2. THE SPECIES OF K R n L
19
in determining the genus to which a species belongs, are the different
states of development of the thoracic legs, especially the sixth, seventh,
and eighth pairs. It must be remembered that much of the material
examined by zoologists is preserved, usually in formaldehyde, because
these animals are caught in plankton nets with very many other
organisms and it is often impossible to pick them all out from the fresh
catch because the next haul is ready for sorting before the previous one
is finished with. Furthermore, many species of euphausiids, and of
course their larvae, are small and sorting has to be done slowly with the
aid of low-power microscopes. Consequently, unless live or fresh
animals are required for a specific purpose, most hauls are preserved
and then examined in detail a t a later date. Preservation affects the
euphausiids, causing them to become brittle if preserved in alcohol, or
to lose their colour if preserved in either alcohol or formaldehyde.
Colour variation among individuals of a species is frequently large but
nevertheless an experienced worker can often identify species simply
by the distribution of pigment in or on the body. Euphausiids are
transparent when alive and the movements of the stomach and gut
and of the heart can be observed through the relatively thin cuticle.
The hepatopancreas is often coloured-in Meganyctiphanes norvegica
it is green and red-and, of course, the photophores are a deep red
colour. Red chromatophores are present on the thoracic limbs and
the mouthparts of several species. On the other hand, although the
pigmentation of several species is brilliant, colour is not a diagnostic
feature mentioned in the key which is given later in this chapter.
General body shape also varies considerably within species of
euphausiids and several scientists have examined such variation. There
are, of course, always a few specimens in hauls of several thousand
specimens which are " thin " or " out of condition ". Just what this
actually means is often hard to decide. For example, a few specimens
of' Thysanoessa raschii will have no oils stored within the body cavities
of' the cephalotliorax in the late autumn in the Clyde when the majority
of' specimens have so much stored that the carapace is swollen. The
specimens without oils are very thin but as lively as their heavier
neighbours. Then again there are geographical forms of some species,
notably Stybocheiron species. Brinton (1 962a) describes the different
forms of S . afline Hansen which occur in the Pacific Ocean and are
characterized by variations in the proportions of different regions of the
body, in the form of the petasma of the males and by the shape and size
of the eyes (Fig. 4). He also compares the length of the sixth abdominal
segment divided by its depth against the width of the lower section of
the eye divided by the width of the upper section in the different forms
19
in determining the genus to which a species belongs, are the different
states of development of the thoracic legs, especially the sixth, seventh,
and eighth pairs. It must be remembered that much of the material
examined by zoologists is preserved, usually in formaldehyde, because
these animals are caught in plankton nets with very many other
organisms and it is often impossible to pick them all out from the fresh
catch because the next haul is ready for sorting before the previous one
is finished with. Furthermore, many species of euphausiids, and of
course their larvae, are small and sorting has to be done slowly with the
aid of low-power microscopes. Consequently, unless live or fresh
animals are required for a specific purpose, most hauls are preserved
and then examined in detail a t a later date. Preservation affects the
euphausiids, causing them to become brittle if preserved in alcohol, or
to lose their colour if preserved in either alcohol or formaldehyde.
Colour variation among individuals of a species is frequently large but
nevertheless an experienced worker can often identify species simply
by the distribution of pigment in or on the body. Euphausiids are
transparent when alive and the movements of the stomach and gut
and of the heart can be observed through the relatively thin cuticle.
The hepatopancreas is often coloured-in Meganyctiphanes norvegica
it is green and red-and, of course, the photophores are a deep red
colour. Red chromatophores are present on the thoracic limbs and
the mouthparts of several species. On the other hand, although the
pigmentation of several species is brilliant, colour is not a diagnostic
feature mentioned in the key which is given later in this chapter.
General body shape also varies considerably within species of
euphausiids and several scientists have examined such variation. There
are, of course, always a few specimens in hauls of several thousand
specimens which are " thin " or " out of condition ". Just what this
actually means is often hard to decide. For example, a few specimens
of' Thysanoessa raschii will have no oils stored within the body cavities
of' the cephalotliorax in the late autumn in the Clyde when the majority
of' specimens have so much stored that the carapace is swollen. The
specimens without oils are very thin but as lively as their heavier
neighbours. Then again there are geographical forms of some species,
notably Stybocheiron species. Brinton (1 962a) describes the different
forms of S . afline Hansen which occur in the Pacific Ocean and are
characterized by variations in the proportions of different regions of the
body, in the form of the petasma of the males and by the shape and size
of the eyes (Fig. 4). He also compares the length of the sixth abdominal
segment divided by its depth against the width of the lower section of
the eye divided by the width of the upper section in the different forms