106 Chapter 4 Euphausiids
point of serrated margin. Process curved as well,
and forming a similar obtuse angle between basal and distal portions. Distal portion rather
straight and finely serrated on externa! edge.
Spiniform process slender, straight, stiff and
reaching mid-angle of terminal process. Lateral
process implanted near base of median lobe sinuous, longer than spine-shaped process, and
shorter than proxima! process. Setiferous lobe
bearing 2 setae on externa! margin, 3 on interna!
margin and 2 at tip. Ear-like large lappet extending beyond distal margin of lobe. According to
Costanzo and Guglielmo (1976a), proceeding
from the posterior margin of the 6th thoracic
segment, the sternite extends forward, protruding in a finger-like process and forming a cavity
which is anteriorly more accentuated, so that a
groove is formed. In this groove arise the stems
of the two spermatophores. The coxal plates are
highly developed; they bear some setae on the
inner surface.
Remarks
The rostrum varies largely in specimens of both
sexes. In females collected off the coast of Chile,
it is long and slender, and in males short to absent, but the opposite can also be found exceptionally. The rostrum is rarely reduced in males
from Atlantic South African (Boden 1954) and
Mediterranean (Boden 1954; Soulier 1963) waters. Gopalakrishnan (1975) remarked that the
rostrum of males of the genus Nematoscelis is
only rarely variable.
The side of the carapace has a sinuous and
delicate cervical groove overlapping the bucal
one. This latter groove is also sinuous but deeper
than the cervical one. This is best observed from
a dorsal view. This feature could be an additional
character to distinguish N. megalops from the
very similar species N. difficilis Hansen. Another
character of similar importance is the presence
of low mid-dorsal keels ending in posterior
spines along the 4th and 5th abdominal segments. This has also been observed in specimens
offNew Zealand (Bary 1956). The preanal spine,
bifurcated in females and simple in males, can
vary to agreat extent. N. megalops may be separated from N. difficilis (Hansen 1911) by the
structure of the petasma. Females and subadults
are difficult to differentiate. In fact, Einarsson
(1942), Karedin (1971) and Costanzo an Guglielmo ( 1980) considered both as identica! species
based on their petasma and thelycum. However,
Boden (1954), Boden et al. (1955), Bary (1956),
Mayer (1969) and Gopalakrishnan (1975) have
all recognized the validity ofboth species.
Distribution
N. megalops inhabits tropical and subtropical regions of the Indian Ocean (Illig 1930) and Mediterranean Sea (Ruud 1936; Soulier 1963). In the
North Atlantic, it is found in sub-Antarctic and
subtropical waters (Moore 1952). In the South
Atlantic, it occurs between sub-Antarctic and
subtropical waters of the eastern sector (Boden
1955; Nepgen 1957) and between 45° and 49° S
off the coast of Argentina (Ramirez 1971; Antezana and Brinton 1981). In the South Pacific, N.
megalops is found in the Transition Zone, a transoceanic belt between equatorial and sub-Antarctic waters, extending from the South American coast to New Zealand and Australia (Brinton
1962b ). In the Eastern South Pacific off Chile, N.
megalops is found in the oceanic waters of the
Humboldt Current and in the most coasta! subAntarctic waters of the Ca pe Horn Current. The
geographical distribution and overalllatitudinal
range of N. megalops is similar to the other Transition Zone species T. gregaria, but it is somewhat more restricted, encompassing a still wider
(24°-55°$) latitudinal range (Antezana 1978,
1981). N. megalops has also been found along the
entire Chilean fjord region, with the exception of
a few locations (e.g. those of the Straits ofMagellan; Antezana 1976). Brinton (1962b) suggested
that the tip of South America is a laud barrier
preventing the passage of most antitropical species, with some exceptions such as T. gregaria.
The above mentioned records provide evidence
of the connection between Pacific and Atlantic
populations.
In the present study, N. megalops was the second most abundant species sampled in the
Straits after E. vallentini. It was present at almost
all stations even though the number of adults
sampled was small. Most specimens were juvenile or furcilia stages (4-6 mm). In the Pacific
and Central areas, where it was more common,
the species was sampled from the surface to
300m.
point of serrated margin. Process curved as well,
and forming a similar obtuse angle between basal and distal portions. Distal portion rather
straight and finely serrated on externa! edge.
