Description: Body relatively thin, long, and narrow with a medium eye and a terminal, medium-sized mouth and often thick lips. Pectoral fins long, reaching to vent, pelvic fins long and form a large obvious cup extending about two-thirds of the way to the vent. Dorsal and anal fins relatively short, caudal peduncle long and narrow, procurrent caudal-fin rays 6-8 (6-7 spindly). Lightly marked along the lower body: melanophores along the ventral midline at the isthmus (sometimes missing) and the pelvic-fin insertion, along the anal-fin base (paired, one per side) and extending along the ventral peduncle ending well before the start of the procurrent caudal-fin rays. Internal melanophores at the dorsal surface of the swim bladder and sometimes around the gut near the vent. Melanophores sometimes present on the base of several of the lower caudal-fin segmented rays and extending out a short distance along the rays. Series of transitional larvae show the eye remains round. Transitional larvae develop a blunt snout profile and develop bars radiating from the eye and bands across the top of the head, connected by a large X pattern over the brain. Patches of melanophores develop on the preopercle, on the base of the pectoral fin, and in a prominent row along the dorsal midline of the body as well as along the lateral midline and around the base of the caudal-fin rays.
Diagnosis: Fused pelvic fins and modal fin-ray counts of D-VII,11 A-10 and Pect-16 overlaps many species of the numerous Gobiosoma/Tigrigobius/Elacatinus group, but matches the mode only for Tigrigobius saucrus, T. macrodon (Florida to Haiti), and Gobiosoma hildebrandi (Panama). Tigrigobius dilepis shares most features with T. saucrus, but has a mode of 17 or 18 pectoral-fin rays; individuals may be included in the larval type where the species overlap in geographical distribution (but this larval type in Panama has 15-17 pectoral-fin rays with a strong mode at 16). Transitional recruits with a prominent row of black spots along the side of the body confirms T. saucrus. G7
Analogues: (ventral midline series x3: thorax, anal-fin, caudal peduncle streak) Larval T. saucrus share this melanophore pattern with several congeners: T. dilepis larvae are likely identical, but with one or two more pectoral-fin rays (modal 17-18); T. multifasciatus have a much shorter cup-shaped pelvic fin, less than half-way to the vent, and 20-21 pectoral-fin rays; and the cleaner gobies have a larger eye and D-12, A-11 fin-ray counts. Larval T. saucrus and congeners can be separated from the very common six-spined gobies with the same VMSx3 melanophore pattern primarily by the length of the caudal peduncle streak. Most larval Coryphopterus and Lythrypnus (all six-spined) have their caudal peduncle streak extending to the start of the procurrent caudal-fin rays vs. about half-way for the seven-spined gobies. Many of those six-spined species also have a prominent melanophore at the corner of the jaw, absent on the seven-spined larvae. Also distinctive is the large cup-shaped pelvic fin on larval T. saucrus; the seven-spined gobies tend to have flat pelvic fins.
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