A new deep-water catshark, Bythaelurus stewarti, is described based on 121 examined specimens caught on the Error Seamount (Mount Error Guyot) in the northwestern Indian Ocean. The new species differs from all congeners in the restricted distribution, a higher spiral valve turn count and in the morphology of the dermal denticles. It is distinguished from its morphologically and geographically closest congener, B. hispidus (Alcock), by the larger size (maximum size 44 vs. 39 cm TL, maturity size of males 35–39 vs. 21–28 cm TL), darker fresh coloration and dark grayish-brown mottling of the ventral head (vs. ventral head typically uniformly yellowish or whitish). Furthermore, it has a strongly different morphology of dermal denticles, in particular smaller and less elongate branchial, trunk and lateral caudal denticles that are set much less densely and have a surface that is very strongly and fully structured by reticulations (vs. structured by reticulations only in basal fourth). In addition, the new species differs from B. hispidus in having more slender claspers that are gradually narrowing to the bluntly pointed tip without knob-like apex (vs. claspers broader and with distinct knob-like apex), more spiral valve turns (11–12 vs. 8–10) and numerous statistical differences in morphometrics. A review of and a key to the species of Bythaelurus are given.

Recent sampling efforts in Madagascar and Zanzibar, as well as examinations of six-gilled sawsharks in several museum collections provided evidence for a complex of species within Pliotrema warreni Regan. The present manuscript contains a redescription of P. warreni involving the syntypes and additional material, as well as formal descriptions of two new species of Pliotrema Regan. All specimens of both new species were found in the western Indian Ocean. Individuals of the first new species, hereafter referred to as P. kajae sp. nov., were identified originating from Madagascar and the Mascarene Ridge. Specimens of the second new species, hereafter referred to as P. annae sp. nov., were only found off Zanzibar. Pliotrema kajae sp. nov. appears to inhabit upper insular slopes and submarine ridges at depths of 214–320 m, P. annae sp. nov. so far is only known from shallow waters (20–35 m). Both new species differ from P. warreni in a number of characteristics including the known distribution range and fresh coloration. Taxonomical differences include barbels that are situated approximately half way from rostral tip to mouth, with prebarbel length equidistant from barbel origin to symphysis of the upper jaw in P. kajae sp. nov. and P. annae sp. nov. (vs. about two thirds way from rostral tip to mouth, with prebarbel length about twice the distance from barbel origin to symphysis of upper jaw in P. warreni) and rostra that are clearly and slightly constricted between barbel origin and nostrils, respectively (vs. rostrum not constricted). Pliotrema kajae sp. nov. differs from P. annae sp. nov. in a longer snout, more numerous large lateral rostral teeth and upper jaw tooth rows, jaw teeth with (vs. without) sharp basal folds, and coloration, particularly pale to light brown (vs. medium to dark brown) dorsal coloration with (vs. without) two indistinct yellowish stripes. A revised diagnosis of Pliotrema and a key to the species are provided.

Members of the genus Brevitrygon are small, locally abundant tropical stingrays (family Dasyatidae) occurring in soft sedimentary habitats of inner continental shelves of the Indo-West Pacific from the Red Sea to Indonesia. Formerly members of the genus Himantura, whose members lack dorsal and ventral skin folds on the tail (typical of most dasyatid genera), folds are present or rudimentary in some Brevitrygon. Important to artisanal fisheries and known to consist of at least five species, these fishes are possibly the most frequently misidentified of all stingrays. Most were inadequately described in the 19th century, and they are often taxonomically confused due to morphological similarity, ontogenetic variability, and sexual dimorphism. Their nomenclatural history is complex with four of the known species represented within the type series of one species, B. walga (Müller & Henle). Also, the type of the species with which B. walga is most often confused, B. imbricata (Bloch & Schneider) from off southern India and Sri Lanka, is in very poor condition. A lectotype has been designated for B. walga (confined to the Bay of Bengal). The genus also contains B. heterura (Bleeker) from the Indo-Malay Archipelago, B. javaensis (Last & White) from off southern Indonesia, and a new species, B. manjajiae sp. nov., from the western Indian Ocean. The former species are redescribed and redefined based largely on a combination of morphometrics, tail morphology, squamation, and molecular data. Molecular divergences were detected within lineages of B. heterura, B. walga and B. manjajiae sp. nov., requiring further investigation.