206
THE BIOLOGY OF EUPHAUSIIDS
300 and 800, the number being partly dependent on the size of the
species ; the majority of them probably lay between 300 and 500 eggs.
The estimated fecundity of these species is as follows : Stylocheiron spp.,
3-20 ; Pseudeuphausia spp., 20-100 ; Nyctiphanes spp., 50-100 ; Nematoscelis spp. and Nematobruchion spp., probably 200-400. These figures
agree with the published counts of eggs produced by these species, and
must, of course be multiplied by two if the females live for 2 years and
breed twice, as for example do Meganyctiphanes norvegica, Thysanoessa
spp. and probably many Euphausia spp.
No proof has so far been obtained to show that a species of euphausiid
produces several broods in the one breeding season but there are some
indications that this can be true. Eggs of Meganyctiphanes norvegica
occur in the plankton of the Clyde during the period April to July but
breeding does not take place at the same intensity throughout the whole
period. There are usually two peak periods of breeding, the first in
early April, the second at the end of June or in early July. It is likely
that many females spawning in the second period are doing so for a
second time that year, thus producing two broods. Sheard (1953) states
that Nyctiphanes australis has a protracted breeding season of from
8 to 11 months and, consequently, since it is probable that these
animals, like Meganyctiphanes norvegica, require a year to mature
sexually, it is possible that several broods are produced by this species.
Populations of Euphausia lucens and Nyctiphanes couchii, according
to Nepgen (1957) and Einarsson (1945), may produce eggs from the
beginning of spring to the end of summer but may also produce a few
eggs throughout the winter months ; consequently, several broods may
be produced by these species. Pearcy (personal communication) finds
that Euphausia pacijca breeds throughout the year off the Oregon
coast of the United States but has no evidence, as yet, that an individual
female produces more than one brood. Euphausia krohnii and Nematoscelis megalops and N . atlantica may have very long breeding seasons in
the Mediterranean ; Ruud (1936) further suggests that all species which
carry their eggs may have Iong breeding seasons and so two or more
broods may be produced. Thysanoessa species can also have protracted
spawning seasons which result both from gradual maturation of subgroups within the population and from intermittent spawnings of
individuals (Ponomareva, 1963). According to Macdonald (1928),
T . raschii in the Clyde has two periods of spawning in the year, the first
from the end of February to the middle of May and the second from the
middle of August to the middle of September but Mauchline (1966a)
found only one period of breeding, April to May and, to a lesser extent,
June. Thus, there may be annual variations in the number of broods
THE BIOLOGY OF EUPHAUSIIDS
300 and 800, the number being partly dependent on the size of the
species ; the majority of them probably lay between 300 and 500 eggs.
The estimated fecundity of these species is as follows : Stylocheiron spp.,
3-20 ; Pseudeuphausia spp., 20-100 ; Nyctiphanes spp., 50-100 ; Nematoscelis spp. and Nematobruchion spp., probably 200-400. These figures
agree with the published counts of eggs produced by these species, and
must, of course be multiplied by two if the females live for 2 years and
breed twice, as for example do Meganyctiphanes norvegica, Thysanoessa
spp. and probably many Euphausia spp.
No proof has so far been obtained to show that a species of euphausiid
produces several broods in the one breeding season but there are some
indications that this can be true. Eggs of Meganyctiphanes norvegica
occur in the plankton of the Clyde during the period April to July but
breeding does not take place at the same intensity throughout the whole
period. There are usually two peak periods of breeding, the first in
early April, the second at the end of June or in early July. It is likely
that many females spawning in the second period are doing so for a
second time that year, thus producing two broods. Sheard (1953) states
that Nyctiphanes australis has a protracted breeding season of from
8 to 11 months and, consequently, since it is probable that these
animals, like Meganyctiphanes norvegica, require a year to mature
sexually, it is possible that several broods are produced by this species.
Populations of Euphausia lucens and Nyctiphanes couchii, according
to Nepgen (1957) and Einarsson (1945), may produce eggs from the
beginning of spring to the end of summer but may also produce a few
eggs throughout the winter months ; consequently, several broods may
be produced by these species. Pearcy (personal communication) finds
that Euphausia pacijca breeds throughout the year off the Oregon
coast of the United States but has no evidence, as yet, that an individual
female produces more than one brood. Euphausia krohnii and Nematoscelis megalops and N . atlantica may have very long breeding seasons in
the Mediterranean ; Ruud (1936) further suggests that all species which
carry their eggs may have Iong breeding seasons and so two or more
broods may be produced. Thysanoessa species can also have protracted
spawning seasons which result both from gradual maturation of subgroups within the population and from intermittent spawnings of
individuals (Ponomareva, 1963). According to Macdonald (1928),
T . raschii in the Clyde has two periods of spawning in the year, the first
from the end of February to the middle of May and the second from the
middle of August to the middle of September but Mauchline (1966a)
found only one period of breeding, April to May and, to a lesser extent,
June. Thus, there may be annual variations in the number of broods