Bryconops caudomaculatus
Creators: name: Barry Chernoff
name: Antonio Machado-Allison
Publisher: name: Zenodo
Year: 2005
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Characiformes
Iguanodectidae
Bryconops
Bryconops caudomaculatus

description: Comments on Bryconops caudomaculatus
Tetragonopterus caudomaculatus was described by Günther (1864) from a single specimen (BMNH 1852.9.13.74), which is in very good condition given its age, though the caudal fin is broken. The type locality was given as South America. We believe that the specimen originated from Guyana (formerly British Guiana) for two reasons. First, and most importantly, the holotype corresponds most closely to historic and recent material from the Essequibo River drainage. Second, Günther described many other species of freshwater fishes from Guyana, including Tetragonopterus affinis (= B. affinis), that live sympatrically with B. caudomaculatus.
In the original description, Günther (1864: 330) wrote that the holotype had “… caudal fin black, with a pair of large red spots on its basal half.” This statement raises three important issues: i) that “spot” could be consistent with a non-circular color mark; ii) that the term “spot” is consistent with the term “ocellus” as applied to a clear area on the caudal fin; and iii) that there is a “spot” of color on the ventral lobe of the caudal fin.
First, while “spot” implies circularity, a somewhat round or elliptical mark, as well as any blotch, may have been referred to as a “spot” by Günther (1864), Eigenmann (1912), and other authors. If present, the color “spot” found on the caudal fins of species of Bryconops are rarely exact circles. Rather they are quasi-circular, elliptical, or irregular in shape (e.g., Fig. 2). Günther (1864) did not observe the fishes in the field but depended upon notes from naturalist collectors, in this case those associated with the Zoological Society. We interpret “spot” as a general description rather than as a strict indicator of circularity.
Second, the caudal fin coloration in Bryconops is most often associated with a relatively clear area over the fin membranes and rays, termed an ocellus, in which the color is displayed prominently. We restrict use of the term “ocellus” to a well circumscribed area lacking melanophores. In a number of species of Bryconops, including B. collettei, the postero-dorsal margin of the unpigmented area is fuzzy -lacking a clearly differentiated border with the melanophores. In the case of species with a fuzzy ocellus, the color extends posteriorly beyond the ocellus on the interradial membranes. The surviving portion of the upper caudal lobe of the holotype of B. caudomaculatus shows a clearly circumscribed ocellus (Fig. 2a). The lower caudal lobe is not present and we cannot ascertain if there was an unpigmented area close to the fin base in which the color was displayed. However, such a condition with red color corresponds to recent and historic material from the Essequibo River drainage of Guyana.
Third, in Bryconops a “spot” of color on the dorsal lobe of the caudal fin is possessed by most species; a color “spot” on the ventral lobe, however, is relatively rare.
Other than B. caudomaculatus, only B. melanurus and B. affinis have such markings, but these two taxa belong to the subgenus Creatochanes. Recent material of B. caudomaculatus from the Essequibo River has an irregular clear area across the base of the ventral caudal fin lobe in which the red color is expressed (Fig. 3a). This color pattern distinguishes the true B. caudomaculatus from all other members of the subgenus. Despite the fact that Eigenmann (1912) had collected many of the true B. caudomaculatus in the Essequibo River basin, he failed to observe the color mark or the clear unpigmented area near the base of the lower caudal lobe, and only mentions the single red mark on the dorsal lobe (Eigenmann, 1912; Eigenmann and Myers, 1929). This has led to much confusion about the identity of B. caudomaculatus.
The holotype has 41 lateral scales and 41 pored scales; the pored scales do not extend beyond the hypural plate onto the scales attached to the caudal-fin rays. The type’s pored and lateral scales represent a somewhat ambiguous condition with respect to other individuals. There are two large phenotypic groups comprising individuals formerly referred to B. caudomaculatus. One group has the number of pored scales exceeding 41, usually 42 or more while the other group is usually 41 or less. The type sits on the boundary. Similarly, the former group has the pored scales extending beyond the hypural plate and onto the caudal fin such that the number of pored scales exceeds the number of lateral scales by at least one scale. We refer to this as the complete condition. The latter group usually has one or fewer pored scales than lateral scales, referred to as the incomplete condition. Approximately 10% of all individuals have equal numbers of lateral and pored scales. Most of the populations with individuals having equal numbers of pored and lateral scales also possess relatively high numbers of scales; thus, they are considered to possess the complete condition. There is one collection from the lower Potaro River, Essequibo drainage, at Tumatumari (FMNH 69672, n=122) that matches the type almost exactly. This collection has a relatively low number of lateral and pored scales in comparison to other populations with the complete condition: 40-42, modally 40, and 41-45, modally 42, respectively. There is one individual in this collection with equal numbers of pored and lateral scales. Thus, we consider the type specimen of B. caudomaculatus to possess the complete condition.
The type is also slightly unusual with respect to body depth. The type is very deep bodied (28.7%) relative to all other measured specimens that could be identified as B. caudomaculatus from across the continent (n>600 examined). The collection from near Tumatumari, Guyana is also relatively deep bodied, 24.5-27.9%, mean = 26.3%, n=75. Though apparently shallower than the type, we note that body depth is strongly dependent upon size, and that the type specimen is larger than the specimens in FMNH 69672. Thus, we conclude that the typical Bryconops caudomaculatus possesses the following traits: (i) a caudal fin with a well developed ocellus containing red color in the dorsal lobe and a clear area in the ventral lobe also with red color; (ii) the complete condition with relatively few lateral scales, 40-42; and (iv) relatively deep body. This phenotype is best found in the Essequibo River basin and other watersheds draining the Guyana Shield.
description: Published as part of Barry Chernoff & Antonio Machado-Allison, 2005, Bryconops magoi and Bryconops collettei (Characiformes: Characidae), two new freshwater fish species from Venezuela, with comments on B. caudomaculatus (Gunther)., pp. 1-23 in Zootaxa 1094 on pages 7-8