Description: Body relatively thin, long and narrow with a large eye and a terminal mouth. Pectoral and pelvic fins long, reaching almost to the vent, with a obvious pelvic frenum. Dorsal and anal-fin bases medium-length and caudal peduncle medium-length and sharply narrowing, 7-9 procurrent caudal-fin rays (7-8 spindly). Heavily marked mostly along the lower and midbody: there is a large melanophore at the tip of the lower jaw and one at the angle of the jaw. Along the ventral midline there are large stellate or streak melanophores at the isthmus, the pelvic-fin insertion, and one to three along the mid-abdomen, then variably paired on either side of the ventral midline at the anal-fin base and then extending along the ventral peduncle ending at the start of the procurrent caudal-fin rays. Internal melanophores occur around the sacculus and along the dorsal surface of the swim bladder and around the gut near the vent. Melanophores along the dorsal midline are limited to the rear body (vs. B. curacao); as one to three variably paired large stellate melanophores on either side of the dorsal midline at the base of the mid to rear soft dorsal fin. Long streak melanophores are present along the membranes of the second to fifth fin rays on both the soft dorsal and anal fins. There is a single prominent stellate internal vertebral melanophore at the lateral midline at about the level of the mid soft dorsal fin that ramifies around the vertebral bodies and extends between and around myomeres and often up to the surface. Series of transitional larvae show development of the eye from round with dorsal and ventral indentations in the iris (mostly on the dorsal-anterior to ventral-posterior axis, but can vary) to fully round (most pre-transitional larvae captured have no indentations and some transitional larvae have iris indentations). Early transitional larvae develop a stripe of melanophores from the eye forward to the mid-upper jaw. As transition continues, the melanophores become essentially a stripe from the tip of the lower jaw across the mid-upper jaw to the eye, over the iris and onto the operculum, continuing internally from the sacculus to the dorsal surface of the swim bladder and around the gut near the vent and along the anal fin to the tail. Melanophores also develop at the end of the caudal peduncle, primarily at the base of the central and lower segmented caudal-fin rays. Series of transitional larvae show the eye remaining round, but becoming larger with the iris developing a dark surface pigmentation layer. Late transitional larvae develop an additional scattering of large discrete melanophores on the dorsal half of the head and the operculum, extending to the base of the pectoral fin. Small iridophores occur in patches behind the eye and in a stripe out onto the middle rays of the pectoral fin. Patches of small melanophores develop around the base of the spinous dorsal fin.
Diagnosis: Modal fin-ray counts of D-VI,10 A-9 and Pect-19 with fused pelvic fins indicate Bathygobius soporator or B. mystacium. The two species are usually separated by the former having 37-41 and the latter 35 (33-36) scale rows later in development. The genus is known for having the dorsal-most pectoral-fin rays separate from the rest and filamentous, however this feature is clearly not apparent on larvae. The DNA sequence of this larval type (the "spot" type, i.e. a single large vertebral melanophore) matches adult B. soporator. Other six-dorsal-spined gobies with the same median-fin ray counts (but fewer pectoral-fin rays) include the congener B. curacao with 16-17, as well as Lythrypnus (14-16), Coryphopterus alloides (16-17), Lophogobius cyprinoides (17-18), and Priolepis hipoliti (18). (DNA) G14
Analogues: (heavy ventral markings)
