Comprehensive DescriptionRead full entry
Early juvenile markings: This species is distinct in the field in having magenta stripes and spots on the head, not strictly blue as in other juvenile damselfishes. This important feature is rarely mentioned in books and guides (in Randall's 2nd edition the
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New recruits (10-15 mm SL) of S. diencaeus have a pattern of magenta stripes and spots on the head and upper body with two spots and a stripe on the iris. There is no additional row of spots between the top stripe on the head and the upper-eye stripe at this stage (this additional row emerges after about 15-20 mm SL, often just three spots in a row). There is a large black ocellus ringed in blue centered on the last three dorsal-fin spines and first three dorsal-fin soft rays (about 50% on the fin, 50% on the body). Although the top of the head and anterior body may be duskier than the rest of the body in new recruits, it is not blue. Ontogenetic homologies include a transient darkening of the scales at the saddle area of the caudal peduncle, a dark spot at the top of the pectoral-fin base, and some darkening of the anterior upper body (but not clearly blue).
Later juveniles (over 20 mm SL) are characterized by magenta stripes on the head and upper body, a relatively uniform yellow body (without obviously blue shading on the anterior upper body and head), a dorsal-fin ocellus distinctly over both the body and fin, no caudal-peduncle saddle spot, and no black spot at the top of the pectoral-fin base.
Occasional problematic individuals do have some blue shading over the top of the head and more than the usual blue spotting and these may be confused with (or hybrids of) S. leucostictus (both species missing the caudal-peduncle saddle). DNA sequence comparisons underway at present should resolve the line of separation.