Filter-feeding, near-field flows, and the morphologies of colonial choanoflagellates

TL;DR

This study investigates how the morphology and behavior of choanoflagellates influence their nutrient uptake efficiency, revealing that swimming unicells optimize filter feeding in still environments, while sessile forms are favored in flow conditions.

Contribution

It provides a detailed analysis of how different choanoflagellate morphologies and behaviors affect nutrient uptake, linking physical form to ecological advantages in various flow conditions.

Findings

Swimming unicells maximize filter feeding in still water.

Sessile thecate cells are advantageous in large external flows.

Prey diffusion favors swimming unicells over other forms.

Abstract

Efficient uptake of nutrients from the environment is an important component in the fitness of all microorganisms, and its dependence on size may reveal clues to the origins of evolutionary transitions to multicellularity. Because potential benefits in uptake rates must be viewed in the context of other costs and benefits of size, such as varying predation rates and the increased metabolic costs associated with larger and more complex body plans, the uptake rate itself is not necessarily that which is optimized by evolution. Uptake rates can be strongly dependent on local organism geometry and its swimming speed, providing selective pressure for particular arrangements. Here we examine these issues for choanoflagellates, filter-feeding microorganisms that are the closest relatives of the animals. We explore the different morphological variations of the choanoflagellete $Salpingoeca~rosetta$, which can exist as a swimming cell, a sessile thecate cell, and as colonies of cells in various shapes. In the absence of other requirements and in a homogeneously nutritious environment, we find that the optimal strategy to maximize filter feeding by the collar of microvilli is to swim fast, which favours swimming unicells. In large external flows, the sessile thecate cell becomes advantageous. Effects of prey diffusion are discussed and also found to be to the advantage of the swimming unicell.