The Badwater Snail (Assiminea infima) was described by Berry (1947) from Badwater Spring, Death Valley, California (U.S.A.). Hershler (1987) described the shell and soft parts of this snail in much greater detail and identified 3 new localities where it occurred in the area. Assimlnea infima typically occurs either under a salt-crust roof fringing the water's edge or on moistened riparian vegetation. Snails only occur in the vicinity of spring sources, where they are often found fully submerged. (Hershler 1987). Assiminea infima occurs at low elevations where mean annual precipitation is less than 3 cm and temperatures range from a maximum of 55 C during summer to a winter minimum of −5 C (Sada 2001 and references therein).
Hersher and Liu (2008) undertook a study to test the hypothesis that Assimina snails living in association with highly mineralized springs in the Death Valley–lower Colorado River region are relicts of the Bouse Embayment, a putative late Miocene–early Pliocene transgression of the ancestral Gulf of California along the lower Colorado River valley. Results of their biogeographic analysis, including an attempt to infer species relationships and approximate divergence times using mitochondrial DNA, appear to be incompatible with this hypothesis, suggesting instead that the Assiminea fauna of this inland area was more likely founded by coastal colonists transported on water birds than through a direct connection with the sea.
The Badwater Snail (Assiminea infima) is endemic to low-elevation, spring-fed habitats in Death Valley, where its abundance is believed to be adversely affected by municipal diversions and habitat trampling by Park visitors. Sada (2001) examined effects on demography and habitat at sites highly, lightly, and unaffected by these activities. Snail density ranged from 0 to 19000 per square meter along the banks of seven springs sampled at Badwater and Cottonball Marsh. Springbrooks with high, steep, and overhanging banks were preferred A. infima habitat. Mean distance of snails from water ranged from 1 mm at Badwater to 39 mm at Cottonball Marsh, and distance from water was greater in autumn than spring. Frequency distributions of shell height showed each population was comprised of several cohorts during spring and autumn, suggesting that reproduction occurs several times a year.
Cottonball Marsh is extremely isolated and is visited by just a handful of people each year--according to Sada, it is one of the few aquatic habitats in western North America minimally affected by cultural disturbance. In contrast, Badwater is the lowest point in North America and is visited by hundreds of thousands of people each year. Field experiments demonstrated that trampling reduces bank heights, angles and overhangs, and A. infima population density (Sada 2001). Sada suggested that actions are required to manage public use at Badwater to stop declines in abundance and distribution of A. infima and to allow for recovery.
A small (shell height, 2.5-3.5 mm) species, distinguished from North American coastal Assiminea by its broadly conical shell with moderate to highly convex whorls. Other distinguishing features include the presence of a single, enlarged ctenidial filament anterior to the more typical, stubby filaments; a thickened pallial swelling anterior to the rectum; and the looping condition of the anterior vas deferens (see Hershler 1987).
Hershler (1987) described the known Assiminea infima sites in some detail, summarized below; additional information is available in Sada (2001):
Badwater: Small seeps at Badwater discharge into two shallow (less than 5 cm) pools, 70 meters apart, occupying approximately 2000 square meters. The salt-crust-covered sump that Badwater occupies is moist throughout, with the water table lying at or just beneath ground level. Spring sources are particularly noticeable at the northeastern corner of the south pool. During most of the year Badwater has no outflow. Badwater is virtually saline, with total dissolved solids averaging about 23000 ppm. Aquatic vegetation consists of dense stands of ditch-grass (Ruppia sp.), which are most common in the north pool. Pickleweed (Allensolfia occidentalis) fringes parts of both pools. The perimeter of the north pool is largely fringed by a salt-crust roof overhanging (sometimes just touching) the water's edge by 5 to 10 cm. The south pool perimeter has been trampled down over the years by tourist activity and a salt-crust roof is currently restricted to the southeastern portion of the pool and seep inflows to the pool. Assiminea infima is most common on the undersides of the salt-crust roof, where the snails are moistened and sometimes submerged. Snails were removed and counted from a few measured pieces of salt crust fringing the north pool (23 February 1985), yielding densities ranging up to 6748 per square meter. Snails also occur under emergent offshore salt-crust pieces, on submerged driftgrass, and on moistened pickleweed roots in shaded situations. As much as 70 to 80% of the total Badwater population dwelt in the north pool, with the density highest on the northern side of the pool where algal growth was low and spring inflow probably occurs. (Hershler 1987 and references therein)
Travertine Springs: Travertine Springs emerge from low travertine (a form of limestone deposited by mineral springs) mounds as a series of thermal (30 to 32°C) rheocrenes (flowing springs emerging from the ground) draining into Furnace Creek Wash. The springs are fairly small, with outflows typically 0.5 meters across and 1 to 5 cm deep, The spring water is much softer than that at Badwater, with total dissolved solids of 640 ppm. Dense sedge growth fringes the outflows, shading much of their length. One of the larger springheads has been partly capped and the lower reaches of the streams now collect into diversion works. Assiminea infima is uncommon at Travertine Springs, with occasional individuals found in moistened masses of living and dead sedges along the stream outflows. This habitat is quite limited at Travertine Springs as the extent of sloping banks, which provide a moistened riparian environment, is minimal.
