Analogues: Tail with full ventral streak
Diagnosis: Modal fin-ray counts of D-VI,10 A-9 and Pect-15 (often 16) match Lythrypnus as well as several other goby species. Bathygobius curacao can overlap the fin-ray counts but has a heavily-marked larval stage. Coryphopterus alloides shares the median-fin-ray count but has an equal incidence of 16 and 17 pectoral-fin rays (adults have divided pelvic fins with no frenum, but that is likely not apparent on larvae). C. kuna shares the 15 pectoral-fin rays but has equal numbers of dorsal and anal soft rays (D-VI,9 A-9) and often dorsal head and body markings. Other gobies which share the median-fin-ray count but have more pectoral-fin rays include Lophogobius cyprinoides 17-19 (16-20), Priolepis hipoliti (17-19), Bathygobius mystacium and B. soporator (both with mode 19-20). The only seven-spined goby coming close to the fin-ray count is Gobiosoma grosvenori (16-18).
There are many regional Lythrypnus species and their ranges are not well-understood and they may include geographic variants of the widespread L. nesiotes. It is likely that these species all appear similar or identical as larvae. Up to seven species could be present at most locations, making DNA sequencing necessary to reliably identify larvae to species. L. phorellus is primarily found from Florida northwards and L. mowbrayi was first described as the Bermuda sibling of L. nesiotes. L. nesiotes and L. crocodilus are widespread, co-occur with some habitat isolation, and differ only slightly in markings (L. nesiotes has dark bars wider than light bars). Other species with a mode of 15 pectoral-fin rays (but often having 14) include L. heterochroma, L. elasson, and the tiny and elusive L. minimus. L. okapia has fewer rays, i.e. a mode of 9 dorsal-fin elements, 8 anal-fin elements and 14 pectoral-fin rays (but rare individuals of the other species also have this fin-ray count). L. spilus has a mode of 16 pectoral-fin rays.
Note: There is extensive variation in body shape and melanophore patterns in this laval type. Since all fin-ray counts can overlap extensively in this genus, it is unclear whether this variation indicates different species or variation during transition. There are three basic forms of these larvae, but intermediates occur and I cannot separate them as larval types. The most common form has four or five melanophores in the row along the anal-fin base and about seven melanophores in the row along the ventral midline of the caudal peduncle. The anal-fin row is about half the length of the caudal peduncle row. Less common is a "short anal" variant with a shorter row of anal-fin base melanophores, two or three, but often just a short streak that is clearly less than half the length of the caudal peduncle row. The caudal peduncle row is also often reduced in this type. Lastly, there is a "five spot" variant with only about five total melanophores along the anal fin and the caudal peduncle, and the anal-fin base melanophores are widely-spaced. Some of these have typical medium-sized round eyes but some are hunched-over and have larger eyes. It is possible that the reduction in melanophores is part of transition, but it does not seem to correlate with the development of metamorphic melanophores (however this would not be unusual in larval gobies).
Description: Body relatively thin, long and narrow with a medium to large eye and a terminal mouth. Pectoral and pelvic fins long, reaching beyond the vent as transition approaches, dorsal and anal-fin bases relatively short, caudal peduncle long and narrowing rapidly, procurrent caudal-fin rays 5-7 (5, sometimes 6, spindly). Lightly marked mostly along the lower body: melanophores on the ventral midline at the isthmus and the pelvic-fin insertion (usually streaks). The latter melanophore often appears to extend deep into the body. Then there is a row along the anal-fin base, usually four or five, paired and one per side, between the third and eighth element, often merged into a streak on each side (less common forms have only two or three melanophores along the anal-fin base, but there are intermediate individuals). Then, after a space, there is a row of midline melanophores, usually seven unpaired (but often merged into a streak), extending along the caudal peduncle ending near the start of the procurrent caudal-fin rays (the reduced-melanophore forms have only three or four). In addition, all individuals have a melanophore at the angle of the jaw, at very least on one side. Rare individuals have a single midline melanophore just after the last dorsal-fin ray or a single melanophore on the base of the lower segmented caudal-fin rays. Internal melanophores are present in the head at the base of the saccule and at the dorsal surface of the swim bladder and around the gut near the vent. There is a characteristic deep thoracic melanophore beneath the base of the pelvic fin, often merged into the surface melanophore at the pelvic-fin base. Series of transitional larvae show development of the eye from a markedly narrowed vertical oval, often tilted forward and often with a dorsal indentation in the iris, with a small posterior-inferior extension of the iris (usually covered with surface melanophores) to medium-sized and round and then becoming much larger at the end of transition. On the oval eyes, there is often a small speckled "eyebrow" membrane over the anterior-superior lobe of the eyeball that appears detached from the pigmented iris below. Unlike several other goby larval types, this membrane is not obvious on the later rounded eyeball. The pectoral and pelvic fins become distinctly longer through transition. Transitional larvae develop seven bars of melanophores radiating outwards from the orbit, beginning with the bar to the mid-upper jaw.
