Austrolebias accorsii Nielsen & Pillet, 2015

Drawert, Heinz Arno & Ergueta, Carlos, 2024, Redescription of Austrolebias accorsii (Cyprinodontiformes: Rivulidae) and description of a new species of the genus from the upper Paraguay River basin, Neotropical Ichthyology (e 240001) 22 (2), pp. 1-29 : 5-12

Austrolebias accorsii Nielsen & Pillet, 2015

( Figs. 1–2 ; Tab. 1)

Austrolebias accorsii Nielsen, Pillet, 2015a:174 , figs. 1–2 (photos, original description). —Nielsen, Pillet, 2015b:186 (list of syntopical species). —Alonso et al., 2016:5 (comparison). —Alonso et al., 2018:2 (mention). —Serra, Loureiro, 2018:547 (mention). —Volcan, Severo-Neto, 2019:542 (mention). — Zamorano, 2019:38 (list of new species for Bolivia). —Alonso et al., 2023:2 (mention).

Austrolebias vandenbergi Huber, 1995 . —Montaña et al., 2012:589, fig. 1 (photo, misidentified specimens from Santa Cruz, Bolivia).

Austrolebias vandengerbi . —Montaña et al., 2012:590 (misspelled).

Diagnosis. Males of Austrolebias accorsii differ from all congeners, except A. vandenbergi , by absence of whitish to light blue dots or bars on flanks (vs. presence of vertical rows of dots in Austrolebias n. sp. (described below), A. bellottii , and A. univentripinnis ; vertical bars in A. ephemerus , A. melanoorus , and A. queguay ) and by urogenital papilla not attached to anal fin; and, except Austrolebias n. sp., A. ephemerus , and A. vandenbergi , by presence of tree or more transverse rows of scales on basal anterior and median portion of anal fin. It can be distinguished from A. vandenbergi by the absence of white or whitish dots on unpaired fins (vs. presence), presence of well-defined black marginal line on anal fin (vs. if dark margin present, broader and attenuated), and dark blotch on dorsum between end of head and dorsal-fin origin (vs. absent or inconspicuous). Males of A. accorsii , as well, differ from those of A. queguay by having more anal-fin rays (28–36 vs. 24–27); and from A. melanoorus by more longitudinal series of lateral scales (30–34 vs. 26–29) and more posterior dorsal-fin origin (vertical between 4 th and 6 th anal-fin ray vs. anterior to anal-fin origin). Females of A. accorsii can be distinguished from all other congeners, except Austrolebias n. sp. and A. ephemerus , by a more posterior dorsal-fin origin (vertical between 5 th and 10 th anal-fin ray vs. anterior to 5 th anal-fin ray); from A.melanoorus , A. queguay , A. univentripinnis , and A. vandenbergi by posterior tip of pectoral fins reaching vertical between 3 rd and 7 th anal-fin ray (vs. anterior to 2 nd anal-fin ray); from A. melanoorus and A. univentripinnis by a longer prepelvic length (41.3–48.5% SL vs. 52.6–59.2%, 52.5–57.7%, respectively); from A. queguay by having more anal-fin rays (25–35 vs. 21–24).

Description. Morphometric and meristic data presented in Tab. 1. Males larger than females; largest examined male 55.2 mm SL, largest female 42.6 mm SL. Body elongated, moderately deep and laterally compressed. Highest body depth approximately on vertical at anal-fin origin. On lateral view, dorsal profile of head straight to slightly concave from snout to vertical between eye and superior limit of operculum, from this point to posterior end of dorsal-fin base convex, and straight to slightly concave on caudal peduncle; ventral profile slightly convex from lower jaw to pelvic-fin base, from there to end of anal-fin base straight to slightly convex, and straight to slightly concave on caudal peduncle. Snout blunt and jaws short.

Tip of dorsal and anal fins rounded in both sexes; in females, anteromedial anal-fin rays lengthened resulting in triangular shaped anal fin; dorsal-fin origin on vertical between 4 th and 6 th anal-fin ray in males, and 5 th to 10 th in females; dorsal-fin origin between neural spines of 9 th to 10 th vertebrae in males and 10 th to 11 th in females; anal-fin origin between pleural ribs of 6 th to 8 th vertebrae in males and 5 th to 6 th in females. Caudal fin rounded. Pectoral fin elliptical; posterior margin on vertical between 4 th and 7 th anal-fin ray in males, between 3 rd and 7 th in females. Pelvic fin membrane medially coalesced 50–75% of length, posterior tip reaching between 1 st and 4 th anal-fin ray in males, and between 2 nd and 3 rd in females. Urogenital papilla in males attached to anal fin only basally and maximum 10% of length, prominent in females. Dorsal-fin rays 22–32 in males and 20–27 in females; anal-fin rays 28–36 in males and 25–35 in females; caudal-fin rays 20–26 in males and 24–28 in females; pectoral-fin rays 12–14 in males and 11–13 in females; pelvic-fin rays 4–5 (one male with 3 and two with 7, one female with 6) in both sexes.

