Why Homoscleromorph Sponges Have Ciliated Epithelia: Evidence for an Ancestral Role in Mucociliary Driven Particle Flux

TL;DR

Homoscleromorph sponges have ciliated epithelia that help move mucus and particles, suggesting this function is ancestral in animals.

Contribution

The study provides evidence that ciliated epithelia in Homoscleromorph sponges function in mucociliary transport, a trait likely lost in other sponge lineages.

Findings

Ciliary beat frequencies in Homoscleromorph sponges are similar to those in other invertebrates used for mucus transport.

Cilia on Homoscleromorph sponges are homogeneously oriented and move particles in a coordinated direction.

Nocodazole treatment reduced particle movement, confirming ciliary activity drives mucociliary transport.

Abstract

Cilia are found on the epithelia of almost all metazoans, so their absence from the epithelia of all but one class of Porifera is puzzling. Homoscleromorph sponges possess ciliated epithelia, but their function and evolutionary history within Porifera are unclear. We compared the ciliary beat frequencies (CBFs) of cilia on outer epithelia of the homoscleromorph sponge Oscarella sp. with those of other animals to suggest possible functions for the cilia. Settled Stage 4 buds, or juveniles, were found to have a higher CBF than free‐moving Stage 1 buds, and CBF was within the range of cilia that function in mucus transport in other aquatic invertebrates. Scanning Electron Microscopy (SEM) images of buds fixed with ruthenium red to detect the presence of mucus showed that mucus was associated with the cilia of the exopinacoderm and both SEM and immunofluorescence images revealed fields of homogeneously oriented cilia. Confocal imaging of fluorescent beads also showed that cilia beat in the same direction. Movement of beads was reduced by nocodazole treatment indicating that the movement of particles over the surface was caused by ciliary beat. These results suggest that cilia on the epithelia of Homoscleromorph sponges are involved in mucociliary‐driven particle flux, and may be used to clean the surface using mucus.

Epithelia are typically ciliated, except in sponges. Of all Porifera only Homoscleromorphs have motile cilia on their epithelia. Our data highlight the presence of cilia and mucociliary particle transport as a common feature of metazoa and a secondary loss in other sponge lineages.

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