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Australia: The Land Where Time Began |
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Jellyfish Anatomy – Locomotion Structures
Most jellyfish are essentially passive drifters carried along by
currents; therefore even large jellyfish are classified as planktonic.
Though mostly purely planktonic, most have structures that allow them to
change direction and move up or down in the water column and some can
even swim against weak currents.
Medusae, Siphonophores and Salps – locomotion
The pulsation of the bell used by the medusae is the most familiar type
of locomotion among jellyfish. This method is also employed by the
lesser known salps and siphonophores. Medusae can contract their bell as
they have well developed muscles. The contraction of the bell by these
muscles use half as much energy for the same amount of propulsion as any
other organism would, which makes their swimming action the most energy
efficient mechanism of locomotion known among animals. When the bell
contracts, the power stroke, the water trapped under the bell is forced
rapidly out through an opening which has been narrowed by a thin shelf
of tissue, a velum or velarium, which results in a thrust of jet
propulsion. When the bell expands out again it does so through
elasticity and “memory” of the bell, so no energy expenditure is
required for the counterstroke.
The cubozoans (box jellyfish) and related species have swimming
abilities that are well developed. Some of the larger coastal species,
such as
Chironex fleckeri, can
swim against powerful currents at up to 4 knots (about 5 miles/hr).
Species of the genus
Alatina, which are open
ocean box jelly species, have not been timed, but have been observed to
be swimming very fast and Gershwin suggests they may be faster than
Chironex.
Special structures known as swimming bells, or nectophores, are present
on many types of siphonophores which are used for locomotion. According
to Gershwin these are basically medusae that have been highly modified
and are fixed to colony, so not dispersing away from the colony to
become free-floating. Nectophores contract and expand as do normal
medusae, but in order to accomplish forward motion all the individuals
of the colony must coordinate their pulsations.
Salps and related forms also swim by using a pulsing motion of their
barrel-like bodies. In each salp species there is a characteristic
number and pattern of muscle bands that encircle their bodies, in the
same manner as the metal hoops of a wine barrel. A whole-body pulsation
is produced by the stimulation of these muscle bands which moves the
animal through the water. Salps, like siphonophores, need to coordinate
their pulsations in their aggregate stage to accomplish forward motion.
Ctenophore locomotion
There is only 1 ctenophore family that has the ability to pulsate.
Species of the family Ocyropsidae are able to violently flap their body
in the manner of hand-clapping, or similar to a disturbed scallop, so
are capable of moving rapidly from an unwanted stimulus. Gershwin
suggests also that many predators may be frightened by the wild flapping
motion.
Some drifting species of ctenophores move by means of a type of tractor
motion involving the rhythmic beating of ciliary plates. Ctenophores
have 8 comb rows, which are tracks of cilia. The composition of these
rows is actually clusters of cilia, of which there are dozens to
hundreds of cilia, that adhere together in paddles or plates known as
ctenes (pronounced teens). These plates are located, more or less, along
the length of each of the 8 rows. The ctenes are coordinated so that
waves of motion pass down the rows together, all of which is coordinated
by a tiny primitive version of a centralised nervous system, and by this
means is accomplished forward movement while producing very little
vibration. These paddles can be reversed to allow the ctenophore go into
reverse.
Benthic forms – movement
The benthic species of jellies have evolved strategies and structures to
help them move around. Platyctenes have cilia on their undersurface,
though they lack the comb rows and paddles that are present on drifting
ctenophores. They can glide at “a good clip” if necessary, though
normally they creep fairly slowly. Similarly, Stauromedusae can move
when necessary, though they spend most of their time fixed to rocks or
algae. When motion is necessary they bend to one side then discharge
some nematocysts to anchor an arm to the ground, then release their
sticky foot, and somersault away.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |