Austrolebias

Wilson J. E. M. Costa, 2006, The South American annual killifish genus Austrolebias (Teleostei: Cyprinodontiformes: Rivulidae): phylogenetic relationships, descriptive morphology and taxonomic revision., Zootaxa 1213, pp. 1-162 : 10-21

Austrolebias Costa

Austrolebias Costa, 1998a: 75 (type species: Cynolebias bellottii Steindachner, by original designation).

Megalebias Costa, 1998a: 76 (type species: Cynolebias wolterstorffi Ahl, by original designation).

Diagnosis

Similar to Cynolebias and distinguished from other cynolebiatine genera in possessing the following synapomorphic features: tip of pterotic posterolateral rim thickened and directed laterally, presence of a process on the dorsomedial portion of the palatine, dorsal portion of opercle about as long as ventral portion, or narrower; medial and distal radials of dorsal and anal fins poorly ossified to completely cartilaginous; and cartilaginous space of shoulder girdle broad; radials discoid, thin, the uppermost radial usually absent or extremely reduced. Other synapomorphies shared by Cynolebias and Austrolebias , but homoplastically occurring in other cynolebiatines, are a) dermosphenotic absent, and b) a narrowed dorsal portion of metapterygoid. Austrolebias differs from all other cynolebiatines in possessing the following synapomorphies: a) absence of scales between corner of mouth and anterior portion of preopercular region and ventral portion of opercular region, b) a deep urohyal, and c) a dark gray to black infraorbital bar and supraorbital spot. Other synapomorphies of Austrolebias , but independently acquired by other cynolebiatines are: a) dorsal and anal fins rounded in males, b) a long urogenital papilla in male, and c) a reduced ventral process of angulo-articular.

Osteology

The description of osteological structures below focuses on features considered phylogenetically informative among rivulids (e. g., Parenti, 1981; Costa, 1998a, 1998c, 2001, 2002a, in press, and present paper) or important for diagnosing taxa (i. e., autapomorphies). For a recent detailed description of osteological structures in cynolebiatines, see Costa (in press).

Superficial dermal bones and neurocranium (Fig. 2). Nasal scale-like, thin, with short ventral keel. Lacrymal thin and twisted, almost cylindrical, the dorsal portion widened and anteriorly directed; the lacrymal distinctively curved in A. elongatus species group. Dermosphenotic absent; sometimes present in A. bellottii species group, one or two minute ossifications on dermosphenotic region.

Neurocranium slightly depressed. Dorsal surface bones thin, the limits scarcely visible. Frontal approximately rectangular, with concave lateral borders, occupying about anterior two thirds of neurocranium roof. Parietal short, subtriangular, sometimes possessing a distinctive transverse keel. Supraoccipital with short paired posterior process. Sphenotic with narrow lateral process in most species, but broad with anterolateral process in A. elongatus species group. Vomer subtriangular, without teeth. Lateral ethmoid compact, with distinctive anterior retrorse process, but shorter in A. elongatus species group; anteromedian portion of lateral ethmoid slightly overlapping lateral portion of vomer, medial margin not reaching lateral surface of anterior process of parasphenoid, or sometimes abutting it in A. elongatus species group. Parasphenoid cross-shaped, the anterior process dorsally overlapping posterior process of vomer, the median portion sometimes widened, the posterior process firmly attached ventrally to basioccipital, each short lateral process attached to prootic; and posterior process distinctively widened in A. elongatus species group. Pterotic with thickened, laterally-directed tip.

Jaws, jaw suspensorium and opercular apparatus (Fig. 3). Premaxilla elongate, with teeth on medial half of anterior border, and trapezoidal anterior process of alveolar arm; premaxilla narrower and longer, the alveolar arm reduced almost imperceptibly in some species of A. elongatus group; ascending process rectangular, flattened, shorter in A. elongatus species group; some species of A. elongatus group with distinct concavity on anteromedial portion of ascending process. Maxilla placed dorsolaterally to premaxilla, elongate, slightly curved, bifid on anterior medial portion. Rostral cartilage large and ovoid. Dentary with teeth on anterior two thirds of anterodorsal surface; ventroposterior process prominent; dentary distinctively longer in some species of A. elongatus group. Angulo-articular with reduced ventral process. Retro-articular small, often with sharp anterior process. Premaxillary and dentary teeth arranged in irregular rows; a few distinctively larger fang-like teeth on most external row; in some species of A. elongatus group teeth of outer row proportionally smaller, more curved and numerous.