Scales cycloid, large. Trunk and head completely scaled, except ventral surface of head. Frontal squamation H-patterned or sometimes G-patterned (two males), E-scales usually overlapping medially; scales transversally arranged on trunk. No scales surpassing base of dorsal, pectoral, and pelvic fins in both sexes; in males 3 to 6, and in females 1 to

3, rows of scales over-passing anal-fin base on anterior and median portion; 3 rows of scales on caudal-fin base in both sexes. Longitudinal series of scales 30–34, transversal

series of scales 14–18, and circumpeduncular rows of scales 16–20 (one female with 21), in both sexes. Males with 1–3 contact organs per scale on anterior central and

ventral section of flanks and opercular region; rows of contact organs on 3–4 uppermost pectoral-fin rays and on anterior distal anal-fin rays; no contact organs on other fins and not present on females.

Cephalic neuromasts: supraorbital 14–19 (usually 16–18), parietal 1–4 (usually 2), anterior rostral 1–2 (usually 2, sometimes 3 adding both sides of head), posterior rostral 1–2 (usually 1), infraorbital 2 + 24–26, preorbital 2–3 (usually 2), otic 1–4 (usually 3), post-otic 4–5, lateral supratemporal 1–2 (usually 1), median opercular 1, ventral opercular 1–4 (usually 2–3), preopercular 21–25, mandibular 13–16 (usually 14), lateral mandibular 4–8 (usually 7), post-temporal 1–2 (usually 1). Lateral line complete, with one neuromast per scale.

Total number of vertebrae 30–32. Hypurals ankylosed in a single plate without visible fissure. Six branchiostegal rays, arranged in 3 pairs. Gill-rakers on 1 st branchial arch 6–8 + 14.

Coloration in life. Males ( Fig. 1 ). Head greenish gray, opercular region bluish gray. Black bar crosses vertically the eye; subrectangular at suborbital portion with tip directed slightly posteroventrally, and subtriangular at supraocular portion slightly surpassing dorsally the line of parietal neuromasts. Iris whitish gray to pale yellow with black bar through center of eye. Trunk and caudal peduncle ground color greenish gray to greenish pale brown, darker on dorsal region and paler on anteroventral region. Neck notoriously dark. Subadults usually with golden-metallic shine on anterior dorsal region. Flanks and caudal peduncle sides with 8–17 dark vertical bars of same color as most dorsal portion of flanks, usually curved on posterior region of flanks and caudal peduncle; conspicuous on juveniles and fading with age, almost disappearing in senescent specimens. Sometimes one or two dark greenish metallic blue, vertically elongated, spots on anteromedial region of flanks, approximately on or some anterior to center of imaginary axis between origins of dorsal and anal fins, fading with age and absent in older males. Pectoral fins hyaline, ventrally, and sometimes also distally, with black margin. Pelvic fins whitish to gray, usually with black tips. Dorsal fin greenish gray on basal half and more bluish gray on distal, sometimes with rosy hue, and usually with black posterior distal margin; in subadults distal half hyaline to pale gray. Anal fin gray to bluish gray with conspicuous black distal margin. Caudal fin light blue to pale turquoise, sometimes hyaline distally, especially in subadults.

Females ( Fig. 2 ). Head greenish yellow to yellowish brown, opercular region pale bluish gray to greenish gray. Sometimes inconspicuous dark vertical bar through eye. Iris whitish gray to pale yellow with dark bar through center of eye. Trunk and caudal peduncle ground color greenish pale yellow to yellowish brown, anteroventral region pale yellow to whitish. Flanks and caudal peduncle sides with dark, sometimes bluish metallic shining, vertically elongated spots, in some cases at medial flanks coalesced forming vertical bars between dorsal and anal fin bases. On anteromedial flanks, some anterior to the center of the linear axis between anal- and dorsal-fin bases, 1–4 black, sometimes greenish to bluish iridescent, vertical elongated blotches. Fins hyaline; dorsal fin sometimes pale yellowish at basal half and/or with some black dots; anal fin sometimes with pale light blue shine, and/or pale yellowish at anterior and median basal section and with some black dots on posterior basal section.

