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Early juvenile markings: New recruits (10-15 mm SL) of S. leucostictus have a pattern of blue stripes and spots on the head and upper body with a broad stripe and two spots on the iris. The upper part of the head and anterior body have a dusky blue background color. The blue stripes and spots are heavier than in other conspecifics and usually there is a third line of blue spots between the top stripe on the head and the stripe directly off the upper eye. Furthermore, the stripes typically break up into a pattern of numerous bright spots on the anterior dorsal fin. There is a large black ocellus ringed in blue centered on the last two dorsal-fin spines and first three dorsal-fin soft rays. The ocellus is typically mostly on the fin (usually 70% or more) and often higher than wide. There is usually one unmarked scale row between the lateral line and the blue ring. In addition, the scales with blue below the ocellus on the body are darkly-outlined rather than merged into a blue line. The spot migrates rapidly onto the fin and, by the time the juveniles reach 20 mm SL, almost all of the black spot is over the dorsal fin. There is a dark spot at the top edge of the pectoral-fin base. There is no caudal-peduncle saddle spot.
Later juveniles (over 20 mm SL) are yellow with a blue (often bright blue) upper head and anterior body and dorsal fin with numerous iridescent blue stripes and spots, a dorsal-fin ocellus mostly off of the body (often completely over the fin rays and subsequently the edges becoming indistinct and disappearing), a spot at the top of the pectoral-fin base, and no caudal-peduncle saddle. Some juvenile damselfishes are identical in all respects to these S. leucostictus, but do have a caudal-peduncle saddle, and these are automatically assigned to S. variabilis (DNA sequencing should confirm this conclusion).