Pyrosomes and Salps are pelagic (free-swimming) tunicates or sea
squirts.
All species are open ocean animals that rarely come close to
shore, and all are colonial, although many Salps can also be solitary.
Pyrosomes
are colonies of tiny animals that form hollow tubes sealed at one end -
the species in the first part of the video is giant Pyrosome
Pyrostremma Spinosum - it can reach 30m in length!
Salps have much
larger individuals than Pyrosomes, individuals pump water through
themselves.
Colonies are formed of chains of individuals.
Salps can form
very high densities under good conditions, and are an important oceanic
food source for fish.
These animals were filmed off the Tasman
Peninsula in Tasmania, Australia - one of the few areas in the world
where a wide range of oceanic gelatinous plankton, including Ctenophores
and jellyfish, comes close to shore, and is easily seen while diving.
They are otherworldly creatures who glow in the dark, without brains or
bones, some reaching 100 feet long.
And they live just off California's
coast.
Join two top marine biologists who have devoted their careers to
unlocking the mysteries of jellyfish and alien-like siphonophores.
Jellyfish or jellies[1] are the major non-polyp form of individuals of the phylumCnidaria.
They are typified as free-swimming marine animals consisting of a gelatinous umbrella-shaped bell and trailing tentacles.
The bell can pulsate for locomotion, while stinging tentacles can be used to capture prey.
Jellyfish are found in every ocean, from the surface to the deep sea. Scyphozoans are exclusively marine, but some hydrozoans live in freshwater.
Large, often colorful, jellyfish are common in coastal zones worldwide.
Jellyfish have roamed the seas for at least 500 million years,[2] and possibly 700 million years or more, making them the oldest multi-organ animal.[3]
The English popular name jellyfish has been in use since 1796.[4]
It has traditionally also been applied to other animals sharing a superficial resemblance, for example ctenophores
(members from another phylum of common, gelatinous and generally
transparent or translucent, free-swimming planktonic carnivores now
known as comb jellies) were included as "jellyfishes".[5]
Even some scientists include the phylum ctenophora when they are referring to jellyfish.[6] Other scientists prefer to use the more all-encompassing term gelatinous zooplankton, when referring to these, together with other soft-bodied animals in the water column.[7]
As jellyfish are not true fish (which are vertebrates, unlike jellyfish), the word jellyfish is considered by some to be a misnomer.
Public aquariums may use the terms jellies or sea jellies instead.[8]
The term "jellies" may have become more popular than "jellyfish".[1]
In scientific literature, "jelly" and "jellyfish" are often used interchangeably.[9] Some sources may use the term "jelly" to refer to organisms in this taxon, as "jellyfish" may be considered inappropriate.[10]
Many textbooks and sources refer to only scyphozoa as "true jellyfish".[11][12]
A group of jellyfish is sometimes called a bloom or a swarm.[13]
"Bloom"
is usually used for a large group of jellyfish that gather in a small
area, but may also have a time component, referring to seasonal
increases, or numbers beyond what was expected.[14]
Another collective name for a group of jellyfish is a smack,[15]
although this term is not commonly used by scientists who study
jellyfish.
Jellyfish are "bloomy" by nature of their life cycles, being
produced by their benthic polyps
usually in the spring when sunshine and plankton increase, so they
appear rather suddenly and often in large numbers, even when an
ecosystem is in balance.[16]
Using "swarm" usually implies some kind of active ability to stay together, which a few species such as Aurelia, the moon jelly, demonstrate.[17]
Medusa jellyfish may be classified as scyphomedusae ("true" jellyfish), stauromedusae (stalked jellyfish), cubomedusae (box jellyfish), or hydromedusae, according to which clade their species belongs.[18]
This term refers exclusively to the non-polyp life-stage which occurs
in many cnidarians, which is typified by a large pulsating gelatinous
bell with long trailing tentacles.
All medusa-producing species belong
to the sub-phylum Medusozoa.
In biology, a medusa (plural: medusae) is a form of cnidarian in which the body is shaped like an umbrella, in contrast with polyps.
Medusae vary from bell-shaped to the shape of a thin disk, scarcely
convex above and only slightly concave below.
The upper or aboral
surface is called the exumbrella and the lower surface is called the subumbrella;
the mouth is located on the lower surface, which may be partially
closed by a membrane extending inward from the margin (called the velum).
