Introduction

Cochemiea (K. Brandegee) Walton (Cactaceae Juss., Cacteae Rchb.) is a small genus of five currently accepted species that occur in Mexico (e.g., ^{Vázquez-Sánchez et al., 2013}; ^{Hind, 2018}). This genus is morphologically characterized by cylindrical decumbent to prostrate stems and by a long red-scarlet zygomorphic perianth, presumably specialized for hummingbird pollination (^{Anderson, 2001}).

Although Cochemiea was originally proposed at subgenus level of Mammillaria Haw. by ^{Brandegee (1897)}, recent molecular data (^{Butterworth and Wallace, 2004}; ^{Hernández-Hernández et al., 2011}; ^{Vázquez-Sánchez et al., 2013}) supported the proposal by ^{Walton (1899)} to consider this taxon at generic level. Many authors subsequently accepted this treatment (e.g., ^{Britton and Rose, 1923}; ^{Backeberg and Knuth, 1935}; ^{Backeberg, 1966}; ^{Bravo-Hollis and Sánchez-Mejorada, 1991}; ^{Barthlott and Hunt, 1993}; ^{Guzmán et al., 1993}), whereas other botanists (e.g., ^{Schumann, 1899}; ^{Hunt, 1971}, ¹⁹⁸⁷, ²⁰⁰⁶; ^{Lüthy, 1995}; ^{Hernández and Gómez-Hinostroza, 2015}) still recognized Cochemiea within Mammillaria.

As part of the ongoing taxonomic studies in Mexican territory (e.g., ^{García-Morales et al., 2014a}, ^2014b, ^2019a, ^2019b), we realized that a population discovered by Thomas Linzen in 2012 in central Sinaloa, which was identified as Mammillaria sp., actually refers to a Cochemiea species. The plants cannot be morphologically ascribed to any of the known species of the genus, and we here propose to describe this population as a new species for science, including a diagnostic key of the known species of Cochemiea

Material and Methods

The work is based on both field surveys carried out in autumn 2018 and spring 2019 at the locality previously visited by Thomas Linzen in Sinaloa, Mexico, examination of specimens deposited at GBH, HFLA, ITCV, MEXU, NY, and UC (acronyms according to ^{Thiers, 2020+}), and analysis of relevant literature (^{Brandegee, 1897}; ^{Walton, 1899}; ^{Bravo-Hollis and Sánchez-Mejorada, 1991}; ^{Anderson, 2001}). Plants (both live and exsiccata) were examined using a stereomicroscope (Carl Zeiss Stemi DV4, Göttingen, Germany), whereas seeds were studied using a scanning electron microscope (SEM Phillips XL30 ESEM at 20 kV, Eindhoven, Netherlands). Spines and seeds were coated with gold before SEM observation. Its conservation status was assessed following the guidelines of ^{IUCN (2014)}. The Area of Occupancy (AOO) and Extent of Occurrence (EOO) were calculated based on the known collections of the new species by using the program GeoCAT (^{Bachman and Moat, 2012}). Coordinates are not included because of conservation risks.

Results

Cochemiea thomasii García-Mor., Rodr. González, J. García-Jim. & Iamonico, sp. nov. Fig. 1.

Figure 1: Cochemiea thomasii García-Mor., Rodr. González, J. García-Jim. & Iamonico. A. plant (scale bar=10 cm); B. spines (scale bar=1cm); C. flowers (scale bar=1 cm); D1. seed; D2. seed surface magnified (photos A-C by T. Linzen). 

TYPE: MEXICO. Sinaloa, municipality Cosalá, North Cosalá, 300 m, low decidous TRopical Forest, 15.III.2019, L. García-Morales and T. Linzen 6226 (holotype: ITCV!, isotypes: CIDIIR!, GBH!, HFLA!, IEB!).

Diagnosis: Cochemiea thomasii differs from C. halei (Brandegee) Walton by the hanging stems, the larger conical tubercles, less numerous and shorter radial spines, less numerous and shorter central spines, the ovoid fruits and the isolated continental distribution.

