Zygonopus Ryder

Shear, William A., 2010, 2385, Zootaxa 2385, pp. 1-62 : 28-30

Genus Zygonopus Ryder

Zygonopus Ryder 1881:527 . Packard, 1883:194. McNeill, 1888:9.

Bollman 1893:158. Cook & Collins, 1895:59. Causey, 1960:70.

Trichopetalum (in part), Shear, 1972:277 ( Zygonopus incorrectly synonymized with Trichopetalum ).

Type species: Zygonopus whitei Ryder , by monotypy.

Included species: In addition to the genotype, Z. krekeleri Causey 1960 , Z. packardi Causey 1960 , and Z. weyeriensis Causey 1960 .

Diagnosis: Distinct from Trichopetalum and Nannopetalum in having 30, rather than 28, trunk diplosegments, lacking eyes, in the very robust sixth legpair of males, and in the poorly sclerotized, lobe-like colpocoxites; from Trigenotyla in the form of the male ninth legpair, which have the telopodites apically attached on the coxae, rather than on the sides; from Causeyella in having the telopodite of the ninth legpair swollen and never with vestigial distal segments; from Scoterpes in the shorter segmental setae and in not having the fimbriate branch of the gonopod arising apically on the colpocoxite. Scoterpes lacks microtrichia on the gonopod angiocoxites; these are present in Zygonopus .

Eyeless, depigmented, legs and antennae elongate in typical troglomorphic syndrome. Legpairs 3–7 of males enlarged ( Figs. 43, 47–50), but pairs 3–5 and 7 only slightly so, pair 6 at least twice as large as postgonopodal legs, separated by a distinct gap from legpair 5, femora swollen, almost globular, and with ventroapical hook ( Figs. 33, 35, 44, 49). Male legpair 2 with coxal hook above seminal duct opening ( Fig. 42). Gonopods with angiocoxite deeply divided into lateral and mesal branches, branches simple, acuminate. Colpocoxites rounded to subquadrate, poorly sclerotized.

The name Zygonopus is compounded from Greek words, and means “feet that make a yoke,” a reference to the much enlarged sixth legpair in males ( Ryder 1881).

Distribution: Found only in limestone caves along the boundary between the states of Virginia and West Virginia, from eastern Wythe Co., Virginia, in the south, to southern Shenandoah and Page Cos., Virginia, in the north; in West Virginia from Hardy, Mineral and Randolph Cos. in the north, to Mercer Co. in the south. It would appear that few caves in this well-defined area are without Zygonopus populations. The recent compendium of West Virginia cave fauna ( Fong et al. 2007) presents maps of the distribution of Zygonopus species in West Virginia that are based, unfortunately, on some erroneous identifications. Although I am credited in the compendium as having provided the identifications, in fact only a few, relatively recently collected specimens were seen by me; the majority of the localities mapped are from the literature and based on determinations provided by others. A comparison of the map (Map 3) presented here with the maps in Fong et al. (2007) and the detailed records given will illustrate the differences. A key point is that none of the species of Zygonopus appear to be sympatric, much less syntopic, anywhere in the generic range. However, I cannot claim to have seen all the specimens on which the maps in Fong et al. (2007) are based.

The generic distribution encompasses the Ridge and Valley physiographic region in Virginia and northern West Virginia (the “panhandle”), and the southwest-northeast trending valleys lying between the westernmost Ridge and Valley ridge and the escarpment of the Alleghany Plateau. The ridges are sandstone and shale, but erosion in the valleys has exposed Paleozoic limestones which are richly cavernous. While there are anomalies, the four species of Zygonopus are each more or less limited to specific drainage systems. Even terrestrial troglobionts may have distributions which follow drainage patterns, because the caves in which they live are formed by water and often contain active streams. In this region, stream valleys follow the softer limestone exposed under the hard siliceous caprock of the ridges, so individual valleys tend to be closed systems; the caprock limits cave formation in the underlying limestone, making underground dispersal between valleys impossible.

