142
THE BIOLOGY O F EUPHAUSIIDS
larvae of Thysanoessa macrura and T . vicina. The evidence of migration
of larvae of the latter two species is not very conclusive but the data
do show that the furciliae of these oceanic species have their layer of
maximum abundance in the upper 100-250m layer of the water
column although they do occur in smaller numbers down to depths of
at least 1000 m. Boden and Brinton (1957) state that furciliae of
Thysanopoda aequalis and T . subaequalis show no tendency to perform
a diurnal vertical migration but according to the data in their Fig. 4
they caught more of them in the surface layers at night than during
the day, thus suggesting that a migration of small vertical extent may
take place.
Einarsson (1 945) tried to define distributional types of euphausiid
larvae on the basis of the ontogenetic migrations of the different groups
of species. The types he proposed are as follows :
These species usually live close to the
bottom and lay their eggs there ; these drift towards the upper layers
where the larvae develop. This group contains Thysanoessa inermis,
T . raschii, and Meganyctiphanes norvegica.
2. The “ longicaudata ) ) type. These species usually live in the
surface layers, spawn there, and the larvae develop there. This group
contains Thysanoessa longicaudata.
3. The “acutifrons” type. These species live a t great depth,
spawn there, but the early larvae move upwards and most of the
larval development takes place in the surface layers but the juveniles
and adolescents move deeper. This group contains Thysanopoda
acutifrons .
4. The “ cornuta ’’ type. Spawning, larval, and adult life take place
a t very great depths and these species are never, a t any stage of their
life, present in the surface layers. This group contains Thysanopoda
cornuta, T . egregia and Bentheuphausia amblyops.
Einarsson suggests that species in the “ inermis ” type lay their
eggs close to the bottom ; this is not true for the majority of the populations of these species, the eggs in most cases being laid near the surface.
From the foregoing discussions in this chapter there appears to be little
distinction between species in types 1 and 2. The most useful distinction is probably to restrict coastal and shelf species to type 1 and
reserve type 2 for species with a more oceanic distribution like
Euphausia superba, E. triacantha, and Thysanoessa Zongicaudata. The
majority of euphausiid species, the epipelagic species, belong to these
two types, the mesopelagic species to type 3, and the bathypelagic
species to type 4.
The periods of the year during which larvae of any one species occur
1. The “ inermis ’’ type.
THE BIOLOGY O F EUPHAUSIIDS
larvae of Thysanoessa macrura and T . vicina. The evidence of migration
of larvae of the latter two species is not very conclusive but the data
do show that the furciliae of these oceanic species have their layer of
maximum abundance in the upper 100-250m layer of the water
column although they do occur in smaller numbers down to depths of
at least 1000 m. Boden and Brinton (1957) state that furciliae of
Thysanopoda aequalis and T . subaequalis show no tendency to perform
a diurnal vertical migration but according to the data in their Fig. 4
they caught more of them in the surface layers at night than during
the day, thus suggesting that a migration of small vertical extent may
take place.
Einarsson (1 945) tried to define distributional types of euphausiid
larvae on the basis of the ontogenetic migrations of the different groups
of species. The types he proposed are as follows :
These species usually live close to the
bottom and lay their eggs there ; these drift towards the upper layers
where the larvae develop. This group contains Thysanoessa inermis,
T . raschii, and Meganyctiphanes norvegica.
2. The “ longicaudata ) ) type. These species usually live in the
surface layers, spawn there, and the larvae develop there. This group
contains Thysanoessa longicaudata.
3. The “acutifrons” type. These species live a t great depth,
spawn there, but the early larvae move upwards and most of the
larval development takes place in the surface layers but the juveniles
and adolescents move deeper. This group contains Thysanopoda
acutifrons .
4. The “ cornuta ’’ type. Spawning, larval, and adult life take place
a t very great depths and these species are never, a t any stage of their
life, present in the surface layers. This group contains Thysanopoda
cornuta, T . egregia and Bentheuphausia amblyops.
Einarsson suggests that species in the “ inermis ” type lay their
eggs close to the bottom ; this is not true for the majority of the populations of these species, the eggs in most cases being laid near the surface.
From the foregoing discussions in this chapter there appears to be little
distinction between species in types 1 and 2. The most useful distinction is probably to restrict coastal and shelf species to type 1 and
reserve type 2 for species with a more oceanic distribution like
Euphausia superba, E. triacantha, and Thysanoessa Zongicaudata. The
majority of euphausiid species, the epipelagic species, belong to these
two types, the mesopelagic species to type 3, and the bathypelagic
species to type 4.
The periods of the year during which larvae of any one species occur
1. The “ inermis ’’ type.