Pimelodella bockmanni, Slobodian & Pastana, 2018

New distribution records of Pimelodella bockmanni and P. serrata

The Colombian records of Pimelodella bockmanni come from terra firme forest streams, tributaries to the main channel of the Amazon River in Colombia, in the southernmost extreme of the country, known regionally as Trapecio Amazónico ( Fig. 1 View FIGURE 1 ). These specimens were originally identified as P. geryi Hoedeman, 1961 by Arbeláez et al. (2008), possibly due to their similar general appearance and pigmentation pattern ( Fig. 2 View FIGURE 2 ). However , Colombian specimens differ from P. geryi by their supraoccipital process not reaching the nuchal plate (vs. in contact), adipose fin longer (35.2–40.1% in SL vs. 23.8–25.1%), and upper lobe of caudal fin longer (vs. lower lobe longer) ( Hoedeman 1961; Slobodian et al. 2017). Pimelodella bockmanni was recently described from main right tributaries of the Madeira River in Brazil ( Slobodian & Pastana 2018). Although the type series of P. bockmanni was not examined directly, we found that our specimens agree with the morphological information provided in its original description ( Slobodian & Pastana 2018), except by having a longer upper lobe of the caudal fin (39.0–51.0% of SL vs. 25.8–34.8%), which can be attributed to the larger size of the examined specimens from Colombia (SL: 86.0– 113.7 mm vs. 39.8–76.9 mm of the type series) ( Table 1 View TABLE 1 ).

The geographic distribution of Pimelodella bockmanni seems to conform to the Central Blackwater Amazon pattern proposed by Dagosta & de Pinna (2019), which connects the two hydrographic systems with known records of the species, i.e. Madeira and western Amazon (above the mouth of the Rio Negro). Although the Amazon River is a typically white-water system in the Amazonas / Solimıes region , the streams where Colombian specimens were collected drain heavily leached soils, and their waters are poor in nutrients and dissolved solids ( Arbeláez et al. 2008). Detailed data on the type of water corresponding to the localities where the type series originated will allow confirmation of this biogeographic categorization.

Pimelodella serrata ( Fig. 3 View FIGURE 3 ), is recorded for the first time from Colombia, based on specimens coming from flooded areas (laguna Yahuarcaca) and terra firme streams (quebrada La Arenosa), draining directly to the main channel of the Amazon River in Colombia ( Fig. 1 View FIGURE 1 ). This species was described for San Joaquín, a locality drained by the Machupo River, a tributary of the Iténez River (upper Guaporé drainage) in Bolivia, and was distinguished by four characters in the dichotomous key provided in Eigenmann (1917): 1) absence of conspicuous cutaneous pores associated to the lateral-line sensory canals in the head, whose presence has been useful in the recognition of other species of Pimelodella (e.g. P. longibarbata , P. mucosa ) ( Cortés-Hernández et al. 2020); 2) adipose fin long (<3 times in SL); 3) anterior and posterior margins of pectoral-fin spine fully serrated ( Fig. 4 View FIGURE 4 ); and 4) maxillary barbel extending from the posterior end of the anal-fin base to beyond the caudal-fin base. All these conditions were corroborated in the specimens from Colombia, as well as all morphometric and meristic characters indicated in P. serrata description and/or verified in the radiographs of its holotype ( Table 2 View TABLE 2 ). Pimelodella serrata is perhaps one of the most easily recognized species in the genus, due to its peculiar pectoral-fin spine ornamentation, consisting of completely serrated anterior and posterior margins. This condition is also verified in P. chaparae , a species showing a similarly wide distribution in the Amazon basin, albeit extending further north to the separate Colombian Orinoco basin ( Cortés-Hernández et al. 2020). Nonetheless, P. chaparae can be differentiated from P. serrata by: adipose-fin base shorter (27.4–30.8% of SL vs. 40.2–43.8%), greater interdorsal distance (7.4–14.0% of SL vs. 1.5–2.4%), longer preadipose length (56.6–64.2% of SL vs. 50.7–53.0%), shallower head (head depth: 26.4–32.2% of SL vs. 42.0–45.0%), larger orbital diameter (19.6–25.1% of SL vs. 15.0–17.7%), narrower interorbital (interorbital width: 12.6–18.3% of SL vs. 19.8–21.4%), shorter maxillary barbel (extending between the middle and the posterior end of the adipose fin vs. surpassing base of caudal fin in P. serrata ), and fewer vertebrate (41 vs. 43).

In addition to records restricted to the Madeira River drainage ( Machado-Allison et al. 1999; Chernoff et al. 2000; Sarmiento, 2000; Bockmann & Slobodian, 2013; Oliveira et al., 2020; Miranda-Chumacero et al. 2022), P. serrata has also been recorded for the Peruvian Amazon, in the Loreto department (Mesa-Vargas et al. 2021), and along with records reported here, show a wide distribution of this species in the piedmont and lowlands of the western Amazon.