Apogon pillionatus Bohlke and Randall

Apogon pillionatus Böhlke and Randall

Identification. Adult Apogon pillionatus (Fig. 5) is diagnosed by the combination of eight segmented anal-fin rays, the body and lateral-line scales of similar size, a dark bar just behind the second dorsal fin that does not reach the ventral midline, and a very broad bar on the posterior part of the caudal peduncle (also does not reach the ventral midline). The distance between the two body bars is considerably less than the width of the posterior bar (Böhlke & Chaplin 1993; Gon 2002). No adult specimens of A. pillionatus were collected in this study, and no COl sequences for the species were found in GenBank. The specimen featured in Figure 5 was collected on Saba Bank Atoll prior to our study and is not a DNA voucher specimen. Juveniles identified in this study as A. pillionatus (Appendix 1) have a relatively narrower bar of dark pigment on the posterior part of caudal peduncle than adult A. pillionatus (see “Juveniles,” below), and the anterior dark bar is situated behind the posterior base of the second dorsal fin. The identification of those juveniles as A. pillionatus was accomplished by process of elimination and comparative morphological examination. Six western Atlantic Apogon species, Apogon planifrons, A. phenax, A. robinsi, A. townsendi, A. gouldi, and A. pillionatus, have two dark bars on the posterior part of the body: one in the area of the posterior portion of the second dorsal fin and the other on the caudal peduncle (Böhlke & Randall, 1993). In A. townsendi, A. planifrons, and A. gouldi the anterior bar is situated entirely beneath the second dorsal-fin base. Apogon phenax has a wedge-shaped bar situated below and just behind the second dorsal-fin base. Only two Apogon species have the anterior bar well behind the end of the second dorsal fin: A. robinsi and A. pillionatus. Apogon robinsi is easily recognized by the lateral extensions of the premaxillary tooth patches (see “Identification” under “ A. robinsi ”), such that a portion of the dentition lies outside the confines of the mouth (Böhlke & Randall, 1968; Böhlke & Chaplin 1993; Gon 2002). Furthermore, the anterior body bar in A. robinsi extends to the ventral midline, whereas in A. pillionatus it falls short of the ventral midline. The juveniles identified herein as A. pillionatus (Fig. 1) have the anterior bar well behind the second dorsal-fin base, that bar terminating well short of the ventral midline, and no dentition outside of the mouth.

Juveniles (Fig. 6). Five juveniles identified as described above range from 15.0 to 17.0 mm SL. The body is pale orange with darker orange coloration on the head and anterior rays of the first dorsal fin. There are melanophores on top of head, behind the eye on the cheek and temporal regions, and on the gut. There are melanophores on the posterior part of the second dorsal- and anal-fin bases, as well as on the distal tips of the middle rays of the first dorsal and anal fins. There are numerous melanophores on the outer rays of the caudal fin. The anterior bar of the body is entirely behind the posterior end of the second dorsal-fin base and does not reach the ventral midline of the body. The bar on the caudal peduncle is broad. As noted above, this bar is narrower in juveniles than in adults, and the space between the anterior and posterior body bars in juveniles is equal to or greater than the width of the posterior bar (this space smaller in adults).

Comparisons Among Juveniles. Characters used to separate juvenile A. pillionatus from other Apogon species having two body bars are discussed above (see “Identification”).