Cephalopoda
Geographic and vertical distribution

Planktonic cephalopods, and planktonic developmental stages of nektonic, nektobenthic, and benthic cephalopods, are distributed through the World Ocean, from the North Polar Basin to the Antarctic. But, except for larvae of nearshore benthic octopuses, they are nearly absent over the inner shelf and rare over the outer shelf. Their main habitat is oceanic waters. Meridional ranges of most species of neritic and nerito-oceanic (living over the continental slope) cephalopods extend along shelves and slopes, sometimes across climatic belts. On the other hand, most oceanic cephalopods are zonally distributed, their ranges closely associated with the large oceanic circulation systems. Their range boundaries in many cases coincide with global oceanic fronts and convergences, such as (in the South Atlantic) the Southern Tropical Front, the Southern Subtropical Convergence, and the Antarctic Convergence. The geographic distribution of oceanic cephalopods and their types of ranges are listed and mapped by Nesis (1985).

Planktonic cephalopods are distributed from the surface to abyssal depths, and include epipelagic, mesopelagic, and bathypelagic species, as well as various intermediate types. Most species perform diurnal vertical migrations, and many descend during ontogeny. Larvae of almost all species live on the average closer to the surface than adults. Even late larvae may perform diurnal vertical migrations (usually within the epipelagic realm). Diurnal migrations are of the usual type, with nighttime ascent. Only the juveniles (not larvae nor adults) of some oceanic ommastrephid squids (mostly Ommastrephes and Sthenoteuthis) perform reverse migrations, with daytime ascent. The vertical distribution of oceanic cephalopods is discussed by Roper and Young (1975) and Nesis (1985).

Quantitative distributional data on planktonic cephalopods are scanty, first because no device catches all developmental stages with equal efficiency, and second because trawls that effectively catch adult cephalopods are non-quantitative. The only available information is estimates of relative abundance (biomass) of pelagic cephalopods, and of their relative distribution in the World Ocean (Clarke, 1987; Nesis, 1985).

The South Atlantic is not as rich in cephalopods as, for example, the subarctic North Pacific and the Okhotsk and Bering seas. But rich commercial squid fisheries are known on both sides of the South Atlantic: off Argentina and the Malvinas (=Falkland) Islands in the west, and off Namibia in the east. Larval and juvenile cephalopods are generally at maximum abundance in epipelagic layers, decreasing at greater depths. Adults are usually most abundant in mesopelagic layers, particularly where they contact the bottom along slopes and rises. In terms of the concentration of cephalopods under one square kilometer of ocean surface, maximum abundances always occur over the shelf break and continental slope, and oceanwards from trade wind upwelling areas, while they are low over the inner shelf and in the central part of anticyclonic gyres (Nesis, 1985).

Two typical examples illustrate cephalopod abundance in pelagic areas. In large Bongo net (mouth diameter 60 cm, mesh 0.7 mm) samples from the upper epipelagic layer (0 to 50-55 m) of the open Gulf of Guinea during June-August of 1985 there were, in different water masses, from 10 to 36 cephalopod larvae per 1000 mö3 (Arkhipkin et al., 1988). In midwater trawl samples from the outer shelf and slope off Namibia (17-27°S), the biomass (wet weight) of cephalopods was 80.1 g per trawling hour in autumn (April-June), and 31.5 g hö-1 in summer (January), or 8.1 and 2.4% of the total biomass of fish, cephalopods, decapod crustaceans and euphausiids, respectively (5-10 times less than fish biomass). These were Isaacs-Kidd and Aseyev-Samyshev trawls towed horizontally at levels from 50 to 1000 m over bottom depths of 105-4700 m, 15 to 200 nautical miles from shore (Nezlin and Nesis, 1988).

The biomass of cephalopod larvae is always very significantly lower than that of, say, planktonic crustaceans, and usually may be considered insignificant. But the biomass of adult cephalopods in mesopelagic layers in some very productive areas of the World Ocean may be comparable to that of fish (Clarke, 1996). Furthermore, the life span of an average cephalopod is much shorter than that of an average fish, and the growth rates, food rations and production:biomass (P:B) coefficients are correspondingly much higher (Nesis, 1985). Thus, the role of cephalopods in total production and consumption of food in the ocean is much more important than their share of biomass may suggest (Boyle, 1983-1987; Nesis, 1985; Clarke, 1996).

See Ce. 7 Tables for the tables that summarize the known horizontal and vertical distribution range of all planktonic cephalopods in the South Atlantic.

Ce table 1A-E (species A-E)
Ce table 1G-N (species G-N)
Ce table 1O-Z (species O-Z)
Ce table 1abbrev (abbrevations used in table 1)