Comprehensive Description

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[[ Genus Myoglanis ZBK ]]

Discussion

Eigenmann (1912) created the genera Brachyglanis ZBK , Myoglanis ZBK and Leptoglanis ZBK for new species of small catfishes from Guyana. The last of these, being preoccupied by Leptoglanis Boulenger ZBK (1902), was replaced by Leptorhamdia Eigenmann ZBK (1918). The fishes in Eigenmann’s genera are similar in having a thick layer of jaw adductor muscle on the skull roof, strong pectoral-fin spines and subcutaneous eyes. As diagnosed by Eigenmann, Brachyglanis ZBK , Myoglanis ZBK and Leptorhamdia ZBK differ primarily in degree of spine-like ossification of the dorsal spine, length of the adipose fin and its contact with the caudal, and caudal-fin shape. The taxonomic histories of Myoglanis ZBK and Leptorhamdia ZBK are checkered with their synonymy by Gosline (1941) followed by renewed recognition by Lundberg et al. (1991), Bockmann (1998) and Bockmann and Guazelli (2003).

Brachyglanis ZBK , Leptorhamdia ZBK and Myoglanis ZBK along with Gladioglanis ZBK and the members of the Nemuroglanis ZBK subclade (Ferraris 1988) were assigned by Lundberg et al. (1991) to a large unnamed monophyletic group characterized by the loss of a free orbital rim around the eye. This group excludes the genera Brachyrhamdia ZBK , Pimelodella ZBK (with its synonyms Caecorhamdella ZBK and Typhlobagrus ZBK ), Rhamdella and Rhamdia (including Caecorhamdia ZBK ). Because of their plesiomorphic retention of a strong pectoral spine Gladioglanis ZBK , Brachyglanis ZBK , Leptorhamdia ZBK and Myoglanis ZBK are excluded from the Nemuroglanis ZBK subclade in which the spine is reduced to a segmented, unbranched ray. Moreover, Gladioglanis ZBK , Leptorhamdia ZBK and Myoglanis ZBK each share different synapomorphic characters with the Nemuroglanis ZBK subclade, the resulting pattern of incongruence resulted in an unresolved arrangement at this level with Brachyglanis ZBK in a lower level because of its plesiomorphic condition for all of those characters. Lundberg et al. (1991) also advanced a hypothesis of possible monophyly of Brachyglanis ZBK , Leptorhamdia ZBK and Myoglanis ZBK based on the invasion of the adductor mandibulae muscle onto the skull roof, an obvious derived condition for Siluriformes (Lundberg 1982; Grande & Lundberg 1988).

More recently Bockmann (1998) presented a comprehensive hypotheses of phylogenetic relationships and diagnoses for heptapterid genera and higher subgroups. Bockmann (1998) presented phylogenetic evidence for a sister group relationship between Leptorhamdia ZBK and Myoglanis ZBK within a clade also including Brachyglanis ZBK . In our study we located two additional informative characters that corroborate the Leptorhamdia ZBK - Myoglanis ZBK pair. First, the anteriormost cutaneous sensory pores of supraorbital (SO1), infraorbital (IO1) and mandibular branch (MA1-3 in Leptorhamdia ZBK and Myoglanis aspredinoides ZBK ; MA1-5 in Myoglanis ZBK sp) are wider than remaining pores. In the case of Myoglanis potaroensis ZBK only SO1 and IO1 are considerably wider than the others and the pores of preoperculomandibular canal are relatively wide but of similar diameter. A widened condition of cutaneous sensory pores in the head is also found in Gladioglanis ZBK , but in this case all the pores of each canal exhibit the same diameter and are therefore considered different from the asymmetric condition of Leptorhamdia ZBK and Myoglanis ZBK . Second, the anterior margin of first and second pelvic-fin rays are pinnate (Fig. 4), with long anterior extensions of the lepidotrich segments. In Heptapteridae a similar but less developed condition is present in Cetopsorhamdia ZBK sp., where the lepidotrich segments are restricted to the distal margin. Elsewhere in Siluriformes the glyptosternoid fishes of the family Sisoridae exhibit an extreme pinnate condition both in pectoral and pelvic fins that has apparently evolved independently and is associated with the torrential stream habitat of glyptosternoids (Hora & Silas 1952). Table 2 summarizes these and other characters varying among Myoglanis ZBK , Leptorhamdia ZBK and Brachyglanis ZBK . Thus, in the latest authoritative treatments, Myoglanis ZBK is considered both valid and monophyletic. Within this framework, the diagnostic synapomorphies of Myoglanis ZBK are: ossified portion of dorsal spine greatly reduced; first dorsal-fin basal radial inserted posterior to neural spine of vertebra 6; 46 or more total vertebrae (vs. 47 or fewer); 16 or more anal-fin rays (vs. 17 or fewer); lower half or more of adipose fin thickened (vs. base only thickened); adipose fin reaching and broadly confluent with caudal fin (vs. adipose fin not reaching caudal fin or if close adipose retains a free posterior lobe); upper procurrent rays of the caudal fin reduced to 8-13 (vs. 12 or more). Within the genus, M. aspredinoides ZBK exhibits the most highly derived conditions of total vertebral, dorsal procurrent caudal-fin ray and anal-fin ray counts, and matches its congeners in the dorsal and adipose fin synapomorphies.

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