The digestive cavity consists of the gastrovascular cavity
and radiating canals which extend toward the margin; these canals may
be simple or branching, and vary in number from few to many.
The margin
of the disk bears sensory organs and tentacles.
The major surfaces and axes of a scyphozoan jellyfish
Most jellyfish do not have specialized digestive, osmoregulatory, central nervous, respiratory, or circulatory
systems. The manubrium is a stalk-like structure hanging down from the
centre of the underside, often surrounded by oral arms, which connects
with the mouth/anus at the base of the bell.[19]
This opens into the gastrovascular cavity,
where digestion takes place and nutrients are absorbed. It is joined to
the radial canals which extend to the margin of the bell, where
tentacles are attached.[20]
Nematocysts, which deliver the sting, are located mostly on the tentacles; Scyphozoans also have them around the mouth and stomach.[21]
Jellyfish do not need a respiratory system since their skin is thin enough that the body is oxygenated by diffusion.
They have limited control over movement, but can use their hydrostatic skeleton
to navigate through contraction-pulsations of the bell-like body; some
species actively swim most of the time, while others are mostly passive.[22]
Depending on the species, the body contains between 95 and 98% water.[23]
Most of the umbrella mass is a gelatinous material — the jelly — called mesoglea
which is surrounded by two layers of protective skin.
The top layer is
called the epidermis, and the inner layer is referred to as
gastrodermis, which lines the gut.
Nervous system
Jellyfish employ a loose network of nerves, located in the epidermis, which is called a "nerve net".[24]
Although traditionally thought not to have a central nervous system, nerve net concentration and ganglion-like structures could be considered to constitute one in most species.[25]
A jellyfish detects various stimuli including the touch of other
animals via this nerve net, which then transmits impulses both
throughout the nerve net and around a circular nerve ring, through the rhopalial lappet, located at the rim of the jellyfish body, to other nerve cells.
Vision
Some jellyfish have ocelli: light-sensitive organs
that do not form images but which can detect light and are used to
determine up from down, responding to sunlight shining on the water's
surface.
These are generally pigment spot ocelli, which have some cells
(not all) pigmented.
Certain species of jellyfish, such as the box jellyfish, have more advanced vision than their counterparts.
The box jellyfish has 24 eyes, two of which are capable of seeing color, and four parallel information processing areas or rhopalia that act in competition,[26] supposedly making it one of the few creatures to have a 360-degree view of its environment.[27]
The eyes are suspended on stalks with heavy crystals on one end, acting like a gyroscope to orient the eyes skyward.
They look upward to navigate from roots in mangrove swamps to the open lagoon and back, watching for the mangrove canopy, where they feed.[3]
Size
Jellyfish range from about one millimeter in bell height and diameter
to nearly 2 metres (6.6 ft) in bell height and diameter; the tentacles
and mouth parts usually extend beyond this bell dimension.
The smallest jellyfish are the peculiar creeping jellyfish in the genera Staurocladia and Eleutheria,
which have bell disks from 0.5 mm to a few millimeters in diameter,
with short tentacles that extend out beyond this, which these jellyfish
use to move across the surface of seaweed or the bottoms of rocky pools.[28]
Many of these tiny creeping jellyfish cannot be seen in the field
without a hand lens or microscope; they can reproduce asexually by
splitting in half (called fission).
Other very small jellyfish, which
have bells about one millimeter, are the hydromedusae of many species
that have just been released from their parent polyps;[29]
some of these live only a few minutes before shedding their gametes in
the plankton and then dying, while others will grow in the plankton for
weeks or months.
The hydromedusae Cladonema radiatum and Cladonema californicum are also very small, living for months, yet never growing beyond a few mm in bell height and diameter.[30]
Another small species of jellyfish is the Australian Irukandji, which is about the size of a fingernail.[3]
The lion's mane jellyfish, Cyanea capillata,
was long-cited as the largest jellyfish, and arguably the longest
animal in the world, with fine, thread-like tentacles that may extend up
to 36.5 metres (120 ft) long (though most are nowhere near that large).[31][32]
They have a moderately painful, but rarely fatal, sting.
