A new species of the subterranean amphipod crustacean genus Stygobromus ( Crangonyctidae ) from a cave in Nevada , USA

Stygobromus albapinus, a new stygobiotic amphipod crustacean species in the family Crangonyctidae, is described from two pools in Model Cave in Great Basin National Park, White Pine County, Nevada, USA. The type specimens were collected on two different visits to the cave. The new species is assigned to the hubbsi group, bringing the number of described species in this group to 45, but many other provisionally recognized species assigned to this group remain undescribed. With exception of a single species from deep wells in southeastern Wisconsin, all other members of the hubbsi group are recorded from a wide variety of subterranean groundwater habitats (e.g., caves, springs, wells, etc.) in western North America, west of the Great Plains. Although the taxonomic affi nities of Stygobromus albapinus, n. sp. need further study, the species does appear to share several important morphological characters with a species from a cave in western Utah located approximately 300 km east-northeast of Model Cave.


INRODUCTION
The genus Stygobromus is comprised exclusively of stygomorphic species (typically lacking eyes and pigment) that inhabit a variety of subterranean groundwater habitats, including cave pools and streams, phreatic water in wells, seeps (hypotelminorheic) and springs, the underfl ow (hyporheic) of surface streams, and rarely deep lakes (Ward and Holsinger 1981;Holsinger 2009, see also Culver and Pipan 2009).In North America, the genus is represented by 129 described species, although numerous new species have been discovered and are either undergoing description (22 in manuscript by JRH) or are provisionally recognized.In addition, four (and possibly fi ve) species are recorded from Eurasia, bringing the total number of described species in the genus to 133 or 134 (Sidorov et al 2010).In North America more than 50 species are found in the western United States and southwestern Canada (Wang and Holsinger 2001), the majority of which lack sternal gills or processes and have been assigned to the hubbsi group (see Holsinger 1974;Wang & Holsinger 2001) primarily on the basis of this diagnostic character.The systematics of the genus in North America are treated in a series of major papers produced over several decades (e.g.Holsinger 1974Holsinger , 1978Holsinger , 2009;;Wang and Holsinger 2001), along with several others more restricted in scope (Holsinger 1980;Bousfi eld and Holsinger 1981;Holsinger andShaw 1986, 1987;Ward 1977).The most recent paper in this series by Sidorov et al (2010) includes descriptions of two new species from Siberia.These papers should be consulted for keys, taxonomic changes, and specifi c distributional information for the genus.
In the course of a survey of the cave fauna of Great Basin National Park (White Pine County, Nevada), an undescribed Stygobromus species of the hubbsi group was collected from two different pools in Model Cave in Great Basin National Park, and it is described herein.The description of S. albapinus n. sp.increases the number of species assigned to the hubbsi group to 45, all but one of which occur in the western Cordillera of North America.

METHODS AND MATERIALS
Specimens were collected directly from cave pools and preserved in 70% ethanol.In the laboratory permanent slide preparations were made by mounting dissected appendages in Faure's mounting medium (a modifi cation of the conventional Hoyer's medium [Upton 1993]) on glass slides.Slide-mounted appendages were photographed using a Leica DM 2500 compound microscope, and stacked digital images were created using Image-Pro Express (version 5.1.1.14, ©2005, Media Cybernetics, Inc.).Images were imported into Adobe Illustrator CS2 (version 12.0.1,©2005 Adobe Systems, Inc.), where final line drawings were produced.Drawings are of female specimens unless otherwise indicated.
In the description, usage of the descriptive terms "seta" and "spine" are in the original conventional sense and do not agree with terminology suggested by Watling (1989).We use the term "spine" for thick and stiff setae and the term "seta" for thin and fl exible structures.No-menclature for setal patterns on segment 3 of the mandibular palp follows the system introduced by Stock (1974).The term "defi ning angle" of the gnathopod propodi refers to the "angle" formed by the end of the palm and beginning of the posterior margin or the point at which the tip of the dactyl closes on the propodus.

