Macrodon atricauda Gunther 1880

Macrodon atricauda (Günther, 1880).

Pescada-Foguete (Pt); Pescadilla Real (Sp); Southern King Weakfish (En) (Figures 1 to 4)

Ancylodon atricauda Günther, 1880.

Sagenichthys ancylodon Berg, 1895; Ihering, 1897; Jordan & Evermann, 1898; Ribeiro, 1915; Devincenzi, 1924. Perca mollis Larrañaga, 1923; in Divicenzi, 1925 (synonymized with Sagenichthys ancylodon). Perca vigintoctoradiata Larrañaga, 1923; in Divicenzi, 1925 (synonymized with Sagenichthys ancylodon).

Holotype: BMNH 1879.5.14.258 (1, 94.2 mm SL), Mouth of the Rio de la Plata, Argentina, Station 321, 13 fathoms (Günther, 1880). Jar with two labels: label 1: Ancylodon atricauda no. 406, Stat.321, Rio de La Plata – Challenger, y9.5.111.258; label 2: 32.495, Macrodon ancylodon.

Diagnosis: a species of Macrodon as defined by Chao (1978) and Sasaki (1989), with the following character set: lateral line pored scales 49 to 63 (compared to 66–78 in M. ancylodon); anal fin soft rays 9 or 10, rarely 8 (usually 8, occasionally 9 in M. ancylodon); dorsal fin soft rays 27 or 28, rarely 29 (28–29, rarely 27 in M. ancylodon); pectoral fin rays 16 or 17 (15–16 in M. ancylodon); largest front canines 2.5 to 3.2 in eye length, proportionally smaller and thinner when compared to M. ancylodon (1.8 to 2.2).

Description (based on the holotype, 39 other museum specimens and the present sample of 70 fresh individuals): Dorsal fin XI, 27–29 (rarely 29); anal fin II, 8–10 (rarely 8); pectoral fins 16 or 17; pelvic fin I,5; lateral line pored scales 49–63; 9–12 gill rakers on upper limb of first arch (2 or 3) and on lower limb (7–9).

Body elongate, moderately compressed. Mouth large, strongly oblique, the lower jaw projecting; maxilla extending beyond eye; teeth very sharp, arrow-headed, set in narrow ridges in both jaws; upper jaw with a pair of large, elongated, fang-like, narrow, canines at tip, and an outer row of sharp teeth; lower jaws overlaying upper jaw with several canines at the tip, larger than the other mandibular teeth, but much smaller than the upper canines, and also with a widely spaced inner row of sharp teeth, the middle ones usually largest. Largest upper canine contained 2.5 to 3.2 (modally 2.7–2.9) times in eye horizontal diameter (vs. 1.8 to 2.2 – modally 1.8–2.0 in M. ancylodon), see Figure 6; this proportion does not vary with growth for specimens between 70 and 250 mm SL. Scales small, cycloid, often overlapping; soft dorsal fin and anal fin covered with small scales; lateral line in a smooth arch from upper operculum to below soft dorsal fin and straight from there almost to the end of the caudal fin. Caudal fin with the central rays longer, more pointed in young, and rhombic in large adults.

Color silvery-gray to golden yellow, darker on back, paler ventrally, with an iridescent greenish cast on the sides of the body in very fresh specimens; pectoral fin with a dusky elongate blotch on its upper distal border, more evident in the young, and a dusky spot on its upper base; caudal fin yellowish to grayish, dusky to blackish distally; dorsal fin dusky-yellowish, anal and pelvic fin pale yellow to white.

Largest specimen analyzed: 343.0 mm SL, 420.3 mm TL.

Largest specimen reported in literature: 460 mm TL (Carneiro & Castro, 2005).

Comparisons: The most obvious difference between M. ancylodon and M. atricauda is the upper jaw canine size, which is larger in M. ancylodon (FIG. 6). This concurs with Günther’s (1880) original description, that “the canine teeth are comparatively smaller than in Ancylodon jaculidens [= Macrodon ancylodon]”.

While there is some overlap between M. ancylodon and M. atricauda in the number of soft rays, there are also strong regional divergences, and scale counts and canine length are quite distinct. Individuals from the northern population have more soft rays in the dorsal fin, and fewer in the anal and pectoral fins. In addition, whereas in the northern population all individuals had at least 66 pored lateral line scales, none of the specimens from the southern population had more than 63 (Table 3).

Overall, then, these differences in the morphological features of the two populations clearly support their classification as distinct, albeit closely-related species, as suggested by the genetic data.

Distribution: M. atricauda occurs between the Brazilian state of Espírito Santo and northern Argentina. The closely related M. ancylodon occurs between Venezuela and the Brazilian state of Bahia (FIG. 5).

Conservation: At first glance, the fish fauna of the western Atlantic is relatively homogeneous, although a growing number of recent studies indicate that the southwestern Atlantic is home to a substantial number of endemic species. Over the past years several new species were described or revalidated, while at least another 40 species have been identified as unknown and are being described (e.g. Figueiredo et al., 2002; Rocha, 2004; Bernardes et al., 2005; Melo, 2007; Floeter et al., 2008; Mincarone & Anderson, 2008; Sazima et al., 2008; Carvalho-Filho & Floeter, in preparation).

The revalidation of M. atricauda represents one more study where genetics aids taxonomy. Both Atlantic species are important fishery resources. The main practical difference between them is the fact that the distribution of M. atricauda coincides with the most densely-populated stretch of coastline in South America, whereas M. ancylodon ranges over a much wider and sparsely populated area, especially in northern Brazil and the Guyanas. In addition, while M. atricauda is heavily exploited throughout its range, M. ancylodon is commercially important mainly in Venezuela (Cervigón, 1993) and the Brazilian states of Amapá, Maranhão and Pará (Fernandes, 1981 /82; Haimovici et al., 1996; Haimovici, 1998; Piorsky et al., 2004).

While there are few available data on the Brazilian population of M. ancylodon, M. atricauda is under intense pressure. Whereas 8,000 tons of this species were landed in 1986 (Carneiro and Castro, 2005, as M. ancylodon), the catch decreased to 4,000 tons in 2002, indicating a decline in commercial stocks of at least 50% over this 16-year period. Demersal resource stocks evaluation in southern Brazil obtained from 1975 to 1994 show a decreasing total biomass of M. atricauda since the beginning of exploitation and that the resource is heavily exploited in that region (Haimovici, 1998, as M. ancylodon). Similarly, stock evaluations of M. ancylodon based in recruitment yield model-based and Thompson and Bell’s predictive models in the period from 1997 to 2000, in Pará and Maranhão, Brazil’s north coast, indicate that the species is overexploited or in the maximum limit of exploitation (Ikeda et al., 2003).

Ultimately, an important question to be considered is whether and to what extent the presence of two different species of Macrodon in the southwestern Atlantic should redefine conservation and management strategies for these populations, especially considering that one species is clearly under much greater pressure than the other. Despite the relative morphological similarity between them, those species might have accumulated differences in physiological and behavioral features due to adaptations to local environmental characteristics, and should be managed as distinct units.