Spiniform process slender, straight, stiff and
reaching mid-angle of terminal process. Lateral
process implanted near base of median lobe sinuous, longer than spine-shaped process, and
shorter than proxima! process. Setiferous lobe
bearing 2 setae on externa! margin, 3 on interna!
margin and 2 at tip. Ear-like large lappet extending beyond distal margin of lobe. According to
Costanzo and Guglielmo (1976a), proceeding
from the posterior margin of the 6th thoracic
segment, the sternite extends forward, protruding in a finger-like process and forming a cavity
which is anteriorly more accentuated, so that a
groove is formed. In this groove arise the stems
of the two spermatophores. The coxal plates are
highly developed; they bear some setae on the
inner surface.
Remarks
The rostrum varies largely in specimens of both
sexes. In females collected off the coast of Chile,
it is long and slender, and in males short to absent, but the opposite can also be found exceptionally. The rostrum is rarely reduced in males
from Atlantic South African (Boden 1954) and
Mediterranean (Boden 1954; Soulier 1963) waters. Gopalakrishnan (1975) remarked that the
rostrum of males of the genus Nematoscelis is
only rarely variable.
The side of the carapace has a sinuous and
delicate cervical groove overlapping the bucal
one. This latter groove is also sinuous but deeper
than the cervical one. This is best observed from
a dorsal view. This feature could be an additional
character to distinguish N. megalops from the
very similar species N. difficilis Hansen. Another
character of similar importance is the presence
of low mid-dorsal keels ending in posterior
spines along the 4th and 5th abdominal segments. This has also been observed in specimens
offNew Zealand (Bary 1956). The preanal spine,
bifurcated in females and simple in males, can
vary to agreat extent. N. megalops may be separated from N. difficilis (Hansen 1911) by the
structure of the petasma. Females and subadults
are difficult to differentiate. In fact, Einarsson
(1942), Karedin (1971) and Costanzo an Guglielmo ( 1980) considered both as identica! species
based on their petasma and thelycum. However,
Boden (1954), Boden et al. (1955), Bary (1956),
Mayer (1969) and Gopalakrishnan (1975) have
all recognized the validity ofboth species.
Distribution
N. megalops inhabits tropical and subtropical regions of the Indian Ocean (Illig 1930) and Mediterranean Sea (Ruud 1936; Soulier 1963). In the
North Atlantic, it is found in sub-Antarctic and
subtropical waters (Moore 1952). In the South
Atlantic, it occurs between sub-Antarctic and
subtropical waters of the eastern sector (Boden
1955; Nepgen 1957) and between 45° and 49° S
off the coast of Argentina (Ramirez 1971; Antezana and Brinton 1981). In the South Pacific, N.
megalops is found in the Transition Zone, a transoceanic belt between equatorial and sub-Antarctic waters, extending from the South American coast to New Zealand and Australia (Brinton
1962b ). In the Eastern South Pacific off Chile, N.
megalops is found in the oceanic waters of the
Humboldt Current and in the most coasta! subAntarctic waters of the Ca pe Horn Current. The
geographical distribution and overalllatitudinal
range of N. megalops is similar to the other Transition Zone species T. gregaria, but it is somewhat more restricted, encompassing a still wider
(24°-55°$) latitudinal range (Antezana 1978,
1981). N. megalops has also been found along the
entire Chilean fjord region, with the exception of
a few locations (e.g. those of the Straits ofMagellan; Antezana 1976). Brinton (1962b) suggested
that the tip of South America is a laud barrier
preventing the passage of most antitropical species, with some exceptions such as T. gregaria.
The above mentioned records provide evidence
of the connection between Pacific and Atlantic
populations.
In the present study, N. megalops was the second most abundant species sampled in the
Straits after E. vallentini. It was present at almost
all stations even though the number of adults
sampled was small. Most specimens were juvenile or furcilia stages (4-6 mm). In the Pacific
and Central areas, where it was more common,
the species was sampled from the surface to
300m.