Coloration in alcohol. Overall color pattern similar to that of live specimens, but less intense and faded, without iridescence and shines, or color hue. Body background color yellowish gray to pale yellowish brown in both sexes.

Geographical distribution. Austrolebias accorsii is the only species of the genus found in the Amazon basin, and with distribution outside the La Plata basin. The distribution is restricted to the upper Iténez/Guaporé River drainage (hydrologic unit HU 62268), southern of the Chiquitos montain range, in and around the “Bañados del Izozog” wetlands in the watersheds of the Tunas (HU 6226858) and Quimome (HU 622689 and 622688) rivers. Therefore, the species is endemic to this area in the Cordillera Province of Santa Cruz Department in Bolivia ( Fig. 3 ).

Ecological notes. The species is exclusively found in temporary pools with muddy waters in the dry Chaco forests of the northern section of the Chacoan biogeographic province, in the Izozog district ( Fig. 4 ). The depth of these water bodies does not exceed 50 cm, and the substratum typically consists of a thin layer of plant litter and debris (1 to 2 cm) on a clay bottom. The pools lack submerged aquatic vegetation but feature floating and emergent vegetation, mainly from the Poaceae and Polygoniaceae families, typically limited to the riparian zone. The only sympatric fish species found in these aquatic ecosystems are other rivulids, such as Neofundulus aff. paraguayensis (Eigenmann & Kennedy, 1903) , Titanolebias monstrosus (Huber, 1995) , Trigonectes aplocheiloides (Huber, 1995) , and in the northeasternmost locations, also Spectrolebias brousseaui Nielsen, 2013 . The annual minimum and maximum temperatures range from 1.9–9.9°C to 34.8–43.8°C (averages of 5.7–6.2°C and 39.2–40.6°C for different locations), with minimums concentrated in the months of May to August and maximums in September to November. Annual accumulated precipitation in different locations varies from 564– 633 mm to 1366–1566 mm (averages of 886–1018 mm), predominantly concentrated in the months of December to March.

Conservation status. The species has an estimated extent of occurrence (EOO) of less than 20,000 km 2. The estimation of the area of occupancy (AOO) is complex, considering that this species inhabits small ephemeral and temporary aquatic ecosystems (often less than 100 m 2) in the dry forest of the northern Chacoan biogeographic province, which are generally scattered, isolated, and challenging to quantify using telemetric methods. This indicates a severely fragmented distribution, further exacerbated by the fact that there are only three known subpopulations of the species in two locations. Although approximately 50% of the EOO is within the Kaa-Iya del Gran Chaco National Park and Integrated Management Natural Area, the other half faces significant pressure on natural ecosystems. In the last 25 years, and more intensely in the last 10 years, these ecosystems have been replaced by large-scale agro-industrial crops over extensive areas. It is inferred that this trend of advancing agricultural frontiers, especially in the western and northern parts of the EOO, will continue or even accelerate in the coming years due to recent government policies promoting the expansion of agricultural activities in the region. The announced introduction of genetically modified drought-resistant crops in the near future poses a significant threat, as adverse climatic conditions for conventional crops have so far been the primary obstacle to the expansion of industrialized agriculture in the Dry Chaco ecoregion of Bolivia. On the other hand, the direct and indirect effects of climate change, such as changes in hydrological regimes or an increased incidence of wildfires, also represent potential threats to the conservation status of this species. Following the guidelines for categories and criteria of the International Union for Conservation of Nature (IUCN), the species may be considered Vulnerable (VU) as it meets criteria A3c, A4c, B1ab, and D2 (IUCN Standards and Petitions Committee, 2022).

