THE BIOLOGY OF ASOIDIANS
29
Oka and Usui (1 944) showed that colonies of Polycitor mutabilis Oka
divide repeatedly. Division takes place both by simple fission, and by
colonial budding in which zooids extruded from the parent colony give
rise to new colonies. Moreover, colonies of this species undergo cyclical
expansion and contraction. By considering all the daughter colonies
derived from one original colony as a composite unit Oka and Usui were
able to show that the area plotted on a logarithmic scale bears a linear
relationship to time (Fig. 13). Division of the colony also occurs in
Didemnum candidum Savigny (Carlisle, 1961) and Archidistoma aggregatum Garstang (Nakauchi, 1966a), and here, too, the area of all the
daughter colonies would have to be used in growth studies. Sabbadin
(1960) measured the growth of Botryllus schlosseri, not by the area of
colonies, but by the number of zooids, and found this to vary so widely
that in the fifteenth generation of blastozooids different colonies contained between 1 and 97 zooids. Growth is further complicated by
periodic rejuvenescence, with parts of the colony proliferating while
other parts degenerate. Earlier, it had been shown for the same species
that the zooids may double in number every two or three days and that a
fast-growing colony attains 1 000-2 000 zooids within a month of its
establishment (Grave, 1933).
Growth may be interrupted and later resumed, and Nakauchi
( 1966b) observed that colonies of Aplidium multiplicatum (Sluiter)
increased in size until, at a temperature of about 3OoC, breeding took
place. At this time growth of the colony ceased, and the zooids regressed
and divided ; only later, when the sea temperature had fallen to 25OC
did the buds develop into new zooids. Then active life and growth of
the colonies were resumed, followed by further breeding. It is not clear
whether interruption of growth was associated primarily with breeding
or with the high temperature.
The involved questions of the physiological control and mechanics
of budding are beyond the scope of this review and have been discussed
by Berrill (1951), but it may be stated that in general " the phases of
maximum sexual and asexual reproduction alternate " (Berrill, 1935b),
with the sexual process confined to summer. After the breeding season
the zooids usually undergo a process of budding and the colonies may
pass the winter in a relatively inactive condition, in some cases being
reduced to masses of dormant buds. Temperature affects the balance
between regression of zooids and subsequent differentiation into new
zooids, as Barth and Barth (1 966) showed in Perophora viridis Verrill.
It appears, however, that in some species low temperature merely
retards recovery of the colony, and Vernay (1955) found that the buds
of Synoicum argue slowly develop into functional zooids throughout the
29
Oka and Usui (1 944) showed that colonies of Polycitor mutabilis Oka
divide repeatedly. Division takes place both by simple fission, and by
colonial budding in which zooids extruded from the parent colony give
rise to new colonies. Moreover, colonies of this species undergo cyclical
expansion and contraction. By considering all the daughter colonies
derived from one original colony as a composite unit Oka and Usui were
able to show that the area plotted on a logarithmic scale bears a linear
relationship to time (Fig. 13). Division of the colony also occurs in
Didemnum candidum Savigny (Carlisle, 1961) and Archidistoma aggregatum Garstang (Nakauchi, 1966a), and here, too, the area of all the
daughter colonies would have to be used in growth studies. Sabbadin
(1960) measured the growth of Botryllus schlosseri, not by the area of
colonies, but by the number of zooids, and found this to vary so widely
that in the fifteenth generation of blastozooids different colonies contained between 1 and 97 zooids. Growth is further complicated by
periodic rejuvenescence, with parts of the colony proliferating while
other parts degenerate. Earlier, it had been shown for the same species
that the zooids may double in number every two or three days and that a
fast-growing colony attains 1 000-2 000 zooids within a month of its
establishment (Grave, 1933).
Growth may be interrupted and later resumed, and Nakauchi
( 1966b) observed that colonies of Aplidium multiplicatum (Sluiter)
increased in size until, at a temperature of about 3OoC, breeding took
place. At this time growth of the colony ceased, and the zooids regressed
and divided ; only later, when the sea temperature had fallen to 25OC
did the buds develop into new zooids. Then active life and growth of
the colonies were resumed, followed by further breeding. It is not clear
whether interruption of growth was associated primarily with breeding
or with the high temperature.
The involved questions of the physiological control and mechanics
of budding are beyond the scope of this review and have been discussed
by Berrill (1951), but it may be stated that in general " the phases of
maximum sexual and asexual reproduction alternate " (Berrill, 1935b),
with the sexual process confined to summer. After the breeding season
the zooids usually undergo a process of budding and the colonies may
pass the winter in a relatively inactive condition, in some cases being
reduced to masses of dormant buds. Temperature affects the balance
between regression of zooids and subsequent differentiation into new
zooids, as Barth and Barth (1 966) showed in Perophora viridis Verrill.
It appears, however, that in some species low temperature merely
retards recovery of the colony, and Vernay (1955) found that the buds
of Synoicum argue slowly develop into functional zooids throughout the