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Populations of the Sea
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Yellow-Green Algae

There is considerable disagreement as to the proper grouping and status of divisions within this heterogeneous assemblage of organisms, some of which, as indicated below, are animal in nature. As a matter of convenience in discussing the more important marine members, we shall here employ only names of more or less familiar usage in biology and oceanography. Many of the members included are classified as animals in zoological texts, but in consideration of their holophytic nature (faculties of photosynthesis) it is most convenient for oceanographic studies to include them a priori among the producers. For more detailed treatment of the systematics of the various divisions, the reader is referred to Fritsch (1935) and the relevant works included in the discussions under the separate groups.

In contrast to the algae previously discussed, the members of this assemblage of plants and plantlike animals are primarily floating forms and will be taken up in the order of their importance in the economy of the sea.

Diatoms. The plants here included are all microscopic in size, the larger species viewed individually appearing only as tiny points. Some earlier authors of marine botany included them with the brown algae. A comprehensive treatment of the group is given by Hustedt (1930). In structure they are unicellular, but individuals may form chains or groups of various types. Examples of types representing the common genera are

given in fig. 70. A characteristic feature of diatoms is the shell, or frustule, which is composed of translucent silica, and the variety of form and sculpturing shown by striae, points, and pits is truly remarkable. These shells are of considerable importance in siliceous sediments and have formed great fossil deposits known as diatomaceous earth.


Characteristic types of diatoms. a, Corethron; b, Nitzschia closterium; c, Planktoniella; e, Coscinodiscus; f, Fragilaria; g, Chaetoceros; h, Thalassiosira; i, Asterionella; j, Biddulphia; k, Ditylum, l, Thalassiothrix; m, Navicula; n, o, Rhizosolenia semispina, summer and winter forms.

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Since these plants as a group may be considered the most important in the economy of the sea, it is imperative that we treat them in considerable

detail in order to facilitate understanding of their mode of maintenance, numerical fluctuations, and requirements in the sea as set forth in the following chapters, in which organisms are considered in the light of ecological factors.

The shell structure of diatoms (fig. 71) may be likened to a box with a telescoping lid, because it consists of two nearly equal halves fitted one over the other. The pieces corresponding to the top and bottom of the box are known as the valves, and these are each joined by connecting bands that overlap and together form the girdle. The larger half of the shell is known as the epitheca, and the smaller half, which fits into it, as the hypotheca. The protoplasm lies wholly within the shell, but for exchange of metabolic products it is exposed by a slit (raphae) in the valve of some types and by small pores in others.


The gross structure of a simple diatom (Coscinodiscus). a, valvular view; b, girdle-view section of cell wall.

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Reproduction in diatoms. a,b, cell division; c, diminution of size resulting from cell division in three generations.

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Diatoms may possess only one or many chromatophores, which may vary in color from yellow to olive-green or brown. Authorities are in poor agreement as to the nature of the pigments present, but there is some indication that the common pigments are masked by the accessory brown pigment diatomin, which may be identical with fucoxanthin of the brown algae. An important product of assimilation is an oil that is frequently visible as droplets within the diatom.

Methods of Reproduction. The most common method of propagation among the diatoms is by simple cell division (fig. 72a). This method has a far-reaching effect on the population in two distinct ways. First, it is conducive to a rapid production of enormous numbers when

