Apteronotus caudimaculosus, Carlos David de Santana, 2003

Carlos David de Santana, 2003, Apteronotus caudimaculosus n. sp. (Gymnotiformes: Apteronotidae), a sexually dimorphic black ghost knifefish from the Pantanal, Western Brazil, with a note on the monophyly of the A. albifrons species complex., Zootaxa 252, pp. 1-11 : 2-5

Apteronotus caudimaculosus - new species

Figs 1 - 2; Table 1

Holotype- MZUSP 79359, 286.8 mm TL, Mato Grosso do Sul. AQUARAP Pantanal 19° 36.64’ S; 56°24.87’ W, Aquidauna, Rio Novo, Brejo de Santa Sofia; A. Machado & B. Chernoff, 3 September 1998.

Paratypes- ANSP 178659 (1 specimen, 145.9 mm) ; CAS 216788 (1, 127.6 mm) ; INHS 94260 (1, 137.5 mm) ; GMNH 3435 (1, 149.2 mm) ; INPA 20066 (1, 100.1 mm) ; MCZ 161831 (1, 126.0 mm) ; MZUSP 59295 (22, 71.7-165.8 mm) ; MZUSP 59296 (4, 175.4-231.1 mm) ; UMMZ 240259 (1, 107.1 mm) ; USNM 371234 (1, 106.2 mm); Locality data as for holotype. FMNH 108537 (2, 114.3-159.2 mm), Rio do Peixe on MS 080, 19° 23.15’ S; 54° 48.47’ W. 23 October 1998 .

Diagnosis- A. caudimaculosus is included in the Apteronotidae by the presence of a dorsal-thong and caudal fin (Mago-Leccia, 1994; Albert & Campos-da-Paz, 1998; Albert, 2001). The new species shares the following diagnostic features within the Apteronotinae (Albert & Campos-da-Paz, 1998; Albert, 2001): gape of mouth large, extending to the edge of the eyes; maxilla rhomboid in lateral view; anterior surface of mesethmoid concave and lateral process of ventral ethmoid robust. It was included in the genus Apteronotus by the presence of three of the five synapomorphies proposed by Albert (2001): body surface deep brown or black with a white mid-dorsal stripe and white mental and caudal patches; elongated posterior limb of the anguloarticular; and surface of cranial bones solid, not pitted. A. caudimaculosus was included in A. albifrons species-group by the presence of a pale pigment patch on the caudal peduncle and a highly contrasting pale or white mid-dorsal stripe and mental patch. The new species can by recognized within the genus by the following derived characters: (1) two clear bands encircling the caudal peduncle (vs. one clear band encircling the base of caudal fin, except A. albifrons ); (2) first caudal band with irregular dark spots throughout its development (vs. without spots in A. albifrons ). Furthermore, shorter tail length can be used to separate A. caudimaculosus from A. albifrons , (10.4-19.2% vs. 22.3-23.1% of LEA).

Description- Morphometrics for holotype and paratypes are presented in Table 1. Body laterally compressed, with greatest body depth at abdominal cavity or a little posterior to this region. Dorsal profile straight. Lateral line extending to tail, absent in caudal fin. First perforated scale above pectoral fin origin.

Head laterally compressed, widest at opercular region and deepest at dorsal region; profile straight in males and slightly convex in females. Snout long in males, shorter in females and immature specimens. Eyes small, located laterally on head, completely covered by a thin membrane. Mesethmoid short and narrow, its anterior tip reduced, flexed ventrally, and concave. Lateral ethmoid present, ossified, small and slender, shaped as a tube. Premaxilla of moderate size, with two irregular rows of teeth. Maxilla crescentshaped, with an ossified anterodorsal head and anteroventral shelf, the ventral margin of its descending blade curved. Dentary longer than deep, bearing two rows of teeth. Anguloarticular elongated. Mouth large, terminal, rictus passing a vertical through posterior border of eyes in both sexes. Endopterygoid narrow, without teeth, with ascending process connecting to the orbitosphenoid, its base ossified, and the remaining portion unossified; anterior portion narrow, extending to the midlength of dentary. Metapterigoid triangular, the posterior portion as an ascending process. Sympletic shorter than hyomandibula and articulated with it by a cartilage. Four branchiostegal rays, first and second almost filamentous, the remaining large and laminar. Anterior naris at the end of a small tube, close to the tip of snout; posterior naris ellipsoid, without a tube, closer to the tip of the snout than to the eyes. Branchial opening anterior to pectoral fin origin; branchial membranes joined to isthmus. Anus and urogenital papilla adjacent, ventral, located in a ventral posterior to eyes, but with a noticeable forward displacement with age. Pectoral fin somewhat elongated, broad and pointed distally, with ii + 13-14 [13]. Origin of anal fin slightly anterior to opercular region, anal fin rays 146-154 [151], with the 11-16 anterior rays unbranched. Small cycloid scales covering the body, 11 to 14 above lateral line. Number of black perforated scales on the lateral line before the first white scale equal to 64-66 [66].

