THE BIOLOGY OF CORAL REEFS
237
For the animal the significance of the association has in the past
been considered in relation to the possibility that the algae represent
(1) a possible source of food, (2) a significant source of oxygen, (3) a
significant contribution to excretion, (4) an aid to calcification.
It is no longer disputed that individual hermatypic coral colonies
can live in darkness and so effectively without zooxanthellae (Goreau,
1959a). There is no sound evidence that hermatypic corals are not
specialized carnivores (like possibly all coelenterates with the exception
of the Xeniidae and related Octocorallia mentioned below). While
there has been no recent work on the subject, it was shown (Yonge,
1940) that corals collect exclusively animal prey by means of (a)
tentacles, (b) tentacles aided by temporary reversal of cilia, (c) orally
directed cilia probably aided by extruded mesenterial filaments. There
has been no further work on digestive enzymes but all that is known
about those of coelenterates generally, notably sea anemones (Nicol,
1959), indicates action exclusively on animal matter.
(1) It is now known that in cultures algae excrete a wide range of
organic substances, much of it mucilaginous but including peptides and
glycolic acid possibly amounting in all to 50% of the carbon aoquired
in photosynthesis (Allen, 1956). The " juvenile " stages in the life
history of the zooxanthellae (Fig. 11A) have a thin wall which would
facilitate diffusion. They might even, it is suggested by Zahl and
McLaughlin (1959), be actually digested by the animal. This seems
improbable, the former the more likely although in this early stage
little material may be extruded. It has, however, been demonstrated by
Muscatine and Hand (1958), using the anemone Anthopleura elegantissima, that material can pass from the zooxanthellae into the animal
tissues. They exposed the anemones to sea water labelled with "CO,
and by the use of autoradiographs showed the initial fixation of the
labelled carbon in the zooxanthellae (confined to the endoderm) and its
later presence widely dispersed throughout the body. Passage of
material from the zooxanthellae is thus demonstrated but neither its
nature nor its significance. Goreau and Goreau (1960a) have taken the
matter further using the common Atlantic hermatypic corals Manicinu
areolata and Montastrea annuhris. They ran experiments both in
light and in darkness with corals containing zooxanthellae and with
other corals which had lost their zooxanthellae following 3 months
in complete darkness. After exposure for 50 hr to water containing
Na,Cl4O0,, they found no significant uptake of radio-carbon in the dark
or in the light where zooxanthellae were absent. Where these were
present the tissue background activity was about five times higher than
in corals kept in the dark. But they considered the level of the transfer
237
For the animal the significance of the association has in the past
been considered in relation to the possibility that the algae represent
(1) a possible source of food, (2) a significant source of oxygen, (3) a
significant contribution to excretion, (4) an aid to calcification.
It is no longer disputed that individual hermatypic coral colonies
can live in darkness and so effectively without zooxanthellae (Goreau,
1959a). There is no sound evidence that hermatypic corals are not
specialized carnivores (like possibly all coelenterates with the exception
of the Xeniidae and related Octocorallia mentioned below). While
there has been no recent work on the subject, it was shown (Yonge,
1940) that corals collect exclusively animal prey by means of (a)
tentacles, (b) tentacles aided by temporary reversal of cilia, (c) orally
directed cilia probably aided by extruded mesenterial filaments. There
has been no further work on digestive enzymes but all that is known
about those of coelenterates generally, notably sea anemones (Nicol,
1959), indicates action exclusively on animal matter.
(1) It is now known that in cultures algae excrete a wide range of
organic substances, much of it mucilaginous but including peptides and
glycolic acid possibly amounting in all to 50% of the carbon aoquired
in photosynthesis (Allen, 1956). The " juvenile " stages in the life
history of the zooxanthellae (Fig. 11A) have a thin wall which would
facilitate diffusion. They might even, it is suggested by Zahl and
McLaughlin (1959), be actually digested by the animal. This seems
improbable, the former the more likely although in this early stage
little material may be extruded. It has, however, been demonstrated by
Muscatine and Hand (1958), using the anemone Anthopleura elegantissima, that material can pass from the zooxanthellae into the animal
tissues. They exposed the anemones to sea water labelled with "CO,
and by the use of autoradiographs showed the initial fixation of the
labelled carbon in the zooxanthellae (confined to the endoderm) and its
later presence widely dispersed throughout the body. Passage of
material from the zooxanthellae is thus demonstrated but neither its
nature nor its significance. Goreau and Goreau (1960a) have taken the
matter further using the common Atlantic hermatypic corals Manicinu
areolata and Montastrea annuhris. They ran experiments both in
light and in darkness with corals containing zooxanthellae and with
other corals which had lost their zooxanthellae following 3 months
in complete darkness. After exposure for 50 hr to water containing
Na,Cl4O0,, they found no significant uptake of radio-carbon in the dark
or in the light where zooxanthellae were absent. Where these were
present the tissue background activity was about five times higher than
in corals kept in the dark. But they considered the level of the transfer