Body slender cylindrical, stems pendulous, sprouting from the base and later also from the body sides, rarely sprouting from the apices or nearly so when damaged, shoots erect at first, later on the soil or hanging, 3.5-5 cm diameter, up to 60 cm long or longer, older body parts with corky texture partly without spines, with watery juice; roots fibrous, spreading, tubercles conical, rounded at the apex, areolar groove absent, slightly tapered at the base, the sides somewhat less rounded, the areole tip slightly upwards, 11-13 mm wide and high, 9-10 mm long, green colored; areoles rounded, in young plants 2-3 mm diameter, with white wool, later naked; axils between the young tubercles, from which the flowers develop, with white wool dots, later naked; radial spines 10-15, relatively uniform, radially and horizontally radiating, stiffly acicular, white, partly with a short brown tip, 7-9 mm long; central spines 1(-2)-(3-)4, if 4 then nearly cross-shaped, or when 3, two pointing toward the apex and one deflected, this last being slightly longer, stiff needle-like, slighly thicker than the radial spines, 7-13 mm long, whitish, brown to black, darkening to apex, later gray, all straight; radial spines 10-15, acicular, slender, white with reddish tips, radiate around the areole, 5-10 mm long; flowers zygomorph-tubular, close to the apex, 30-42 × 12-15 mm at the apex, more or less apically campanulate, all flower parts scarlet red; sepals in three series, the upper lanceolate, 6-8, 20-25 × 5-7 mm, apex entire, irregularly rounded, incurved outwards; the middle segments 4-6, located at middle of the tube, 4-12 × 4-5 mm, incurved apically outwards, the lower segments squamiform, 2-5 mm long and wide, appressed to the tube; filaments 40-60, 25-32 mm long, scarlet red, protruding over the upper sepals and these in turn clearly surpassed by the pistil, anthers 1 mm long, 0.4 mm wide, dark red; stigma pale pink, pistil 28-38 long, 0.5 mm wide, stigma lobes lanceolate-oblong, 5-6, 1.0 mm × 0.4 mm, fimbriate, glutinose, scarlet red; ovary ovoid, 2.5-3 mm diameter, ovary walls 0-8-1 mm thick; fruit ovoid, dehiscent by a lateral slit, initially green, later reddish-brownish, juicy, 7-9 × 6-8 mm, dried perianth segments sometimes remaining attached; seed pear-shaped, 1.3 × 0.8 mm, 0.65 mm thick, black; hylum micropylar region subbasal, oval-shaped; testa with tabular-concave roundish to isodiametric cells whose sunken central area is roundish, the non-sunken peripheral wall portions are structured in a weakly wart-like manner, the anticline borders on them are barely recognizable, sunken in uneven honeycomb form.

Etymology: we dedicate this new species to our friend and colleague Thomas Linzen (Irxleben, Germany), discoverer of this interesting new species and great scholar of the genus Mammillaria and relatives.

Phenology: flowering in February-March; fruiting in July-August.

Distribution and habitat: Cochemiea thomasii is known from a single narrow location north of the town of Cosalá, Sinaloa, at elevations between 300 to 325 m, in gentle slopes near the transition of the Pacific Coastland into the Sierra Madre Occidental. According to ^{González-Elizondo et al. (2012)}, the habitat corresponds to deciduous tropical forest. The observed plants are sporadically distributed and they always grow on vertical rock walls. Most of the plants are inaccessible at a height of 5-10 m above ground. Older specimens are particularly noticeable by their hanging stems. The geographically closest representatives of the genus Cochemiea occur in Baja California, more than 300 km from this location. From Mammillaria halei Brandegee, the morphologically closest species, the geographic distance extends more than 550 km.

Conservation status: only one population (type locality) was found and a total of 150 individuals was counted. On the basis of the criteria B2a (geographic range) and C (small population) of the ^{IUCN (2014)}, we assessed this species within the categories CR (Critically Endangered, AOO is 4 km², whereas EOO is about 1 km²) and VU (Vulnerable, by counting less than 500 mature individuals). According to the ^{IUCN (2014)} guidelines, “In situations where the spread of plausible values ... qualifies a taxon for two or more categories of threat, the precautionary approach would recommend that the taxon be listed under the higher (more threatened) category”, and thus we here assess Cochemiea thomasii as Critically Endangered (CR).

Revised specimens of Cochemiea halei (Brandegee) Walton: MEXICO. Baja California Sur, Magdalena Island, I.1889, T. S. Brandegee s.n. (UC108174); loc. cit., III.1917, C. R. Orcutt 054 (NY00385890, NY00385891, NY1188383); loc. cit., 01.V.1924, C. R. Orcutt 054 (K000062947); Santa Margarita Island, 20.III.1911, J. N. Rose 16301 (NY03858753, US-00171189); Santa María bay, 18.III.1911, J. N. Rose 16275 (NY03858752, US-00171188); Mainland between Cd. Constitución and San Carlos, Ciudad Constitución, 42 km SW, IV.1983, N. P. Taylor 65 (K29047.722).

Taxonomic notes: Cochemiea thomasii is morphologically similar to C. halei, based on characters of tubercules and spines (Table 1). Molecular studies are in process to verify the relationships between C. thomasii and the remaining Cochemiea taxa (García-Morales et al., in prep.).

By the addition of our new species, Cochemiea now includes six species. A diagnostic key is proposed below.

Diagnostic key of Cochemiea species