Diagnosis: The modal fin-ray count of D-XVIII,11 A-II,18 and P-13 indicates Labrisomus albigenys and L. nigricinctus. This fin-ray count falls within the lower range for Malacoctenus aurolineatus and M. versicolor, as well as for many of the remaining 18 and 19-spined Labrisomus. (DNA) Ecology: The white-cheek blenny is a rare species, with few collections and no live photographs. They are found sporadically throughout the Caribbean Sea, as well as in the Bahamas and the Yucatan, but not farther into the Gulf of Mexico or in Florida, Bermuda, NE Venezuela, or Brazil and its offshore islands. Their larvae are rare in collections. Description: Pre-transitional larvae: Body long, moderately narrow, and thin with a large round eye, pointed snout, and medium-sized terminal mouth. Long continuous dorsal and anal fins with a very short and relatively narrow caudal peduncle. Pectoral fins long, reaching past the vent, and pelvic fins long and thread-like. On the head there are a pair of large side-by-side melanophores overlying the midbrain lobes and smaller melanophores, either one off-center or a pair, over the forebrain lobes. The cheek melanophores are apparently absent on the three larvae identified. There are no melanophores along the base of the dorsal or caudal fins, although one specimen has one and two melanophores on the sides of the body just lateral to the base of the last dorsal-fin spines (perhaps transitional). Along the ventral midline there is a melanophore at the isthmus and deep at the pelvic-fin base. Along the anal fin there is a melanophore at the base of each anal-fin soft ray (sometimes at the second spine as well). In addition, there is a variably-placed large melanophore near the base of the 14th anal-fin soft ray, sometimes at the base of the ray and extending into the musculature, but sometimes located subsurface in the body some distance away from the base of the fin ray. There are no melanophores along the ventral midline of the caudal peduncle. Internal melanophores comprise only the basic complement: the nuchal midline, otic capsule, and overlying the abdominal organs. Transitional stage: L. albigenys larvae in transition develop diffuse areas of fine surface melanophores over the top and side of the head, not in defined patches. A pale bar develops across the cheek from 4-5 o'clock on the orbital rim. The species apparently develops metamorphic melanophores over the head first, unlike most Labrisomus species. Cirri develop on each side of the head, on the nape (relatively stubby), over the eye (delayed), and over the nasal tube. One larva caught at a light at night had a fully-scaled body before developing metamorphic melanophores. Juveniles: L. albigenys juveniles have no opercular ocellus (or dark spot) and usually an anterior dorsal-fin spot. They have a prominent pale patch on the cheek below the eye (4-6 o'clock from the orbit) outlined by two dark lines. Analogues: Larval L. albigenys can be distinguished from congeners by the prominent enlarged melanophore in or near the posterior anal-fin base series (internal into the musculature or sub-surface on the body near the series). A similar kind of melanophore can be found in the anal-fin base series of some Starksia larvae, however they all have fewer than 9 dorsal-fin soft rays. L. albigenys larvae share their small size, relatively small terminal mouth, and dorsal-fin outline (short first spine and short posterior spines), with the larvae of L. nigricinctus and some Malacoctenus. L. albigenys larvae are very similar to larval L. nigricinctus in both morphology and fin-ray counts, and can best be separated by the enlarged or deep anal-fin melanophore and perhaps by having no melanophores on the ventral caudal peduncle (vs. one). L. albigenys larvae are separated from Malacoctenus by melanophore pattern and fin-ray counts. Only M. gilli larvae occasionally share the pattern of a pair over the midbrain lobes and one or a pair over the forebrain lobes, but M. gilli larvae have no isthmus melanophores. M. erdmani and M. macropus can have similarly few head spots, but they are over the midbrain lobes only and they have no anterior ventral midline spots. Only rare individuals of M. aurolineatus and M. versicolor share the low median and pectoral-fin ray counts of L. albigenys and both have different arrangements of head melanophores: the former with a midline melanophore over the midbrain lobes and additional melanophores along the soft dorsal and caudal-fin bases and the latter with many more head spots. Larvae of the remaining Labrisomus species have more head spots, at least five in a U- or V-shape, relatively longer posterior dorsal-fin spines (vs. soft rays), and are generally larger at transition. Transitional L. albigenys larvae are distinguished by fin-ray counts, persistent larval melanophores, and their metamorphic melanophore pattern, i.e. the metamorphic melanophores on the head are diffusely distributed with only a pale bar across the cheek. Transitional L. nigricinctus are unknown, but probably do not share the persistent enlarged or deep anal-fin melanophore and the head melanophore distribution. L. albigenys are separated from transitional Malacoctenus by having stubs of nuchal cirri and not having clearly delineated patches of metamorphic melanophores on the head. The remaining Labrisomus species develop metamorphic melanophores over the head and body simultaneously, have more larval head melanophores, and relatively longer posterior dorsal-fin spines. L. albigenys juveniles have a uniform dark head with a pale patch on the cheek under the eye, usually outlined by two dark lines, although a similar pattern is common to lesser degrees among many Labrisomus species. Juvenile Malacoctenus all have well-outlined and distinctive head markings. The dorsal-fin outline, with the short first spine and very short penultimate spine relative to the soft rays, separates L. albigenys juveniles from several other Labrisomus species (except L. nigricinctus, L. guppyi and, to a degree, L. gobio). The XVIII,11 or fewer dorsal-fin elements also helps to distinguish less-marked L. albigenys juveniles from occasional lightly-marked juveniles of other Labrisomus species, although a few can range down to overlap. The absence of a dark spot or ocellus on the operculum separates L. albigenys from the other 18-spined species: L. nigricinctus and L. nuchipinnis, L. conditus, and L. cricota (which all also have longer snouts, more than two-thirds the bony orbit diameter). The 19- and 20-spined Labrisomus species share the blunter snout with juvenile L. albigenys (less than two-thirds the bony orbit diameter), but only rarely have so few median fin-rays. L. albigenys early juveniles also have been described to have only two nasal cirri and one long simple orbital cirrus per side (more or branched in the other Labrisomus and Malacoctenus species, except M. macropus).
