Life in the Open Ocean. Joseph J. Torres

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Life in the Open Ocean - Joseph J. Torres


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target="_blank" rel="nofollow" href="#ulink_cb68336b-0b7a-57f5-a19f-1a640475b66a">Figure 3.22 Nerve net in a hydromedusa.

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

      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.

Schematic illustration of sensory mechanisms: rhopalia.

      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.

      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.

Schematic illustration of 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


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