2.5
General Features of Dinoflagellate Material
Collected by the "Anton Bruun" during the
International Indian Ocean Expedition
F. J. R. TAYLOR
I. Biology of Dinoflagellates and their Role in the Food Web
Dinoflagellates are a nutritionally diverse group and cannot be totally included with
the diatoms, coccolithophorids and blue-green algae as primary producers. Many are
photosynthetic autotrophs, although typically these have strong requirements for organic
micro-nutrients such as vitamin B12 (auxotrophs), a characteristic they share with many
planktonic diatoms. Secondly, some of the autotrophs are also capable of ingesting other
cells (BIECHELER, 1936), presumably as a supplement to their photosynthetic nutrition
(myxotrophs). This phenomenon appears to be relatively rare although the large ventral
opening in the thecae of the marine ceratia may be an adaptation for this phenomenon
(NORRIS, 1969) in addition to rarely observed sexual conjugation (VON STOSCH, 1964).
Whether or not this phenomenon is ecologically significant on a large scale has not
been accurately evaluated. Thirdly, a large number of dinoflagellates lack any photosynthetic ability and it is assumed that these are phagotrophic grazers. Although the
largest of these colourless dinoflagellates may regularly be observed to contain ingested
cells (e.g. Noctiluca scintillans), many of the dinoflagellates below 40 or 50 fl in size, such
as many neritic species of Peridinium, cannot be seen to contain ingested cells. Two
large, specialized vacuoles termed "pusules" are usually present in these smaller nonpigmented forms (and also in some pigmented species), typically one being much larger
than the other and sometimes occupying a great part of the epicone. The content of these
pusules is a fluid which is sometimes distinctly pinkish or violet in colour. The function of
the pusules is unknown at present but it is tempting to assign a nutritional role to them. It
seems possible that many of these small, colourless dinoflagellates are feeders on very
small particulate organic material, perhaps including bacteria, or even large organic
molecules. Whether or not the pusule participates, this type of nutrition would explain the
absence of observed food vacuoles containing recognizable ingested cells.
There is a further type of nutritional mechanism which, although usually considered
rare, may be less so among tropical dinoflagellates and may be significant in assessing the
food-web roles of these species. This is the formation of symbiotic consortia between
photosynthetic and non-photosynthetic organisms. It is now well known that
dinoflagellate autotrophs are among the commonest endosymbionts of marine
invertebrates, and particularly the coelenterates. Consequently their role in the
ecosystems of coral reefs is evident although not quantitatively assessed as yet. Much less
well known is a reverse situation found in the tropical plankton involving nonphotosynthetic dinoflagellates and blue-green algae. This seems to be particularly common
among the morphologically elaborate dinophysoid genera, Ornithocercus (Fig. 5 c),
General Features of Dinoflagellate Material
Collected by the "Anton Bruun" during the
International Indian Ocean Expedition
F. J. R. TAYLOR
I. Biology of Dinoflagellates and their Role in the Food Web
Dinoflagellates are a nutritionally diverse group and cannot be totally included with
the diatoms, coccolithophorids and blue-green algae as primary producers. Many are
photosynthetic autotrophs, although typically these have strong requirements for organic
micro-nutrients such as vitamin B12 (auxotrophs), a characteristic they share with many
planktonic diatoms. Secondly, some of the autotrophs are also capable of ingesting other
cells (BIECHELER, 1936), presumably as a supplement to their photosynthetic nutrition
(myxotrophs). This phenomenon appears to be relatively rare although the large ventral
opening in the thecae of the marine ceratia may be an adaptation for this phenomenon
(NORRIS, 1969) in addition to rarely observed sexual conjugation (VON STOSCH, 1964).
Whether or not this phenomenon is ecologically significant on a large scale has not
been accurately evaluated. Thirdly, a large number of dinoflagellates lack any photosynthetic ability and it is assumed that these are phagotrophic grazers. Although the
largest of these colourless dinoflagellates may regularly be observed to contain ingested
cells (e.g. Noctiluca scintillans), many of the dinoflagellates below 40 or 50 fl in size, such
as many neritic species of Peridinium, cannot be seen to contain ingested cells. Two
large, specialized vacuoles termed "pusules" are usually present in these smaller nonpigmented forms (and also in some pigmented species), typically one being much larger
than the other and sometimes occupying a great part of the epicone. The content of these
pusules is a fluid which is sometimes distinctly pinkish or violet in colour. The function of
the pusules is unknown at present but it is tempting to assign a nutritional role to them. It
seems possible that many of these small, colourless dinoflagellates are feeders on very
small particulate organic material, perhaps including bacteria, or even large organic
molecules. Whether or not the pusule participates, this type of nutrition would explain the
absence of observed food vacuoles containing recognizable ingested cells.
There is a further type of nutritional mechanism which, although usually considered
rare, may be less so among tropical dinoflagellates and may be significant in assessing the
food-web roles of these species. This is the formation of symbiotic consortia between
photosynthetic and non-photosynthetic organisms. It is now well known that
dinoflagellate autotrophs are among the commonest endosymbionts of marine
invertebrates, and particularly the coelenterates. Consequently their role in the
ecosystems of coral reefs is evident although not quantitatively assessed as yet. Much less
well known is a reverse situation found in the tropical plankton involving nonphotosynthetic dinoflagellates and blue-green algae. This seems to be particularly common
among the morphologically elaborate dinophysoid genera, Ornithocercus (Fig. 5 c),