Jellyfish Anatomy – Locomotion Structures

Australia: The Land Where Time Began

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.

Sources & Further reading