Dorsal thong origin on posterior half of body, and inserted into a narrow middorsal groove, almost extending to, or slightly passing the end of the anal fin. Tail compressed and short, ending in a small and somewhat rounded caudal fin, with 18-22 rays [22].

Coloration in alcohol- Body, pectoral and anal fins black; caudal fin white on base and black posteriorly. Clear band from chin to the beginning of the dorsal thong. Two clear bands surrounding the caudal peduncle, the first with irregular dark spots during all stages of development.

Secondary sexual dimorphism (SSD)- Secondary sexual dimorphism is an important issue in understanding intraspecific variation and species identification among apteronotids(e.g., Cox-Fernandes, 1998; Cox-Fernandes et al., 2002). Historically, populations of A. albifrons were treated as sexually monomorphic. For example, Ellis (1913) presented a taxonomic key using traits such as, short and blunt snout to diagnose A. albifrons and “A”. bonapartii (Castelnau, 1855) from the remaining Apteronotus species. Mago-Leccia (1994) recognized two groups of species according to snout length, including A. albifrons among the species with a short snout. Dunlap et al. (1998), in a study of sexual dimorphism in A. albifrons and A. leptorhynchus , observed that “… A. albifrons shows relative little sexual dimorphism in both electrocommunication signals and morphology” and concluded that “…We found no obvious dimorphism in the body shape of A. albifrons .” Based on my observation and previous studies (Cox-Fernandes, 1998; Cox-Fernandes et al., 2002), it is possible to verify that males of A. caudimaculosus show SSD in head morphology and probably in body size as well (see Table 1 and Figure 2): head profile straight in males and slightly convex in females; snout straight and long in matures males, slightly curved in females and immature males and females. Based on gonad inspection, only four of 15 adults were males. It is possible that the greatly elongated snout in mature males of A. caudimaculosus offers some biological advantages in courtship and agonistics encounters as already suggested by Cox-Fernandes et al. (2002) in related apteronotids.

Distribution and habitat- A. caudimaculosus is known from the Rio Novo in the Middle Rio Negro and from Rio do Peixe in the Upper Rio Negro, Paraguay River basin in the state of Mato Grosso do Sul, Pantanal wetland, western Brazil (Willink, et al., 2000: map 3: 118).

Following Strussmann et al. (2000), the type-locality showed the following characteristics: it is a small channel in a swamp, with turbid water, mud and sand at the bottom and moderate current. The borders of the channel are covered by thick aquatic vegetation extending across the rest of the marsh. The Middle Rio Negro swamp was characterized by abundant water hyacinths and other rooted and floating aquatic plants (Willink et al., 2000). The water characteristics in August and September of 1998 were: temperature23.2-23.5°C; pH 5.85-5.82; conductivity 53.8 -55.0 µS/cm; dissolved oxygen 7.67-7.93 mg/cm. The specimens of A. caudimaculosus (cited as A. albifrons ) were captured among water hyacinths ( Eichornia sp.), which included also the following gymnotiforms: Eigenmannia trilineata , E. virescens , “A”. bonapartii ; Rhamphichthys sp., Brachyhypopomus sp. and Gymnotus sp. (Willink et al., 2000). The following fish genera were caught in the same locality: Apistogramma , Crenicichla , Cichlasoma and Bujurquina (Cichlidae); Hypostomus , Loricariichthys , Rineloricaria and Farlowella (Loricariidae); Pimelodella (Pimelodidae).

Etymology- Caudimaculosus, with spots on tail. From the Latin, caudi and maculosus meaning, tail and spots, respectively.