THE BIOLOGY OF ASCIDIANS
7
(ii) Plankton samples give direct evidence of breeding, but in
practice the method seldom has been used, owing to the difficulty of
identifying ascidian larvae and to their brief appearance in the plankton,
Brewin (1946), Lutzen (1960) and Dybern (1965) are amongst the few
who have used plankton samples in this way, and the value of this
approach is illustrated in Fig. 1 which clearly shows the breeding season
of a population of Ciona intestinalis in the Gullmar Fjord, Sweden
(Dybern, 1965).
1 9 5 9
1 9 6 0
1 9 6 1
FIG. 1. Seasonal occurrence of larvae of Ciona inhfinali.9 a t Stromnara, Sweden, as
shown by the number of larvae per plankton scrmple (redrawn after Dybern, 1965).
(iii) The commonest approach has been that used in fouling studies ;
test panels are placed in the sea at intervals and examined periodically
for the presence of attached animals. Owing to the difficulty of identifying very young specimens, however, species are often recorded only
some time after settlement, and in areas where growth is slow the
breeding season may be considerably longer than that recorded.
Another source of error arises from the requirement of certain species
for a clean surface and others for an already fouled surface, before
larval attachment will take place (Scheer, 1945; Goodbody, 1962), and
the consequent doubt whether the absence of a species from test panels
may have resulted from the unsuitable condition of the surface rather
than from the absence of breeding. Any heavy mortality amongst the
very young settled ascidians may also conceal the occurrence of
breeding.
(iv) Studies of size-distributions in natural populations may indicate
the breeding period, by showing the appearance of new generations
through the identification of peaks in the histograms (Allen, 1953;
7
(ii) Plankton samples give direct evidence of breeding, but in
practice the method seldom has been used, owing to the difficulty of
identifying ascidian larvae and to their brief appearance in the plankton,
Brewin (1946), Lutzen (1960) and Dybern (1965) are amongst the few
who have used plankton samples in this way, and the value of this
approach is illustrated in Fig. 1 which clearly shows the breeding season
of a population of Ciona intestinalis in the Gullmar Fjord, Sweden
(Dybern, 1965).
1 9 5 9
1 9 6 0
1 9 6 1
FIG. 1. Seasonal occurrence of larvae of Ciona inhfinali.9 a t Stromnara, Sweden, as
shown by the number of larvae per plankton scrmple (redrawn after Dybern, 1965).
(iii) The commonest approach has been that used in fouling studies ;
test panels are placed in the sea at intervals and examined periodically
for the presence of attached animals. Owing to the difficulty of identifying very young specimens, however, species are often recorded only
some time after settlement, and in areas where growth is slow the
breeding season may be considerably longer than that recorded.
Another source of error arises from the requirement of certain species
for a clean surface and others for an already fouled surface, before
larval attachment will take place (Scheer, 1945; Goodbody, 1962), and
the consequent doubt whether the absence of a species from test panels
may have resulted from the unsuitable condition of the surface rather
than from the absence of breeding. Any heavy mortality amongst the
very young settled ascidians may also conceal the occurrence of
breeding.
(iv) Studies of size-distributions in natural populations may indicate
the breeding period, by showing the appearance of new generations
through the identification of peaks in the histograms (Allen, 1953;