Pseudecheneis Blyth

[[Pseudecheneis Blyth]]

Discussion

Pseudecheneis intermedius Chu, 1982 is here considered a junior synonym of P. paviei. Pseudecheneis intermedius was distinguished from P. paviei by the relative position of the adipose and anal fins (Chu, 1982). Our examination of material identified as P. intermedius and P. paviei from northern Vietnam and southern China revealed no significant differences in the relative position of the adipose and anal fins, nor could we find any other morphological or biometric differences to distinguish between the two. Chu et al. (1990) indicated that the outline of the snout tip can be used to distinguish between the two species, but this is a variable character that changes with the position of the maxillary barbels (i.e. whether they are projected sideways or pressed against the head) and is therefore not considered a useful diagnostic feature here. We tentatively follow Zhou & Chu (1992) in considering P. tchangi (Hora, 1937) a junior synonym of P. sulcata. The status of P. tchangi is being investigated in a separate study by HHN.

The relative height of the neural spines readily distinguishes P. serracula from its congeners, especially P. sulcata (which occurs sympatrically and syntopically with it; see below). The difference is readily apparent in radiographs (Fig. 5), but even without recourse to such techniques, it is possible to see a difference in external morphology caused by the difference in height of the neural spines. The increase in body depth in the region between the dorsal and adipose fins caused by the neural spines in P. serracula imparts a somewhat hunched appearance compared to P. sulcata (compare Figs. 3 and 6).

Biplots of the length of the adipose-fin base for P. serracula vs. P. sulcata (Fig. 7) and caudal peduncle depth against SL for P. crassicauda vs. P. serracula and P. sulcata (Fig. 8) show that the regression lines are significantly different (ANCOVA; P<0.05). Likewise, biplots of eye diameter against SL for for P. crassicauda vs. P. serracula and P. sulcata (Fig. 9) are also significantly different (ANCOVA; P<0.05).

Pseudecheneis serracula was found syntopically with P. sulcata in the Seti River (the differences among the three species of Pseudecheneis known from the Ganges River drainage are highlighted in Table 3). Sisorid congeners occurring syntopically have been noted in several genera, e.g. Bagarius, Gagata, Glyptothorax, Nangra (DeWitt, 1960; Roberts, 1983; Edds, 1993; Roberts & Ferraris, 1998). It is common to find coexisting mixed-species groups of related fishes (Matthews, 1998), although hypotheses of resource partitioning (e.g. Zaret & Rand, 1971; Werner et al, 1977) would predict otherwise. Matthews (1998) suggested that fishes in heterospecific schools may gain information or other benefits. Such groupings might increase foraging efficiency or enhance detection and avoidance of predators (Boucher et al., 1982; Allan, 1986; Allan and Pitcher, 1986; Pitcher, 1986). Gorman (1988) reported feeding associations between two species of Notropis (Cyprinidae), and Greenberg (1991) noted similar interactions among three species of Percina (Percidae). Mendelson (1975) concluded that multispecific schooling by four species of cyprinids might gain them anti-predator benefit. Moyle & Li (1979) and Allan (1986) suggested that rare fish species might benefit by hiding in groups of one or more other species that were more abundant. A few fishes enter into more complex symbioses via nest associations, some of which may be mutualistic (Matthews, 1998).

It is possible that the material we identify as P. sulcata from the Seti River is not conspecific with P. sulcata s. str., given that Pseudecheneis species appear to have restricted distributions (they are not found below elevations of ca. 150 m asl) and that the type locality of P. sulcata is in the tributaries of the Brahmaputra River in the Khasi Hills, India, a considerable distance to the southeast. A separate study by HHN is examining variation in P. sulcata throughout its range, and will include a redescription of the species and discussion of its diagnostic characters.