Rissoids

Rissoids are small marine gastropod mollusks that belong to the family Rissoidae, within the superfamily Rissoidea. They include numerous genera, with Rissoa being among the best known. Commonly referred to as rissoids or rissoid snails, these mollusks occur worldwide, from shallow coastal waters to deeper continental shelf habitats. Most species are micromollusks, typically less than 5 mm in adult size.

Rissoid shells are usually minute, with high-spired, elongated shapes ranging from conical to ovoid. The surface often shows fine sculpture, such as growth lines or delicate spiral ridges, though some species are nearly smooth. The aperture is generally rounded or oval, with a thickened outer lip and a short to moderate siphonal notch. The shell and operculum are calcified and small, reflecting their diminutive size.

Rissoids inhabit a variety of marine environments, including rocky shores, coral reefs, seagrass beds, algal mats, and sandy or muddy substrates. They feed mainly by grazing microalgae, biofilm, and detritus on surfaces. They are typically slow-moving and short-lived, contributing to the meiofaunal and microherbivorous components of coastal ecosystems.

Reproductive strategies vary; many rissoids have direct development or short larval stages, which limits dispersal compared with species with free-swimming larvae. With small size and high population turnover, rissoids can rapidly colonize available microhabitats.

The small, durable shells of rissoids fossilize well, and fossil rissoids are widely used in paleontological studies and biostratigraphy to interpret paleoenvironments and shifts in marine communities.