Scanning electron microscopy of the oral apparatus and buccopharyngeal cavity of Atelognathus salai larvae (Anura, Neobatrachia)

ECHEVERRÍA, D. D; UBEDA, C. A .; BASSO, N. G.

PHYLLOMEDUSA

ESALQ - USP

Lugar: Sao Paulo; Año: 2006 vol. 5 p. 77 - 81

1519-1397

Frogs of the genus Atelognathus form a clade of nine narrowly distributed species restricted to Patagonia, southern Argentina and Chile, characterized by a large, exposed frontoparietal fontanelle, short palatines, large nasals and by the absence of quadratojugals and middle ear elements. The genus is currently classified within the Telmatobiinae, a subfamily that has been considered the stem group of the family Leptodactylidae (Lynch 1978) Atelognathus salai Cei, 1984 is known only from its type locality: Laguna de los Gendarmes and nearby ponds, north of Lake Buenos Aires, Santa Cruz Province, Argentina. Of the nine Atelognathus species known today (Basso 1998, Meriggio et al. 2004), only the tadpoles of A. patagonicus (Cei 1965), A. reverberii (Cei 1969), A. nitoi (Basso and Úbeda 1997), A. salai (Úbeda and Basso 2003), and A. jeinimenensis (Meriggio et al. 2004) have been described. Wassersug and Heyer (1988) described the microanatomy of the buccopharyngeal cavity of Atelognathus reverberii and A. patagonicus. Echeverría et al. (2001a) summarized the general features of the oral morphology of the genus Atelognathus in their description of the fine surface structure of the buccopharyngeal cavity of Atelognathus nitoi. The aim of this study is to describe the horny structures of the buccal apparatus and buccopharyngeal cavity of A. salai by means of scanning electron microscopy (SEM), and to compare them to those of the other known species of Atelognathus and related genera. The morphological descriptions are based on two tadpoles of  A. salai collected from the type locality, Laguna de los Gendarmes (46°6`S, 71°41`W), Santa Cruz Province, Argentina. The specimens, at stages 37 and 38 of Gosner’s normal development table (Gosner 1960), are deposited at  the collection of the  Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires  (LARV-DDE-FCEN- 228-229).  The specimens were  treated according to the techniques for scanning electron microscopy (SEM) described by Wassersug (1980) and Echeverría (1995), which included a rapid dehydration in ethyl alcohol (80%, 8 h; 96%, 2 h; 100%, 1 h), ethyl alcohol-acetone (3:1, 1 h; 1:1, 1 h; 1:3, 1 h) and pure acetone not more than two hours before the final dehydration. Critical-point dehydration was done in a Balzers® 030 vacuum dryer. Specimens were coated in gold using an ION Balzers® CPD 040 sputter coater. A Philips® 505 scanning electron microscope was used for observations and photographs. Terminology in the descriptions of the horny structures of the buccal apparatus and buccopharyngeal cavity follows Deunff and Beaumont (1959),Viertel (1982) and McDiarmid and Altig (1999). The dental formula terminology is based on Altig (1970). The labial teeth form continuous, homogeneous rows (labial tooth row formula: 2(2)/3(1)). The number of teeth in a row at stage 37 is approximately 5 to 7 per 100 µm. Each tooth comprises a base, a neck, and a paddle with 14 to 16 short, subequal marginal serrations (denticles). The total length of a labial tooth is 25–30 µm, with maximum paddle width of 15 µm (Figure 1A). The jaw sheath teeth are arranged in a palisade, with a density of 4–5 teeth per 100 µm; the total tooth length is approximately 30–35 µm and the maximum width is 20 µm (Figure 1B). On the buccopharyngeal cavity floor, the prelingual region has six infralabial papillae: two ventral and four lateral (Figure 1C). Four simple lingual papillae are present on the lingual anlage; the central papillae are longer and closer to the anterior lingual margin (Figure 1C). The buccal floor arena is limited laterally and posteriorly by peripheral papillae (Figure 1D). The buccal pockets are elongated, located transversally to the medial line. The internal lateral area of the prepocket is preceded by laminar projections, with two tall, well developed, digitiform tips pointing towards the interior of the buccopharyngeal cavity. Near the anterior edge of the pocket, there are 2 to 3 low papillae oriented towards the buccal pocket (Figure 1D). The ventral velum has four pronounced, widely separated marginal projections on each side of the medial notch. There are glandular pits on the velum margin and its projections. On the buccopharyngeal cavity roof, there are elliptical choanae oriented transversally to the cephalocaudal axis. In the postnarial area, there are two pairs of postnarial papillae of different sizes, the medial pair being the most developed (Figure 1E). The lateral ridges are triangular-shaped projections, compressed anteroposteriorly, and with irregular edges. The median ridge is triangular, with smooth walls and irregular edges. The buccal roof arena is delimited by 6 pairs of tall, conical marginal papillae, and numerous pustules. The glandular area is well developed, arranged in an open U-shaped band. The secretory pits are circular or irregular shaped (Figure 1F).             Figure 2 shows the buccopharyngeal cavity features for A. salai at stage 37. There is a large morphological and morphometric similarity between the labial teeth of A. salai and A.  nitoi (Echeverría et al. 2001a), although A. salai has more denticles per paddle. Some of the anatomical features of the buccopharyngeal cavity of A. salai are common to other leptodactilid tadpoles. The four lingual papillae present in A. salai are also present in the other known species in the genus Atelognathus and in the tadpoles of the leptodactylid genera Alsodes, Batrachyla, Caudiverbera, Hylorina, Pleurodema and Odontophrynus (Brieva Vásquez 1988, Wassersug and Heyer 1988, Echeverría et al. 2001a,b). The morphology of the buccopharyngeal cavity of the tadpoles of A. salai resembles most A. patagonicus, A. reverberii (Wassersug and Heyer 1988) and A. nitoi tadpoles (Echeverría et al. 2001a) in having four lingual papillae; four of the six infralabial papillae enlarged; a triangular median ridge; and simple, well-developed lateral ridges. The dental formula of A. salai [2(2)/3(1), Úbeda and Basso 2003] is generalized and widespread among different types of tadpoles, and cannot be related to a particular aquatic environment. It is present in Patagonian leptodactylid frogs inhabiting temporary or permanent, lotic or lentic environments with a variety of ecomorphological characteristics (i.e. Pleurodema thaul, P. bufonina, Hylorina sylvatica, Atelognathus spp., Alsodes spp., Batrachyla spp., Caudiverbera caudiverbera, and Odontophrynus occidentalis).   The morphology of the oral apparatus and buccopharyngeal cavity observed in A. salai matches the general descriptions for other Atelognathus species. The characteristics in common with other Patagonian leptodactylid frogs of different habits may be attributed mainly to phylogenetic constraint rather than to convergent ecological adaptations.