THE BIOLOGY OF ASCIDIANS
27
winter growth checks, and Huntsman (1921) found that C . macleayanum
Broderip and Sowerby continues t o grow for more than four years in
Alaskan waters, C . productum Stimpson for more than three years and
G. columbianum Huntsman for only one year.
Although species with a wide latitudinal range show great variation
in tho life cyclc, which can be compressed into a few months in the
warni part of tlic range, warm-water species of restricted distribution
tnny not havc such a rapid tuni-over of generations or such a short life.
Thus Ascidia nigra in Jamaica attains an age of 18-22 months (Goodbody, 1962). Styela plicata (Lesueur), a warni-water species of rather
wide distribution, appears t o live for 5-6 months in warm years and
7-8 months in cold years (Kanatani et al., 1964).
Est,imates of the life span of a species do not, of course, imply that
t he whole population dies more or less simultaneously, although
catmast,ropliic niortalitics have been recorded, as in three species in
,Jamaica which were decimated by a sudden influx of fresh water
following heavy rain (Goodbody, 196lc). Sudden high mortalities are
soinctimcs due to the death of the organisms on which the ascidians
liavc scttlcd, McDougall (1943) having noted cases of the rapid disappearance of Styela plicata and Atlolgula manhuttensis when their
supporting Bugula and Tubularia died. Amongst other causes of mass
niortality is low winter temperature, and the fate of Ciona intestinalis,
Jlolgula inanhattensis and Styela plicata, in the lagoon of Venice is
incntionccl clscwhere (Sabbadin, 1957).
In the absence of such dramatic events, much of the mortality can
be ascribed to the activity of predators and competitors, and ultimately
to t'lic onset- of senescence (Goodbody, 1962). Goodbody (1961a, 1963a)
was able to follow the course of events in populations of Ascidia nigra.
During the six wccks after settlement 97-08% of the young ascidians
usiiiilly died, with ext.rcmes of lOOyo and 82.5%. The different survival
rii.t,cs iirc related t.0 differences in the associated plant and animal
c~otnriiuiiit,ics, sonic members of which may have competed with the
young :iscidi;ins or physically displaced them. After this critical
cwly period the prcssurcs appear to be much less, and the individuals
which survive it have a good chance of completing their potential lifespan. Kot undil 18-22 months after settlemcnt is there another sharp
tlrcliiie in nuinbers, and a t this t>ime the animals probably die of old age
(Fig. 12).
In cornpound forins, as already noted, thc existence of a colony with
it,s niiiny zooitls undergoing t'hcir own cycles of growth, budding and
rcgenen~t~ion, ninkes the assessment of growth more difficult than in the
solitary form. The simplest approach relies on successive measure-
27
winter growth checks, and Huntsman (1921) found that C . macleayanum
Broderip and Sowerby continues t o grow for more than four years in
Alaskan waters, C . productum Stimpson for more than three years and
G. columbianum Huntsman for only one year.
Although species with a wide latitudinal range show great variation
in tho life cyclc, which can be compressed into a few months in the
warni part of tlic range, warm-water species of restricted distribution
tnny not havc such a rapid tuni-over of generations or such a short life.
Thus Ascidia nigra in Jamaica attains an age of 18-22 months (Goodbody, 1962). Styela plicata (Lesueur), a warni-water species of rather
wide distribution, appears t o live for 5-6 months in warm years and
7-8 months in cold years (Kanatani et al., 1964).
Est,imates of the life span of a species do not, of course, imply that
t he whole population dies more or less simultaneously, although
catmast,ropliic niortalitics have been recorded, as in three species in
,Jamaica which were decimated by a sudden influx of fresh water
following heavy rain (Goodbody, 196lc). Sudden high mortalities are
soinctimcs due to the death of the organisms on which the ascidians
liavc scttlcd, McDougall (1943) having noted cases of the rapid disappearance of Styela plicata and Atlolgula manhuttensis when their
supporting Bugula and Tubularia died. Amongst other causes of mass
niortality is low winter temperature, and the fate of Ciona intestinalis,
Jlolgula inanhattensis and Styela plicata, in the lagoon of Venice is
incntionccl clscwhere (Sabbadin, 1957).
In the absence of such dramatic events, much of the mortality can
be ascribed to the activity of predators and competitors, and ultimately
to t'lic onset- of senescence (Goodbody, 1962). Goodbody (1961a, 1963a)
was able to follow the course of events in populations of Ascidia nigra.
During the six wccks after settlement 97-08% of the young ascidians
usiiiilly died, with ext.rcmes of lOOyo and 82.5%. The different survival
rii.t,cs iirc related t.0 differences in the associated plant and animal
c~otnriiuiiit,ics, sonic members of which may have competed with the
young :iscidi;ins or physically displaced them. After this critical
cwly period the prcssurcs appear to be much less, and the individuals
which survive it have a good chance of completing their potential lifespan. Kot undil 18-22 months after settlemcnt is there another sharp
tlrcliiie in nuinbers, and a t this t>ime the animals probably die of old age
(Fig. 12).
In cornpound forins, as already noted, thc existence of a colony with
it,s niiiny zooitls undergoing t'hcir own cycles of growth, budding and
rcgenen~t~ion, ninkes the assessment of growth more difficult than in the
solitary form. The simplest approach relies on successive measure-