Material examined. ZUEC 10792 , holotype (photo only), male, 37.7 mm SL, Bolivia, Santa Cruz, [Charagua Iyambae Guarani Autonomous Territory], temporary pool, [hydrologic unit 6226858 (Pfafstetter), Parapetí watershed , Iténez / Guaporé drainage, Amazon basin (corrected)], 18°39’26.2”S 62°58’00.2”W, 10 Apr 2014 , V. Chaverou , B. Accorsi, M. Beuchey, S. Blois, E. Vandekerkhove & D. Pillet . ZUEC 10793 , paratype (photo only), 1 female, 39.2 mm SL, collected with holotype. Non-types : MNKP 16550 , 3 (juveniles), Bolivia, Santa Cruz, Charagua Iyambae Guarani Autonomous Territory, temporary pool near Izozog on road from Abapó to Pailón , hydrologic unit 6226858 ( Pfafstetter ), Parapetí watershed , Iténez / Guaporé drainage, Amazon basin, 18°46’42”S 63°07’44”W, 30 Apr 2021, H. A. Drawert & J. C. Catari . MNKP 16551 , 3 (2 males, 1 female), Bolivia, Santa Cruz, Charagua Iyambae Guarani Autonomous Territory, temporary pool near Izozog on road from Abapó to Pailón , hydrologic unit 6226858 ( Pfafstetter ), Parapetí watershed , Iténez / Guaporé drainage, Amazon basin, 18°44’24”S 63°03’57”W, 30 Apr 2021, H. A. Drawert & J. C. Catari . MNKP 16552 , topotypes, 23 (7 males, 16 females), 39.3–48. 6 mm SL (males) 33.7– 42.6 mm SL (females), Bolivia, Santa Cruz, Charagua Iyambae Guarani Autonomous Territory, temporary pool near Izozog on road from Abapó to Pailón , hydrologic unit 6226858 ( Pfafstetter ), Parapetí watershed , Iténez / Guaporé drainage, Amazon basin, 18°39’27”S 62°58’00”W, 30 Apr 2021, H. A. Drawert & J. C. Catari . MNKP 16553 , topotypes, 4 (1 male, 3 females), collected with MNKP 16552 . MNKP 16556 , 16 [7 (2 c&s) males, 9 (1 c&s) females], 32.4–42.3 mm SL (males) 29.2–31.6 mm SL (females), Bolivia, Santa Cruz, Charagua Iyambae Guarani Autonomous Territory, temporary pool near Izozog on road from Abapó to Pailón , hydrologic unit 6226858 ( Pfafstetter ), Parapetí watershed , Iténez / Guaporé drainage, Amazon basin, 18°39’49”S 62°59’31”W, 30 Apr 2021, H. A. Drawert & J. C. Catari . MNKP 16583 , 6 , Bolivia, Santa Cruz, Charagua Iyambae Guarani Autonomous Territory, Ñembi Guasu Conservation and Ecological Importance Area , temporary pool near Tucabaca Camp on road to San José de Chiquitos , hydrologic unit 6226882 ( Pfafstetter ), Parapetí watershed , Iténez / Guaporé drainage, Amazon basin, 18°23’15”S 60°49’48”W, 4 Jun 2021, C. Ergueta & H. A. Drawert . MNKP 16584 , 3 (2 males, 1 female), 43.8–55.2 mm SL (males), 39.8 mm SL (female), Bolivia, Santa Cruz, Charagua Iyambae Guarani Autonomous Territory, Ñembi Guasu Conservation and Ecological Importance Area , temporary pool near Tucabaca Camp on road to San José de Chiquitos , hydrologic unit 6226882 ( Pfafstetter ), Parapetí watershed , Iténez / Guaporé drainage, Amazon basin, 18°23’38”S 60°49’38”W, 4 Jun 2021, C. Ergueta & H. A. Drawert .

Figures

FIGURE 1 | Austrolebias accorsii. Males (1 day after collection). Left: MNKP 16556, 32.4–42.3 mm SL, Bolivia, Santa Cruz, Cordillera, Charagua Iyambae Guarani Autonomous Territory. Right: MNKP 16552, topotypes, 39.3–48.6 mm SL, Bolivia, Santa Cruz, Cordillera, Charagua Iyambae Guarani Autonomous Territory.

FIGURE 2 | Austrolebias accorsii. Females (1 day after collection). MNKP 16552, topotypes, 33.72–42.61 mm SL, Bolivia, Santa Cruz, Cordillera, Charagua Iyambae Guarani Autonomous Territory.

FIGURE 3 | Occurrence localities of Austrolebias species in the northern Chacoan province. Stars represent type-localities, colors differentiate the species (A. accorsii red, Austrolebias n. sp. yellow, A. ephemerus green, A. vandenbergi blue). Blue numbers indicate the Hydrologic Unit (Pfafstetter, level 5). Sources: Ecoregions from Olson et al. (2001); Hydrologic Units from Lehner, Grill (2013); Basemap ESRI World Topo (2022).

FIGURE 4 | Biotope at type-locality of Austrolebias accorsii. Bolivia, Santa Cruz, Cordillera, Charagua Iyambae Guarani Autonomous Territory, 18°39’26.2”S 62°58’00.2”W, 30 April 2021.