is warmer than the equatorial Pacific because the westward phase of the
monsoonal circulation produces equator-ward flow, and thus a stable lens of
heated surface water. Probably that explains the extension of C. arcuicornis
across the equator in the Indian Ocean, but not the Pacific Ocean. Habitat
differences explaining exclusion of Indo-Pacific species from the Atlantic
remain obscure. There are massive intrusions of water at times around the
Cape of Good Hope from east to west. Possibly, the warm-water species
cannot transit the full extent of the cold Benguela Current.Pacific Patterns
(^) Work at Scripps Institution of Oceanography, led by Martin Johnson in the 1950s,
provided distributions for species from a variety of groups in just the Pacific Ocean as
far south as the subantarctic convergence. The British Discovery Expeditions
extended the analysis to Antarctica. Other work, particularly Japanese studies (see
Nishida 1985), has added more species distribution results, but remarkably few new
patterns. Species patterns shown were selected as typical.
(^) Pacific subarctic
(^) There are two variants:
(^) All across the subarctic gyre north of 40°N, not carried south in the California
Current:
(^) Sagitta elegans: Chaetognath (Fig. 10.4a)
Fig. 10.4 Subarctic Pacific distribution patterns of (a) the chaetognath Sagitta elegans
(^) (after Bieri 1959),
and (b) the euthecosome pteropod Limacina helicina
(^) (after McGowan 1963).
Dots are sampling sites. Variations in shading represent factors of 10 in population,
darker for higher abundance. The pteropod pattern illustrates extension of the pattern
southward in the California Current – a feature of some subarctic species.