Aphonopelma atomicum Hamilton

Hamilton, Chris A., Hendrixson, Brent E. & Bond, Jason E., 2016, Taxonomic revision of the tarantula genus Aphonopelma Pocock, 1901 (Araneae, Mygalomorphae, Theraphosidae) within the United States, ZooKeys 560, pp. 1-340 : 48-51

Taxon classification Animalia Araneae Theraphosidae

Aphonopelma atomicum Hamilton sp. n. Figures 20, 21, 22, 23, 24

Aphonopelma mojave Prentice, 1997 (in part): 161.

Types.

Male holotype (APH_2727-2) collected at the Nevada Testing Site, Nye Co., Nevada, 37.025579 -116.023865 6, elev. 3990ft., viii.1961, coll. Gertsch; deposited in AMNH. Paratype female (AUMS_3267-2) from 10 miles W of Mercury, Mercury Valley, Nye Co., Nevada, 36.691928 -116.174483 5, elev. 3470ft., 3.xi.1972, coll. Wendell Icenogle; deposited in AUMNH. Paratype male (AUMS_2638) from Nros Rd., 20 miles W of Mercury, Nye Co., Nevada, 36.66182 -116.368 5, elev. 2744ft., 3.xi.1972, coll. C.S.L.; deposited in AUMNH.

Etymology.

The specific epithet is a neuter noun meaning "of or relating to atoms". The name references the Nevada Test Site, constructed by the Atomic Energy Commission for the testing of nuclear devices, where this species was originally collected. The name is in homage to the famous sci-fi B movies of the 1950's, of which Tarantula (1955) was the most entertaining, and slightly ironic given that this species is one of the smallest tarantulas in the United States.

Diagnosis.

Aphonopelma atomicum (Fig. 20) is a member of the paloma species group and can be distinguished by a combination of morphological, molecular, and geographic characteristics. Nuclear and mitochondrial DNA identifies Aphonopelma atomicum as a strongly supported monophyletic lineage (Figs 7-8) that is a sister lineage to Aphonopelma mojave , Aphonopelma icenoglei sp. n., Aphonopelma prenticei sp. n., and Aphonopelma joshua . Aphonopelma atomicum can easily be differentiated from larger syntopic species (e.g., Aphonopelma iodius ) due to their much smaller size and the extent of metatarsal scopulation on legs III and IV, and from all other miniaturized species of Aphonopelma by their locality (with the exception of Aphonopelma prenticei ). Molecular data is needed to differentiate this species from Aphonopelma prenticei (see Figs 7, 8). The most significant measurements that distinguish male Aphonopelma atomicum from its closely related phylogenetic and syntopic species are F3 and the extent of metatarsus IV scopulation. Male Aphonopelma atomicum can be distinguished by possessing a larger F3/M3 (≥0.99; 0.99-1.03) than Aphonopelma joshua (≤0.98; 0.91-0.98) and Aphonopelma xwalxwal sp. n. (≤0.95; 0.91-0.95); a smaller F3L/W (≤3.40; 2.92-3.40) than Aphonopelma icenoglei (≥3.51; 3.51-3.90); and a smaller L4 scopulation extent (26%-41%) than Aphonopelma iodius (62%-88%). There are no significant measurements that separate male Aphonopelma atomicum from Aphonopelma mojave or Aphonopelma prenticei ( Aphonopelma atomicum males possess a swollen F3 femur like Aphonopelma prenticei , whereas Aphonopelma mojave males do not). The most significant measurements that distinguish female Aphonopelma atomicum from its closely related phylogenetic and syntopic species are F3 and the extent of metatarsus IV scopulation. Female Aphonopelma atomicum can be distinguished by possessing a smaller A1/F3 (≤0.56; 0.53-0.56) than Aphonopelma mojave (>0.56; 0.56-0.67), Aphonopelma icenoglei (≥0.59; 0.59-0.67), and Aphonopelma joshua (≥0.57; 0.57-0.64); a smaller Cl/F3 (≤1.38; 1.31-1.38) than Aphonopelma prenticei (≥1.39; 1.39-1.64); and smaller L4 scopulation extent (37%-49%) than Aphonopelma iodius (59%-83%).

