Diagnosis: The modal fin-ray count of D-XIX,11 A-II,19 and P-13 is shared by the 19-spined trio of L. guppyi, L. gobio, and L. kalisherae, as well as by Malacoctenus aurolineatus (but usually with 14 pectoral-fin rays). Note that almost 90% of L. guppyi individuals have 19 dorsal-fin spines. The fin-ray count falls within the upper or lower range for most other Labrisomus species and both M. delalandii and M. gilli. (DNA) Ecology: The mimic blenny is a somewhat common small blenny found mostly under rocks and in shallow complex limestone and mixed coral habitats. The species is widespread in the region, found in Florida, the S. Gulf of Mexico, the Bahamas, and across the Caribbean Sea except NE Venezuela. Unfortunately, the 19, 20, and 21-spined Labrisomus are very similar morphologically and are frequently confused in guidebooks and photographs. All six species share the blunt-headed and goggle-eyed appearance, at least as juveniles, i.e. the snout length (bony orbit to jaw tip) is less than two-thirds of the orbit diameter. Adult mimic blennies are recognized in the field mostly by a distinct opercular ocellus, well-outlined and typically with an orange posterior rim, and a relatively short first dorsal-fin spine. Juveniles are particularly difficult to separate, since the distinguishing markings for each species are either not well-developed or shared by other species at this early stage (but see description below). L. guppyi larvae are uncommon in collections. Description: Pre-transitional larvae: Body long, narrow, and thin with a medium eye, pointed snout, and terminal medium-sized mouth. Larvae can have the normal body shape or develop a hunched-over appearance (tip of snout below the lateral midline of the body). Long continuous dorsal and anal fins with a short and narrow caudal peduncle. Pectoral fins long, reaching past the vent, and pelvic fins long and thread-like; the third pelvic-fin ray is about 3/4 the length of the second. There are some small spines along the rim of the preoperculum that no longer project as larvae approach transition. The complement of melanophores on the top of the head is usually a narrow-U, i.e. a row of three on each side of the head with the spots closer to the dorsal midline anteriorly, plus a near-surface melanophore at the midline behind the mid-brain lobes (not the deep nuchal midline melanophore), completing a narrow-O (note that the rear side-by-side pair are typically more widely spaced than the middle pair). Usually there are additional equally large melanophores over some or all four quadrants, i.e. each lobe of the fore- and mid-brain. On more marked larvae, there is a prominent melanophore on each side just behind the tip of the upper jaw (this pair is often absent on lightly marked larvae). There is a cheek melanophore on each side. Melanophores typically run along the base of all of the soft dorsal-fin rays, and, on more heavily marked larvae, along some of the posterior dorsal-fin spines as well; however some lightly-marked larvae have only a few soft-dorsal fin base spots or even rarely none. A vertical line of melanophores develops along the base of the caudal-fin segmented rays (first proximal, then distal), sometimes few and mostly on the upper half, or rarely none, on lightly marked larvae. Some larvae can have one or two linear melanophore streaks outlining the mid-length of one or two lower caudal-fin segmented rays (most have none). Along the ventral midline there are melanophores at the isthmus and deep behind the pelvic-fin base. There is no extensive speckling of the peritoneum visible through the ventral abdominal wall, but some more heavily marked larvae can show some peritoneal speckling on the upper sidewalls of the abdomen. Along the anal fin there is a melanophore at the base of each anal-fin soft ray, followed by none to three along the ventral midline of the caudal peduncle. A row of internal melanophores overlies the vertebral column, spaced about every third vertebra, along the mid- and rear body, continuing onto the caudal peduncle. Additional internal melanophores include those at the nuchal midline, otic capsule, and overlying the abdominal organs. Transitional stage: L. guppyi larvae in transition develop metamorphic melanophores over the head and body simultaneously, a mostly uniformly speckling of fine melanophores, although some transitional larvae develop an indistinct pattern of dark bars.The full complement of larval melanophores, especially multiple spots over each quadrant of the forebrain and midbrain as well as the pair behind the tip of the upper jaw are present at transition. Juveniles: L. guppyi juveniles have dark bars on the body that extend onto the dorsal-fin membranes, with the second body bar (under the 10-15th dorsal-fin spines) covering the bases of about 4 spines. The last dark bar on the tail ends as a straight vertical line or a slight crescent. Early juveniles may have only a diffuse dark spot on the operculum, but they rapidly develop an ocellus with a thin orange rim on the posterior edge. The first dorsal-fin spine is notably short, only a third or less of the body depth at the dorsal-fin origin and well