342
THE BIOLOQY O F EUPHAUSIIDS
Euphausia diomedeae, E . paragibba, Nematoscelis gracilis, Stylocheiron afine " Western Equatorial Form ", and X. microphthalma are
the species most restricted to the equatorial water masses. Thysanopoda
tricuspidata and Euphausia tenera are two equatorial species whose
distributional range extends into western central waters and their total
area of occurrence is, according to Brinton, determined by the ranges
of temperature tolerance of their surface living larvae. A group of
tropical species have localized distributions, being restricted to the
Eastern Equatorial Basin : these are Euphausia eximia, E . distinguenda,
E . lamelligera, Nyctiphanes simplex, and Stylocheiron afine '' Eastern
Equatorial Form ". A further group live in the tropical regions of the
western Pacific : Euphausia sibogae, E. fallax, Pseudeuphausia latifrons,
P. sinica, Nematoscelis lobata, Xtylocheiron insulare, and S. afine " Indo
Australian Form ", and Brinton includes Euphausia pseudogibba with
this Indo-Australian group. Roger ( 1 9 6 7 ~ )
discusses the possible effects
of the equatorial currents, including the Cromwell counter current,
on the equatorial distribution of Thysanopoda rnonacantha, T . aequalis,
T . tricuspidata, Euphausia paragibba, E. eximia, E . gibboides, Nernatoscelis tenella and N . gracilis.
A last group of species is found throughout most of the Pacific
between 40"N and 40"s except, in many cases, in the eastern area
comprising the California Current, the wedge of oxygen poor water
extending westwards from Mexico between 10"N and 20"N, and the
Peru Current and its extension westwards beyond the Galapagos
Islands. These species are the bathypelagic Bentheuphausia amblyops,
the meso pelagic Thysanopoda monacantha, T . orientalis, T . pectinata,
Nematoscelis tenella, Stylocheiron elongatum, 8. longicorne, and S.
maximum and the epipelagic Nematoscelis microps, Nematobrachion
jlexipes, Xtylocheiron carinatum, and S. abbreviatum. The information
on latitudinal and approximate bathymetrical distribution of Pacific
species is summarized in Fig. 127.
Indian Ocean
Forty species are known to occur in the Indian Ocean but the full
results of the surveys made by the International Indian Ocean Expedition are not yet known and the list of species may be added to in the
future, especially as regards the mesopelagic species. The Indian Ocean
has, of course, no northern cold area but has a warm northern area
with surface salinities of 36x0 or more; very few species live in this
area. Thus the Indian Ocean can be divided into the following
latitudinal regions : the area north of about 12"N and the area between
12"N and about 40"s. The tropical convergence is located near Z O O S ,
THE BIOLOQY O F EUPHAUSIIDS
Euphausia diomedeae, E . paragibba, Nematoscelis gracilis, Stylocheiron afine " Western Equatorial Form ", and X. microphthalma are
the species most restricted to the equatorial water masses. Thysanopoda
tricuspidata and Euphausia tenera are two equatorial species whose
distributional range extends into western central waters and their total
area of occurrence is, according to Brinton, determined by the ranges
of temperature tolerance of their surface living larvae. A group of
tropical species have localized distributions, being restricted to the
Eastern Equatorial Basin : these are Euphausia eximia, E . distinguenda,
E . lamelligera, Nyctiphanes simplex, and Stylocheiron afine '' Eastern
Equatorial Form ". A further group live in the tropical regions of the
western Pacific : Euphausia sibogae, E. fallax, Pseudeuphausia latifrons,
P. sinica, Nematoscelis lobata, Xtylocheiron insulare, and S. afine " Indo
Australian Form ", and Brinton includes Euphausia pseudogibba with
this Indo-Australian group. Roger ( 1 9 6 7 ~ )
discusses the possible effects
of the equatorial currents, including the Cromwell counter current,
on the equatorial distribution of Thysanopoda rnonacantha, T . aequalis,
T . tricuspidata, Euphausia paragibba, E. eximia, E . gibboides, Nernatoscelis tenella and N . gracilis.
A last group of species is found throughout most of the Pacific
between 40"N and 40"s except, in many cases, in the eastern area
comprising the California Current, the wedge of oxygen poor water
extending westwards from Mexico between 10"N and 20"N, and the
Peru Current and its extension westwards beyond the Galapagos
Islands. These species are the bathypelagic Bentheuphausia amblyops,
the meso pelagic Thysanopoda monacantha, T . orientalis, T . pectinata,
Nematoscelis tenella, Stylocheiron elongatum, 8. longicorne, and S.
maximum and the epipelagic Nematoscelis microps, Nematobrachion
jlexipes, Xtylocheiron carinatum, and S. abbreviatum. The information
on latitudinal and approximate bathymetrical distribution of Pacific
species is summarized in Fig. 127.
Indian Ocean
Forty species are known to occur in the Indian Ocean but the full
results of the surveys made by the International Indian Ocean Expedition are not yet known and the list of species may be added to in the
future, especially as regards the mesopelagic species. The Indian Ocean
has, of course, no northern cold area but has a warm northern area
with surface salinities of 36x0 or more; very few species live in this
area. Thus the Indian Ocean can be divided into the following
latitudinal regions : the area north of about 12"N and the area between
12"N and about 40"s. The tropical convergence is located near Z O O S ,