Description of male holotype

(AUMS_2727-2; Fig. 21). Specimen preparation and condition: Specimen originally preserved in unknown percentage of ethanol; original coloration faded due to preservation. Missing leg IV right side. Left legs I, III, IV, and left pedipalp removed for measurements and photographs; stored in vial with specimen. No tissue for DNA. General coloration: Brown/faded brown. Cephalothorax: Carapace 6.523 mm long, 6.382 mm wide; densely clothed with brown/faded brown pubescence, appressed to surface; fringe covered in long setae not closely appressed to surface; foveal groove medium deep and straight; pars cephalica region rises gradually from foveal groove on a straight plane towards the ocular area; AER slightly procurved, PER slightly recurved; normal sized chelicerae; clypeus mostly straight; LBl 0.918, LBw 1.107; sternum hirsute, clothed with faded brown, densely packed, short setae. Abdomen: Densely clothed in short brown pubescence with numerous longer, lighter setae interspersed (generally red or orange in situ); dense dorsal patch of black Type I urticating bristles ( Cooke et al. 1972) - smaller and distinct from large species. Legs: Hirsute; densely clothed in faded brown pubescence. Metatarsus I straight/very slightly curved. F1 7.691; F1w 1.62; P1 3.012; T1 6.618; M1 5.767; A1 3.83; F3 6.665; F3w 1.958; P3 2.436; T3 5.294; M3 6.438; A3 4.207; F4 7.922; F4w 1.596; P4 2.656; T4 6.615; M4 8.407; A4 4.358; femur III is swollen/slightly swollen. All tarsi fully scopulate. Extent of metatarsal scopulation: leg III (SC3) = 61.4%; leg IV (SC4) = 37.3%. No distinct ventral spinose setae on metatarsus III; three ventral spinose setae on metatarsus IV; two prolateral spinose setae on tibia I; one megaspine present on the retrolateral tibia, at the apex of the mating clasper; three megaspines on the apex of the retrolateral branch of the tibial apophyses. Coxa I: Prolateral surface covered by fine, hair-like setae. Pedipalps: Hirsute; densely clothed in the same setal color as the other legs, with numerous longer ventral setae; one spinose seta at the apical, prolateral femur and two prolateral spinose setae on the palpal tibia; PTl 4.486, PTw 1.51. When extended, embolus tapers with a curve to the retrolateral side; embolus slender, no keels; distinct dorsal and ventral transition from bulb to embolus.

Variation (5).Cl 6.523-7.917 (7.157 ± 0.23), Cw 6.382-6.798 (6.64 ± 0.09), LBl 0.832-0.988 (0.927 ± 0.03), LBw 1.098-1.394 (1.231 ± 0.06), F1 7.434-8.185 (7.761 ± 0.12), F1w 1.565-1.914 (1.783 ± 0.08), P1 2.859-3.481 (3.12 ± 0.11), T1 6.618-7.496 (7.053 ± 0.16), M1 5.767-6.402 (6.083 ± 0.12), A1 3.768-4.149 (3.976 ± 0.08), L1 length 26.653-28.926 (27.993 ± 0.5), F3 6.665-7.388 (6.962 ± 0.12), F3w 1.958-2.454 (2.223 ± 0.09), P3 2.355-2.84 (2.562 ± 0.08), T3 5.294-5.812 (5.58 ± 0.1), M3 6.438-7.349 (6.88 ± 0.15), A3 4.204-4.298 (4.252 ± 0.02), L3 length 25.04-27.374 (26.236 ± 0.39), F4 7.773-8.406 (8.073 ± 0.11), F4w 1.596-2.045 (1.816 ± 0.08), P4 2.621-2.9 (2.786 ± 0.06), T4 6.615-7.533 (7.11 ± 0.17), M4 8.197-9.072 (8.573 ± 0.15), A4 4.358-4.937 (4.64 ± 0.1), L4 length 29.958-32.832 (31.181 ± 0.54), PTl 4.486-4.966 (4.689 ± 0.08), PTw 1.433-1.736 (1.564 ± 0.06), SC 3 ratio 0.552-0.725 (0.626 ± 0.03), SC4 ratio 0.264-0.409 (0.356 ± 0.03), Coxa I setae = very thin tapered, F3 condition = slightly swollen/swollen.