Diagnosis
A relatively small stygobiotic species of the hubbsi group distinguished by structure of pereopods 6 and 7, which have relatively long, narrow bases lacking distinct distoposterior lobes; uropod 3, which has a very short ramus bearing 2 small, apical spines; and sub-rectangularshaped telson with small V-shaped apical notch and 7 to 8 apical spines.This species appears to be morphologically more similar to S. utahensis, Wang and Holsinger (2001) from Pole Creek Cave in Duchesne Co., Utah and possibly S. blinni, Wang and Holsinger (2001) from Roaring Springs Cave in Coconino Co., Arizona than to any other species in the hubbsi group known to date.Largest ♀, 5.0 mm; largest ♂, 3.5 mm Female (Fig. 1) Antenna 1 (Fig. 2B) approximately 45% percent length of body, 25 % longer than antenna 2 (Fig. 2C) primary fl agellum with 11 segments, most bearing aesthetascs (not shown); accessory fl agellum approximately equal in length to 1 st fl agellar segment.Antenna 2  (Fig 2C ): peduncular segments approximately equal in length, fl agellum with 5 segments.Mandibles (Fig. 2D,E) closely similar but differing in a few minor ways: spine row of both with 5 lightly plumose spines; lacinia mobilis of left with 2 or 3 tiny teeth, that of right apically forked with 3 tiny spines.; incisors normal for genus; mandibles cone shaped, protruding, each with single seta.Palp segments subequal; segment 2 bearing 2 or 3 setae on inner margin distally; segment 3 with 4 long E setae, 5 or 6 shorter D setae and 1 B seta; lacking both A and C setae.Lower Lip: inner lobes present.Maxilla 1 (Fig. 2F): inner plate with 6 apical plumose setae; outer plate with 6 apical spines (2 or 3 with serrated margins); palp with 4 short, apical setae.Maxilla 2 (Fig 2G ): inner plate with 4 relatively long naked setae on distal half, followed by approximately 8 shorter setae apically; outer plate with 9 or 10 apical, plumose setae.Maxilliped (Fig. 2H): inner plate with 2 bladelike spines, 1 plumose spine, and 2 naked setae apically; outer plate with 7 or 8 short setae on upper, inner margin; palp segment 2 broader than and more than 2X longer than palp segment 1, inner margin with row of about 8 rather long setae; nail of dactyl (palp segment 4) relatively long and sharply pointed.
Gnathopod 1 (Fig. 3A): propodus smaller than that of gnathopod 2, palm straight and about 25 % longer than posterior margin, armed with double row of about 15 spine teeth in unequal double row; defi ning angle rounded, with 3 spine teeth on outside, 2 shorter ones on inside; posterior margin lacking setae; 1 superior medial setae, 3 inferior medial setae; dactyl nail short; coxa little broader than deep, ventral margin with 1 seta.Gnathopod 2 (Fig 3B ): propodus longer than broad, palm nearly straight, margin irregular, armed with 11 or 12 spine teeth in double row; defi ning angle with 1 long spine tooth on outside, 2 shorter spine teeth on inside; posterior margin approximately 50 % length of palm, with 2 sets of doubly inserted setae; row of 4 superior medial setae, and 2 inferior medial setae.Coxa of gnathopod 2 slightly broader than deep, margin with 1 seta.Brood plates well developed in sexually mature females (Fig 3C).

Male
Closely similar to the female but apparently reaching sexual maturity at a slightly smaller size and differing in the morphology of uropod 1 (Fig. 5F) as follows: inner ramus equal in length to outer ramus, about 70 percent length of peduncle, armed with 7or 8 spines; outer ramus with 5 spines; peduncle with 4 small spines on dorsal margin; possession of distinct peduncular process approximately 20 % percent length of peduncle and bearing 5 tiny peg-like spines.

Etymology
The specifi c epithet albapinus is a contraction of alba (Latin for white) and pinus (Latin for pine), in reference to White Pine County, Nevada-the type-locality and only known population of this species.

Distribution & Ecology
The species is known only from two pools in Model Cave, Great Basin National Park, White Pine County, Nevada, where specimens were collected from both a mud/silt bottom pool with temperature of 7°C and a sump pool with temperature of 4°C.The majority of specimens were collected from the mud/silt bottom pool.Of the 47 specimens sampled, only one is a mature male, suggesting a tendency for parthenogenesis; a trend previously noted for several other species of Stygobromus (Culver and Holsinger 1969).

