Life in the Open Ocean. Joseph J. Torres

Figure 3.22 Nerve net in a hydromedusa.

      Source: Kaestner (1967), figure 4‐20 (p. 61). Reproduced with the permission of John Wiley & Sons.

The marginal centers are important junctions for integrating sensory inputs and conveying them as needed to the swimming muscle. They are located directly behind the rhopalia on the bell margin, most easily visualized in the coronates (Figure 3.23a). Their precise location is unknown, but the fact that the centers are located very close to, but not within, the rhopalia itself was determined by methodical and highly localized ablation experiments in the 1980s (Passano 1982). The criterion for determining the location of the marginal center was the presence or absence of a pacemaker signal to the swimming musculature.

      Senses and Sensory Mechanisms

      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.

The Siphonophores

The Siphonophora comprise an order in the class Hydrozoa, subclass Hydroidolina, as shown in the classification scheme. They are quite distinct, differing from their hydrozoan brethren and the remainder of the pelagic Cnidaria in general morphology and basic organization. Three major biological groups are widely recognized within the Siphonophora, but there are disagreements about their relative taxonomic rank within the order. In this case we are following the World Register of Marine Species (WoRMS) and many decades of history in placing them each in their own suborder. The three suborders are the Cystonectae the Physonectae, and the Calycophorae (Figure 3.25). Altogether, the three suborders contain about 190 species, with the lion’s share of them, about 60%, in the suborder Calycophorae. Suborders within the siphonophores are discriminated initially by the presence or absence of a gas‐filled float or pneumophore. The suborders Cystonectae and Physonectae (Figure 3.25a–d) both have a gas‐filled float, whereas the calycophorans do not (Figure 3.25e and f). The second division for classification is in the presence of swimming bells beneath the float. The physonects have swimming bells, or nectophores, beneath the float; cystonects do not. Table 3.5 is a synopsis of siphonophore classification. The siphonophores are more diverse at the family level than the rest of the Cnidaria.

Figure 3.24 Basic statocyst structure showing the calcium carbonate statolith resting on sensitive sensory cells, which respond to changes in position of the statolith.

      Source: Tschachotin (1908), text figure 5 (p. 358).

      Siphonophores are possibly the most confusing group in the animal kingdom. A free‐floating individual siphonophore is considered to be a colony of various individuals working together to feed, reproduce, and move about, and within each siphonophore colony are