How Opuntia Species Are Identified

Introduction

Opuntia polyacantha, NM
Opuntia polyacantha, New Mexico

The first iteration of Opuntia Web was built in 2005. Our scientists identified Opuntia species in the field. We visited the western states to study cacti in habitat. We also visited Florida, the southern Atlantic states, and the Midwest to observe cacti in those regions (e.g., O. macrarthra). We made a visit to Pennsylvania to verify the presence of O. humifusa. In addition to extensive field studies, we consulted herbarium specimens, old black and white photographs, compendia, the current literature, original citations and descriptions. 

Bentonii with Fruit
Opuntia bentonii with Fruit, Galveston, TX

We compared our field observations of opuntias with the information we found from these sources to identify the plants that we observed. Additionally, we consulted with other specialists and scientists on an ad hoc basis. Herein, we provide evidence to support our findings in the form of descriptions, published literature, photo-reproductions of herbarium samples, and in situ photographs. Opuntia Web does not provide new taxonomic decisions. Rather, it uses published descriptions of species and identifies the plants in the field. Taxonomy is a separate discipline from identification of plants with published materials. 

Our understanding of Opuntia continues to evolve, and there are many names available for some species. A single citation is used herein to provide an Opuntia species name. The precise citation chosen provides the earliest identifying name for the taxon as we treat it and a place for the reader to start if more information is desired. Tropicos is a good place to expand the search for alternate treatments.  

We anticipate that some names will change in the future. We also understand that currently undescribed taxa will be reported in the future. We will change our treatments accordingly as new Opuntia information is published. For instance, one of our editors (Nancy Hussey) recently co-published the description of O. diploursina with her colleagues.

Some species of Opuntia have been lost to science because they were assumed to be synonymous with other taxa. For instance, O. cespitosa was lost because it was assumed to be synonymous with O. humifusa. However, more recently, botanists have shown O. cespitosa to be separate and distinct from O. humifusa. Likewise O. mesacantha ssp. mesacantha and O. mesacantha ssp. lata were assumed to be synonymous with O. humifusa but are now known to be different. Thus, there are four species known were it was thought there was only one. 

What is a species?

Opuntia sp. aff. valida, AZ
Opuntia aff valida, AZ

Identification of an Opuntia species requires a working sense of what a species is. We use the following considerations as our guide, “clusters of monotypic or polytypic biological entities, identified using morphology, genetics, or molecular biology, forming groups that have few or no intermediates with other groups when in contact.” We also accept that Opuntia species are “groups of organisms that inherit characters from each other…and are a reproductive community and unit.” 

We know that future phylogenetic hypotheses supported by molecular comparisons and rigorous morphometric and ecological analyses (see the treatment of O. humifusa s.l.) will supplement our current observations and help further illuminate our  knowledge of opuntias. Field studies of Opuntia species will also be essential for our understanding and we will adapt and change this Website to reflect new findings. 

We consider varieties as populations within a species that differ from the type in some details and that blend into one another anywhere their distributions meet or overlap. It is often difficult to label an Opuntia type to a proper variety because, unlike hybrids, varietal intermediates may occur with regularity and may be only subtly different from the type. Our thought is that widespread and variable Opuntia species (e.g., O. phaeacantha or O. engelmannii) may contain undescribed varieties or even subspecies and species. For example O. gregoriana is now considered to be a form of O. engelmannii, but it was once treated as a separate species. However, perhaps it could be described as a variety? 

Opuntia wootonii flower
Opuntia wootonii flower, Belen, NM

Details

We consider gross morphological information of opuntias such as spine colors, sizes, numbers, and arrangements, etc; glochid arrangements, sizes, and colors; cladode shape, color, and overall aspect; areole details; fruit and seed characteristics, general plant appearance, size, and shape; etc. Even edaphics, elevation, geography, and climate are considered as we identify species. Sometimes Opuntia bloom season is considered. If observable, we also consider phenotypic clines, introgressions, and other types of data in making decisions about the limits of a taxon. Ploidies are determined by our Editor-in-Chief or are reported in the literature

Other data, such as phenology; pollination ecology; ecosystem position, and proposed phylogenies, etc., are mostly not available for opuntias at the specific level. However, where published such information is used to inform our treatments and descriptions of opuntias.

Because opuntias are notoriously plastic in gross morphology, we do not rely upon single plants, observations, or locations, etc., to inform our decisions. Rather, we observe multiple plants, in multiple locations, under different conditions, and over different seasons to make determinations.