Nevares Springs: Nevares Springs emerge from a large travertine mound as thermal (30 to 35°C) rheocrenes, coalescing to form a single stream flowing along a wash. The shallow (less than 6 cm) spring outflows occupy either narrow (about 0.5 m) incised channels or fan out as broad (3 meter) streams. Riparian sedges are common along the streams on the mound, but are absent from the well-scoured wash. The largest spring has been capped, and at least one springhead was dug out in the past in an effort to increase discharge. Assiminea infima is moderately common in dense, moistened riparian vegetation lining the sides of upper spring outflows; no specimens were found along the stream in the wash,
Cottonball Marsh: Cottonball Marsh occupies a large (about 2.56 square kilometers) area on the west side of Death Valley. Salt Springs emerge west of the marsh and flow into large pools, which drain into the marsh farther out on the salt pan. Additional springs emerge on the salt pan in the middle of Cottonball Marsh. Spring inflows to the marsh are thermal (31°C) and the marsh is saline, with total dissolved solids ranging from 14000 to 160000 ppm. Water depth varies from less than 1 cm to nearly 1 meter in the deep pools. As at Badwater, a salt crust covers much of the marsh and forms a fringing roof at the water's edge in many places. Saltgrass (Distichlis sp.) and pickleweed are common only at Salt Springs; the remainder of the marsh is almost entirely devoid of vegetation. Cottonball Marsh is in a remote portion of Death Valley rarely visited by people and remains in pristine condition. At Salt Springs Assiminea infima is common on the bases of moistened riparian vegetation and on the undersides of the salt-crust roof. Snails were not found between Salt Springs and the point where additional springs emerge, about 1.6 km out onto the salt pan. At those latter springs, A. infima is very common under the salt-crust roof (moistened or submerged) fringing stream outflows, especially on algal-covered salt-crust pieces.
Description (from Hershler 1987, which should be consulted for more detail):
Shell: Adult shells have 4.0 to 5.0 whorls, with measured shell heights varying from 2.5 mm to 3.5 mm. Females are typically slightly larger than males. Shells are fairly thin and translucent, with only a slight thickening at the aperture. Shell color is very light to dark amber. The aperture is longer than wide, rounded below, and angled above. The inner lip is fully formed only in the largest specimens, and in these it often forms a broadened callus. Separation of the inner lip from the body whorl is rare. The umbilicus is often restricted due to peristome overlap and appears chinklike. The protoconch, with 1.25 whorls, is smooth or with a few faint spiral lines. Teleoconch sculpture consists of strong collabral growth lines. The paucispiral is thin, amber-colored, and without unusual features.
Head-foot: The broadly bilobed snout is dorsoventrally flattened, longer than wide, and creased along most of its length. The distal end of the snout flares outward to form broad, fleshy oral lappets. The tentacles are short, thickened, rounded at the tips, and without cilia.
The eyespots are near the distal ends of the tentacles. A ciliated groove extends along each side of the head-foot, originating where the snout joins the anterior foot and ending on the side of the "neck”. The right lateral (or omniphoric) groove, along which fecal pellets frequently pass, is much wider than the left groove. The grooves are well inset and surrounded by angular, unciliated ridges. The operculigerous lobe is quite swollen and fleshy, bulging outward from the surrounding portion of the headfoot. The broad, thickened foot is rounded anteriorly and posteriorly. The center of the anterior pedal crease has a large pore through which pedal glands discharge. There is no suprapedal fold.
The genus Assiminea as currently delineated includes mainly tropical and subtropical species, but is represented in North America by several coastal species, as well as an inland group of Assiminea occurring in spring-fed habitats in the arid southwest United States. Recent work suggests that species currently placed in the genus do not form a monophyletic group and that the genus is in need of major revision. (Hershler and Liu 2008 and references therein).
