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Channel Islands and California Desert Snail Fauna

The land snail faunas of the Channel Islands and the California deserts include many helminthoglyptid species (desert snails, shoulderbands, island snails, and allies). These are relatively well known, and their classifications are well documented and easy to describe. These regions provide only two examples of the diversity of California's land snail fauna. Snails and slugs found elsewhere in California are equally deserving of attention and conservation.

   

Channel Islands Snails

 

Southern California's Channel Islands are sometimes separated into two groups. The northern group (including San Miguel, Santa Rosa, Santa Cruz, and Anacapa islands) has been considered a westward extension of the Santa Monica Mountains on the mainland, although existence of a Pleistocene or Holocene connection to the mainland is now doubted (Junger and Johnson 1980). The southern group (Santa Barbara, San Nicolas, Santa Catalina, and San Clemente) is generally more remote from the mainland than the northern group.

   

The San Miguel shoulderband is endemic to the northern group of islands. This species has its closest known affinities with two coastal species from north of Point Conception in Santa Barbara and San Luis Obispo counties: the surf shoulderband and the endangered Morro shoulderband (Roth 1973; U.S. Fish and Wildlife 1994). Even without an earlier land bridge connection to the mainland, the ancestors of these snails could have reached these islands through over-water dispersal. Many land snails of arid or semiarid areas have features (reviewed by Chambers 1991) that permit them successful island colonization by dispersal on floating rafts of woody debris.

   

The southern group of islands is occupied by a much more diverse fauna that includes island snails and cactus snails. The island snails are single-island endemics on these islands and on Guadalupe Island, Baja California Norte, except the San Nicolas island snail, which also occurs as fossils on San Clemente Island (Roth 1975a,b). The cactus snails (formerly included with island snails by Pilsbry 1939) are made up of four living species. The plain cactus snail and the wreathed cactus snail are extant, single-island endemics that occur on San Clemente Island. The Catalina cactus snail has also been reported by Pilsbry (1939) from the Palos Verdes Peninsula, Los Angeles County, which is the only report of the genus outside of the southern group of Channel Islands. The co-occurrence of species of these two groups on each of these islands is another notable feature of their land snail fauna.

   

The Santa Barbara shelled slug has been found living only on Santa Barbara Island, although fossil shells have been found on San Nicolas Island (Roth 1975a; Hochberg 1979). The only other occurrences of this small group are the Guadalupe shelled slug of Guadalupe Island, Baja California Norte, and forms of limited known occurrence on the mainland of Baja California Norte (Roth 1975b).

   

An extremely curious element of the snail fauna of these islands is the Catalina mountain snail, which is endemic to Santa Catalina Island. This population occurs several hundred miles away from the nearest living related populations in southeastern Arizona and Baja California Sur. Because the original collector (Henry Hemphill in 1905) was not known for his care in handling specimens, and because subsequent workers failed to locate the species on Santa Catalina Island, this seemingly anomalous record was doubted for many years. Hochberg et al. (1987) provide a fascinating account of their rediscovery and verification of the identity of the species, along with documentation of its likely status as a relict of the group's formerly wider distribution.

   

The high incidence of endemism of land snails in the Channel Islands has probably resulted from the low frequency of inter-island dispersal and the age of the islands. Formerly larger island areas during periods of lower sea levels (Roth 1975b) may have contributed to the high diversity seen today in the southern group of islands.

   

California Desert Snails

 

A diverse helminthoglyptid fauna has evolved in the Mojave and Colorado deserts of southeastern California (Pilsbry 1939; Bequaert and Miller 1973). Most distinctive are two desert snail groups limited to these California deserts and extending only to western Arizona and northern Mexico. Species of these groups mainly occupy rockslides in the bordering mountain ranges and the smaller, isolated ranges that are scattered throughout these deserts. Some of these ranges support endemic desert snails.

   

Two additional groups endemic to the northern Mojave Desert are each represented by a single species. The El Paso shoulderband is endemic to the El Paso Mountains, and the Argus desert snail is endemic to the Argus and Slate mountains (Pilsbry 1939).

   

The land snails of this desert fauna have probably evolved from ancestors that were more widespread during prolonged periods of moister climate (Wells and Berger 1967). They have probably survived in this most inhospitable (for snails) dry climate by remaining inactive deep within rockslides between rains. Isolated because of poor dispersal capabilities and lack of refuge from desiccation between rockslide areas, they have differentiated into the relict fauna observed today.

   

The shoulderband group is also represented by several localized species in the Mojave Desert. Endemic species occur in the El Paso (mimic shoulderband) and Panamint mountains (Panamint shoulderband). Two additional species occur near the Mojave River, which receives flow from snowmelt from the San Bernardino Mountains. Other desert shoulderbands occur at the edges of the deserts, such as near the San Gabriel (Soledad shoulderband) and Tehachapi mountains (Mojave shoulderband).

   
  Author
Steven M. Chambers
Division of Endangered Species
U.S. Fish and Wildlife Service
P.O. Box 1306
Albuquerque, New Mexico 87103

References


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