METHODS OF FLOTATION
It is clear that if plant life is to exist at all in the offshore waters of more than very moderate depths, it must in some manner be maintained in the upper layers where it can float freely suspended near the surface, for only there can sunlight penetrate sufficiently to meet the requirements of photosynthesis. Once a diatom or any other autotrophic organism requiring light has sunk below this zone, it is doomed to disintegration in the lower layers unless there be prevailing in the area some sort of vertical circulation which will bring it quickly back to the surface. This may occur in some few instances, but there is a strong tendency for water to remain stratified with the lighter layers on top until strong forces overcome this stability (p. 497).
Living protoplasm is heavier than pure water, its specific weight ranging from 1.02 to 1.06, and the shells are even heavier; hence, adaptations are required to prevent or retard sinking. The rate of sinking of a body heavier than water depends upon the ratio of surplus weight to friction. Friction in turn is determined mainly by the surface area. The simplest way to obtain a relatively large surface area is to reduce the absolute size. However, adaptations have gone far beyond this simple limitation to minute size. Special structures, shapes, or other means presently to be considered have evolved to enable the small plants and animals to remain in suspension and thus to populate the vast pelagial expanses of the sea that would otherwise be sterile. Doubtless, the most important adaptation to a pelagic existence in plants is a reduction of absolute size in order to increase the ratio of surface area to volume. In plants the advantage is twofold, for it facilitates not only flotation but also absorption of plant nutrients, which in the sea are in very dilute solutions at best. As a further response to the requirements of this passive floating existence, the plants in most instances also possess structures considered to be special adaptations which aid them in keeping in suspension sufficiently long to grow and reproduce. The characteristic morphological adaptations represent forms which are diametrically opposite to streamline.
Diatoms. Let us first consider the pelagic diatoms, since they are wholly unable by voluntary means to adjust themselves with respect to depth.
Bladder type, in which the cell is relatively large and the cell wall and protoplasm form a thin layer inside the shell or test, and the remainder of the cell is filled with a light cell fluid or sap. Examples are found in species of Coscinodiscus. The shape may approach that of a disc as in Planktoniella, which sinks in a zigzag course, covering a much greater distance than if it were to follow a direct line from the surface to the bottom.
The needle or hair type is long and slender and is typified by Rhizosolenia and Thalassiothrix, which sink slowly when suspended with the long axis horizontal to the pull of gravity but with increased rate when in a vertical position. The cells also may be curved or provided with ends that are beveled in such a manner that they are soon brought back into the horizontal position, once they have been displaced. In this way the sinking is accomplished in great wide circles. The same applies to Nitzschia serriata when in chains with overlapping ends.
The ribbon type is illustrated by Fragillaria and Climacodium. Here the cells are broad and flat and are attached to each other in chains in which the individual cells lie side by side in a single layer. They are not numerous in the sea.
The branched type is illustrated especially by the genus Chaetoceros, and by Corethron. Here a great many spines are grown as projections to resist sinking, and the cells are often in chains and are frequently spiraled for greater effect. They are extremely abundant in the sea.
All of the pelagic species are thin-shelled as compared with the bottom or littoral forms, and some may secrete a coating of light mucus to aid in tiding over unfavorable conditions. There are also summer and winter forms in some species. The summer forms usually have lighter shells, which is in keeping with the reduced viscosity of warmer water.
The specific gravity of diatoms may also be lessened by the presence of oil in the cells. Oil has been noted to accumulate in diatoms late in the vegetative period and, in addition to providing a food reserve, may also result in keeping them afloat while waiting return of conditions favorable for multiplication.
Many diatom species, though conforming in cell structure to either of the above types, also unite the separate cells into chains of various types. This is accomplished by siliceous cements or by mucous pads or threads (fig. 70).
Dinoflagellates. Unlike diatoms, the dinoflagellates are not strictly passive, for they are provided with two minute whiplike flagella which by almost constant undulating motion provide a means of feeble
In some dinoflagellates, for example, Triposolenia spp. and Amphisolenia spp. (fig. 74), an asymmetry is developed, resulting in a deflection of the ends of the antapical horns. During periods of rest or weak swimming, these deflected horns tend to orient the sinking body so that the long axis lies horizontal and thus provides the maximum surface and resistance against passive sinking. In Triposolenia, a descent of about ten times its body length is sufficient to swerve the organism slowly into this advantageous passive position. Here it remains at least momentarily and the frequent alternation of positions leads to a slow wavering descent.
In referring to the asymmetry discussed above, Kofoid (1906) remarks,
Had it appeared as one of the characters of a single species, or even of several, it would be dismissed as one of those usual chance vagaries of structure which so often crop out in the diagnostic features of a species. Its occurrence in the two genera named and suggestions of an analogous structural feature in some other genera of dinoflagellates is indicative of a more profound relationship to the welfare of the organisms in which it appears.