Palatine and ectopterygoid completely fused, with prominent dorsomedial process. Mesopterygoid thin, the posterior tip separated from metapterygoid by a cartilaginous interspace, ventral portion abutting or not contacting dorsal portion of quadrate. Quadrate subtriangular, with anterior and ventral margins approximately straight, posterior margin rounded; and posterior process short. Metapterygoid slender, sometimes ventral portion widened, becoming abruptly narrower dorsally. Opercular bones thin, dorsal portion of opercle attenuated; dorsal portion of preopercle short and pointed, medial flap reduced.

Hyoid and branchial arches (Fig. 4). Basihyal usually triangular and flattened, often wide; basihyal cartilage short in A. elongatus species group, but broad, occupying most portion of basihyal in other congeners. Dorsal and ventral hypohyals small, always ossified. Anterior ceratohyal usually wide and short, to distinctively elongated in some species of A. elongatus group; two narrow branchiostegal rays attached to ventral border of anterior ceratohyal, except in A. wolterstorffi , which has three rays. Anterior and posterior ceratohyals separated by broad space of cartilage, and supporting four branchiostegal rays. Posterior ceratohyal compressed, subtriangular; oval and cartilaginous interhyal attached to posterior end of posterior ceratohyal. Urohyal thin and deep, with short ventrolateral flap and long anterodorsal process; in some species of A. elongatus group the urohyal is slender, and the dorsal process is short and anterodorsally directed.

Epibranchials and interarcual cartilage long. Distal tip of first epibranchial with elongate cartilage, often with minute ossification. Second pharyngobranchial approximately triangular, shortened in A. elongatus species group; often two to six teeth of second pharyngobranchial, sometimes teeth absent; teeth numerous (8-12) in A. bellottii , A. apaii and A. vandenbergi (see species descriptions below); medial condyle perpendicular to dorsal condyle, except in species of the A. robustus group, with medial condyle obliquely positioned. Basibranchials 1-3 ossified, rectangular; in species of the A. robustus group, basibranchial 2 wider, with pronounced lateral tips; basibranchial 4 cartilaginous, large, approximately lozenge-shaped. Hypobranchials 1-3 thin, rounded, with broad marginal cartilages; proximal edge of hypobranchial 1 without vestige of branching; ceratobranchials 1-4 narrow, without teeth; ceratobranchial 5 boomerangshaped, anterior portion shorter than posterolateral portion, except in some species of the A. bellottii group and in A. cinereus ; ceratobranchial 5 with well-developed dentigerous plate and triangular laminar process on ventral surface.

Vertebrae and fin support (Fig. 5). First vertebra with long neural spine bearing anterior and posterior laminar extensions, longer and narrower in male; neural prezygapophysis absent, neural postzygapophysis reduced or absent; neural spine of second vertebrae similar, neural spine of third vertebra distinctively narrower. Pleural ribs robust, firmly attached to parapophyses; epipleural ribs thin and short. Neural prezygapophyses of caudal vertebrae vestigial or absent, neural postzygapophyses short or absent. Total vertebrae 26-32 in most species of Austrolebias , but 30-35 in A. robustus and A. nonoiuliensis and 32-39 in A. elongatus species group (see species descriptions below).

Epural and parhypural similar in shape, proximal portion narrowed, often slightly curved with tip anteriorly directed. All hypurals ankylosed, without vestige of gaps. Accessory cartilages absent. Caudal-fin rays 20-35 (see species descriptions below).

Males with more dorsal and anal-fin rays than females in most species, but both sexes with approximately same number of rays in A. elongatus species group, except in A. cheradophilus . Dorsal-fin rays 17-28 in males, 15-23 in females; anal-fin rays 19-32 in males, 14-29 in females (see species descriptions below). Most dorsal and anal-fin rays branched. Two rays associated with first proximal radial of dorsal fin, and two or three associated with first proximal radial of anal fin; first anal-fin proximal radials widened; median and the distal radials of dorsal and anal fin usually cartilaginous, sometimes poorly ossified.

Posttemporal usually elongated and forked; ventral process often reduced or absent. Supracleithrum elongate. Dorsal portion of cleithrum long and nearly triangular. Ventral tip of cleithrum and ventral tip of coracoid in close proximity. Scapula rounded, separated from coracoid by broad cartilage; coracoid subtriangular, with broad anterior condyle; narrow posterior process on coracoid directed to ventralmost proximal radial, sometimes inconspicuous. Proximal radials rounded and thin, surrounded by broad cartilage, uppermost radial vestigial or absent. Third postcleithrum narrow and long, often contacting first pleural rib; first and second postcleithra absent. Pelvic bone narrow, without a distinct ischial process, small in A. elongatus species group, sometimes poorly ossified; sometimes pelvic bones in contact or medially coalesced. Pectoral-fin rays 10-14; pelvic-fin rays 3-6 (see species descriptions below).