Description of female paratype

(AUMS_3267-2; Figs 22-23). Specimen preparation and condition: Specimen originally preserved in unknown percentage of ethanol; original coloration faded due to preservation. Left legs I, II, III, IV, and pedipalp removed for photographs and measurements; stored in vial with specimen. No tissue for DNA. Genital plate with spermathecae removed and cleared, stored in vial with specimen. General coloration: Brown and faded brown. Cephalothorax: Carapace 7.291 mm long, 6.814 mm wide; Hirsute, densely clothed with short faded brown pubescence closely appressed to surface; fringe densely covered in slightly longer setae; foveal groove medium deep and procurved; pars cephalica region gently rises from thoracic furrow, arching anteriorly toward ocular area; AER slightly procurved, PER very slightly recurved - mostly straight; chelicerae robust, clypeus extends forward on a slight curve; LBl 1.058, LBw 1.416; sternum hirsute, clothed with short faded brown setae. Abdomen: Densely clothed dorsally in short faded brown setae with longer, lighter setae (generally red or orange in situ) focused near the urticating patch; dense dorsal patch of black Type I urticating bristles ( Cooke et al. 1972) - smaller and distinct from large species. Spermathecae: Paired and separate, short, with capitate bulbs widening towards the bases; not fused. Legs: Hirsute; densely clothed in short faded brown pubescence; F1 6.49; F1w 1.891; P1 2.912; T1 5.232; M1 3.952; A1 3.102; F3 5.539; F3w 1.789; P3 2.418; T3 4.188; M3 4.363; A3 3.418; F4 6.898; F4w 1.766; P4 2.753; T4 5.716; M4 5.993; A4 3.839. All tarsi fully scopulate. Extent of metatarsal scopulation: leg III (SC3) = 68.3%; leg IV (SC4) = 37.2%. One ventral spinose setae on metatarsus III; four ventral spinose setae on metatarsus IV, with numerous thickened setae throughout. Coxa I: Prolateral surface covered by very thin tapered and fine, hair-like setae. Pedipalps: Densely clothed in the same setal color as the other legs; one spinose seta on the apical, prolateral femur, five prolateral (three at the apical, prolateral border with the tarsus) spinose setae and one ventral spinose seta on the tibia.

Variation (3).Cl 7.081-7.978 (7.45 ± 0.27), Cw 5.951-7.105 (6.623 ± 0.35), LBl 0.922-1.218 (1.066 ± 0.09), LBw 1.105-1.621 (1.381 ± 0.15), F1 5.656-7.25 (6.465 ± 0.46), F1w 1.663-2.032 (1.862 ± 0.11), P1 2.912-3.253 (3.026 ± 0.11), T1 5.232-5.714 (5.394 ± 0.16), M1 3.461-4.532 (3.982 ± 0.31), A1 2.787-3.179 (3.023 ± 0.12), L1 length 20.052-23.928 (21.889 ± 1.12), F3 5.11-5.984 (5.544 ± 0.25), F3w 1.493-1.907 (1.73 ± 0.12), P3 2.046-2.81 (2.425 ± 0.22), T3 3.603-4.223 (4.005 ± 0.2), M3 3.981-4.61 (4.318 ± 0.18), A3 3.305-3.641 (3.455 ± 0.1), L3 length 18.045-21.268 (19.746 ± 0.93), F4 6.234-7.413 (6.848 ± 0.34), F4w 1.473-1.956 (1.732 ± 0.14), P4 2.541-2.92 (2.738 ± 0.11), T4 5.234-6.039 (5.663 ± 0.23), M4 5.687-6.245 (5.975 ± 0.16), A4 3.446-3.86 (3.715 ± 0.13), L4 length 23.142-26.477 (24.939 ± 0.97), SC3 ratio 0.683-0.755 (0.726 ± 0.02), SC4 ratio 0.372-0.49 (0.439 ± 0.03), Coxa I setae = thin tapered. Spermathecae variation can be seen in Figure 23.

Material examined.