conditions for growth are favorable (p. 767). Second, the maximum size attainable by individuals in a portion of the population is constantly being reduced by each successive division. This reduction follows from the fact that as a result of division of the protoplast (living portion of the diatom) one of the new daughter cells retains the larger of the valves (epitheca), whereas the other daughter retains the smaller valve (hypotheca). During the process of division, new complementary valves are laid down, but in such a manner that they fit into the old valves of the parent; that is, they become hypothecae, and the hypotheca of the parent diatom must now be looked upon as the epitheca of the smaller of the two daughter diatoms formed. Thus, through many successive generations of uninterrupted cell division a marked reduction in width must occur in many individuals. This change is schematically illustrated for three generations in fig. 72b. It appears that this diminution of size can go on only to a certain point, when a return to maximum size must be accomplished by the formation of auxospores (fig. 73a). If auxospores are not formed at a certain minimum size, the decrease in size continues, with ultimate abnormalities and death. In auxospore formation the contents of the diminutive, rigid, siliceous shell escape from the parted valves inclosed in a distensible pectin membrane. It is then possible for them to grow to larger size with the formation of new full-sized valves. A number of variations occur in the method of auxospore formation, dependent upon the group. For example, several auxospores may result, and these may fuse with yet others. In general, however, some type of rejuvenation seems to take place. Auxospore formation has been shown in various species, but in nature the spores are found only in small numbers.


Reproduction in diatoms. a, auxospore formation in Thalassiosira aestivalis (after Gran and Angst); b, increase in cell size following auxospore formation in Melosira nummuloides (after Fritsch); c, resting spores in mother cells, Chaetoceros vanhurckii; d, resting spore of Chaetoceros diatema; e, resting spore of Chaetoceros radicans; f, microspores in Ditylum; g, microspores in Chaetoceros didymus (after Gran and Angst).

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Diatoms may also produce what are known as microspores (fig. 73b). These were early observed by Murray, Gran, and others. They consist of small protoplasmic spheres occupying the shell, and may escape as biciliated spores. The significance of these bodies is not fully known.

Resting spores of characteristic structure (fig. 73c) are also formed in most pelagic neritic species, especially of the centric types, by the cell contents becoming condensed and surrounded by a heavy, siliceous wall. They may be produced at the initial appearance of unfavorable living conditions, and may drift for some time within the old frustule or sink to the bottom to survive the unfavorable seasons of inadequate nutrients, cold, or of varying salinity so characteristic of many coastal areas. Gran (1912) has reported them from Arctic collections in which they were enclosed in ice.

Winter and summer forms of oceanic diatom species have been reported. These are cases of marked dimorphism in which the coarse winter forms have been looked upon as a means of survival from one favorable season to another. However, the dimorphism may be only an adjustment to changes of viscosity inherent with seasonal temperature changes.

Many diatoms grow normally on the bottom in the littoral zone, where they may or may not be attached by stalks or glide freely over the bottom. These benthic forms produce the heavily shelled types with most exquisite designs. Diatoms may also grow in profusion on other plants and animals. The littoral genus Licmophora frequently occurs on pelagic copepods, and the massed growth of Cocconeis ceticola flourishing on the skin of whales that have spent considerable time in the cold antarctic waters has, by its yellow color, given rise to the name “sulphur-bottom” for the blue whale.

Dinoflagellata. These are frequently spoken of collectively as the dinoflagellates (fig. 74). Space will not permit the amount of discussion that this diverse group of organisms requires for adequate treatment (see Kofoid and Swezy, 1921, Kofoid and Skogsberg, 1928, Fritsch, 1935). It is a group concerning which it is not easy to make generalizations without the danger of introducing errors. The members are of great importance in the economy of the sea. A large number are holophytic and rank second to the diatoms as producers in the marine plankton. They are therefore best studied with the phytoplankton. Others are holozoic or animal-like in nutritional requirements, ingesting particulate food and possessing other characteristics that place them clearly with the animals. Some are saprophytic, living upon dead organic matter. All are important as food to filter- and detritus-feeding animals.

Typically, the dinoflagellates are unicellular, some being armored with plates of cellulose, others unarmored or naked. All possess two flagella for locomotion, an important feature in the holophytic forms, for

they can thus, perhaps, adjust themselves in some degree to water strata most favorable with respect to light and dissolved plant nutrients. Many dinoflagellates are luminescent, being responsible for much of the brilliant luminescent display so characteristic of the sea.


