Analogues: (heavy ventral markings)
Description: Body relatively thin, long and narrow with a large eye and a terminal mouth. Pectoral fins long, reaching to the vent. Pelvic fins long, reaching almost to the vent, with an 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 with markedly dendritic melanophores: there is a large melanophore at the tip of the lower jaw and at the angle of the jaw. Along the ventral midline there are large stellate or streak melanophores at the isthmus, forward and behind of the pelvic-fin insertion, then a variable row (paired, one per side) at the anal-fin base and then unpaired extending along the caudal peduncle ending at the start of the procurrent caudal-fin rays. Internal melanophores occur around the lower brain case and around the sacculus continuing along the dorsal surface of the peritoneum and swim bladder extending to the gut near the vent (often all of these merge into a dark streak arcing through the body). There is a row of internal melanophores surrounding the vertebral bodies and extending for most of the spine from the level of the vent to the mid-caudal peduncle, often with a discrete row of deep melanophores along the dorsal vertebral spines as well. There is a prominent and characteristic matching row of dendritic surface melanophores along the lateral midline. Melanophores along the dorsal midline are limited to the rear body (vs. B. curacao ), as two or three variably-paired large stellate melanophores on either side of the dorsal midline at the base of the mid to rear soft dorsal fin. 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).with melanophores extending in patches across the surface of the iris. Transitional larvae develop a stripe of melanophores from the eye forward to the mid-upper jaw and across to the mid-lower jaw. As transition continues, the melanophores become essentially a stripe from the tip of the lower jaw back across the mid-upper jaw to the eye, over the iris, continuing internally over the base of the braincase to the sacculus continuing internally to the dorsal surface of the swim bladder then to the vertebral row of melanophores. A branch stripe extends bilaterally along the internal lateral abdominal wall to the vent and along the base of the anal fin to the tail. A scattering of large melanophores and some leukophores develops on the top of the head.
Diagnosis: Modal fin-ray counts of D-VI,10 A-9 and Pect-19 with fused pelvic fins indicate Bathygobius mystacium or B. soporator. The two species are usually separated by the former having 35 (33-36) and the latter 37-41 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 not apparent on larvae. This larval type has a mode of 19 pectoral-fin rays, consistent with either B. soporator or B. mystacium. Since DNA sequences match the "spot" type Bathygobius larva to B. soporator, this larval type matches B. mystacium. 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). (ML) G14b
