Life in the Open Ocean. Joseph J. Torres

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Life in the Open Ocean - Joseph J. Torres


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      Pugh (1999) observed that calycophophoran species tend to dominate in net samples, but that over 70% of the specimens collected by submersibles are the larger, more delicate, and more highly pigmented physonects. An interesting, though disturbing, corollary to his observations on relative numbers was that about half of the physonects collected by submersibles were new to science. Clearly there remains a lot to learn about the siphonophores.

      Source: From the data in Pugh (1999).

Primary depth range Number of species
Order Cystonectae Order Physonectae Order Calycophorae
Epipelagic (0–250 m) 3 4 34
Epi‐upper Mesopelagic (<100 m to >250 m) 0 5 12
Mesopelagic (200–1000 m) 0 7 24
Bathypelagic (>1000 m) 0 0 4

      Diurnal Vertical Migration

      As discussed, swimming ability within the siphonophores varies widely. The idea that siphonophore populations move up to the surface at dusk and back to depth at dawn in response to the waning and waxing illumination in near‐surface waters is not a compelling one, especially for the suborders with more limited mobility. Nonetheless, there is a substantial amount of data for many species that suggest precisely that. Moore (1949, 1953) reported vertical excursions of 30–40 m for many species of calycophorans in both the Florida current and in the vicinity of Bermuda on a day–night basis. Similar results were obtained by Musayeva (1976) for calycophorans in the Sulu Sea.

      Without question, a changing vertical distribution over the diel cycle is a characteristic of many siphonophore species. However, even among the calycophorans the vertical excursions are usually quite limited in scope (<50 m), a situation to be expected in an order that is morphologically adapted more for ambush predation than long‐distance swimming. Because of their float, the physonects are not only good acoustic targets, they face the same problems that fish with swimbladders do when moving vertically: expansion and compression of gas in their flotation system when moving up and down in the water column.

      Geographical Distribution

      Mackie et al. (1987) provide a summary of diversity and numbers for 21 common species of calycophoran siphonophores in the North Atlantic. The data show a peak in both numbers and diversity at about 18 °N with a gradual decline in species numbers further north. A second peak in abundance is obvious between 40 and 53 °N.

      Organization and Sensory Mechanisms

      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).

Schematic illustration of the four major temperate zones of the ocean surface.