Capsulus

Capsulus is a genus of small, sessile marine invertebrates distinguished by a cup-shaped calcareous or chitinous shell. The genus was established to classify several soft-bodied organisms found attached to rocky substrates in shallow coastal waters. The name derives from Latin capsula, meaning small box or capsule, a reference to the cup-like morphology.

Individual Capsulus measure roughly 0.3 to 2.0 millimeters in diameter. The tests form a hollow cup with a rim and a sealed base, enclosing a simple gut and a crown of ciliated ridges that aid feeding. The exterior may bear spines or attachment structures depending on species.

Capsulus species have a cosmopolitan distribution in temperate to tropical coastal zones, frequently inhabiting kelp forests, coral reefs, and patchy rocky substrates. Fossil records indicate a long-standing lineage with sporadic appearances in marine sediments since the Cretaceous.

Reproduction occurs both asexually by budding and sexually through free-swimming larvae. Adults are generally sessile, relying on water flow to deliver microorganisms and detritus to their feeding apparatus.

Capsulus cups provide microhabitats for epibiotic algae and tiny invertebrates and can influence local microhabitat structure. They are studied as models for biomineralization and the evolution of cup-shaped body plans in marine invertebrates. The genus is placed within a broader grouping of Lophotrochozoa in conventional classifications; however, taxonomic placement remains uncertain for some species due to limited molecular data.