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
Photograph by Gregory Taylor saltyzoo.com
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.
Juvenile analogues: New recruits of S. diencaeus are separated from S. planifrons and S. adustus by having stripes along with the spots on the head and iris and from S. partitus by having a dorsal-fin ocellus. New recruits of S. diencaeus are most easily separated from the other striped species when the S. diencaeus have no blue wash over the anterior upper body and head, but some individuals do show a dusky anterior coloration and these individuals can be problematic. Other features separating the new recruits from S. leucostictus are having the ocellus half onto the body, the blue ring below the ocellus a continuous line, no additional row of spots between the top stripe on the head and the upper-eye stripe, and sparser spotting on the anterior dorsal fin. New recruits of S. diencaeus can also be separated from S. variabilis by having only a faint caudal-peduncle saddle (if any) but, when compared to recruits of S. variabilis that have not yet developed their saddle, the distinction is difficult without life colors. Unfortunately, some S. diencaeus recruits up to 20 mm SL have some form of faint caudal-peduncle saddle and some S. variabilis of the same size can have similar unformed saddles. In that case, the distinction can become more subtle: S. variabilis have more dusky blue shading over the head and body, especially including the posterior dorsal fin and the anal fin which are not dusky in S. diencaeus. Unfortunately, degrees of duskiness are poor characters to separate individuals. DNA sequence comparisons underway at present should resolve the line of separation.
Diagnosis: Damselfishes with 12 dorsal-fin spines and a mode of 15-16 dorsal-fin soft rays indicate Stegastes and Microspathodon chrysurus. Fin-ray counts broadly overlap among Stegastes with most species having 13-14 soft anal-fin rays and 18-20 pectoral-fin rays (S. adustus and M. chrysurus have a mode of 21 pectoral-fin rays). Given this overlap, larvae and even early juveniles can require DNA sequencing to reliably distinguish the species.
Stegastes diencaeus, the longfin damselfish, is a damselfish in the family Pomacentridae from the Western Atlantic. It occasionally makes its way into the aquarium trade. It grows to a size of 12.5 cm in length.[2] Longfin damselfish have been reported to have a mutualistic relationship with mysid shrimp of the species Mysidium integrum. The interaction is described as a form of domestication with the shrimp providing nutrients for the algae farms the fish feed on and the fish providing protection from predators.[3][4]
Stegastes diencaeus, the longfin damselfish, is a damselfish in the family Pomacentridae from the Western Atlantic. It occasionally makes its way into the aquarium trade. It grows to a size of 12.5 cm in length. Longfin damselfish have been reported to have a mutualistic relationship with mysid shrimp of the species Mysidium integrum. The interaction is described as a form of domestication with the shrimp providing nutrients for the algae farms the fish feed on and the fish providing protection from predators.
