THE BIOLOGY OF CORAL REEFS
239
This is in striking contrast to parallel experiments carried out over
much longer periods on hermatypic corals at Low Isles (Yonge and
Nicholls, 1931b). Colonies kept in the light and fed daily with plankton
lived normally for up to 228 days (when the experiment terminated).
Those starved in the light immediately began to extrude zooxanthellae.
There were clear signs of starvation after only 9 days, the tissues
obviously retreating. This was most clearly shown in Fungia danai
where after 73 days the greater part of the upper surface of the skeleton
was exposed, the “ mouth ” having about half the diameter of the disc.
Colonies fed in the dark remained perfectly healthy for up to 228 days
but grew paler with the continuous loss of zooxanthellae, starved for
lack of light. Other colonies starved in the dark appeared much as those
starved in the light except that more zooxanthellae were extruded and
these were all obviously dead. More recently Goreau (1959a, 1961a) has
made a regular practice of ridding hermatypic corals such aa Manicina
areolda of their zooxanthellae by keeping them for appropriate periods
in darkness and without harm to the animals.
Thus in the hermatypic corals it is food, usually in the form of zooplankton, which ensures survival. Light is less relevant. In the Xeniidae,
on the other hand, owing to needs of the contained zooxantheUae,
light is essential but the presence or absence of zooplankton is immaterial.
In neither Scleractinia nor Alcyonacea, it may be noted, can the
a l p survive for long in the dark. They do not have the capacity
possessed by the zoochlorellae in Paramecium burwrk and Chlorohydra
of living chemotrophically at the expense of the host animal (see Yonge
(1944) for references). Even when cultured, Symbiodinium microadriuticum shows no capacity for chemotrophy (McLaughlin and Zahl,
1959). Droop (1963) notes that “ poor permeability-one of the factora
responsible for obligate phototrophy-could be an advantage for life
in a medium as rich in possibly toxic substrates as the interior of an
animal cell.”
The Alcyonacea (possibly also the Zoenthidea, according to Goreau
and Goreau (1960b) ), do appear to be more omnivorous than other
coelenterates. In tropical, although not in temperate, species there is a
range of feeding habit correlated in part with the presence of zooxanthellee. The temperate Alcyonium digitatum appears to be a typical
coelenterate in its feeding reactions (Romhdy, 1962) although it
possibly also accepts phytoplankton (Roushdy and Hansen, 1960).
Nothing definite is known about the feeding habits of the numerom
tropical species of Alcyonium but, since the work of Pratt (1905), it haa
been known that the ventral (i.e. digestive) mesenterial filaments of
239
This is in striking contrast to parallel experiments carried out over
much longer periods on hermatypic corals at Low Isles (Yonge and
Nicholls, 1931b). Colonies kept in the light and fed daily with plankton
lived normally for up to 228 days (when the experiment terminated).
Those starved in the light immediately began to extrude zooxanthellae.
There were clear signs of starvation after only 9 days, the tissues
obviously retreating. This was most clearly shown in Fungia danai
where after 73 days the greater part of the upper surface of the skeleton
was exposed, the “ mouth ” having about half the diameter of the disc.
Colonies fed in the dark remained perfectly healthy for up to 228 days
but grew paler with the continuous loss of zooxanthellae, starved for
lack of light. Other colonies starved in the dark appeared much as those
starved in the light except that more zooxanthellae were extruded and
these were all obviously dead. More recently Goreau (1959a, 1961a) has
made a regular practice of ridding hermatypic corals such aa Manicina
areolda of their zooxanthellae by keeping them for appropriate periods
in darkness and without harm to the animals.
Thus in the hermatypic corals it is food, usually in the form of zooplankton, which ensures survival. Light is less relevant. In the Xeniidae,
on the other hand, owing to needs of the contained zooxantheUae,
light is essential but the presence or absence of zooplankton is immaterial.
In neither Scleractinia nor Alcyonacea, it may be noted, can the
a l p survive for long in the dark. They do not have the capacity
possessed by the zoochlorellae in Paramecium burwrk and Chlorohydra
of living chemotrophically at the expense of the host animal (see Yonge
(1944) for references). Even when cultured, Symbiodinium microadriuticum shows no capacity for chemotrophy (McLaughlin and Zahl,
1959). Droop (1963) notes that “ poor permeability-one of the factora
responsible for obligate phototrophy-could be an advantage for life
in a medium as rich in possibly toxic substrates as the interior of an
animal cell.”
The Alcyonacea (possibly also the Zoenthidea, according to Goreau
and Goreau (1960b) ), do appear to be more omnivorous than other
coelenterates. In tropical, although not in temperate, species there is a
range of feeding habit correlated in part with the presence of zooxanthellee. The temperate Alcyonium digitatum appears to be a typical
coelenterate in its feeding reactions (Romhdy, 1962) although it
possibly also accepts phytoplankton (Roushdy and Hansen, 1960).
Nothing definite is known about the feeding habits of the numerom
tropical species of Alcyonium but, since the work of Pratt (1905), it haa
been known that the ventral (i.e. digestive) mesenterial filaments of