Comprehensive DescriptionRead full entry
(Figures 1-4; Tables 1, 8, 9)
Holotype: USNM 371386, 86 mm SL; Mona Passage, off the west coast of Puerto Rico; 18°13’ N, 67°20’ W; 384 m; OREGON station 2645; 5 October 1959.
Diagnosis. A species of Symphysanodon ZBK (Figure 1) distinguishable from all other species of the genus in having fewer gillrakers on first arch (4 or 5 + 19-21 = 24 or 25 total vs. 8-14 + 20-29 = 28-42 total). In addition, it differs from other Atlantic species in having a ventral branch of obliquus dorsalis 3 muscle (Figure 2, vs. its absence in the other two species) and in having a posteriorly projecting extension of the cartilaginous lateral end of ceratobranchial 4 (Figure 3A, vs. its absence in the other two species; S. berryi ZBK has a small accessory cartilage, Figure 3B, associated with the lateral end of ceratobranchial 4; the specimen of S. octoactinus ZBK examined may have had a very small accessory cartilage that was lost during dissection).
Description. The characters included in the combined description of S. mona ZBK , S. parini ZBK , and S. rhax ZBK and those presented in the species diagnosis form part of the species description. Tubed scales in lateral line ca. 47 (many missing). Dorsal-fin rays IX, 10. Pectoral-fin rays 17. Procurrent caudal-fin rays 13 dorsally, 12 ventrally. Trisegmental pterygiophores: 3 associated with dorsal fin, 2 with anal fin. Epineurals associated with first 9 vertebrae. Uroneurals probably 2 pairs, not seen clearly on radiograph. (Johnson, 1984, reported 2 uroneurals for the genus and found 2 in a 15-mm cleared and stained larva of another species of Symphysanodon ZBK , pers. comm. 21 February 2003, and we found 2 uroneurals in radiographed specimens of each of the other species in the genus.) Internarial distance contained ca. 5 times in snout length. Pelvic fins and both caudal-fin lobes damaged; measurements of other body parts are presented in percentages of SL in Table 1.
Comparisons of Atlantic species of Symphysanodon ZBK . Symphysanodon mona ZBK is easily distinguished from the other Atlantic Symphysanodon ZBK on the basis of gillraker counts: S mona ZBK with 4 or 5 + 19-21 = 24 or 25 total; S. berryi ZBK with 9-12 + 24-28 = 34-39 total; S. octoactinus ZBK with 12-14 + 26-29 = 38-42 total. Selected morphometric data for the Atlantic species of Symphysandon are compared in Table 1. In addition there are notable differences among the Atlantic species in the musculature and skeleton of the branchial region (see Figures 2 & 3). Symphysanodon mona ZBK has a ventral branch of the obliquus dorsalis 3 muscle (Figure 2, vs. its absence in the other two Atlantic species) and has a posteriorly projecting fingerlike extension of the cartilaginous lateral end of ceratobranchial 4 (Figure 3A, vs. its absence in the other Atlantic species; S. berryi ZBK has a small accessory cartilage, Figure 3B, associated with the lateral end of ceratobranchial 4, which is not present in S. mona ZBK and was not observed in S. octoactinus ZBK ).
Distribution. Known only from the type locality: Mona Passage off the west coast of Puerto Rico in 384 m (see Figure 4). The holotype of S. mona ZBK was collected with a specimen of S. berryi ZBK (USNM 289922).
Etymology. The name mona refers to the type locality (Mona Passage) and is a feminine noun in apposition to Symphysanodon ZBK . The common name Akarnax Slopefish is derived from a Greek word meaning “a basslike fish” and from the fact that species of Symphysanodon ZBK have been collected frequently in slope waters.