Erithalis includes perhaps 9 species of shrubs and small tress found on limestone in the Caribbean. This genus is characterized by regular branching; medium-sized to small, decussate, often thick-textured leaves with the venation generally not visible and the apex often rounded; distinctive tubular, persistent, shortly cuspidate stipules; terminal, cymose, often lax inflorescences, or the flowers 1-3 and subsessile in Erithalis orbiculatum; rather small, 4-8-merous flowers; shortly funneform to rotate or salverform, white to pink corollas with narrow lobes; stamens inserted at the base of the corolla with the exserted anthers that are held together with the stigmas in the corolla throat; small, subglobose, fleshy, drupaceous fruits, and numerous small, flattened, half-moon-shaped pyrenes that each contain 1 seed. The flowers were noted by Negrón-Ortiz (2005) to be fragrant and produce "abundant nectar", and two species were documented as pollinated by various different insects by Landry et al. (2014) and the remaining species have similar flowers and are assumed to have similar pollination biology.  Negrón-Ortiz (2005) noted that fruit color varies among plants in populations in some species in some areas, with the fruits variously blue-black or white flushed with pink. Erithalis fruticosa is the most widespread and commonly collected species, and is frequent along the coasts of many Caribbean islands. Erithalis species are usually thought of as characteristic coastal plants, but several species also range inland to middle elevations and Erithalis angustifolia is found in montane forests. 

Negrón-Ortiz (2005) presented a heroic taxonomic treatment of Erithalis, which involved study of an unusually large number of specimens, the problem of morpholgically very variable plants living in seasonal habitats, extensive field work, and the early days of molecular data (Negrón-Ortiz & Watson, 2002, 2003). Their molecular analyses were done to find the biogeography and diversification history of Erithalis, but also to try to eludicate species circumscriptions, which have long been problematic in this group. Their molecular analyses were not entirely conclusive but found two general clades, one with Erithalis fruticosa and Erithalis harrisii and the other with the remaining species they sampled; they were not able to include Erithalis acuminata. Their molecular analyses included multiple samples of several species, while later analyses (e.g., Pauydal et al. 2018) included only one sample per species and did lacked the problematic Erithalis odorifera so these provided little information about the separation of species in this group. Franck et al. (2017) presented a molecular analysis with multiple accessions of several Erithalis species, but found little resolution within their cladogram; their analysis was aimed at understanding the relationships of the Jamaica Erithalis orbiculata, which was grouped with two other Jamaican endemics. To date, molecular data has not been employed adequately to understand systematics within the genus, but hopefully future studies will provide more data. 

The morphological details given by Negrón-Ortiz (2005) are extensive but not totally clear. and her species separations seem to be well taken conceptually but the characters that separate these are subtle and some of her identifications of individual specimens have since been re-evaluated. Negrón-Ortiz described the ovary as "2-5 locules, 5-20-celled", with no anatomical illustrations or additional discussion or details; the meaning here, or distinction between "locules" and "cells" is not entirely clear. The citation of 2-locular ovaries may be connected with her documentation of two distinct stigma forms in the group, with bilobed stigmas in some species and 5-8-lobed stigmas in other species, or included based on earlier genus descriptions when Erithalis was confused with the bilocular Chiococca. Other authors have cited Erithalis as having 5-20 pyrenes and, presumably, ovary locules. Both the consistency and developmental sequence of the stigma forms and the arrangement of the ovaries probably deserves further evaluation. Negrón-Ortiz morphologically distinguished some species based on the development of foliaceous bracts, but that is a variable character in many Rubiaceaee and perhaps not entirely diagnostic at the species level. 

Several species of Erithalis are quite similar, especially as specimens with young fruit, and the species-level taxonomy of this group has varied among authors. In particular, authors have differed on the separation and identity of Erithalis odorifera, and the importance of floral fragrance as a species-level character. Adams (1975) included these plants within Erithalis fruticosa, while Howard (1989) separated these and noted that they are distinct in flower in the field; Howard gave distinct flower sizes and ovary characters for these species in his region. Negrón-Ortiz & Watson (2002) found plants from Puerto Rico and the Lesser Antilles that they identified as Erithalis odorifera mostly separated from Erithalis fruticosa in their molecular analysis, and Negrón-Ortiz (2005) separated these by overlapping anther and leaf size, supposedly distinct inflorescence sizes (8+ cm long vs. less than 8 cm), and the degree of development of foliaceous bracts. Her circumscription of Erithalis fruticosa here, however, included morphologically distinct plants now excluded from the species as Chiococca insularis. See the web page for Erithalis fruticosa for more detailed discussion of its separation from Erithalis odorifera here.  

Erithalis is similar in aspect and many character to Chiococca, and these have been confused especially until recently when Paudyal et al. (2018) clarified the circumscription of Chiococca with their separation of Ramonadoxa and expansion of Salzmannia. Paudyal et al.'s molecular analysis recovered a monophyletic genus Erithalis that was sister to a clade with Chiococca, Salzmannia, Ramonadoxa, and Scolosanthus. They regarded Erithalis as clearly delimited and related to these other genera, and did not present any more discussion of it. Chiococca differs in its 2-locular ovaries, and many species differ in their spongy white fruits. 

Erithalis is unusual in Rubiaceae in its corollas with 4-10 thinly imbricated lobes, multilocular ovary, and similar drupaceous fruits with numerous separate, flattened pyrenes. The name Erithalis was historically used for some species of Timonius, which is a distinct geenus found in Asia and the Pacific; Timonius shares this unusual combination of features, but differs from Erithalis in its lineolate higher-order venation, unisexual flowers on dioecious plants, and stamens inserted in the corolla throat. Some authors have considered Forster's use of the name Erithalis  to be intended as the description of a new genus that accidentally had the same name as Browne's, but Forster's protologue seems to only intend to describe some new species in Browne's established genus. 

Author: C.M. Taylor 
The content of this web page was last revised on 21 September 2021.
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.shtml

• Negrón-Ortiz, V. & L. E. Watson. 2002. Molecular phylogeny and biogeography of Erithalis (Rubiaceae), an endemic of the Caribbean basin. Pl. Syst. Evol. 234: 71–83.
• Negrón-Ortiz, V. & L. E. Watson. 2003. Hypotheses for the colonization of the Caribbean Basin by two genera of the Rubiaceae: Erithalis and Ernodea. Syst. Bot. 28(2): 442–451.
• Negrón-Ortiz, V. 2005. Taxonomic revision of the neotropical genus Erithalis (Rubiaceae: Chiococceae). Sida 21(3): 1565–1598.
• Franck, A. R., Lewis, A. Oberli, A. Haynes-Sutton, P.E. Rose & K. C. St. E. Campbell. 2017. The Jamaican endemic Exostema orbiculatum belongs to Erithalis (Rubiaceae). Phytotaxa 267(3): 273–281.
• Paudyal, S.K., P. G. Delprete, S. Neupane & T. J. Motley. 2018. Molecular phylogenetic analysis ad generic delimitations in tribe Chioccceae (Cinchonoideae, Rubiaceae). Bot. J. Linn. Soc. 187: 365-396.
• Landry, C. L., N. L. Elliott, A. J. Finkle & L. B. Kass. 2014. Plant-pollinator interactions in Bahamian coastal communities. Caribbean Naturalist 9: 1–16.