United States: California: Inyo: Death Valley National Park, Hwy-178, Salsberry Pass, 35.923118 -116.431747 1, 3204ft., [APH_1478, 30/7/2012, 1♀, Brent E. Hendrixson, Brendon Barnes, Austin Deskewies, AUMNH]; Death Valley National Park, ~7.5 miles S Hwy-190 on Dante’s View Rd, 36.267976 -116.665576 1, 3242ft., [APH_1479, 30/7/2012, 1 juv, Brent E. Hendrixson, Brendon Barnes, Austin Deskewies, AUMNH]; Nevada: Nye: Rock Valley Wash, off Hwy-95, 36.622963 -116.309464 1, 2784ft., [APH_1549, 24/10/2012, 1♀, Brent E. Hendrixson, AUMNH]; Rock Valley, 36.632732 -116.313933 5, 2884ft., [AUMS_2637, 20/10/1972, 1♀, E.L. Sleeper, AUMNH]; Mercury, 36.66051 -115.994475 5, 3796ft., [APH_2725-1, 6/9/1960, 2♀, 14♂, Gertsch, AMNH]; Nevada Testing Site, 37.025579 -116.023865 6, 3990ft., [APH_2725-2, 1/10/1959, 3♂, JRM, AMNH]; [APH_2727, 29/6/1961, 3♀, 15♂, Gertsch, AMNH]; [APH_2727-2, 8/1961, 1♂, Gertsch, AMNH]; [APH_2730, 16/8/1964, 1♀, unknown, AMNH]; Nros Rd., 20 miles W of Mercury, 36.66182 -116.368 5, 2744ft., [AUMS_2638, 3/11/1972, 1♂, C. S. L., AUMNH]; 10 miles W of Mercury, Mercury Valley, 36.691928 -116.174483 5, 3470ft., [AUMS_3267 & 3267-2, 3/11/1972, 2♀, W. Icenogle, AUMNH].

Distribution and natural history.

Aphonopelma atomicum is only known from a handful of specimens surrounding the Amargosa Desert in southern Nye County (Nevada) and southeastern Inyo County (California), including the Amargosa Range and Nevada Test Site (Fig. 24). Specimens have been collected at elevations between 850 and 1220 meters, inhabiting the Mojave Basin and Range Level III Ecoregion. This species is likely syntopic with Aphonopelma iodius throughout its range and may be found near populations of Aphonopelma prenticei . Burrow entrances are generally surrounded by a distinct mound or turret made of excavated soil and silk (Fig. 2 D–E). Mating most likely occurs during daylight hours in autumn (October-November).

Conservation status.

Aphonopelma atomicum has a highly restricted distribution limited to the mountains and foothills surrounding the Amargosa Desert and Death Valley. While this species is not dramatically different from Aphonopelma prenticei , it is genetically unique and should be considered important. The species is most likely secure.

Remarks.

Aphonopelma atomicum is unique because it was quite possibly the first miniature tarantula species collected in the United States (although it was never described and sat on a shelf in the AMNH collection). It is somewhat puzzling that Gertsch never described this species given the number of individuals he collected and how radically different (i.e., small in size) specimens are from all other Aphonopelma in that region. Other important ratios that distinguish males: Aphonopelma atomicum possess a smaller A1/F3 (≤0.58; 0.55-0.58) than Aphonopelma mojave (≥0.58; 0.58-0.63) and Aphonopelma prenticei (≥0.58; 0.58-0.65). Certain morphometrics have potential to be useful, though due to the amounts of variation, small number of specimens, and the small differences between species, no other are claimed to be significant at this time (see Suppl. material 2). During evaluation of traditional two-dimensional PCA morphospace and three-dimensional PCA morphospace (PC1~PC2~PC3), males of Aphonopelma atomicum separate from Aphonopelma iodius , Aphonopelma joshua , and Aphonopelma xwalxwal along PC1~2, but do not separate from Aphonopelma mojave , Aphonopelma icenoglei , or Aphonopelma prenticei . Female atomicum separate from Aphonopelma iodius in morphological space, but do not separate from the other miniature species ( Aphonopelma mojave , Aphonopelma icenoglei , and Aphonopelma prenticei ). There are no known female Aphonopelma xwalxwal at this time to compare. PC1, PC2, and PC3 explain ≥97% of the variation in all analyses.