Читать книгу Life in the Open Ocean - Joseph J. Torres - Страница 114

No sensory apparatus has been detected in the siphonophores, i.e. no ocelli, statocysts, or mechanoreceptors such as those observed in the medusae. However, siphonophores are sensitive to touch, light, chemicals, and, in some cases, to waterborne vibration. How? It is likely that the nerves themselves act as receptors although mechanisms effecting the receptor‐like responses are undescribed.

Two types of conduction are recognized in the siphonophores, epithelial and neural. Both contribute to coordinated movement and responses. Epithelial conduction is similar to the spread of depolarization in myogenic hearts and is present in the nectophores of physonects and calycophorans. Epithelial conduction was effectively demonstrated in Nanomia when its nectophores remained coordinated after severing their nervous connection to the stem (Mackie 1964).

Epithelial conduction as a mechanism for propagating impulses is confusing at best. There is no obvious morphological distinction between epithelia that are capable of conduction and those that are not. Before the techniques of neurophysiology were available, the presence of epithelial conduction was inferred by the absence of nerves coupled with the presence of coordinated activity (Mackie et al. 1987). Presumably, the epithelial cells themselves possess the membrane channels that are necessary for ion movement and signal propagation. Since all animal cells maintain an ionic disequilibrium with the medium bathing them, whether that medium is seawater or blood, the basic ion transport “equipment” is already in place within the membrane. The signal must spread from cell to cell to function as a conductive pathway. How it happens is still undescribed.

Figure 3.36 The four major temperate zones of the ocean surface. The Tropical Zone (TR) is delimited by the 20 °C isotherm for the coldest month. It is bordered by a Warm‐Temperate Zone (W‐T), a Cold‐Temperate Zone (C‐T), and a Cold or Polar Zone (C).

Source: Adapted from Briggs (1995), figure 55 (p. 209).