Stygobromus obscurus Holsinger 1974

Stygobromus obscurus

MATERIAL EXAMINED.—MONTANA. Ravalli Co.: well at Victor Crossing, holotypes female (USNM 142796) and 1 paratype (USNM 142797), Ocie Hessling, 11 Oct. 1952.

DIAGNOSIS.—A rather unusual, medium-sized subterranean species, apparently unrelated to other known species of Stygobromus in the western United States and readily distinguished by the gnathopodal propods being subequal in size; shallow coxal plates of gnathopod 2 and pereopods 3 and 4; narrow bases of pereopods 5–7 which lack distoposterior lobes; possession of 2 pairs of simple, lateral sternal processes; absence of ventral spines on the pleonal plates; numerous long spines on the uropods; and by the apical margin of the telson which is narrowly incised about one-fourth the length to base and armed with 14 rather long spines. Largest female, 7.0 mm.

DESCRIPTION.—Antenna 1: about 50 percent as long as body, 35 percent longer than antenna 2; primary flagellum with 15 segments. Antenna 2: peduncular segments 4 and 5 with a number of slender spines and stiff setae; flagellum with 5 segments. Mandibles subequal; spine row with 5 spines; palpal segment 2 with 8 long, inner marginal setae; palpal segment 3 with row of short setae on inner margin and 5 long setae near apex. Maxilla 1: inner plate with 5 apical, plumose setae; outer plate with 4 spines and 2 setae apically. Maxilla 2: inner plate with oblique row of 6 plumose setae on inner margin; outer plate with 8 to 10 stiff setae. Maxilliped: inner plate with 4 thick spines and 1 stiff seta apically; outer plate reaching only about one-fourth the length of palpal segment 2, apex with 2 spines and 1 seta, inner margin with 4 or 5 setae. Lower lip with prominent outer lobes and small but distinct inner lobes.

Gnathopod 1: Propod equal to or perhaps a little longer than 2nd propod; palm long, oblique, nearly straight, armed with double row of 13 spine teeth; posterior angle not well defined; posterior margin comparatively short, with 1 long seta; medial setae few in number, singly inserted. Coxal plate of gnathopod 1 longer than broad, with 3 marginal setae. Gnathopodal propod 2 about as long as, but not as broad as, 1st propod; palm convex, armed with double row of 7 or 8 spine teeth; posterior angle with 1 long and 3 short spine teeth and 2 long setae on outside, 3 short spine teeth on inside; posterior margin nearly as long as palm, with 2 sets of 4 long setae each; medial setae mostly singly inserted; dactyl nail rather short. Coxal plate of gnathopod 2 about as broad as long, with 3 marginal setae; coxal plates of pereopods 3 and 4 shallow, broader than long, with 3 or 4 marginal setae. Pereopods 6 and 7 about equal in length, about 45 percent as long as body, about 35 percent longer than pereopod 5. Bases of pereopods 5–7 narrow, about as broad proximally as distally; distoposterior lobes indistinct; posterior margins with 5 or 6 short setae; anterior margins with 2 or 3 long, stiff setae. Dactyls of pereopods 6 and 7 comparatively short, only 15 to 20 percent as long as corresponding propods. Coxal gills on pereopods 2–6. Small, simple, paired lateral sternal processes on pereonites 6 and 7. Brood plates of female small and narrow.

Pleonal Plates: Posterior margins of 1 and 3 slightly convex, that of 2 concave; posterior corners small, distinct, rounded; ventral margins without spines. Uropod 1: inner ramus shorter and thicker than outer ramus, about one-half the length of peduncle, armed with 14 spines; outer ramus with 14 or 15 spines; peduncle with 17 spines. Uropod 2: inner ramus shorter and thicker than outer ramus, about two-thirds the length of peduncle, armed with 14 spines; outer ramus with 7 spines; peduncle with 7 spines. Uropod 3: ramus about one-half the length of peduncle, armed with 6 long, apical spines. Telson about as broad as long; apical margin incised about one-fourth the distance to base; apical lobes with 7 long spines each.

TYPE-LOCALITY.—A well as Victor Crossing, Ravalli County, Montana. See notes under the description of S. tritus for further details.

DISTRIBUTION AND ECOLOGY.—This species is known only from its type-locality on the basis of two females collected on 11 October 1952. The two specimens were found in a sample containing 11 specimens of S. tritus and 5 specimens of S. montanensis. The brood plates of the larger female (7.0 mm long) were fringed with setae, indicating sexual maturity.

COMMENTS.—This species differs from members of the hubbsi group by having gnathopodal propod 1 equal to or slightly larger than propod 2, possession of setae on the posterior margin of gnathopodal propod 1, possession of lateral sternal processes, equal length of pereopods 6 and 7, and heavily spinose uropods. With S. montanensis, it shares the possession of lateral sternal processes and setae on the posterior margin of gnathopodal propod 1, but in other characters it does not appear to be closely related. Were it not for the free uronites and its geographic distribution, S. obscurus could be assigned to the genus Stygonectes.