shorter than the mid-fin spines. Analogues: The larvae of the 19-spined Labrisomus are likely identical, although, as they approach transition, L. guppyi larvae should develop their shorter first dorsal-fin spine. Lightly-marked larvae of L. guppyi missing the pair of melanophores behind the tip of the upper jaw can be distinguished from larvae of L. nuchipinnis, L. conditus, and L. cricota, by the absence of melanophores along the base of the anterior half of the spinous dorsal fin, especially the first ten spines, and the absence of melanophores outlining the lower segmented rays of the caudal fin (also many of the latter three species have only a single melanophore over the forebrain quadrants (V of 5) vs. more on L. guppyi). L. albigenys and L. nigricinctus larvae are very lightly-marked, missing most of the melanophore complement of L. guppyi. Among the Malacoctenus larvae, only M. aurolineatus and a rare M. triangulatus share the melanophores along the base of the soft dorsal fin; the former have a distinctive head melanophore pattern and the latter are distinguished by a short third pelvic-fin ray, many head melanophores varying from tiny to large, and a dorsal-fin outline with a short third-to-last spine. Other Malacoctenus larvae may resemble the rare L. guppyi without dorsal midline melanophores, but they all have fewer head melanophores not in the U or O pattern and/or dorsal-fin outlines with short posterior spines and often different fin-ray counts. L. guppyi larvae with the pair of melanophores behind the tip of the upper jaw can be distinguished from L. bucciferus larvae by a shorter first dorsal-fin spine (as they approach transition) and usually one fewer dorsal-fin spine (possibly also by additional small head melanophores, if present, over all quadrants vs. only forebrain). L. haitiensis larvae differ in having abdominal speckling, melanophores at the base of dorsal-fin spines 8 or 9, longer pelvic fins with a short third ray, and additional dorsal-fin spines. Transitional L. guppyi larvae are distinguished by fin-ray counts, persistent larval melanophores, and their uniform speckling of metamorphic melanophores. The pair of melanophores behind the tip of the upper jaw is shared only by the other 19- and 20+-spined species, L. gobio, L. kalisherae, L. bucciferus, and L. haitiensis, all of which have a different dorsal-fin outline, with longer first spines (about two to 2.5 times in the body depth at the dorsal-fin origin vs. 3 times or more, and about the same length as the the mid-fin spines vs. well shorter). Transitional L. haitiensis develop a prominent pattern of ovals and bars. The transitional larvae of L. nuchipinnis, L. conditus, and L. cricota develop reticulated lines over the body (vs. uniform speckling). Transitional L. guppyi larvae are separated from transitional Malacoctenus larvae by the uniform markings over the head and body (developing at the same time), short cirri, and the dorsal-fin outline. The juveniles of the 19- and 20-spined Labrisomus are blunt-headed, compared to the 18-spined Labrisomus (other than L. albigenys), i.e. the snout length (bony orbital margin to the tip of the closed jaw) is less than two-thirds the bony orbit diameter. Small juveniles of the blunt-headed Labrisomus are difficult to identify to species, especially since some species described with opercular ocelli can have only a diffuse dark spot on the operculum when young. In addition, those species described without ocelli can often have an outlined dark spot on the operculum (but not a thin-rimmed ocellus) as juveniles. Juvenile L. guppyi who have not yet developed a distinct opercular ocellus are difficult to separate from L. gobio juveniles: the key differences are a shorter first dorsal-fin spine (about a third the body depth at the dorsal-fin origin and well shorter than the mid-fin spines vs. about half and slightly shorter, about 90%), the dark bars on the body extend onto the fins in L. guppyi (vs. not onto the fins) and the mid-body bar covers about 3 or 4 spine bases (vs. widening to reach 5 or 6). Juvenile L. kalisherae, as well as L. bucciferus and L. haitiensis (both with higher fin-ray counts), are separated from L. guppyi by having distinctly longer first dorsal-fin spines (about two into the body depth and the same length or a little shorter than the mid-fin spines vs. well shorter) and the pigment on the caudal peduncle ending by outlining three light spots vs. a mostly straight vertical line in both L. guppyi and L. gobio. Juvenile L. albigenys share the dorsal-fin outline of L. guppyi (relatively short first and penultimate dorsal-fin spines) and are best distinguished by the absence of an opercular spot or ocellus and their lower fin-ray counts. After juvenile L. guppyi develop an opercular ocellus with a thin rim (at least on the posterior edge), they can be distinguished from other ocellated species: from L. nigricinctus and L. nuchipinnis (as well as L. conditus and L. cricota, both with less distinct ocelli) by the blunt snout and from L. haitiensis (which can have a similar opercular spot) and L. filamentosus by the short first dorsal-fin spine (and from all by fin-ray counts).
