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Medusae are among the very few aquatic taxa that swim using jet propulsion. Though “jet propulsion” does not necessarily invoke the mental picture of a slowly swimming medusa, the medusae, the squids and octopi, and the salps and doliolids are major marine taxa that swim using jet propulsion. Scallops also use it to escape from starfish predators by rapidly closing their valves to expel a jet of water, allowing brief forays into the water column.

Table 3.4 Diets of cubomedusae.

Source: Larson (1976), table 2 (p. 242). Reprinted by permission from Springer Nature Customer Service Centre GmbH, Cubomedusae Feeding, author R. J. Larson, in Coelenterate Ecology and Behavior, G.O. Mackie editor, 1976.

Species	Coelenteron contents	References
Carybdea alata	Polychaetes, mysids, crab megalopae	Larson (unpublished)
Carybdea marsupialis	Polychaetes, misc. crustaceans (copepods, isopods, amphipods, stomatopod larvae, mysids, caridean shrimp and larvae, crab zoeae), chaetognaths, fish	Berger (1900), Larson (unpublished)
Carybdea rastoni	Polychaetes, mysids, fish	Gladfelter (1973), Ishida (1936), Larson (unpublished), Uchida (1929)
Chiropsalmus quadrumanus	Misc. crustaceans (amphipods, cumaceans, stomatopod larvae, Lucifer spp., caridean shrimp, crab larvae), fish	Larson (unpublished), Phillips and Burke (1970), Phillips et al. (1969)
Chiropsalmus quadrigatus and Chironex fleckeri	Caridean shrimp (Acetes spp.), other small crustaceans, fish	Barnes (1966)
Tripedalia cystophora	Copepods (Oithona spp.)	Larson (unpublished)

Medusae swim by ejecting the volume enclosed by the bell toward the rear with a forceful contraction, propelling the animal forward (Figure 3.20). Alternate rhythmic contraction of muscles to propel the jet and elastic recoil of mesoglea to refill the jet allow a medusa to make its way forward. The basic locomotory system thus comprises the swimming muscles, the deformable mesoglea that gives the swimming bell its elastic character, and a pacemaker that sets the rhythm and assures that contraction is synchronous over the bell.

The swimming muscles are of two basic types: radial muscles aligned in the same axis as the radial canals, that is, from the center to the periphery of the bell; and the coronal muscles that form a ring parallel to the margin (Figure 3.21). In the Scyphozoa, swimming is usually initiated by a contraction of the radial muscles, causing the bell to shorten, followed by a contraction of the coronal musculature, which cinches up the margin and forces a jet of water out the bell. In the Hydrozoa, swimming is effected in much the same way with the exception that the presence of a velum, or skirt, gives more direction to the stream of the jet. Direction of the swimming hydromedusa can thus be manipulated by differential contraction of the radial muscles.

Figure 3.20 Jet propulsion in medusae. (a) Bell expansion and water intake; (b) bell contraction with water expulsion. Direction of motion is indicated by the large arrows.

Unlike the continuous movement of fishes and shrimp, medusan swimming is intermittent in nature, alternating periods of thrust with the refilling of the jet. We know that in most cases the movement is fairly slow, with velocities of 5 cm s⁻¹ or less, but how efficient is it? The answer is, not very. A few studies have examined the swimming performance of medusae using both modeling (Daniel 1983) and empirical approaches (Daniel 1985). Daniel calculated a Froude efficiency of about 10% for the hydromedusa Gonionemus, which means that the work done by the muscle in creating the jet was about 10 times the drag that the jet needed to overcome in order to propel the medusa through the water (Alexander 2003). A medusa expends about 10 times as much effort to move through water as does a fish.

Refilling the medusan bell is accomplished by the deformable and elastic mesoglea, whose recoil to the relaxed state fills the bell with the volume of water that will be forced out in the next jet. Physical properties of the bell were investigated in the hydromedusa Polyorchis (DeMont and Gosline 1988) using measurements in the swimming medusa and in isolated blocks of tissue. They found that about 60% of the work done in deforming the mesoglea was returned in its recoil to the relaxed state, a respectable number but not at the high end of elastic mechanisms employed in the locomotion of other species, which can exceed 90% (Alexander 2003).

Figure 3.21 Oral surface of the subumbrella of Cyanea capillata. The oral arms and tentacles have been removed.

Source: Gladfelter (1972), figure 1 (p. 151). Reproduced with the permission of Springer‐Verlag.