Above: The living holotype of Pseudotropheus longior, BMNH 1995.8.15: 9 (86.7 mm SL), from Mbamba Bay, Tanzania; about 1000 meters south of Chinyangi Point, collected 14 April 1990 by B. Kilian and L. Seegers. Photographed alive in Germany an unspecified interval after collection. Below at bottom of page: The same specimen (holotype), as preserved. Photos reproduced from Figs. 7 (color) and 9 (black & white) of Seegers (1996); used here with the generous permission of Dr. Lothar Seegers.

Pseudotropheus longior is a recently described relative of P. elongatus — one of several known entities that will probably be named as distinct species eventually. According to the diagnosis in Seegers' description, P. longior belongs to the group of slender species within the P. elongatus complex. (In contrast, P. elongatus itself, although "elongate" compared to most mbuna, is one of the more "compact" or deeper-bodied members of the P. elongatus complex.) The diagnosis goes on to state that P. longior "can be distinguished from other members of this complex by its coloration and color pattern" (but, unfortunately, does not state exactly which features are diagnostic; see complete color description below). Again according to the diagnosis, P. longior has a more slender body (depth 24.6-28.3% of standard length, SL, vs. 27.7-31.4% in P. elongatus), and a less deep head (22.9-26.3% SL, vs. 25.9-29.4%). Also, P. longior has a terminal mouth (vs. more subterminal in P. elongatus) with slightly more teeth in the upper jaw (about 44, vs. about 34-36). Although it is noted only in the description and not in the diagnosis, the slightly concave snout profile with convex forehead, as can be seen above in the color photo, also appears important in distinguishing P. longior from P. elongatus, whose head profile from upper jaw to dorsal fin origin is entirely convex.

In view of the importance of details of the coloration in distinguishing this species, the description of the live coloration of P. longior is quoted here:

Fawn on the body to darker brownish on the head with a more or less distinct bluish hue. In the lake, some very intensely colored territorial males ... had a darker brownish basic color with more distinct blue bars especially on the head (two interorbital bars and one occipital bar) and anterior to the vent. These bars become more faint posterior to the vent. Usually barring is much more faint, especially on caudal peduncle, so that it appears unbarred in many individuals. In several specimens the ground color was dark brown and the light blue bars were reduced to only spots. Dorsal fin with whitish to light bluish lappets, black or with a prominent black submarginal band running most of the fin's length. Proximal parts of the last spines and soft rays sometimes orange, skin between the rays blue. Caudal fin fawn, upper and lower margin whitish to light blue, followed by a black submarginal band. Posterior margin orange. Blue stripes running along the skin between the posterior half of the rays. Anal fin fawn with a faint whitish to bluish margin and a broad black submarginal band. The dorsal and anal fins have orange trailing edges as described for the caudal fin. There is usually one, rarely more, orange to yellow spot, sometimes similar spots are shown in the posterior part of the dorsal fin. Pelvics are black, pectoral fins more or less translucent with a fawn or brownish hue. (Seegers, 1996: 106)