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 Although most people
do not realize it, jellyfish and other gelatinous organisms are among the
top predators in estuaries and oceans. These predators eat mostly
zooplankton, microscopic swimming crustaceans, but they also eat the pelagic
eggs and larvae of many fish species, as well as other jellyfish. Thus, they
can be detrimental to fish populations by eating the same zooplankton prey,
as well as eating the young fish directly. Because they may increase
diversity in the plankton by eating a wide variety of foods, including other
jellyfish, and thereby promote stability in their ecosystems, they can be
considered keystone predators.
The gelatinous predators are diverse. They include several types of stinging
cnidarians - scyphomedusae, the large (several inches in diameter),
pulsating, umbrella-shaped jellyfish most people are familiar with;
hydromedusae, which usually are small, (less than 1 inch in diameter), but
abundant umbrella- or thimble-shaped jellyfish; siphonophores, which are
colonial stingers, of which only the Portuguese man of war is widely seen
because it floats at the water's surface; and cubomedusae, also called box
jellyfish or sea wasps, which are known for their painful stings, which from
some species may be fatal to humans. Other gelatinous predators include
non-stinging ctenophores, also called comb jellies. They are less widely
known than true jellyfish because they don't hurt swimmers, but they can be
even more abundant than jellyfish, and may have even greater effects on the
food web. They can be spherical or ovoid, and they are characterized by
having 8 “comb rows” of ciliary plates that they use for gliding through the
water.
Jellyfish can present several problems for humans. They are best known for
their unpleasant and sometimes dangerous stings. They are deleterious to
fish larvae and fish species that eat zooplankton like the jellies do, and
also to pelagic spawning fish species with eggs and larvae the jellies eat.
Several studies have shown substantial jellyfish predation on the eggs and
larvae of commercially-valuable fish species. Jellyfish can also pose
problems for fishermen by clogging the fishing trawls. Jellyfish also clog
the cooling water intakes of power plants, leading to power shutdowns.
 Ironically, humans
inadvertently may be causing jellyfish population to increase. First,
several studies in temperate and sub-Arctic waters have shown correlations
of greater jellyfish abundances with warmer temperatures. Scientists agree
that burning of fossil fuels contributes to the marked warming trend in the
Earth's climate in recent decades. Thus, jellyfish populations may increase
with global warming. Second, humans have altered ocean food webs by fishing
activities. Populations of zooplanktivorous fish, such as herrings,
anchovies, sardines and menhaden have been heavily fish for human
consumption and for pet and aquaculture feeds. Removal of those
zooplanktivores could leave more zooplankton prey for jellyfish. Third,
human populations add nutrients from fertilizers and sewage to the
near-shore waters. Those nutrients contribute to greater phytoplankton
(microscopic plant) growth and consequently to more zooplankton. This
eutrophication may provide better conditions for jellyfish than for fish.
Eutrophication has been linked to smaller phyto- and zooplankton species,
which are less suitable for visually-feeding fish than tactilely-feeding
jellies. Also, eutrophication can reduce water clarity, which may interfere
with feeding by fish but not jellies. Furthermore, eutrophication can lead
to reduced dissolved oxygen in coastal waters; jellyfish and comb jellies
have been shown to be more tolerant of low oxygen (hypoxia) than are fish.
Fourth, most scyphomedusan and hydromedusan species have polyp stages that
live attached to hard surfaces; by adding structures such as marinas,
jetties, oil rigs and artificial reefs, humans are increasing the amount of
suitable habitat for jellyfish polyps. And fifth, transoceanic shipping has
accidentally transplanted several jellyfish species to new locations around
the world. In some cases, the jellyfish have thrived in their new homes, due
to lack of predators or competitors, and have caused disruptions to the food
webs and to fisheries. |