DISCUSSION
The description of S. albapinus n. sp.brings the total number of species of Stygobromus described from western North America west of the Great Plains to 54, of which 45 are assigned to the hubbsi group.Descriptions of 17 new species of Stygobromus, and redescription of S. hubbsi Shoemaker (1942) from western North America, west of the Great Plains, were provided earlier by Holsinger (1974).This paper was followed more recently by descriptions of an additional 28 new species from the same region by Wang and Holsinger (2001).In addition, there are more than 10 provisionally recognized, undescribed new species of Stygobromus from California and Washington State and several more from southern Alberta, Canada and southeastern Alaska (J.R. Holsinger unpublished data).Most, if not all, of the undescribed species from California and Washington can be assigned to the hubbsi group.It is also of biogeographic interest that only one species (Stygobromus putealis Holmes) assigned to the hubbsi group has been found outside western North America in southeastern Wisconsin (Holsinger 1974;Wang and Holsinger 2001).
Despite relatively close morphological similarities among most of the species, and a preliminary cladistic analysis combined with a list of character states (Wang and Holsinger 2001), the origin, interspecific relationships and biogeography of these species remain in need of further study, especially those assigned to the hubbsi group.In addition, given the cryptic habitats and seclusion of groundwater amphipods in western North America, many undescribed species probably remain undiscovered.Thus, future discovery and additional taxonomic study, including molecular analyses, should provide a better idea of the taxonomic and biogeographic relationships among western species.Wang and Holsinger (2001) suggested that changes in the Pleistocene pluvial lake system of western North America could have affected the distribution of hubbsi group species in the Great Basin.Groundwater levels would have been elevated during glacial maxima, followed by disjunctions when groundwater levels were subsequently lowered, and this in turn may have provided the opportunity for speciation in this area.However, it was also noted that the majority of western species occur outside of the areas impacted by the pluvial lakes (Fig. 6), and that the hubbsi group is split roughly into a western subgroup that occurs in the Cascades-Sierra Nevada-Coastal Range and an eastern group that occurs in the Rocky Mountains and Colorado Plateau.Between these two groups, the hubbsi group is represented by only seven species that are recorded to date from the Great Basin and Columbia Plateau (Wang and Holsinger 2001).Finally, it is perhaps of interest that none of the non-hubbsi group western species occur in the Great Basin or further west (Wang and Holsinger 2001).
What does the occurrence of the new stygobiotic species from Great Basin National Park tell us about the Stygobromus fauna of the Great Basin as well as about other cave and groundwater adapted invertebrates?Recent inventories of cave-adapted invertebrate organisms in Great Basin National Park have resulted in the discovery of new species of millipeds and collembolans (Shear 2007, Shear et al 2009;Zeppelini et al 2009), as well as undescribed diplurans and pseudoscorpions.The description of S. albapinus n. sp.brings the total number of new invertebrate taxa described from caves in this small area as a result of recent bioinventory work to four.Studies of spring-inhabiting hydrobiid and pleurocerid snails in the Great Basin, many of which occur at one or few sites, have shown that high diversity of springsnail species in the Great Basin is a refl ection of their strong association with points of groundwater resurgence and the regional history of aquatic connections (Brown et al 2008).Furthermore, a genetic analysis of amphipods in the epigean genus Hyalella (Talitridae) from numerous spring sites in the Great Basin identifi ed a high level of genetic diversity strongly indicating the presence of a multitude of cryptic, undescribed species of Hyalella in this region (Witt et al 2006).These fi ndings suggest that the cave and spring faunas of the Great Basin region have not been thoroughly sampled to date.Moreover, sampling methods needed to obtain subterranean amphipods have not to our knowledge been widely implemented in surveys of Nevada's subterranean groundwater faunas.Despite extensive surveys for springsnails and epigean amphipods in the Great Basin, cryptic subterranean groundwater habitats commonly inhabited by Stygobromus are not as easily accessed and remain poorly sampled.Utilization of special techniques for sampling small groundwater crustaceans, such as the Bou-Rouch groundwater pump and similar tools (e.g.Leijs et al 2009) for probing relatively deep (~30 cm) into the substrate of spring outlets, may reveal additional undescribed species of Stygobromus.A number of factors have to date precluded rigorous sampling of potential groundwater habitats in the Great Basin, perhaps the most important being the marked physical isolation and relatively remote location of aquatic habitats common to Great Basin topography.The occurrence of numerous isolated springs associated with the bases of mountain ranges offers the potential of fi nding additional populations of Stygobromus species within this region.It is likely that the recent discovery of Stygobromus albapinus n. sp. will be followed by additional discoveries with further exploration of groundwater habitats in the Great Basin region.

Fig. 6 -
Fig. 6 -Generalized distribution of described species of Stygobromus in western North America (closed circles) based in part on Wang & Holsinger (2001), with location of Stygobromus albapinus sp.nov.indicated by a star.Dashed line indicates extent of the Great Basin.