Form resistance

Many plankters are markedly subspherical in shape and theory suggests that, in a majority of instances, the departure results in the organism having a slower passive rate of sinking (or floating) than the sphere of equivalent volume and overall density. As has been suggested earlier, there have really been few attempts to verify that this is true for a majority of species, and then mainly through resort to empirical evaluation of the coefficient of form resistance, Even where significant form resistance is established experimentally (see entries in Table 2.5), it does not prove distortion to be necessarily adaptive in the context of floating and sinking. Nevertheless, the experimental demonstrations of the impact on sinking rate made by the presence of horns or spines, cell elongation in one (or possibly two) axes and the creation of secondary shapes by coenobial formations of chains, filaments and spirals make a fascinating study. In the end, they may provide the key to how larger plankters actually do maximise their suspension opportunities.

Protuberances and spines

The value of distortions to staying in suspension goes back a long way in planktology, certainly to Gran's (1912) interpretation, quoted by Hardy (1964), of a verifiable tendency for Ceratium species of less viscous tropical seas to have longer and, often, more branched horns than the species typical of colder, high-latitude seas. Yet it is only relatively recently that effects were quantitatively demonstrated. Smayda and Boleyn (1966a) investigated several aspects of the variability in sinking rate in the marine diatom Rhizosolenia setigera, including the fact that spineless pre-auxospore cells settle significantly faster than the spined vegetative cells that follow auxospore 'germination' (see p. 64). The spines that occur on the end cells of four-celled coenobia of the freshwater chlorophyte Scenedesmus quadricauda are said to reduce the sinking rate relative to a spineless strain, although there is a possibility that there was a density difference between the two forms (Conway and Trainor, 1972). However, the investigations of the role of the 70-nm chitin fibres that adorn the frustules of Thalassiosira weissflogii (formerly T. fluviatilis) in slowing the sinking speed of cells have been carefully evaluated by Walsby and Xypolyta (1977). Cells from which the fibres had been removed with a fungal chitinase sank almost twice as fast as those not so treated, even though the density of the fibres (1495 kg m-3) was rather greater than that of the fibreless cells. The overall volume of the untreated cells was also larger (1.9-fold) than that of the fibreless cells but the surface area was 2.8 times greater. Only the increased form resistance could have been responsible for the reduced sinking rate.

Chain formation

Joining two or more cells together obviously increases the volume of the settling particle in the same ratio. It also increases the surface area, but for the area of mutual contact between individual cells in the chain. Theory dictates that the chain must sink faster than the individual component cells - were sinking rate the only criterion, joining cells together could not be claimed to be an adaptation to suspension. On the other hand, as pointed out by Walsby and Reynolds (1980), if there is another constraint favouring larger size (say, resistance to grazing), it is equally clear that the linear arrangement preserves much more surface drag than a sphere of the same volume of the aggregate of cells. Hutchinson (1967) invoked the results of some experiments by Kunkel (1948), who had measured sinking rates of identical glass beads, either singly or cemented together in linear chains of one, two, three, four or eight. Hutchinson (1967) calculated the relative form resistance and fitted a linear plot against chain length with the equation:

where b is the number of beads. Superficially, this supported observations of Smayda and Boleyn (1965, 1966a, b) in Thalassiosira, Chaetoceros and other chain-forming marine diatoms that sinking

Table 2.5 Comparison of measured sinking rates (ws) of various freshwater plankters and the rates (ws calc) calculated from Stokes' equation for spheres of identical volume and density

Plankter3

Dynamic shape

ws (ßm s 1)

(ws) calc

(Pr

References

Chlorella vulgaris

± spherical

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