FIG. 15
Some flagellates of the plankton. a–h, Dinoflagellates: a, Ceratium fusus (×180); b, C.macroceros (×200); c, Protoerythropsis vigilans (×320) (note clear spherical lens against dark eye-spot); d, Dinophysis acuta (×400); e, Peridinium granii (×360); f and g, P. ovatum (×320), side and top view; h, Polykrikos schwarzi (×250); i and j, the Silicoflagellate Distephanus speculum living and half of skeleton (×320); k, a very small part of the large gelatinous capsule formed by the tiny cells of Phaeocystis; l and m, Coccolithophores (×1000): Coccolithus huxleyei and Coccosphaera leptopora; n–r, some of the smallest flagellates (×1500): n, Dicrateria inornata; o, Hemiselmis rufescens; p, Isochrysis galbana; q, Pyramimonas grossii; r, Chromulina pleiades. Original drawings except c from Marshall (1925), h from Lebour (1925), m from Murray and Blackman (1898) and n to r from Parke (1949).
All the remainder of the planktonic plants belong to the big assemblage of organisms known as flagellates of which there are many different kinds. A selection of the commoner forms is shown in Fig. 15. They are all characterised by possessing at least one, and often two, of the motile whip-like processes termed flagella, with which they draw or propel themselves through the water and are thus able to keep up in the sunlit surface layers. These flagellates are claimed for study by both botanists and zoologists, for among them are indeed both plants and animals—and some which have the characters of both in one. Some possess green pigments allied to chlorophyll or even chorophyll itself, and so feed as true plants; others lack pigment and may feed either by absorbing organic substances through their surface or actually live as animals by capturing particulate food; yet again, others may combine the methods of plant and animal feeding. In this lowly group of organisms the animal kingdom has not yet become fully separated from the plant kingdom. However, most of the planktonic flagellates are in fact plants and most of them have a small red ‘eye-spot’ or stigma which is sensitive to light and so enables them to tell whether they are moving towards or away from the radiant energy necessary to build up their food. If you are able to obtain a plankton sample very rich in these small green flagellates you will be able to see how readily they are attracted upwards towards the light. Fill a tall narrow glass jar with the sample and cover the lower three-quarters with thick brown paper. Now if you leave it for half an hour in the full light of the window you will find on removing the paper that the top quarter of the jar is distinctly greener than the rest; the little flagellates from the whole jar have become concentrated in the sunlit zone. By standing a sample of sea water in the light you may be able to grow a more abundant culture of these little flagellates and so give a more striking demonstration of this experiment. There are some of the Dinoflagellates (members of the family Pouchetiidae) which have a much more elaborate light-sensory organ furnished with both a lens and a pigment-cup; indeed it might almost be called an eye. One of these is shown in Fig. 15c.
The Dinoflagellates are the most striking members of the phytoplankton after the diatoms and are usually present in large numbers; for a full account of them another excellent volume by Dr. Marie Lebour (1925) should be consulted. They have a cell wall made up of a number of plates of cellulose fitting together to form a mosaic and are characterised by possessing two flagella: one working transversely in a prominent groove which almost completely encircles the body like a girdle, the other projecting behind from out of a small longitudinal groove running backwards from the girdle. This latter groove is often protected by curtain-like membranes so that the flagellum may be withdrawn spirally into a sheath. Typically, as in Peridinium, they have a single spine pointing forward in front and two spines projecting backwards from the half of the body behind the girdle. The flagellum working in the groove sets them waltzing round as they are at the same time driven forwards by the other flagellum behind (Fig. 15); they screw themselves through the water. There are a great many species of Peridinium and closely allied genera with very much the same general appearance (Plate IIIb); one related genus, however, Ceratium, is most striking in having the spines drawn out into long horns, with the two posterior ones usually curving forwards to give the whole body the shape of a little anchor. Sometimes, particularly in late summer and autumn, plankton samples may be full of Ceratium tripos or perhaps another of the many species of the genus distinguished by only small differences (Plate Ib). Two species which are very common in our waters stand out in contrast to the rest: Ceratium furca (Plate 1) in which the posterior spines are rather short and point straight backwards, and C. fusus (Fig. 15) in which there is only one posterior spine, long and only very slightly curved, just like the anterior one. There is a remarkable range of colour in Ceratium from a bright green to a yellow-brown.
Dinoflagellates, like other flagellates, multiply by simple fission into two and, like diatoms, each daughter-cell retains one half of the old cell-wall and forms another half anew; but unlike the diatoms the new halves are not formed within the old cell wall and so there is no gradual diminution in size. Occasionally recently-divided individuals of Ceratium may remain adhering together to form little chains. The cell-wall grows in thickness but when the little plates of armour become too heavy they fall off to be replaced by new and extremely thin ones. The long horns of Ceratium are certainly organs to assist in suspension. Species in the warmer waters generally have much longer ones than those in colder waters. Warmer water is more fluid—or less viscid—than colder water, for the molecules move over one another more freely with greater heat-motion; in the tropics an object will sink twice as fast as will one of similar density and shape in the polar seas. In general plankton organisms, both plants and animals, are more spiny in the tropics to give them a greater surface resistance. It has been observed that species of Ceratium have the power of adjusting the length of their horns to the varying viscosity of the water; on being carried by currents into warmer water they grow longer horns, conversely if carried into a colder area they can shed parts of them. (Gran 1912).
Several species of Dinoflagellates can produce a brilliant phosphorescence. Many plankton animals are luminous and produce the sparks of light we often see in the water at night. In addition to such displays, however, we may also see a more general ghostly light or sometimes when out in a rowing boat our oar as it cuts the water may leave a trail of blue-green flame behind it; and even from the shore we may see the waves breaking in a flash of light. Such displays are caused by countless millions of dinoflagellates each glowing by an oxidation process as it is agitated in the water. Noctiluca is the most celebrated for this, but although a dinoflagellate it is curiously modified to be entirely animal in its mode of life and so will be described in a later chapter (see here); other members of the group, however, particularly Ceratium, give almost as good a display. Once on a fisheries research trawler, having stopped at night to make some observations in the Channel, I looked over the side to see a small shoal of fish, most likely mackerel, lit up by each individual being covered by a coat of fire; they were being chased this way and that by some much larger fish similarly aflame. On putting over a tow-net, which came up brilliantly illuminated, the sea was seen to be full of a very small Peridinium-like