Epizon Campbell & Finger gen. nov.

Type species.

Gila alvordensis Hubbs & Miller, 1972.

Included species.

Gila alvordensis Hubbs & Miller, 1972: Trout Creek, Alvord Desert, Harney County, Oregon, USA. The location given is “ just below the canyon and just below bridge where roads to Denio, Jordan Valley, and Fields meet ” (Hubbs and Miller 1972, pg. 102). Holotype: UMMZ 130495 (image examined, Fig. 6 A). Paratypes: UMMZ 130496 (198, sharing same field identifier M 34-87), UMMZ 130533 (536), UMMZ 130534 (112), UMMZ 130535 (299). Valid as Epizon alvordensis.

Gila boraxobius Williams & Bond, 1980: Borax Lake, Harney County, Oregon, USA. Holotype: UMMZ 203329 (image examined, Fig. 6 B). Other specimens from the type locality: UMMZ 203330 (107), OS 4137 (12), OS 4138 (179), OS 12820 (3). Valid as Epizon boraxobius.

Diagnosis.

Epizon differs from Siphateles by having radii in all fields of the scale whereas radii are found only in the posterior field in Siphateles. Members of Epizon typically have seven dorsal- and anal-fin rays in contrast to the eight or more dorsal- and anal-fin rays in Siphateles. Scales are reduced in size and more embedded in Epizon in comparison to Siphateles with larger and less embedded scales. These attributes are discussed by Hubbs and Miller (1972), Miller (1973) and Williams and Bond (1980). Epizon is noted to exhibit sexual dimorphism with male fins longer than females, a characteristic not reported in Siphateles (Williams and Bond 1983; Williams et al. 1980; Baker et al. 2021; Sigler and Sigler 2014). Epizon may be diagnosed from Siphateles by 63 nucleotide substitutions within the mitochondrial cytochrome b gene (cytb), 46 of which are transitions and 17 of which are transversions (Table 2).

Description, distribution, and ecology.

Epizon is a genus of small to medium-sized minnows in the family Leuciscidae (Fig. 6). Teeth pharyngeal, mouth terminal. The color is variable with habitat occupied, varying from nearly black dorsally with speckled golden sides and a silver belly in canyon habitats to light green dorsally without speckling laterally and a white belly in downstream habitats (Williams and Bond 1983). Epizon boraxobius tends to be moderately dark, olive-green dorsally, silver-colored laterally and ventrally (Williams and Bond 1980). Average number of dorsal-fin rays of 7, anal rays 7, pelvic rays 8, and caudal-fin rays 19. Epizon alvordensis commonly has 13 pectoral-fin rays and E. boraxobius 14 (Williams and Bond 1980).

Epizon is endemic to the Alvord Basin of Eastern Oregon of the Northwest Lakes subregion, of the Great Basin (Houghton 1976). The Alvord Basin is a north-south oriented valley bounded by Steens Mountain to the west and the Owyhee Plateau to the east, with formation of the valley during the Miocene (Orr and Orr 2012). It appears that the ancestral lineage of Epizon was isolated during this time. During the Pleistocene, the pluvial Lake Alvord was ~ 19 km wide and 113 km long during maximal extent (Reheis 1999). Subsequent drying isolated fishes in limited suitable habitats in a cyclical fashion, resulting in E. alvordensis being more broadly distributed within the basin of former Lake Alvord and E. boraxobius restricted to the thermal spring-fed Borax Lake and proximate habitats. Within Borax Lake, fish live typically for a year and are of a smaller size (33–45 mm SL) whereas in the diverse habitats elsewhere in Alvord Basin E. alvordensis fish live longer and grow larger, 4–5 years and ≤ 120 mm SL (Williams and Bond 1983; Sigler and Sigler 2014). A sample of 50 fish from Gridley Springs were reported to be 27–91 mm SL with a peak at 30-38 mm SL and fish distributed into larger sizes ≤ 91 mm SL (Williams and Bond 1983). As such, E. boraxobius had been noted prior to formal taxonomic description as a dwarf form of E. alvordensis. Borax Lake is fed by springs with 35–40 ° C outflows resulting in a lake of typically 29–32 ° C temperatures, with E. boraxobius avoiding temperatures above 34 ° C and a loss of equilibrium at 34.5 ° C (Williams and Bond 1983). Epizon alvordensis occupies a variety of habitats in springs, creeks, ponds, and reservoirs, in seven of eight historically occupied drainages (Scheerer et al. 2015). This taxon is found in cold, cool, and warm waters, but not above 31.1 ° C (Williams and Bond 1983). Differentiation of the two species of Epizon may be driven by adaptation to the unique characteristics of the thermal-spring discharge with possible sympatric speciation of E. alvordensis and E. boraxobius (e. g., Smith et al. 2019). In addition to the characters of Williams and Bond (1980), nuclear genetic data separates the two species (Smith et al. 2019). Remple (2013) reports that larger eye diameter, longer snout length and longer head length differentiating E. alvordensis and E. boraxobius are apparent in early life stages and can be used to differentiate larvae.

Nuptial tubercles are present in males with sexual dimorphism reported in E. boraxobius based on relative length of fins, which are all longer in males than females (Williams and Bond 1980). This characteristic is also apparent in E. alvordensis (Sigler and Sigler 2014). Given the constant temperatures of Borax Lake, year-round spawning occurs with E. boraxobius. Epizon alvordensis in thermally fluctuating habitats, spawns only once a year (Williams and Bond 1983). Epizon are opportunistic omnivores with a diet closely related to the production of their habitats and variable with seasons (e. g., Williams and Williams 1980). Top dietary items are microcrustaceans, chironomids and diatoms (Williams and Bond 1983).

Etymology.

Romanized Greek version of επιζών, meaning survivor in reference to the persistence of this genus in the diverse and challenging desert habitats it has found itself. Gender masculine.