Stylophthalmus paradoxus Brauer 1902

Comparison of the syntypes of Stylophthalmus paradoxus with the illustrations of Brauer (1906, pl. V, figs. 2, 5–7; reproduced in Fig. 1) indicated the following: ZMB 17455 (Fig. 2a) is illustrated in pl.V, fig. 7; ZMB 17458 (Fig. 2b), the largest specimen, is illustrated in pl. V, fig. 6; and the two smallest specimens (Fig. 2c, ZMB 17456) are the specimens of Bathylagus shown in pl. V, figs. 2 and 5. One of the latter two is depicted in a pencil drawing, with “139 Vert 1500 m ” inscribed above in Brauer’s hand writing (Fig. 2d); ‘139’ is the Valdivia station number and ‘Vert’ refers to a vertical net. The syntype of S. paradoxus from Station 136 was originally archived at the Zoological Institute of the Karl Marx University in Leipzig. However, in about 1970, it was apparently transferred to ZMB, along with all its additional Valdivia material. Although it appears on a list of the transferred material, it is not registered in the ZMB catalogue and we have not been able to find it. Another specimen, from station 139 (ZMH 8172), is in the fish collection of the University of Hamburg (Wilkens & Dohse 1993). Although the last portion of the body, including the caudal fin, is missing, this larval specimen (Fig. 2e) is similar in body proportions to the larval specimen from station 135 (SMF 2067, Fig. 2f) and both are confirmed larvae of Idiacanthus.

Comparison of Brauer’s (1902) description with the specimens and the illustrations of Brauer (1906; Fig. 1) indicated that he originally described the fish in his pl. V, fig. 6 (ZMB 17458). This specimen, the largest, has the following characters: dorsal and anal fins present, the latter beginning just below the last fin ray of the former; trailing gut; eyes on long stalks; and body depth about 60 times in body length as measured by Brauer (1902).

Beebe (1934: 165) found that the dark mid-lateral spots on the body of the larval specimen of I. fasciola were arranged “one in the middle of each myomere.” This was confirmed by Kawaguchi & Moser (1984: 181), who found a melanophore on the “posterior margin of each hypaxial myomere.” We counted these dark dots and/or myomeres on the actual syntypes of S. paradoxus and I. fasciola (where specimen condition allowed it), on the illustrated larvae in Brauer (1906; Fig. 1) and on the larva depicted in Beebe (1934, fig. 52). These data were compared with Beebe’s (1934) data, as well as with vertebral counts taken from radiographs of more recent specimens of I. fasciola and I. atlanticus, and with data from the literature (Table 1). Beebe (1934) had the lowest range of counts of dark dots or myomeres; his highest value was 72 for a 45 mm long postflexion larva; his lowest count of 64 dark lateral dots on a 16 mm larva was similar to our count of 65 dots on the fish in his fig. 52 (no length given). These values, the latter two in particular, are lower than the counts obtained for I. atlanticus, I. fasciola and S. paradoxus (Table 1) suggesting that Beebe (1934) may have had more than one species in his larval material.

Examination of the largest type specimen of I. atlanticus (ZMB 17723, Fig. 3a) revealed the dorsal fin positioned slightly in front of the pelvic-fin insertion suggesting it could be a misidentification of I. fasciola, but its myomere count of 83 was in the range of that of I. atlanticus of recent authors (Gon 1990; Duhamel et al. 2005; Kenaley et al. 2008). To resolve these conflicting results we studied the position of the dorsal, pelvic and anal fins relative to each other on radiographs of adult females of both species (Figs. 3b, 4a). Table 2 clearly shows the differences between I. fasciola and I. atlanticus in the position of these three fins along the vertebral column and therefore confirms the validity of I. atlanticus Brauer, 1906. Notably, there was no overlap between the species in the position of all three fins. It is therefore possible to distinguish these two species by counting the number of vertebrae to the origins of these fins. This method circumvents the difficulty of obtaining a total vertebral count due to the small size and poor ossification of the posterior 5–10 vertebrae. Intraspecific variation in the position of these fins was smallest in the pelvic fin, amounting to 3–4 vertebrae, and largest in the dorsal fin of I. atlanticus, reaching 7 vertebrae (Table 2). Geographic variation appeared to be negligible (Table 1), but this should be confirmed with larger sample sizes. Comparison of the fin origins between the two species showed that while all three fins changed positions, the dorsal and anal fins shifted to a greater extent than the pelvic fins.