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The medusae possess at least three sensory modalities: photoreception, equilibrium, or balance – sometimes thought of as gravity reception, and chemoreception. Structures have been described for receptors detecting light and balance but not for waterborne chemicals – equivalent to our senses of taste and smell. The fact that medusae respond to chemicals of various kinds allows us to infer that the sense exists, even if there has been no structure identified to associate with it. Clearly, medusae have well‐developed sensory capabilities.

The rhopalia are multifunctional sensory centers, usually possessing a photoreceptor and equilibrium receptor, or statocyst, within the same general structure (Figure 3.23). The photoreceptors vary in complexity from a simple pigment cup without a lens and a limited number of receptors such as that observed in the ocelli of Aurelia aurita (Figure 3.23d) to the well‐developed ocelli of the cubomedusae that possess a cornea, a lens, and a well‐defined retina (Figure 3.23e). Although structures believed to be photoreceptive in nature have been described in medusae since the 1940s, almost no neurophysiological recording has been completed to directly confirm their sensory role. Fortunately, such recordings have been done in other primitive phyla (e.g. flatworms) from highly similar structures, and we may infer their photoreceptive function from those (Land 1990; Withers 1992). In addition, there are a variety of observed behaviors that require a sensitivity to light. Among those are diurnal vertical migration (Hamner 1995) and the orientation of cubomedusae to a point source of light several meters away. The sensory centers of medusae are distributed liberally around the bell margin. Thus even the sensitivity to light and shadow afforded by simple eyes can aid in navigation or alert the individual to the presence of predator or prey.

Figure 3.23 Sensory mechanisms: rhopalia. (a) Position of the rhopalia of the scyphomedusa Atolla, located between the marginal lappets. (b) Sector of the bell of Rhizostoma, showing nerve plexus and gastrovascular network (stippled); (c) rhopalium and surrounding structures in Aurelia. (d) Section of the rhopalium of Aurelia aurita; diameter at ocelli is 0.4 mm. (e) section of a cubomedusan rhopalium.

Sources: (a) Redrawn from Maas (1904), plate IV; (b and c) Hyman (1940), figure 163 (p. 504); (d) Kaestner (1967), figure 5‐4 (p. 91); (e) Redrawn from Mayer (1910), Vol III, plate 56.

Statocysts, or equilibrium sensors, are an example of a class of sensory receptors known as mechanoreceptors. At their most sophisticated, mechanoreceptors detect vibration and sound using the same basic principles we observe in organs of equilibrium. The basic structure of a statocyst is depicted in Figure 3.24. In it is a dense body, or statolith, which can be thought of as a stone or concretion secreted by the animal. The sensory epithelium is made up of cells with hair‐like projections (“hair cells”) that are sensitive to deformation by the statolith. A change in position of the animal will change the position of the statolith on the sensory epithelium, providing information on attitude and equilibrium of the whole animal. Hair cells are present in virtually all types of mechanoreceptors, including those of our own inner ear.

The last type of sensory modality, chemoreception, has been inferred from the behavior of medusae, specifically by the orientation of medusae toward aggregations of prey or even to water that has been conditioned by the presence of prey (Hamner 1995). No specific structures associated with chemoreception have been identified yet, but the fact that cnidarians will show feeding behavior in response to chemical stimuli such as prey homogenates and the tripeptide reduced glutathione has been known for decades.

Sensory mechanisms provide an animal’s windows into the physical world. The most telling evidence for the presence or absence of a sensory modality is in a species’ behavior. Even if a sense such as touch does not have a discrete, obvious, and easily identified receptor, if a medusa responds to touch, e.g. by suddenly retracting its tentacles, the animal is obviously capable of discriminating touch. Even in more advanced species such as the vertebrates, sensory mechanisms exist that are not easily discriminated, those for heat and cold being two. As different open‐ocean dwellers are described in further chapters, we will observe more sophisticated sensory organs in more sophisticated taxa. Sensory mechanisms, and neural processes in general, are exquisitely complex.