MORPHOMETRIC AND FUNCTIONAL ANALYSIS OF THE GREEN ALGA CHAETOCLADUS (DASYCLADALES)

The relatively simple thallus of the noncalcified dasycladalean alga Chaetocladus comprises an upright cylindrical stipe (main axis) surrounded by cylindrical appendages (laterals) arrayed in whorls. Morphometric analysis of six Chaetocladus species from Ordovician through Devonian strata reveals no straightforward pair-wise correlation between primary thallus dimensions such as lateral length and main axis length, lateral radius and main axis radius, lateral length and radius, and main axis length and radius. Surprisingly, however, a strong correlation was discovered between lateral surface area (SA) and main axis SA. Such a correlation suggests not only that a functional relationship regulated by surface area exists between the laterals and main axis, but that this relationship is optimized at a particular ratio of lateral SA to main axis SA. To explore this hypothesis, functional analysis of the dasyclad thallus was conducted using a biophysical model that simulated growth over time as a function of surface-area-regulated nutrient uptake, an application for which Chaetocladus is well suited owing to its relatively simple thallus morphology. In the model, growth was allowed to proceed in various ways by changing the specifications of the input parameters. The results indicate that a clear and robust optimum exists at a particular ratio of lateral-to-main axis SA. In addition, the ratio identified is consistent with that observed among actual Chaetocladus taxa, thereby supporting the inference that the ratio of lateral-to-main axis surface area observed among these taxa is finely tuned to maximize growth rate in the context of this body plan.