Diet Jellyfish are, like other cnidarians, generally carnivorous (or parasitic), feeding on planktonic organisms, crustaceans, small fish, fish eggs and larvae, and other jellyfish, ingesting food and voiding undigested waste through the mouth. They hunt passively using their tentacles as drift lines, or sink through the water with their tentacles spread widely; the tentacles, which contain nematocysts to stun or kill the
prey, may then flex to help bring it to the mouth. A few species such as
Aglaura hemistoma are omnivorous, feeding on microplankton which is a mixture of
zooplankton and
phytoplankton (microscopic plants) such as
dinoflagellates. Others harbour
mutualistic algae (
Zooxanthellae) in their tissues; The
upside-down jellyfish (
Cassiopea andromeda) also has a symbiotic relationship with
microalgae, but captures tiny animals to supplement their diet. This is done by releasing tiny balls of living cells composed of
mesoglea. These use cilia to drive them through water and stinging cells which stun the prey. The blobs also seem to have digestive capabilities.
Predation Other species of jellyfish are among the most common and important jellyfish predators. Sea anemones may eat jellyfish that drift into their range. Other predators include
tunas, sharks,
swordfish, sea turtles and penguins. Jellyfish washed up on the beach are consumed by foxes, other terrestrial mammals and birds. In general however, few animals prey on jellyfish; they can broadly be considered to be
top predators in the food chain. Once jellyfish have become dominant in an ecosystem, for example through overfishing which removes predators of jellyfish larvae, there may be no obvious way for the previous balance to be restored: they eat fish eggs and juvenile fish, and compete with fish for food, preventing fish stocks from recovering.
Symbiosis Some small fish are immune to the stings of the jellyfish and live among the tentacles, serving as bait in a fish trap; they are safe from potential predators and are able to share the fish caught by the jellyfish. The
cannonball jellyfish has a symbiotic relationship with ten different species of fish, and with the
longnose spider crab, which lives inside the bell, sharing the jellyfish's food and nibbling its tissues.
Blooms Jellyfish form large masses or blooms in certain environmental conditions of
ocean currents,
nutrients, sunshine, temperature, season, prey availability, reduced predation and
oxygen concentration. Currents collect jellyfish together, especially in years with unusually high populations. Jellyfish can detect
marine currents and swim against the current to congregate in blooms. Jellyfish are better able to survive in nutrient-rich, oxygen-poor water than competitors, and thus can feast on plankton without competition. Jellyfish may also benefit from saltier waters, as saltier waters contain more
iodine, which is necessary for polyps to turn into jellyfish. Rising sea temperatures caused by
climate change may also contribute to jellyfish blooms, because many species of jellyfish are able to survive in warmer waters. Increased nutrients from agricultural or urban
runoff with nutrients including nitrogen and phosphorus compounds increase the growth of phytoplankton, causing
eutrophication and
algal blooms. When the phytoplankton die, they may create
dead zones, so-called because they are
hypoxic (low in oxygen). This in turn kills fish and other animals, but not jellyfish, allowing them to bloom. Jellyfish populations may be expanding globally as a result of land runoff and
overfishing of their
natural predators. Jellyfish are well placed to benefit from disturbance of marine ecosystems. They reproduce rapidly; they prey upon many species, while few species prey on them; and they feed via touch rather than visually, so they can feed effectively at night and in turbid waters. It may be difficult for
fish stocks to re-establish themselves in marine ecosystems once they have become dominated by jellyfish, because jellyfish feed on plankton, which includes
fish eggs and
larvae. such as the
Baltic Sea. jellyfish blooms are increasing in frequency. Between 2013 and 2020 the
Mediterranean Science Commission monitored on a weekly basis the frequency of such outbreaks in coastal waters from Morocco to the Black Sea, revealing a relatively high frequency of these blooms nearly all year round, with peaks observed from March to July and often again in the autumn. The blooms are caused by different jellyfish species, depending on their localisation within the Basin: one observes a clear dominance of
Pelagia noctiluca and
Velella velella outbreaks in the western Mediterranean, of
Rhizostoma pulmo and
Rhopilema nomadica outbreaks in the eastern Mediterranean, and of
Aurelia aurita and
Mnemiopsis leidyi outbreaks in the Black Sea. Some jellyfish populations that have shown clear increases in the past few decades are
invasive species, newly arrived from other habitats: examples include the
Black Sea,
Caspian Sea,
Baltic Sea, central and eastern
Mediterranean, Hawaii, and tropical and subtropical parts of the West Atlantic (including the
Caribbean,
Gulf of Mexico and Brazil). Jellyfish blooms can have significant impact on community structure. Some carnivorous jellyfish species prey on zooplankton while others graze on primary producers. Reductions in zooplankton and
ichthyoplankton due to a jellyfish bloom can ripple through the trophic levels. High-density jellyfish populations can outcompete other predators and reduce fish recruitment. Increased grazing on primary producers by jellyfish can also interrupt energy transfer to higher trophic levels. During blooms, jellyfish significantly alter the nutrient availability in their environment. Blooms require large amounts of available organic nutrients in the water column to grow, limiting availability for other organisms. Some jellyfish have a symbiotic relationship with single-celled dinoflagellates, allowing them to assimilate inorganic carbon, phosphorus, and nitrogen creating competition for phytoplankton. The microbes break down the organic matter into inorganic ammonium and phosphate. However, the low carbon availability shifts the process from production to respiration creating low oxygen areas making the dissolved inorganic nitrogen and phosphorus largely unavailable for primary production. These blooms have very real impacts on industries. Jellyfish can outcompete fish by utilizing open niches in over-fished fisheries. Catch of jellyfish can strain fishing gear and lead to expenses relating to damaged gear. Power plants have been shut down due to jellyfish blocking the flow of cooling water. Blooms have also been harmful for tourism, causing a rise in stings and sometimes the closure of beaches. Jellyfish form a component of
jelly-falls, events where gelatinous
zooplankton fall to the seafloor, providing food for the
benthic organisms there. In temperate and subpolar regions, jelly-falls usually follow immediately after a bloom.
Habitats Most jellyfish are marine animals, although a few hydromedusae inhabit
freshwater. The best known freshwater example is the
cosmopolitan hydrozoan jellyfish,
Craspedacusta sowerbii. It is less than an inch (2.5 cm) in diameter, colorless and does not sting. Some jellyfish populations have become restricted to coastal saltwater lakes, such as
Jellyfish Lake in Palau. Jellyfish Lake is a
marine lake where millions of golden jellyfish (
Mastigias spp.) migrate horizontally across the lake daily. Some species explicitly adapt to
tidal flux. In
Roscoe Bay, jellyfish ride the current at ebb tide until they hit a
gravel bar, and then descend below the current. They remain in still waters until the tide rises, ascending and allowing it to sweep them back into the bay. They also actively avoid fresh water from mountain snowmelt, diving until they find enough salt. == Relation to humans ==