The genus and its boundaries
Astrophytum Lem. belongs to Cactaceae. The name was published in 1839 and refers to the star-shaped outline of the ribs viewed from above. In the Plants of the World Online treatment adopted here, the genus comprises six species. These are A. asterias, A. capricorne, A. caput-medusae, A. coahuilense, A. myriostigma and A. ornatum. Within A. myriostigma, subspecies myriostigma and quadricostatum are recognised. Name status was checked on 8 October 2026. [2-WWW] [2-WWW] [2-WWW] [2-WWW] [2-WWW] [2-WWW] [2-WWW] [2-WWW] [2-WWW]
Most astrophytum plants form a single stem, initially globose or flattened and often elongated with age. The stem has distinct ribs bearing areoles. An areole is a short shoot characteristic of cacti. It can produce spines, flowers and branches. White dots between areoles form separate clusters of hairs called trichomes. Their appearance, density and distribution differ among species, populations and horticultural varieties. [Lux and Kopunec 1992] [Montanucci and Kleszewski 2019] [Montanucci and Kleszewski 2020] [Montanucci and Kleszewski 2021] [Montanucci 2024] [Montanucci 2009]
A. caput-medusae is distinguished by long, narrow tubercles in place of the typical ribbed body. Both reproductive features and plastid DNA studies support its inclusion in the genus. The two heavily spined species, A. capricorne and A. ornatum, also differ fundamentally. The former usually has flexible, twisted spines, the latter rigid ones. Adult A. asterias, A. coahuilense and A. myriostigma usually remain spineless. Seedlings of some of these species bear short juvenile spines. [Vázquez-Lobo et al. 2015] [Montanucci and Kleszewski 2019] [Montanucci and Kleszewski 2021] [Montanucci 2024] [Montanucci 2009]
Range and environment
The genus is centred in Mexico. A. asterias crosses the United States border and also grows in southern Texas. Astrophytum occupies various environments, from warm lowland scrub to rocky slopes and uplands. Limestone is common in the habitat, but rock chemistry, fine soil between stones and the water taken up by roots provide three different kinds of information. [2-WWW] [2-WWW] [2-WWW] [2-WWW] [2-WWW] [2-WWW] [2-WWW] [Carrillo-Angeles et al. 2016] [Montanucci 2024]
Carrillo-Angeles and colleagues compared four species' distributions across a broad elevation and climate gradient. Their material included 36 records of A. asterias, 61 of A. capricorne, 119 of A. myriostigma and 56 of A. ornatum. Across the full dataset, sites ranged from near sea level to 2700 m above sea level. Mean annual rainfall ranged from 165 to 934 mm. These are the bounds of the four-species dataset, not a range assigned to each species. [Carrillo-Angeles et al. 2016]
Microhabitat changes the conditions experienced by an individual plant. A shrub, rock crevice or slope inclination affects light, surface temperature and moisture persistence. Plants in open ground occur alongside sheltered individuals. Proportions depend on species and location. Here, a nurse plant means a neighbour that improves growing conditions, for example through partial shade. [Carrillo-Angeles et al. 2016] [Montanucci and Kleszewski 2020] [Montanucci 2024]
Flowers, fruits and seeds
Flowers open during the day. In most species they emerge near the body apex, while in A. caput-medusae they arise from areoles on elongated tubercles. Yellow shades predominate. Red or orange flower centres are common in A. asterias, A. capricorne and A. coahuilense. A. myriostigma and A. ornatum usually lack such a distinct throat. Colour should be assessed together with stem, fruit and seed structure because individuals vary. [Montanucci 2008] [Montanucci and Kleszewski 2019] [Montanucci and Kleszewski 2021] [Montanucci 2024] [Montanucci 2009] [Anonim b.d.e]
The way a fruit opens helps distinguish similar plants. In A. myriostigma, it opens at the apex. In A. coahuilense and A. capricorne, it usually splits near the base. Seed structure includes the shape of the coat and the depression around the attachment site to the fruit. These features have been used in genus classifications and dispersal studies. [Montanucci 2008] [Sánchez-Salas et al. 2012] [Montanucci and Kleszewski 2019] [Montanucci and Kleszewski 2021] [Montanucci 2024] [Montanucci 2009]
Relationships
The 2015 analysis covered three plastid DNA regions and confirmed the shared ancestry of the six Astrophytum species studied. One major lineage consisted of A. myriostigma and A. ornatum. The other comprised A. caput-medusae, A. coahuilense, A. asterias and A. capricorne. Within the second lineage, A. asterias and A. capricorne were the closest pair. A. coahuilense resembles myriostigma, but occupies a different position in this tree. [Vázquez-Lobo et al. 2015]
The plastid tree represents the history of the genome fragments studied. Morphological research in 2024 again raised the possibility of an ancient hybrid origin for A. coahuilense. The combination of features supports this hypothesis, but resolving it requires further data, especially from the nuclear genome. In the atlas, A. coahuilense remains a full species, following the adopted taxonomic database. [Vázquez-Lobo et al. 2015] [Montanucci 2024] [2-WWW]
From a label name to plant identification
A species name indicates botanical affiliation. A subspecies name refers to a recognised part of a species. A former botanical variety may now be a synonym, another name for the same taxon. A cultivar name describes plants selected and maintained in cultivation. Terms such as nudum, cristata, variegata and tricostatum are also sometimes used as descriptions of appearance. Their presence on a label alone does not determine botanical rank. [Szychowski b.d.a] [Montanucci and Kleszewski 2019] [Montanucci and Kleszewski 2020] [Montanucci and Kleszewski 2021] [2-WWW] [2-WWW] [2-WWW] [2-WWW] [2-WWW] [2-WWW] [2-WWW] [2-WWW]
Keep the label's original wording together with plant provenance. Alongside it, the atlas shows the accepted name and explains the older wording. This is particularly relevant to A. myriostigma, whose rib number and growth form change during development. Four ribs in a horticultural plant are insufficient to identify subsp. quadricostatum. [Montanucci and Kleszewski 2019] [Montanucci and Kleszewski 2021]