. The pain varies
considerably based on the species and can be excruciating at times. The most
deadly jellyfish is the box jellyfish (Cubozoa, which is also not a true
jellyfish) common to the coastal waters in the Indo-Pacific region. The venom
of the box jellyfish is 100 times more potent than a cobra and can result in cardiac
arrest, brain haemorrhaging and even death. While the Portuguese man o war is
painful it does not compare to the box jellyfish.
Last
week, hundreds of Portuguese man o wars were washed up on the coast of Manzanilla,
which caught the attention of the media, as it is unusual for this time of
year. This adds to the many more reports of jellyfish blooms occurring
globally, largely reported by fishermen, beachgoers and victims of stings, as
well as impacted industries. The higher number of reports has caught the
attention of scientists who are trying to determine whether the increase in
jellyfish populations is related to climate change or otherwise.
There
are many changes in environmental conditions that can cause jellyfish and their
gelatinous relatives to bloom and wash up on to coastlines. Some blooming
episodes are aligned with their natural reproductive cycle, which can be
seasonal or climatic, while other occasions may be associated with human
activities and environmental degradation. Many jellyfish species have been
known to thrive in conditions that are rich in nutrients, murky or low in oxygen.
In these environments, jellyfish fare better than fish species, especially as
they are not visual predators. Additionally, jellyfish can take up nutrients
directly from the water column, but also excrete nutrients into the water as
well.
Human
activities, such as pollution, construction and land clearing can promote the
conditions that encourage a predominance of jellyfish over other fish species. Areas
of coastal development, especially near river or estuary outflows, as well as
aquaculture farms and power plants set up suitable conditions for jellyfish to
strive. We also indirectly influence jellyfish populations by overharvesting fish
species that feed on them. The release from the predation pressure would often
result in a spike in jellyfish populations. Jellyfish were once thought of as
containing little nutrition, but now we recognise that the diversity of marine
organisms that feed on jellyfish is quite high. Predators of jellyfish include the
ocean sunfish, a range of fish species such as tuna and even penguins.
Trinbagonians are especially familiar with the cannonball jellyfish that are
common in the water around the islands during Easter time; these are a
favourite food of leatherback turtles.
At
a larger scale, regional studies have correlated increases in certain jellyfish
populations with warmer water temperatures, especially in temperate and polar regions,
and have suggested that warmer ocean temperatures encourage jellyfish blooms. Conversely,
a few studies done in the tropics observed reduced populations in places that
are already hot and close to their upper temperature tolerance threshold, such
as in the jellyfish lake in Palau that suffered mass die off in recent years. Unfortunately,
little research has been done over the long-term to obtain a comprehensive
understanding of the impacts of climate change on jellyfish. We are less
certain about the impacts of ocean acidification, which is detrimental to
calcifying marine creatures - including cousins to the jellyfish, the
reef-building corals – to jellyfish. Some species of jellyfish use microscopic
calcium statoliths to orient themselves in the water column and may therefore
be at risk under more acidic ocean conditions. It is very likely that jellyfish
population patterns may shift in areas around the world. Jellyfish blooms will also
be likely to become more common as a result of the interacting impacts of a
warming world and local degradation. The future therefore favours jellyfish.
An
increase in jellyfish can have serious ecological and economic consequences to
us. Jellyfish tend to hunt zooplankton, which largely consist of juvenile and
fish larvae; blooming populations may further reduce fish abundance in places
where they may already be compromised. Higher abundance of jellyfish means a
greater predation pressure on fish species potentially even commercial types. A
high density of jellyfish may also impact industry, clogging industrial water
pipes or overrunning aquaculture farms (where jellyfish find an excess of food
and nutrients).
Special note on the Portuguese man o war
The
Portuguese man o war is a colonial organism, made up of a collaboration of four
main organisms or polyps – the air bladder, the tentacles that contain venomous
cells used to stun prey, the digestive organism and the reproductive organism.
These polyps work together to hunt for prey and to escape predation; and
essentially cannot survive separately. Although different from the jellyfish
they perform similar functions to jellyfish and follow similar lifestyles. The
Portuguese man o war is named because it resembles the ballooning sails of man-o-war ships. The air bladders allow them
to drift along the surface with tentacles that can extend to 30 and even 100 feet,
catching prey like a drift net in the ocean. Be aware that the tentacles, as a
separate organism can still sting.
References:
Purcell, J. E. (2005) Climate effects on formation of
jellyfish and ctenophore blooms: a review, J.
Mar. Biol. Ass. U.K., 85, 461–476
Purcell1, J. E., Uye, S., Wen-Tseng Lo, W., (2007)
Anthropogenic causes of jellyfish blooms and their direct
consequences for humans: a review, Mar.
Ecol. Prog. Ser., Vol. 350: 153–174
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