Brazilian Soft Skate

biology_ecology

Biological notes. All male specimens of M. obscura are sexually immature. The largest male is 505 mm in TL and is clearly a juvenile (albeit late stage), and has small claspers that do not reach beyond pelvic fin length, a disc that is not weakly convex as of level of orbits, and lacks alar and malar thorns. The holotype (680 mm TL) is probably an adult female judging by its large size, but conclusive data on sexual maturity are lacking. Males sexually mature at about 575 – 580 mm TL in M. spinacidermis (South African specimens with elongated but not yet fully calcified claspers; SAM 35454, 572 mm TL), probably under 520 mm TL in M. senta (Bigelow & Schroeder, 1953; cf. Packer et al., 2003, who wrongly estimate size at maturity to be about 560 mm TL), and at least at 505 mm TL in M. kreffti (specimens in ZMH; also Stehmann, 1977, 1993). This indicates that M. obscura matures at a rate closer to that of M. spinacidermis (if indeed our largest female is an adult), and also attains greater sizes than either M. senta or M. kreffti (M. spinacidermis reported to reach 640 mm TL; Hulley, 1986). The holotype of M. obscura is the largest Malacoraja specimen examined or reported to date (at

Description of a new species of skate of the genus Malacoraja Stehmann, 1970: the first species from the southwestern Atlantic Ocean, with notes on generic monophyly and composition (Chondrichthyes: Rajidae)

description

Description. Proportional morphometrics are presented in Table 1, and counts in Table 2 (where they and compared to other species of Malacoraja). The description below is based on all specimens, but salient features of the holotype are separately mentioned. External morphology. Disc cordiform, much wider than long (DW 60.3 - 67.5 % of TL, DL 47.5 - 53.1 % of TL), with greatest width just posterior to its horizontal midline. Snout extremity not greatly elongated but clearly projecting beyond anterior disc margin. Anterior margin of disc relatively straight from disc apices to snout tip, not convex from level of orbits to snout tip. Disc apices broadly rounded, with widely convex and broad posterior margins. Posterior lobe of disc extending caudally to about one-half of length of posterior lobe of pelvic fin. Smaller specimens with more acute snout angle (100 - 101 o, vs. 105 - 107 o in larger specimens; Figs. 1 - 3), appearing more triangular in outline. Disc relatively thin and dorsoventrally compressed. First three gill openings slitlike, horizontal wider than both posterior openings (especially fifth); posterior openings semicircular. Gill openings situated just medial to propter

Description of a new species of skate of the genus Malacoraja Stehmann, 1970: the first species from the southwestern Atlantic Ocean, with notes on generic monophyly and composition (Chondrichthyes: Rajidae)

diagnosis

Comparisons South American Rajidae. Malacoraja obscura is unique among the known Atlantic skate fauna of South America in lacking thorns posterior to scapular area and in presenting intense squamation on ventral tail region (a newly collected Rajella species, presently under study, also has ventral tail denticles, but these are not identical to M. obscura). The most similar western South Atlantic species is Rajella sadowskii (Krefft & Stehmann, 1974), known from the continental slope of southeastern and southern Brazil in 800 - 1320 m (Menni & Stehmann, 2000). Rajella sadowskii also has a dense covering of minute denticles (but not on ventral tail), and is similar in disc shape (albeit not as cordiform) and morphology of the oronasal region (but not in its proportions – oronasal region much greater in R. sadowskii). Rajella sadowskii is lightly colored ventrally (dark brown in M. obscura), has fewer tooth rows (37 - 43 / 34 - 41 vs. 57 - 76 / 55 - 74 in specimens of M. obscura over 350 mm TL), and presents greatly enlarged thorns along middisc and dorsal tail region (Krefft & Stehmann, 1974), which are absent in M. obscura. Congeneric species. Species of Malacoraja diverge only sli

Description of a new species of skate of the genus Malacoraja Stehmann, 1970: the first species from the southwestern Atlantic Ocean, with notes on generic monophyly and composition (Chondrichthyes: Rajidae)

diagnosis

Diagnosis. A species of Malacoraja distinguished from all congeners by presenting small, scattered, and faded whitish spots on dorsal disc and pelvic fins, an irregular row of small thorns along dorsal midline of tail which persists in larger specimens (extending from tail base to two-thirds of tail length), and ventral tail midline devoid of small denticles only at base (naked region not extending beyond pelvic fins). The following characters in combination further distinguish M. obscura, n. sp.: ventral disc and tail with a uniform dark brown coloration, absence of scapular thorns in larger specimens, relatively higher number of tooth rows (64 / 62 tooth rows in subadult male of 505 mm TL, and 76 / 74 in large female of 680 mm TL), relatively higher number of vertebrae (27 - 28 Vtr, 68 - 75 Vpd), paired postventral fenestrae on scapulocoracoid, enlarged posterior postventral fenestra, circular foramen magnum, and paired internal carotid foramina on neurocranial basal plate. The significance of these features among species of Malacoraja is discussed below.

Description of a new species of skate of the genus Malacoraja Stehmann, 1970: the first species from the southwestern Atlantic Ocean, with notes on generic monophyly and composition (Chondrichthyes: Rajidae)