Autor: R. Mottram

≪Crevice cactus≫, referring to the characteristic habitat of the genus, growing in crevices and cracks of rocks.
Monospecific genus of tiny plants, globose flattened not exceeding 0.5-1 cm in diameter in habitat (much more in cultivation, caespitose when grafted), with a napiform root. Areoles having long white trichomes (almost completely occulting the epidermis) and some straight spines. It is said that tiny leaves appear in newly germinating plants (but are they not actually the first tubercles?).
Flowers diurnal but remaining open at night, larger than the plant, funnel-shaped, numerous, appearing in the area of the apex, sulphur yellow, with outer tepals tinged brown, floral remains persistent, probably pollinated by small hymenoptera. The flowers last for a very long time (up to 11 days, also open during the night, in Mottram 2001), as the plant grows in an extremely arid area and pollinators, and therefore the potential for fecundation, are probably rare. Fruits angled, hollow, elongated and dry when ripe, pinkish, remaining on the plant for a long time (in culture).
Seeds black, shiny, pear-shaped, granular, with a sunken hilum. Anemophilous dispersal could be possible, but above all, myrmecophilous.
The monotypic genus Rimacactus grows endemically in northern Chile, between 400 m and 1000 m in altitude, in an extremely hostile desert environment, on a very reduced area (approximately 10 km2) and fragmented, small cryptic populations are found, between rocks, in crevices, together with Copiapoa tocopillana, Neoporteria iquiquensis, Eulychnia iquiquensis. This region receives very little rain, but benefits from the frequent coastal fogs (camanchaca).
Chile (Antofagasta).

Currently only one recognised species:
– Rimacactus laui* (Luthy) Mottram 2001

References: "TAXONOMY of the CACTACEAE" -  ISBN 978-84-617-3692-8 (Vol. 2)