Claims that
this jellyfish may be the longest animal in the world are likely
exaggerated; some other planktonic cnidarians called siphonophores may typically be tens of meters long and seem a more legitimate candidate for longest animal.[33]
The increasingly common giant Nomura's jellyfish, Nemopilema nomurai,
found in some, but not all years in the waters of Japan, Korea and
China in summer and autumn is probably a much better candidate for
"largest jellyfish", since the largest Nomura's jellyfish in late autumn
can reach 200 centimetres (79 in) in bell (body) diameter and about 200
kilograms (440 lb) in weight, with average specimens frequently
reaching 90 centimetres (35 in) in bell diameter and about 150 kilograms
(330 lb) in weight.[34][35]
The large bell mass of the giant Nomura's jellyfish[36] can dwarf a diver and is nearly always much greater than the up-to-100 centimetres (39 in) bell diameter Lion's Mane.[37]
The rarely encountered deep-sea jellyfish Stygiomedusa gigantea
is another solid candidate for "largest jellyfish", with its thick,
massive bell up to 100 centimetres (39 in) wide, and four thick,
"strap-like" oral arms extending up to 6 metres (20 ft) in length,[38]
very different from the typical fine, threadlike tentacles that rim the
umbrella of more-typical-looking jellyfish, including the Lion's Mane.
Jellyfish belong to Medusozoa, the clade of cnidarians which excludes Anthozoa (e.g., corals and anemones).
This suggests that the medusa form evolved after the polyps.[39]
The phylogenetics of this group are complex and evolving.
The Medusozoa and Octocorallia are proposed as sister groups according to research published in 2012.
That research also proposes coronate Scyphozoa and Cubozoa as a sister clade to Hydrozoa and discomedusan Scyphozoa, which are themselves sister groups.
The class Storozoa was the earliest group of Medusozoa to diverge and the Limnomedusae were the earliest Hydrozoa to diverge.[40]
The four major classes of medusozoan Cnidaria are:
Scyphozoa
are sometimes called true jellyfish, though they are no more truly
jellyfish than the others listed here. They have tetra-radial symmetry.
Most have tentacles around the outer margin of the bowl-shaped bell, and
long, oral arms around the mouth in the center of the subumbrella.
Cubozoa
(box jellyfish) have a (rounded) box-shaped bell, and their velarium
assists them to swim more quickly. Box jellyfish may be related more
closely to scyphozoan jellyfish than either are to the Hydrozoa.[39]
Hydrozoa
medusae also have tetra-radial symmetry, nearly always have a velum
attached just inside the bell margin, do not have oral arms, but a much
smaller central manubrium with terminal mouth opening, and are
distinguished by the absence of cells in the mesoglea. The majority of
hydrozoan species maintain the polyp form for their entire life cycle
and do not form medusa at all (such as hydra, which is hence not considered a jellyfish).
Staurozoa
(stalked jellyfish) are characterized by a medusa form that is
generally sessile, oriented upside down and with a stalk emerging from
the apex of the "calyx" (bell), which attaches to the substrate. Some
Staurozoa (all? - it is not known yet) also have a polyp form that
alternates with the medusoid portion of the life cycle. Until recently,
Staurozoa were classified within the Scyphozoa.
Some other animals are frequently associated with or mistaken for medusa jellyfish.
Siphonophorae
are an order of hydrozoa which generally live as colonies (for example,
free-swimming chains of repeated units, some units similar to polyps or
to medusa). They are not considered medusa jellyfish. A well-known
example is the Portuguese man o' war.
Ctenophores
(comb jellies) are a separate phylum from Cnidaria. Their method of
propulsion is coordinated movement of thousands of cilia used as
paddles, rather than a pulsating bell, although a few species of
ctenophores swim by flapping large lobes.
Salps are transparent, gelatinous marine organisms that form pelagic colonies like siphonophores. Salps are chordates, and as such are actually more closely related to humans than they are to cnidarians and comb jellies.[41]
There are over 200 species of Scyphozoa, about 50 species of
Staurozoa, about 20 species of Cubozoa, and the Hydrozoa includes about
1000–1500 species that produce medusae (and many more hydrozoan species
that do not produce medusae).[42][43]
Many scientists who work on relationships between these groups are
reluctant to assign ranks, although there is general agreement on the
different groups, regardless of their absolute rank.
Here is one scheme,
which includes all groups that produce jellyfish, derived from several
expert sources:
Source: Wikipedia.org
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