SOME ASPECTS O F THE BIOLOGY OF THE CHAETOGNATHS
345
related to variations in temperature in the course of the year. The
longest specimens are to be found in the colder periods.
The whole life cycle of S. elegans off Plymouth is, according to
Russell (1932a,b), 43 days, and from this he suggests that there may
even be five generations each year in the waters of southern England.
The reproductive cycle of S. elegans arctica in the Canadian eastern
Arctic has been described by Dunbar (1941, 1962) who has observed
that this species has a life-span of two years. The spawning period is
long, from July into the autumn and winter. The breeding cycle is
two-phase or alternating, such that during the spawning season there
are three broods, the smallest of which is the offspring of the largest,
with an adolescent intermediate group which will spawn the following
year and is normally reproductively isolated from the other (Dunbar,
1962). If this cycle is rigidly followed without any change in rhythm,
a genetic isolation between the individuals born from the different
spawnings might occur, which in the long run could lead to some degree
of differentiation, and make even phenomena of speciation possible.
The breeding cycle is determined by the slow growth rate at low
temperature, to which there has been no adaptation, and not by the
maximum abundance of food for the young.
Hydrographical differences though not great are reflected in the
biology of S. elegans arctica. Atlantic waters seem to be of some
importance in their reaching earlier or later sexual maturity. I n the
specialized environment of Ogac Lake which is typically warm water,
the growth rate is rapid and the breeding cycle is single phase or
unalternating as in cold waters. Maturity is reached at less than half
the body length required in cold water (Dunbar, 1962). Environmental
factors have also a great influence on size and duration of life. McLaren
(1966) gives a new explanation of the known phenomenon that zooplankton organisms of the high latitudes generally develop slowly,
reach a large size and live longer than the related forms of warmer
waters. McLaren states that where generation length is set by marked
seasonality of food supply, as in the Arctic, high fecundity and associated large size and slow development may be selected for. It is shown
from analysis of the functions-length, fecundity and natural mortality
-of S. elegans that large size and the two-year cycle in the Arctic seas
are not signs of poor adaptation but are optimal solutions to living-in
highly seasonal conditions.
Reverting to S. gazellae, David (1955) examined the vertical
distribution of individuals according to their age. Mature specimens
can be found only at depths over 750 m, the first stage is observed
prevalently at depths between 50 and 100 m, the second between 500
345
related to variations in temperature in the course of the year. The
longest specimens are to be found in the colder periods.
The whole life cycle of S. elegans off Plymouth is, according to
Russell (1932a,b), 43 days, and from this he suggests that there may
even be five generations each year in the waters of southern England.
The reproductive cycle of S. elegans arctica in the Canadian eastern
Arctic has been described by Dunbar (1941, 1962) who has observed
that this species has a life-span of two years. The spawning period is
long, from July into the autumn and winter. The breeding cycle is
two-phase or alternating, such that during the spawning season there
are three broods, the smallest of which is the offspring of the largest,
with an adolescent intermediate group which will spawn the following
year and is normally reproductively isolated from the other (Dunbar,
1962). If this cycle is rigidly followed without any change in rhythm,
a genetic isolation between the individuals born from the different
spawnings might occur, which in the long run could lead to some degree
of differentiation, and make even phenomena of speciation possible.
The breeding cycle is determined by the slow growth rate at low
temperature, to which there has been no adaptation, and not by the
maximum abundance of food for the young.
Hydrographical differences though not great are reflected in the
biology of S. elegans arctica. Atlantic waters seem to be of some
importance in their reaching earlier or later sexual maturity. I n the
specialized environment of Ogac Lake which is typically warm water,
the growth rate is rapid and the breeding cycle is single phase or
unalternating as in cold waters. Maturity is reached at less than half
the body length required in cold water (Dunbar, 1962). Environmental
factors have also a great influence on size and duration of life. McLaren
(1966) gives a new explanation of the known phenomenon that zooplankton organisms of the high latitudes generally develop slowly,
reach a large size and live longer than the related forms of warmer
waters. McLaren states that where generation length is set by marked
seasonality of food supply, as in the Arctic, high fecundity and associated large size and slow development may be selected for. It is shown
from analysis of the functions-length, fecundity and natural mortality
-of S. elegans that large size and the two-year cycle in the Arctic seas
are not signs of poor adaptation but are optimal solutions to living-in
highly seasonal conditions.
Reverting to S. gazellae, David (1955) examined the vertical
distribution of individuals according to their age. Mature specimens
can be found only at depths over 750 m, the first stage is observed
prevalently at depths between 50 and 100 m, the second between 500