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THE BIOLOGY OF EUPHAUSIIDS
Rare species whose distribution is only approximately known
Thysanopoda cristata, T . cornuta, T . egregia, T . spinicaudata;
Thysanoessa parva; Nemutobrachion sexspinosum; Stylocheiron
robustum.
The transition zones in mid-Pacific are between subarctic water and
central water (35ON-45"N) and between subantarctic water and central
water (35OS-45OS). The transition zone in the California Current is
between subarctic water and equatorial water while that in the Peru
Current is a northern extension of the subantarctic water in the
offshore regions of the Peru Current. A further transitional zone is in
existence in the northwestern Pacific where the warm Kuroshio Current
and its Extension meet with subarctic water. Nematoscelis dificilis lives
in the transition zone of the North Pacific Drift and California Current
while the closely related species, N . megalops, occupies the corresponding
southern transitional zone ; N . dificilis is endemic t o the North Pacific
and is replaced by N . megalops in the North Atlantic. Thysanobsa
gregaria is present in both the northern and southern transitional areas
including the Kuroshio region and California Current and the southern
regions of the Peru Current. Euphausia gibboides and E. pacijca are
confined to the northern transitional zone including the California
Current but the distributional area of E. pacijca extends far into subarctic waters. The last species, Thysanopoda acutifrons, is bipolar, not
present in the Kuroshio region nor in southern regions of the California
Current but penetrating the Gulf of Alaska in the northern extension
of the North Pacific Drift. Of the six species, T . acutifrons and
Euphausia pacijca have the greatest tolerance for cold water while
E. gibboides and Thysanoessa gregaria have the greatest tolerance for
warm water.
The species living in the central waters of the Pacific, like those in
the transition zones, occur in two latitudinal areas, one to the north,
one to the south, except for Euphausia hemigibba which occurs only in
northern central waters and is replaced by the related species, E. gibba,
in southern central waters. Thysanopoda obtusifrons, Euphausia brevis,
E . gibba, E. hemigibba, Nematoscelis atlantica, Stylocheiron afine
" Central Form ", and 8. suhmii all have their highest population
densities in the central regions of the central water masses while
Euphausia recurva and Thysanopoda aequalis occur most commonly
around their periphery. The three mesopelagic species, Thysanopoda
cristata, Nematobrachion sexspinosum and Stylocheiron robustum, on
present evidence, appear to be restricted to the warmer central water
masses but they are recorded in western equatorial regions.
THE BIOLOGY OF EUPHAUSIIDS
Rare species whose distribution is only approximately known
Thysanopoda cristata, T . cornuta, T . egregia, T . spinicaudata;
Thysanoessa parva; Nemutobrachion sexspinosum; Stylocheiron
robustum.
The transition zones in mid-Pacific are between subarctic water and
central water (35ON-45"N) and between subantarctic water and central
water (35OS-45OS). The transition zone in the California Current is
between subarctic water and equatorial water while that in the Peru
Current is a northern extension of the subantarctic water in the
offshore regions of the Peru Current. A further transitional zone is in
existence in the northwestern Pacific where the warm Kuroshio Current
and its Extension meet with subarctic water. Nematoscelis dificilis lives
in the transition zone of the North Pacific Drift and California Current
while the closely related species, N . megalops, occupies the corresponding
southern transitional zone ; N . dificilis is endemic t o the North Pacific
and is replaced by N . megalops in the North Atlantic. Thysanobsa
gregaria is present in both the northern and southern transitional areas
including the Kuroshio region and California Current and the southern
regions of the Peru Current. Euphausia gibboides and E. pacijca are
confined to the northern transitional zone including the California
Current but the distributional area of E. pacijca extends far into subarctic waters. The last species, Thysanopoda acutifrons, is bipolar, not
present in the Kuroshio region nor in southern regions of the California
Current but penetrating the Gulf of Alaska in the northern extension
of the North Pacific Drift. Of the six species, T . acutifrons and
Euphausia pacijca have the greatest tolerance for cold water while
E. gibboides and Thysanoessa gregaria have the greatest tolerance for
warm water.
The species living in the central waters of the Pacific, like those in
the transition zones, occur in two latitudinal areas, one to the north,
one to the south, except for Euphausia hemigibba which occurs only in
northern central waters and is replaced by the related species, E. gibba,
in southern central waters. Thysanopoda obtusifrons, Euphausia brevis,
E . gibba, E. hemigibba, Nematoscelis atlantica, Stylocheiron afine
" Central Form ", and 8. suhmii all have their highest population
densities in the central regions of the central water masses while
Euphausia recurva and Thysanopoda aequalis occur most commonly
around their periphery. The three mesopelagic species, Thysanopoda
cristata, Nematobrachion sexspinosum and Stylocheiron robustum, on
present evidence, appear to be restricted to the warmer central water
masses but they are recorded in western equatorial regions.