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.
The Mesoglea
The central role of the mesoglea (“or mesenchyme”) in the biology of cnidarians is best expressed by the words of Chapman (1966): “the coelenterate has long been regarded as two layers of epithelium stuck to that something which is the mesoglea.” Because it is largely acellular, the mesoglea has never been accorded the status of a germ layer, and its character, its importance, and even its name, differ among the cnidarian taxa. It can be regarded as the substance between the inner and outer epithelium. It assumes its greatest importance in the medusae where it acts variously as a source of buoyancy, an anchor for muscle fibers, a primitive skeleton, and even a source of nutrition.
The mesoglea consists of two or three components: fibers, a gel matrix, and where present, cells. Consensus on the chemical composition of the fibers is that they are composed of a collagen‐like protein. Evidence supporting the collagen‐like nature of the fibers comes from many sources including appearance in the electron microscope, thermal contractility, X‐ray diffraction patterns, amino‐acid composition, and histochemical staining. Overall, the evidence is quite convincing, and agreement is good among multiple authors (e.g. Arai 1997; Chapman 1966, 1974). The composition of the gel matrix is less well described, but the work of Gross et al. (1958) found mucopolysaccharides (glycosaminoglycans) associated with the collagen fibers, strongly suggesting a mucoprotein (proteoglycan)‐based gel. Similar gels are found in the synovial fluid lubricating the joints in vertebrates and are also found along with collagen in the cornea of the vertebrate eye. Proteoglycans and glycosaminoglycans are often highly transparent and quite resilient (Lehninger 1975).
Collagen fibers of varying diameters cross the mesogleal layer of the medusan bell, conferring a memory and resiliency to its shape that is important to the locomotory process described above. Helical elastic fibers of uncertain composition that aid in maintenance of bell shape and elastic recoil have been described in a few species, e.g. Chrysaora quinquecirrha (Arai 1997). To put the relative stiffness of jellyfish mesoglea in perspective, in a discussion of the comparative stiffness of a variety of biological materials, Vogel (1988) observed that the mesoglea of a sea anemone was about 500 times more deformable than rubber. Rubber was about 1000 times more deformable than collagen (as animal tendon). Clearly, the ratio of collagen fibers to gel matrix is important in determining the overall resilience of the medusan bell.
Nerve Nets and Nervous Control of Swimming
Swimming in medusae is a rhythmic process that is controlled by neural pacemakers and communicated to the swimming musculature via a neural network known as a nerve net. Nerve nets in the cnidaria are at their most basic in polyps and comprise two grids, one net beneath the outer epidermal layer and one beneath the inner gastrodermal layer. Both nets are located between the outer epithelium and the mesoglea and are considered literally to be a diffuse network, with no polarity in signal propagation. That is, signals propagate equally in all directions.
In the hydromedusae, the inner and outer nerve nets have partially been consolidated into marginal nerve rings located at the inside (subumbrellar) and outside (exumbrellar) of the umbrellar margin (Figure 3.22). The inner nerve ring communicates with the swimming musculature as well as with the marginal sense organs and tentacles and governs the rhythm of the swimming musculature.
With the exception of the coronates, the scyphomedusae do not have the well‐defined marginal neural rings of the hydromedusae. Nonetheless most of the action in their nervous system takes place at the umbrellar margin because that is where the sense organs and tentacles are located and, as in the hydromedusae, it is also where the swimming rhythm is generated. The neural networks are a bit more complicated in the scyphomedusae and considerably more is known about them.
The nervous system of scyphomedusae is composed of three parts: the motor nerve net, the diffuse nerve net, and the marginal centers (Arai 1997). The motor nerve net innervates the swimming muscle, the diffuse nerve net conveys sensory information to the marginal centers, and the marginal centers act as pacemakers for the swimming rhythm and integrators for the sensory information provided by the sensory apparatus. Even though only one of the nerve nets is termed “diffuse,” both nerve nets are highly complex and diffuse networks that intercalate through different tissues. The neural tissue is difficult to isolate and even more difficult to map. In fact, the two nerve nets are mainly defined by their function, which was described using physiological methods (Anderson and Schwab 1981).