350
ELVEZIO GHIRARDELLI
that one may find Spadella concealed in the silt on the bottom during
winter months also. More precisely, from the observations by Furnestin
and Brunet it appears that Spadella collected in the winter are smaller
than the spring specimens and somewhat bigger than autumn and
summer ones : this actually confirms thc possibility that individuals
born in autumn or late summer survive the winter.
For Spadella living in the prairies of Posidonia, it can be said with
certainty that a direct conncxion exists between the growth, the
number of eggs and the conditions of the habitat. I n springtime, the
conditions become better and as a consequence there seems to be a
quicker growth of the population and a greater production of eggs
which rapidly repopulates the prairies of Posidonia. These observations also show the importance of laboratory experiments in order to
establish the exact duration of reproductive cycles.
This is also confirmed by the experiments of Dallot (1966) which I
mentioned previously (p. 331). He found that in Sagitta setosa, kept
under good conditions, the first batch of eggs spawned is regularly
followed by another about 24 h later. This interval may increase
considerably if food is insufficient. Actually an ovarian cycle continues
as long as the animal is in good physiological condition. Dallot himself
saw up to six successive spawnings in one individual; one kept for 20
days laid ninety-six eggs in 6 days; another laid thirty-four eggs all at
once. The growth of the ovaries is noticeably quick: one individual,
79 mm long, belonging to the summer generation, showed an increase in
ovary length from 0.25 mm (stage 2) to 1 mm (stage 4) in only 3 days;
during this period the diameter of the oocytcs increased from 30 to
160 p though the total length of the animal remained unchanged.
There might be a connexion between the intensity of predation and
the stage of sexual maturity during a 24-11 cycle. It seems that S. setosa
actually feeds more actively during the night and it is at this same time
that the oocytes grow more speedily (Dallot, personal communication).
These observations show that the fecundity of S. setosa is more
considerable than had been concluded from observations on the animals
in nature. There remain, says Dallot, many questions still to be
answered: the total length of the period of reproduction, whether the
stock of oocytes which undergoes maturation is limited or not, what are
the influences of the quantity of food available, of temperature and of
the other hydrological and biological factors on the reproductive
processes.
As regards the number of spawnings and the duration of the
spawning period, reference should be made to the review by Alvarifio
(1965), who has gathered together data from many observations.
ELVEZIO GHIRARDELLI
that one may find Spadella concealed in the silt on the bottom during
winter months also. More precisely, from the observations by Furnestin
and Brunet it appears that Spadella collected in the winter are smaller
than the spring specimens and somewhat bigger than autumn and
summer ones : this actually confirms thc possibility that individuals
born in autumn or late summer survive the winter.
For Spadella living in the prairies of Posidonia, it can be said with
certainty that a direct conncxion exists between the growth, the
number of eggs and the conditions of the habitat. I n springtime, the
conditions become better and as a consequence there seems to be a
quicker growth of the population and a greater production of eggs
which rapidly repopulates the prairies of Posidonia. These observations also show the importance of laboratory experiments in order to
establish the exact duration of reproductive cycles.
This is also confirmed by the experiments of Dallot (1966) which I
mentioned previously (p. 331). He found that in Sagitta setosa, kept
under good conditions, the first batch of eggs spawned is regularly
followed by another about 24 h later. This interval may increase
considerably if food is insufficient. Actually an ovarian cycle continues
as long as the animal is in good physiological condition. Dallot himself
saw up to six successive spawnings in one individual; one kept for 20
days laid ninety-six eggs in 6 days; another laid thirty-four eggs all at
once. The growth of the ovaries is noticeably quick: one individual,
79 mm long, belonging to the summer generation, showed an increase in
ovary length from 0.25 mm (stage 2) to 1 mm (stage 4) in only 3 days;
during this period the diameter of the oocytcs increased from 30 to
160 p though the total length of the animal remained unchanged.
There might be a connexion between the intensity of predation and
the stage of sexual maturity during a 24-11 cycle. It seems that S. setosa
actually feeds more actively during the night and it is at this same time
that the oocytes grow more speedily (Dallot, personal communication).
These observations show that the fecundity of S. setosa is more
considerable than had been concluded from observations on the animals
in nature. There remain, says Dallot, many questions still to be
answered: the total length of the period of reproduction, whether the
stock of oocytes which undergoes maturation is limited or not, what are
the influences of the quantity of food available, of temperature and of
the other hydrological and biological factors on the reproductive
processes.
As regards the number of spawnings and the duration of the
spawning period, reference should be made to the review by Alvarifio
(1965), who has gathered together data from many observations.