22
THE BIOLOGY OF EUPHAUSIIDS
of S. afine and S. longicorne G. 0. Sars (Fig. 5 ) . A comparison of the
descriptions of Thysanoessa raschii suggests that the body shape of this
species may vary; Einarsson (1945) figures Atlantic T . raschii with
slim abdominal segments while Boden et al. (1955) show Pacific
specimens with a heavier abdomen. There are several forms of
T . inermis; the " neglecta )' form has the second pair of thoracic limbs
elongated while the " inermis " form does not and the two forms can
occur in the same sea area, for example in the Barents Sea. Other forms
of T . inermis are detailed by Nemoto (1966) who found, in individuals
from the North Pacific, that variable numbers of the abdominal
segments had dorsal spines present ; the form with dorsal spines on the
fifth and sixth abdominal segments has now been recorded from the
North Sea (Jones et al., 1967). Hansen (1911) described variations in
the body form of Euphausia diomedeae Ortmann, E. triacantha Holt
and Tattersall and E. similis G. 0. Sars. Einarsson (1942) observed
swellings on the dorsal regions of the abdominal segments of male
Nematoscelis microps G. 0. Sam, N . atlantica Hansen and N . tenella
G. 0. Sars which were correlated with the presence of enlarged photophores; N . microps had the photophore on the second abdominal
segment, enlarged and a dorsal swelling on the first segment, N . atlantica
had the second and third abdominal photophores enlarged and dorsal
swellings on the first and second segments, while N . tenella had the
fourth photophore enlarged and a dorsal swelling on the third segment.
Taniguchi (1966) found abnormal N . microps with a large hump on the
dorsal surface of the first abdominal segment. McLaughlin (1965)
describes two forms of N . dificilis Hansen, the typical form having no
lateral denticle, and a smaller form having a lateral denticle on the
carapace ; all other characters of the two forms were identical. There is
often variation in the degree of development of dorsal spines or keels on
the abdominal segments (Fig. 1) ; Euphausia vallentini Stebbing has a
spine developed on the third abdominal segment in some individuals
and not in others and so, in fact, this species appears at two points in
the key to species of the genus Euphausia. The petasma is frequently
the most diagnostic character used to distinguish closely-related species
and the variation in the form of this organ between the different species
of the genus Thysanopoda is evident in Fig. 6. These drawings,
however, were made from fully adult males but while the petasma is
developing in adolescents its form changes as is shown in Fig. 7.
Further, Banner (1950) found that the form of this organ varied in adult
Stylocheiron maximum Hansen (Fig, 7) and is, therefore, not a good
diagnostic feature of this species. This degree of variation in the
petasma is probably exceptional but the detailed form of the organ does
THE BIOLOGY OF EUPHAUSIIDS
of S. afine and S. longicorne G. 0. Sars (Fig. 5 ) . A comparison of the
descriptions of Thysanoessa raschii suggests that the body shape of this
species may vary; Einarsson (1945) figures Atlantic T . raschii with
slim abdominal segments while Boden et al. (1955) show Pacific
specimens with a heavier abdomen. There are several forms of
T . inermis; the " neglecta )' form has the second pair of thoracic limbs
elongated while the " inermis " form does not and the two forms can
occur in the same sea area, for example in the Barents Sea. Other forms
of T . inermis are detailed by Nemoto (1966) who found, in individuals
from the North Pacific, that variable numbers of the abdominal
segments had dorsal spines present ; the form with dorsal spines on the
fifth and sixth abdominal segments has now been recorded from the
North Sea (Jones et al., 1967). Hansen (1911) described variations in
the body form of Euphausia diomedeae Ortmann, E. triacantha Holt
and Tattersall and E. similis G. 0. Sars. Einarsson (1942) observed
swellings on the dorsal regions of the abdominal segments of male
Nematoscelis microps G. 0. Sam, N . atlantica Hansen and N . tenella
G. 0. Sars which were correlated with the presence of enlarged photophores; N . microps had the photophore on the second abdominal
segment, enlarged and a dorsal swelling on the first segment, N . atlantica
had the second and third abdominal photophores enlarged and dorsal
swellings on the first and second segments, while N . tenella had the
fourth photophore enlarged and a dorsal swelling on the third segment.
Taniguchi (1966) found abnormal N . microps with a large hump on the
dorsal surface of the first abdominal segment. McLaughlin (1965)
describes two forms of N . dificilis Hansen, the typical form having no
lateral denticle, and a smaller form having a lateral denticle on the
carapace ; all other characters of the two forms were identical. There is
often variation in the degree of development of dorsal spines or keels on
the abdominal segments (Fig. 1) ; Euphausia vallentini Stebbing has a
spine developed on the third abdominal segment in some individuals
and not in others and so, in fact, this species appears at two points in
the key to species of the genus Euphausia. The petasma is frequently
the most diagnostic character used to distinguish closely-related species
and the variation in the form of this organ between the different species
of the genus Thysanopoda is evident in Fig. 6. These drawings,
however, were made from fully adult males but while the petasma is
developing in adolescents its form changes as is shown in Fig. 7.
Further, Banner (1950) found that the form of this organ varied in adult
Stylocheiron maximum Hansen (Fig, 7) and is, therefore, not a good
diagnostic feature of this species. This degree of variation in the
petasma is probably exceptional but the detailed form of the organ does