Dinoflagellates and other phytoplankton organisms. a, Ceratium tripos; b, Dinophysis; c, Ornithocercus; d,e, Triposolenia, front and side views; f, Peridinium; g, Amphisolenia; h, Goniaulax; i, Ceratium fusus.

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Methods of Reproduction. Among the dinoflagellates, reproduction is accomplished mainly by processes of cell division, which in some instances result in a chain of individuals clinging loosely together. Temporary structural variations may normally occur in individual cells at opposite ends of the chain. The progressive size reduction that is

characteristic of one daughter cell resulting from simple division in the diatoms does not occur after these divisions.

Dinoflagellates are found in all seas, but the greatest development of species is met with in the warmer waters, where a number of very bizarre forms are to be found. Owing to the destructibility of their cellulose plates by bacteria and other agencies, they are not preserved in bottom deposits. Important genera are Ceratium, Peridinium, Dinophysis, Gonyaulax.

Phaeocystis. Phaeocystis is a brown, flagellated plant, neritic in habit, that forms colonies in gelatinous, lobed globules visible to the naked eye. The large numbers produced may at times render the surface water quite brown and become a serious cause of clogging in silk plankton nets. Reproduction is accomplished by formation of flagellated spores that escape from the colonies.

Coccolithophoridae. Among the smallest (5 to 20 microns) of autotropic organisms of the sea are the biflagellated (some marine forms are not flagellated) forms of this group (fig. 74). Usually they are not caught by the ordinary net, through the meshes of which they readily escape, and when caught special care must be taken that their calcareous protecting armor is not dissolved by the preservative, leaving only an indefinable mass. The soft parts are shielded by tiny calcified circular plates or shields of various design and projections called coccoliths, or rhabdoliths. These shields had been found in enormous numbers in marine bottom deposits before the organisms of which they are a part were discovered by the Challenger and identified from their living habitat in the plankton, where they were found entangled in protoplasmic strands of pelagic protozoa or in the stomachs of salps and pteropods. Typically, the coccolithophoridae belong to the open sea, but they may occasionally reproduce in large numbers in coastal waters; at one time, according to Gran (1912) numbers of 5 to 6 million per liter gave the waters of Oslo Fjord a milky appearance. Some also occur in fresh water.

Though minute in size, they are of great importance as food to filter-feeding organisms, and also as contributors to calcareous bottom sediments. They occur in geological formations dating from the Cambrian period. Common genera among these organisms are Coccolithus, Pontasphaera, and Rhabdosphaera.

Halosphaera. Halosphaera is a unicellular, microscopic plant of the order Heterococcales (fig. 74). Earlier authors have included it with the green algae. It occurs at times in vast numbers in the plankton, floating mostly near the surface. Halosphaera virides occurs over the whole Atlantic and is abundant both in the warmer waters of the Gulf Stream system and in high southerly latitudes, where the Discovery investigations in the Antarctic found it second in importance to the diatoms. Meringosphaera of this order also occurs in marine plankton.


According to Gran, Halosphaera is practically the only open-sea form in which the predominantly green color of land plants is to be found. Notwithstanding the vast numbers that are often found, it does not reproduce by the quick method of simple binary fission, as in diatoms, but, after having grown for some time to its maximum size, the cell contents are transformed into a large number of zoospores. These swimming spores escape and through some unknown method are transformed back into tiny globular forms that gradually increase to normal size by successively shedding their weakly silicified investing membranes. Resting spores may also be produced.

Silicoflagellates. These flagellate organisms (fig. 74) deserve mention only briefly, since they do not usually occur in sufficiently large numbers to enter materially into the economy of the sea. However, they are such persistent members of plankton communities from nearly all colder seas that their starlike, open, siliceous shells attract a good deal of interest. Many occur in bottom sediments, and their development is shown in fossil marine deposits. That they contribute at least in a small way to the food of animals is shown by their frequent occurrence in food vacuoles of tintinnids.

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