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As truly open‐ocean species, siphonophores are found throughout the water column from the pleustonic surface layers to very great depth (8000 m, Vinogradov 1970). Though vertical distributions for individual species can be extensive, most siphonophores exhibit depth ranges over which they are typically most abundant. Those ranges are the same as seen with most other open‐ocean taxa: epipelagic (0–250 m), mesopelagic (250–1000 m), and bathypelagic (>1000 m) (Pugh 1999).

Figure 3.35 Degrees of dependence on gelatinous parts for flotation. (a) Rhizophysa filiformis is completely dependent on the apical float for support and lacks gelatinous parts; (b) Nanomia bijuga has gelatinous nectophores and bracts but is largely dependent on the float; (c) Agalma elegans is buoyed mainly by gelatinous parts with the float only supporting a short anterior portion of the stem.

Source: Jacobs (1937), figure 7 (p. 594).

Siphonophores are quite delicate, particularly prone to damage from midwater trawls and even gentle plankton nets. The damage usually involves breaking the animal into its constituent parts, and with particularly delicate forms, the damage may preclude identification altogether. Delicate forms can literally pass through the meshes of a scientific net. Thus, conclusions drawn from a net sampling survey will tend to be biased toward the more robust species. Surveys done primarily with visual observations, by either SCUBA or submersible, will also have bias, usually toward slower moving, larger species. Both types of sampling have their shortcomings, but without question, far more sampling has been done with nets. Trawling techniques have been around longer, trawls sample greater volumes, and they are far cheaper to execute. It is important to recognize the pros and cons of all sampling methodologies and to take away from each what is most useful.

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.

Data from Pugh (1999) for 93 widely distributed Atlantic siphonophore species are summarized in Table 3.9. Forty‐one species are considered to be mainly epipelagic (0–250 m), 17 are epipelagic–upper mesopelagic (<100 m to >250 m), 31 are mesopelagic (200–1000 m), and 4 are bathypelagic (>1000 m). The best information to date suggests that the majority of siphonophores reside in the most productive upper 250 m of the ocean.

Table 3.9 Vertical distribution of Siphonophore species in the South Atlantic.

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