The rate at which a small object sinks in water varies with the amount by which its weight exceeds that of the water it displaces, and inversely with the viscous forces between the surface of the object and the water. The viscous forces opposing the motion are approximately proportional to the surface area, and therefore, other things being equal, the greater the surface area the slower the sinking rate. Because very small objects have a large surface-to-volume ratio, they are likely to sink more slowly than larger particles of similar density. However, there is a tendency for marine micro-organisms to become aggregated with inorganic particles and tiny fragments of organic debris, into larger clumps, which may increase the sinking rate.
There are a number of structural features of planktonic organisms which increase their surface area and must certainly assist in keeping them afloat. The majority of planktonts are of small size, and therefore have a large surface-to-volume ratio. In many cases, modifications of the body surface increase its area with very little increase in weight. Comparable adaptations are found in wind-dispersed seeds and fruits of land plants, which clearly serve to keep them airborne. These modifications generally take two forms: a flattening of the body, or an expansion of the body surface into spines, bristles, knobs, wings, or fins. Many of these extensions of the surface must also have the effect of setting up water turbulence as the object moves, greatly increasing viscous drag.
Reference has already been made to the range of flattened or elaborately ornamented shapes that occur in diatoms. In dinoflagellates, also, the cell wall is in some cases prolonged into spines (Ceratium) or wings (Dinophysis, see Figure 2.5). In the zooplankton, flattened shapes are common, for example the pelagic polychaete Tomopteris (see Figure 2.14), the copepod Sapphirina and various larvae (phyllosoma of the crawfish Palinurus, cyphonautes of the seamat Membranipora). Arrays of spines and bristles also occur, for example some species of copepods Calocalanus and Oithona and many larvae (mitraria of the polychaete Owenia, crab zoeas). The pluteus larvae of echinoderms have long ciliated arms. The chaetognaths have flat fins, and in the polychaete Tomopteris, the parapodia form a series of flattened, wing-like appendages.
Warm water has a lower density and viscosity than cold, and therefore affords less buoyancy and less resistance to sinking. The plankton of warm water includes several species in which the bristles, wings and other flotation devices are exceptionally elaborate (for example Calocalanus spp., Ornithocercus). In temperate waters, there is a tendency in some species for spines to be relatively longer in the warm season than in the colder months, e.g. the diatom Rhizosolenia hebetata (Figure 4.6).
Reduction of the sinking rate is unlikely to be the only, or even the major, advantage gained from modifications which enlarge the surface area. In the phytoplankton, an extensive surface presumably facilitates the absorption of nutrients present only in very low concentration, and also favours light absorption. The shape and distribution of weight determine the orientation of a passively floating body, and it seems probable that diatoms float in a position which presents the maximum surface to light from above. Irregular shapes and long spines may function also as protective devices which small predators find awkward to grapple with. In certain species there are sexual differences in bristles and plumes which indicate some role in sexual display; for example, the long plume of the males of Calocalanus plumulosus.
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