Form and Function of the Pneumatocysts of Marine Algae. II. Variations in Morphology and Resistance to Hydrostatic Pressure

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Abstract
The size, shape and wall thickness of mature pneumatocysts varied with habitat in 11 spp. of brown algae. In 4 Carpophyllum spp. [C. maschalocarpum, C. plumosum, C. flexuosum and C. angustifolium] size apparently increases with wave exposure but a similar trend was not found in other species [Cystophor torulosa, C. retroflexa, Sargassum undulatum, Macrocystis pyrifera, Marginariella boryana, M. urvilliana]. In a sample of all species taken from the same shore level, mean outer diameter [(W + L)/2] ranged from 4.8-55.1 cm; the ratio of wall thickness (T) to (W + L)/2 only varied from 3.8 to 7.9. This indicates a close relationship between T and (W + L)/2. In all species there was an increase in 2T/(W + L) with depth, and in some species there was also a change in shape to more spherical proportions. These morphometric relationships are discussed in reference to the design criteria for fabricated spherical vessels. The hypotheses that increased wall thickness relative to size and/or increased sphericality provide protection against implosion were verified by laboratory experiments. The actual behavior of pneumatocysts under hydrostatic pressures in the field was also examined and characteristic features of the response and the factors which determine the critical pressure for collapse (failure) are discussed. Young pneumatocysts, although having generally lower internal gas pressures, are not more susceptible to implosion.sbd.this is attributed to the protection afforded by their higher ratio of 2T/(W + L). Density of the mature pneumatocysts ranges from 0.355-0.521 g cm-3 as a result of changes in 2T/(W + L) and the varying contributions of stalks or mucronate projections. Density of the remaining parts is related to water content but is only slightly denser than seawater. Density of intact plants ranges from 0.716-1.147 g cm-3 with some tendency to vary with species, maturity, depth and wave exposure. A flexible life-form has adaptive significance for shallow water plants in that it permits compliance.sbd.thus reducing drag in strong wave action and collapses during emergence and thereby reducing water loss. The pneumatocysts, which represent a minimum expenditure of biomass (some 3-10% of the total dry wt), are thought to overcome some inherent disadvantages of such flexibility such as mutual shading and tangling or mechanical abrasion. Some aspects of the initiation of pneumatocysts are discussed in relation to these postulated functions.