Hemipholis cordifera

Hendler, Gordon, 2011, New light on the nomenclature, taxonomy, and biology of Hemipholis species (Echinodermata: Ophiuroidea: Ophiactidae), Zootaxa 3048, pp. 44-52 : 48

Distinguishing H. cordifera from H. gracilis

Small individuals of H. gracilis and H. cordifera look alike, which could account for a record of “ H. elongata ” from the Eastern Pacific (in Alvarado et al. 2010). Although large individuals of both species also are quite similar, several external features can be used to distinguish them ( Figs. 1 , 2 ). Large individuals of Hemipholis gracilis have radial shields that are more constricted proximally than the shields of H. cordifera ( Fig. 1 A,D). The basal ventral arm plates of H. gracilis , particularly the second ventral arm plate, are relatively shorter and wider than those of H. cordifera ( Figs. 1 B,E & 2). Although there is an overlap in the width/length ratios of the second ventral arm plates of both species, animals larger than 7 mm disk diameter have plates that distinctly differ in their proportions ( Fig. 2 ). In addition, the arm spines of large H. gracilis tend to be more tapered than those of H. cordifera (Fig. C,F).

The Pacific and Atlantic populations of Hemipholis are little differentiated morphologically, and dissimilarities in their external structures only arise late in ontogenesis. This is not surprising since geminate species separated by the Panamanian Isthmus are typically “in the initial, allopatric stage of speciation,” and they can even be reproductively compatible ( Lessios 2008:84). Consistent differences between individuals of Hemipholis from opposite sides of the Isthmian barrier, similarities among individuals throughout their range in each ocean, and the species’ broad ranges in the western Atlantic and Eastern Pacific respectively, all suggest that differences between H. cordifera and H. gracilis are not merely ecophenotypic. However, there is a dearth of comparable data on genotypic, phenotypic, and phenological characteristics of populations from both oceans, which are needed to elucidate the evolution and phylogeny of H. gracilis and H. cordifera .

Figures

FIGURE 1. Hemipholis cordifera and Hemipholis gracilis. A – C, H. cordifera, LACM 1999 - 26.2, 7.8 mm disk diameter, from South Carolina; D – F, H. gracilis, LACM 1997 - 224.001, 7.8 mm disk diameter, from Panama; specimens preserved in ethanol. For each specimen, shown from left to right are: portion of disk in dorsal view, arrowheads indicating pairs of radial shields; basal arm joints on ventral side of disk, arrow indicating second ventral arm plate; basal arm joints in dorsal view, arrowheads indicating middle arm spines. Dorsal arm plates of both species are sometimes fragmented as is the case in Figure C. Scale bars = 1.0 mm.

FIGURE 2. Scatter plot for Hemipholis cordifera and Hemipholis gracilis, showing the relationship between the body size in terms of disk diameter, and shape of the second ventral arm plate characterized by the ratio between the ossicle’s width and length.