Zygonopus packardi occurs in the New and southwestern James River drainages, the James drainage occurrences possibly explicable by headwaters erosion leading to stream capture, specifically the capture by the Roanoke River of the Fincastle River in the early Pleistocene, which switched the Fincastle from the New to the James drainage ( Ross 1969). The northeastern part of the James drainage is occupied by Z. weyeriensis . Zygonopus whitei occurs in the headwaters of the Potomac drainage system, specifically the South Branch and Shenandoah Rivers. The relatively few records of Z. krekeleri are from the region drained by the Shavers Fork and South Fork tributaries of the Cheat River, which in turn flows north into the Monongahela system. The ultimate outlet of the Monongahela and New River systems is the Gulf of Mexico via the Ohio and Mississippi Rivers, of the James and Potomac, Chesapeake Bay.

Given these distributional patterns, it is interesting that the maps for many other terrestrial troglobionts (primarily insects; see especially the maps for the beetle genus Pseudanophthalmus Jeannel, 1920 ) given for West Virginia by Fong et al. (2007) show very similar patterns; essentially a division into the same (or very similar) three to five broad regions congruent for a number of species. However, at least in the case of Pseudanophthalmus , each “zone” may contain more than one, and in some cases several, sympatric if not syntopic species of the genus.

Within this larger pattern it seems reasonable to postulate that each Zygonopus species has achieved its present distribution through relatively rapid underground dispersal within the valleys of the stream systems, based on the lack of significant morphological variation within each. Above-ground dispersal can be ruled out by the delicate nature of these animals, unable to withstand warm temperatures or low humidities. It is possible that an initial colonization by a single ancestral population, or a few ancestral populations, eventually gave rise to the closely related and similar species packardi , weyeriensis and whitei , and that the very different krekeleri represents a separate initial colonization. An alternative would be the nearly simultaneous colonization of multiple cave systems by separate epigean ancestors of each population, but this would be expected to lead to substantial variation and even speciation between those populations, which does not seem to be the case. Distribution through the epikarst, the vast array of microcaverns overlying caves accessible to humans ( Culver & Pipan 2009), seems the most likely route for Zygonopus species.

Molecular phylogenetic studies of Zygonopus would be very interesting and would provide an opportunity to test these hyoptheses.

Notes: In 1972 I argued for the synonymy of Zygonopus with Trichopetalum , largely on the basis of a newly discovered Alabama species, T. syntheticum Shear , which on subsequent examination turned out to be a species of Scoterpes . I could hardly have been more confused! Reconsideration of the many consistent differences between the two genera (admittedly some due to troglomorphy, which could be convergent) now leads me to revalidate Zygonopus as an obviously monophyletic group of species. A close relationship with Trichopetalum and Nannopetalum is strongly indicated by the gonopod anatomy; indeed the gonopods of T. stannardi and T. jerryblatti are very similar to Zygonopus gonopods, but the Trichopetalum and Nannopetalum species have the apomorphic character of sclerotized gonopod colpocoxites and lack the extreme modification of male legpair six—an apomorphy of Zygonopus . The common ancestor of the three genera was very likely to have strongly resembled Z. weyeriensis in the gonopods, had 30 trunk segments, but was an eyed, pigmented epigean species.

Causey (1960) revised the genus and described three of the four species. At that time she stated:

“The differences between Z. packardi , Z. weyeriensis , and Z. whitei are mainly quantitative. When these three species were first studied, it appeared that subsequent collections might yield forms that would connect them. Then Dr. Barr’s large collection with male specimens of three species from 12 additional caves was received. Some variations were found, but they are slight, and in no case can they be regarded as intergrades between the species ( Causey, 1960, p. 73)”

Three years later, Causey (1963) had evidently rethought that conclusion, and wrote: “...the four species that I formerly assigned to it ( Causey, 1960) are two species, of which one is composed of three subspecies.” I assume Causey meant that packardi and weyeriensis were subspecies of whitei .

She was right the first time. In studying many more specimens from many more localities, I found the three species Causey later thought to be subspecies of Z. whitei perfectly and unambiguously distinct from one another, with no evidence of intergradation and with well-defined, separate areas of distribution. The differences between them are by no means “quantitative” as Causey herself demonstrated with her excellent figures of the gonopods ( Causey 1960). The category subspecies has a biological meaning; it designates a population that consistantly differs from other populations of the same species, but which is not reproductively isolated from those populations. The lack of reproductive isolation is demonstrated by the presence of zones of intergradation between the populations. If such intermediates cannot be found, it is better, depending on circumstances, to either describe the variation without nomenclatorial status, or name the populations as species. After all, the proposal of a species is a hypothesis like any other, which we expect to be tested in the future by additional data, and supported or rejected ( Shear 2003).