ETYMOLOGY.—The specific name is from the Latin obscurus, meaning “indistinct” or “obscure,” so named because of the obscure relationship of this species to other members of the genus Stygobromus.

Zoogeography

In previous papers (Holsinger, 1966, 1967, 1971) I have discussed a possible mode of origin for some of the freshwater, subterranean amphipod genera of North America (north of Mexico), pointing out that these groups might have evolved from marine or brackish-water ancestors during periods of marine embayments. The major difficulties in tracing these freshwater groups to marine ancestors are the lack of a fossil record and the absence of a potential marine ancestral group living in the sea at the present time. Another cogent question is, namely, have contemporary subterranean genera like Stygobromus, Stygonectes, Apocrangonyx, Allocrangonyx, etc. evolved directly from blind, depigmented, brackish-water ancestors that were living in interstitial habitats at the onset of their invasion of continental freshwaters, or have these exclusively subterranean genera been derived from eyed, pigmented ancestral forms living in epigean habitats prior to their colonization of subterranean waters? Unfortunately, the present evidence does not allow a definitive answer.

Assuming, a priori, as I have done previously that the exclusively subterranean genera of North America were derived directly from brackish water, interstitial ancestors, such as has been rather convincingly shown to be the case with the evolution of the subterranean Hadzia group genera of the greater Caribbean region (Holsinger and Peck, 1968; Holsinger and Minckley, 1971; Holsinger, 1973), then one must look to past geologic times when parts of the North American land mass were inundated by shallow marine waters. As I have already pointed out, the extensive marine embayments of the Late Cretaceous and possibly the early Tertiary would have provided these conditions. During the inundation of North America in the Late Cretaceous, marine waters covered a significant portion of western and southern North America, with wide arms of the sea stretching from the Caribbean region north to the Arctic Ocean and across the Gulf and Atlantic Coastal plains (Dunbar, 1960; Kummel, 1961).

The ancestral stock of Stygobromus, as well as that of the other members of the generic complex which includes Stygonectes and Apocrangonyx (see Holsinger, 1969a), might have been widespread in these shallow marine embayments. Gradual regression of marine waters from the continental land mass would have been accompanied by the formation of new freshwater habitats, thus affording the opportunity for the invasion of newly opened niches. Access by freshwater invaders to inland massifs such as the Appalachians, Interior Low Plateaus, Ozarks, and the western Cordillera would have been facilitated since none of these areas were far removed from ancient Cretaceous shorelines.

Another hypothesis for the origin of North American, subterranean amphipods, and one favored by some amphipodologists, is that these genera or their ancestors were already established in continental freshwaters by the middle of the Mesozoic (see for example, Bousfield, 1958:56). Acceptance of this hypothesis would assume perforce a marine to freshwater invasion much older than the Cretaceous, perhaps as early as the Late Paleozoic. The attractive features of this theory are that it diminishes the importance of the lack of an extant group of potential marine ancestors and makes the widespread, Holarctic distribution of the entire Crangonyx group easier to comprehend.

As shown in Figures 36 and 37, the geographic distributions of species of Stygobromus in the western United States are widespread and scattered. Many of these species display highly insular ranges and all but four are known only from a single locality. Even those species represented in two or more localities are restricted to small ranges and only one species, S. arizonensis, has a range possibly as wide as 70 to 75 miles. The present distributional patterns, however, may not reflect the true ranges of all species, nor can the number of species now recorded from the western United States be regarded as final. With the exception of many of the caves in the Mother Lode region of California, some of the lava tubes of Washington and other well studied spots such as Lake Tahoe, numerous potential Stygobromus habitats remain to be investigated in the rugged mountainous country of the west. Many habitats such as wells, seeps, and remote caves and springs in the more inaccessible regions of the Sierra Nevada and other mountain ranges may be expected to yield additional populations, if not new species, when they are finally checked. The reported sighting of amphipod crustaceans (presumably subterranean) in Devil’s Hole, Nye County, Nevada, by La Rivers (1962) and other unconfirmed site records of subterranean amphipods strongly indicate that these animals are more common in the West than previously believed or than are borne out by the findings reported in the present paper. Moreover, the biological investigation of lava caves in the northwestern part of the country has just begun in earnest (F. G. Howarth, in litt.; Peck, in press) and continued study of these unique caves will almost certainly reveal additional populations of subterranean crustaceans.

The numerical distribution of species by physiographic province or subprovince is as follows (number of species in parentheses): Northern Rocky Mountains (4), Payette Section of the Columbia Plateaus (1), Northern Cascade Mountains (1), Oregon Coastal Range (1), Klamath Mountains (1), California Coastal Range (1), Sierra Nevada (8), and the Mexican Highland of the Basin and Range (1). Species are, in turn, numerically distributed by major biotope as follows: limestone or marble caves (6), lava caves (3), wells (4 or 5), springs (2), and deep lake (2). Only one species, S. puteanus, occurs east of the Continental Divide, in an area that drains north and east to the Missouri River system. The remaining species occur west of the Continental Divide in drainage systems flowing west to the Pacific Ocean.