Biology

Opuntia is a dynamic genus, and  hybridization, polyploidy, and introgression are important biological processes that affect opuntias as well as our understanding of them. 

Hybrids

Opuntia hybrids do occur, but they are not the norm even though they can be confusing. Given the abundance of Opuntia species, the sheer number of individual plants and their overlapping ranges, there are actually few hybrids observed in Nature. When they do occur they are generally found in hybrid zones or regions where two Opuntia species overlap. Even in such zones, Opuntia hybrids are the exception. Because they are exceptions, and may have unique features, hybrids are often prized in gardens. 

Polyploidy

Polypoidization occurs in Opuntia. The basic chromosome number for Opuntia is 11 (2N = 22) However, many Opuntia species are polyploid; they may be tetraploid (2N = 44), hexaploid (2N = 66), or even octaploid (2N = 88). Triploid (N = 3X) and pentaploid (N = 5X) plants are known, but their contributions to gene exchange are unknown. 

Opuntia phaeacantha
Opuntia phaeacantha

Polyploidy may mask entities that are apparently identical, but which actually have different ploidies. For instance, there are diploid, triploid, and tetraploid examples of O. pusilla. Similarly, there are diploid and tetraploid examples of O. humifusa s.l. Auto- and allopolypoidy occurs in Opuntia, and such rich genetic resources have created wonderfully varied species. 

Introgression

Introgression occurs in Opuntia species. Introgression is different from hybridization because it refers to the transfer of limited genetic information from one species to another through ancient hybridization and repeated backcrossing. Introgression begins with hybridization, but it is so much more. Hybridization may split two genomes equally, whereas introgression is generally the invasion of a few genes into one of the parents–leaving the species essentially the same as before. 

Introgression is a ubiquitous phenomenon in plants and animals and it contributes to adaptation. Modern humans are an example of an introgressed species because many of us carry some genes from Neanderthal humans. An example of introgression in Opuntia might be a pink-flowered population in a species that is generally yellow flowered.

Checklist of differences between O. lindheimeri and O. engelmannii

Opuntia lindheimeri
Opuntia lindheimeri

Introduction

Opuntia engelmannii and O. lindheimeri, are similar prickly pears, but there are also constant differences between these prickly pear cacti. We accept these opuntias as separate and distinct species. However, O. lindheimeri has been described as a variety of O. engelmannii by some authors. However you think of them (one Opuntia species or two), this checklist describes the differences.

See O. engelmannii

See O. lindheimeri

Details

Opuntia engelmannii
Opuntia engelmannii
  • The younger pads of O. lindheimeri are less woody and “softer” or “greener” in appearance. Thus, the two opuntias appear different in overall aspect. 
  • O. engelmannii spines range from white, to tan, to yellow shades and brownish. O. engelmannii spines are seldom a rich canary yellow or shiny yellow. O. lindheimeri spines are almost always shiny yellow, whereas O. engelmannii spines are chalky and if yellow are dull. 
  • The base of O. engelmannii spines may be brown, or blackish, or even dark red-brown. Occasionally, plants are found with spine bases that are not darker than the spines. In contrast, the base of O. lindheimeri spines are seldom true brown or black but may be red-brown, dark plum, rust-brown, pink shades, or dark yellow.
  • The annular markings of O. lindheimeri spines are more apparent than in O. engelmannii. The markings may not even be observable in O. engelmannii
  • O. lindheimeri fruits are narrower at the base or even vaguely pear-shaped; they tend to have a smooth surface and are typically red when newly ripe. O. engelmannii fruits are oval or roundish, darker (more purple or black-red) and less clear red in color, and with a more rugose (bumpy) surface. The umbilicus of O. engelmanni fruits is usually more depressed into the end of fresh fruit. 
  • O. engelmannii fruits are sweet and pleasant tasting, whereas those of O. lindheimeri are sour and not sweet. O.  lindheimeri fruits are sometimes almost noxious. 
  • Plants with orange or red flowers occur occasionally (not often) in O. lindheimeri but are rare in O. engelmannii. Yellow flowers of either species may change to orange shades late in the day of anthesis or on the day after. Thus, the flower colors of these two opuntias can be different. 
  • O. lindheimeri is primarily a plant of Texas east of the Pecos River (extending into LA). In contrast, O. engelmannii is typically found west of the Pecos River to CA. If west of the Pecos River, O. lindheimeri grows in more mesic conditions than O. englemannii such as valley bottoms or partially shaded slopes.