Remarks

Loureiro and de Sá (1998) reported the occurrence of two rows of conical teeth on ceratobranchials 1-3, longer on ceratobranchial 1, and a single row of similar teeth on the ceratobranchial 4 and epibranchial 2 in species of Austrolebias . This would be surprising, since these conditions are not found in other cyprinodontiforms and are rarely present in teleostean fishes. Examinination of the position and arrangement of these putative teeth and the figure 6-O in Loureiro and De Sá (1998) makes it clear, however, that these alleged structures are, in fact, gill-rakers.

Loureiro and de Sá (1998) also reported a tooth on the vomer of A. luteoflammulatus , A. nigripinnis , and A. viarius . However, as a result of careful examination of thousands specimens of all cynolebiatine species, it can be stated that vomerine teeth are always absent in Simpsonichthys and Austrolebias , but are present in Nematolebias and a few species of Cynolebias (e. g., Costa, 1998a, 2001, 2002a, in press, and present study). The supposed vomerine tooth illustrated in Loureiro and de Sá (1998: fig. 4) is a small process commonly found among aplocheiloid fishes.

Laterosensory system

(Fig. 6)

Laterosensory system highly elaborate on head, always represented by exposed neuromasts, which in turn may comprise distinct series that often vary greatly in number among species (see species descriptions below). Dorsal surface of head includes anterior and posterior rostral series, a single supraorbital series, and a parietal series. Lateral surface includes pre-orbital, infraorbital, otic, post-otic, supratemporal, preopercular, median and ventral opercular, and lateral mandibular series; infraorbital series comprises two sections, one short anterior and oblique to eye, and one around eye always continuous along its extension; otic series always continuous to infraorbital series, post-otic and otic sometimes continuous in species with numerous neuromasts, often also continuous to lateral line of trunk; irregular row of neuromasts parallel to preopercular series; always single isolated neuromast near mouth corner (paramandibular neuromast according to Costa, in press). Ventral surface includes mandibular series, sometimes connected to preopercular series. Lateral line of trunk complete, with one neuromast per scale.

Contact organs

Contact organs occur in males of all species of Austrolebias , but distribution and size highly variable among species (see species descriptions below). These organs often on trunk and opercle scales, usually minute to small and one per scale; sometimes prominent and more than one per scale; these usually restricted to ventral region, but sometimes extending to entire flank and other times restricted to anteroventral region. Often small or minute papillate contact organs on inner surface of uppermost pectoral-fin rays, and sometimes on most pectoral rays; these structures sometimes prominent, with osseous support (Fig. 5h), and sometimes also on outer surface of pectoral-fin rays (see species descriptions below). Often minute contact organs on distal portion of anal-fin rays, these sometimes prominent, with osseous support formed by expansion of fin rays, on most anal-fin rays; sometimes on distal portion of dorsal-fin rays and on caudal-fin rays.

Squamation

Scales thin and cycloid. Scales minute and irregularly arranged in most species of A. elongatus group. Scales always absent on ventral portion of head and between corner of mouth and preopercular region; sometimes absent on venter, opercular region and above anal fin; in A. cheradophilus , body squamation vestigial (see species descriptions below).

Body squamation often extends slightly over anal-fin base, but in some species of A. elongatus group and in A. vandenbergi there are some transverse rows of scales on anal-fin base. Pectoral-fin base without scales and caudal-fin base with few transverse rows of scales, except in A. elongatus and A. monstrosus , in which minute scales extend over pectoral-fin base and small scales cover about half of caudal fin.

Frontal squamation (Fig. 6). Highly variable, never forming a circular pattern as in basal rivulids. Frontal scales small in species of A. robustus group, ranging to minute in A. elongatus species group. Anterior frontal scales absent in A. elongatus species group. Usually, two or three overlapped scales between supraorbital series of neuromasts and eye. Frontal scales with E, F, G or H pattern (Fig. 6d) (see species descriptions below). Paired E -scales overlapping or not. In A. elongatus species group, frontal scales minute and irregularly arranged.

Fin morphology

Dorsal and anal fins rounded in both sexes. Filamentous rays never on tip of dorsal and anal fins, as in Cynolebias and Simpsonichthys (Costa, 2001; in press), but short filamentous rays on distal margin of dorsal fin and sometimes anal fin in A. patriciae male. Distal portion of anal fin always thickened in female. Subanterior rays of anal fin elongated in A. bellottii species group female. Caudal fin rounded to subtruncate, without posterior extensions. Pectoral fin elliptical and long. Pelvic fin short, without filamentous extensions. Pelvic-fin bases in close proximity or united; medial membrane sometimes variably coalesced among species (Fig. 6f) (see species descriptions below).

Urogenital papilla

Long and cylindrical in male, pocket-like in female. Attached by thin membrane to anterior portion of anal fin in some species of A. bellottii group (Fig. 6e).