Of added zoogeographic interest is the occurrence of S. putealis, a member of the hubbsi group, in eastern Wisconsin, some 1100 miles east of the range of the western species. If one assumes that species of the hubbsi group were derived from a common ancestral species or a cluster of closely related species and that the similarity of S. putealis to western species is more than coincidental, then the presence in Wisconsin of a species belonging to a group otherwise known only from the far western United States must clearly be regarded as a relict distribution. Since the middle of the Cenozoic, climates have become increasingly arid in the west-central United States (i.e., across the Great Plains), and this area, north of central Texas, is practically devoid of freshwater amphipods, either epigean or hypogean. If there was a continuous distribution of hubbsi group progenitors across the west-central United States then it must have occurred prior to the Miocene while the climate in this region was still humid.

An extensive discussion of speciation within the genus Stygobromus must logically await the completion of the three part generic revision. However, based on knowledge of previously described species and my observations on many of the undescribed species of the genus, the hubbsi group appears to be a distinct evolutionary group within the genus. With the exception of S. putealis from Wisconsin, the hubbsi group (and the other three species described above) is geographically disjunct from all other species of Stygobromus and occupies a position well isolated from all other subterranean amphipod genera of North America.

The present distributional patterns of the western species have probably resulted from fragmentation of previously more widespread populations and concomitant isolation of gene pools. The western United States has been marked by a series of major geological changes and climatic shifts since the Laramide Revolution, but modern landforms and climates are the result of events in the middle to late Tertiary such as the Cascadian Revolution and extensive regional volcanism. According to King (1958), both regional and local relief and climatic contrasts have been greater in the Cordillera since the mid-Tertiary than at any other time. Many habitats presently occupied by species of Stygobromus are no older than the Pleistocene, if not, as in the case of the Oregon and Washington lava tubes, even more recent. As Pennak (1958:224) has pointed out for aquatic invertebrates in general: “… the western states appear to present a set of conditions that should encourage isolation and speciation, especially in certain taxa containing macroscopic forms, and the West should theoretically have a unique population of freshwater invertebrates.”

Although the 15 species of the hubbsi group comprise a relatively homogeneous assemblage, several patterns of speciation are discernible. The most obvious pattern is demonstrated by the four closely related cave species of the Mother Lode region of California. The close morphological similarity of these species (especially in regard to their mouthparts and gnathopods), combined with their geographic proximity and similarity in habitat, is strongly indicative of their derivation from a recent common ancestor. The isolated nature of the limestone lenses containing caves in the Mother Lode region could easily provide dispersal barriers to troglobites (or phreatobites), thus effectively isolating populations to single caves or small clusters of caves.

Three other species—S. hubbsi, S. sierrensis, and S. lacicolus—appear to be closely related to the four cavernicolous species of the Mother Lode region. While these species differ in the structure of their mouthparts (i.e., having fewer plumose setae on the maxillae and fewer spines on the inner plate of the maxilliped), their gnathopodal propods are very similar to the Mother Lode species, and two of these species occupy habitats only 50 to 75 miles north of the California cave region. Stygobromus hubbsi, on the other hand, occurs in a lava cave approximately 350 miles north of the Mother Lode region. Moreover, aside from their smaller size and other minor differences, three other species from California—S. tahoensis, S. mackenziei, and S. sheldoni—also show affinity with the Mother Lode cave species as well as with S. sierrensis and S. lacicolus. The remaining five species of the hubbsi group differ more significantly from the Mother Lode species, are far removed geographically, and occur on the periphery of the range of the group. Thus, it would appear that at least one major center of speciation within the hubbsi group has taken place in the east-central California region, producing species which have successfully colonized a diversity of groundwater habitats, including caves, springs, and at least one deep lake.

Only two instances of sympatry (or syntopy) are so far known among western species of Stygobromus. The first is the curious association of three species in the well at Victor Crossing in Ravalli County, Montana. Of the three species, however, only S. tritus is a member of the hubbsi group. A second member of the trio, S. montanensis, while showing a remote affinity with the hubbsi group, is at best only distantly related to S. tritus. The third member, S. obscurus, is unrelated to any of the other western species of the genus and, in addition, bears some affinity with the genus Stygonectes, a genus unknown from any farther west than central Texas. Unfortunately, very little is known about the well at Victor Crossing and it is not known whether these three species continuously share the same habitat or only occasionally come in contact. On the basis of the two samples removed from this well in October 1952, S. tritus appears to be by far the most common of the three species recorded from this habitat.

The other sympatric association occurs in Lake Tahoe, where both S. tahoensis and S. lacicolus were obtained in periodic samples made in the early 1960s. Of the 16 samples studied, however, the two species were found together only twice, and S. tahoensis appeared to be much more abundant than S. lacicolus.