Scleronema, Eigenmann, 1917
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publication ID |
https://doi.org/ 10.11646/zootaxa.5297.1.1 |
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publication LSID |
lsid:zoobank.org:pub:8D837208-C432-4FB7-A6E7-223C87808B3C |
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DOI |
https://doi.org/10.5281/zenodo.7990939 |
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persistent identifier |
https://treatment.plazi.org/id/605EA82E-6C61-FFA3-FF14-FE0EFD132509 |
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treatment provided by |
Plazi |
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scientific name |
Scleronema |
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Monophyly of Scleronema View in CoL View at ENA
The inclusion of Scleronema auromaculatum , a species quite morphologically divergent from all other members of Scleronema , in the genus required a new, more comprehensive diagnosis ( Costa et al., 2022a). The monophyly of Scleronema in an expanded sense (i.e., including S. auromaculatum ) is currently supported by two characters, one of which is the presence of a skin flap on the posterior margin of the opercle ( Costa et al., 2022a). This character has been widely and traditionally used to diagnose the genus ( Eigenmann, 1917, 1918; de Pinna, 1989a, 1998; Arratia, 1990a; Wosiacki, 2002; Ferrrer & Malabarba, 2020) and is conspicuously present in Scleronema carijo ( Figs. 1–2 View FIGURE 1 View FIGURE 2 ).
The second character was proposed by Costa et al. (2022a) as an osteological synapomorphy of Scleronema in their expanded sense: the anterior portion of the contralateral frontals medially separated from each other by a broad interspace, as illustrated for S. auromaculatum ( Costa et al., 2022a: 91, fig. 5a) and Scleronema cf. guapa ( Costa et al., 2022a: 91, fig. 5d). The plesiomorphic state, the anterior portion of the frontals near or contacting medially, would be present, according to Costa et al. (2022a), in all members of Cambeva Katz, Barbosa, Mattos & Costa, 2018 and Trichomycterus , genera that form with Scleronema the so-called CST-clade ( Costa et al., 2022a), as well as in all other trichomycterines. However, S. carijo does not display the apomorphic state; on the contrary, its frontals are in close contact anteriorly, except for a short and narrow anterior region ( Figs. 3 View FIGURE 3 , 4A View FIGURE 4 ). Furthermore, such a presumably plesiomorphic condition can also be observed in all other species of Scleronema ( Eigenmann, 1918: 282, fig. 2d; and Ferrer, 2016: 171–172, figs. 10–11, for some examples). Therefore, it is necessary to properly and thoroughly reevaluate the definition of this character and its distribution both within Scleronema as well as among other members of Trichomycterinae to corroborate its value as a potential synapomorphy of Scleronema .
Here we propose a new putative synapomorphy for Scleronema , including S. auromaculatum , which, despite being easily observed externally, has been neglected so far: the presence of a more or less developed but distinctive adipose fin posterior to the dorsal fin, confluent posteriorly with the dorsal lobe of the caudal fin. This character is unequivocally present in Scleronema carijo ( Figs. 1 View FIGURE 1 , 15–16 View FIGURE 15 View FIGURE 16 ), as well as in all other species of the genus ( Ferrer, 2016; Ferrer & Malabarba, 2020), including in the recently described S. auromaculatum ( Costa et al., 2022a: 89–90, fig. 2a, 4). This character is also homoplastically present, among trichomycterids, in some members of Glanapteryginae ( Pygidianops Myers, 1944 and Typhlobelus Myers, 1944 ; Myers, 1944; de Pinna, 1989a) and Sarcoglanidinae ( Malacoglanis Myers & Weitzman, 1966 , Sarcoglanis Myers & Weitzman, 1966 , and Stauroglanis de Pinna, 1989 ; Myers & Weitzman, 1966; de Pinna, 1989a), and in all species of the subfamily Copionodontinae ( de Pinna, 1992; de Pinna et al., 2018). Schaefer et al. (2005) treated this structure in Pygidianops and Typhlobelus as part of a dorsal fin-fold that extends from the head to the dorsal margin of the caudal fin. As the adipose fin is a remnant of the larval dorsal fin-fold, there is no reason to reject an initial homology statement between these fins in Glanapteryginae and other trichomycterids on a structural basis ( de Pinna, 1989a; Rizzato et al., 2011), although its occurrence in those taxa is homoplastic in relation to Scleronema , considering the phylogenetic distance between these taxa. Within Trichomycterinae , Cambeva variegata ( Costa, 1992) and Trichomycterus astromycterus also have a vestigial adipose fin ( Costa, 1992: 104, fig. 3; Reis et al., 2019), possibly as independent acquisitions, considering the phylogenies that include these species ( Katz et al., 2018; Costa, 2021; Reis & de Pinna, 2022). In addition, Trichomycterus dali Rizzato , Costa-Jr., Trajano & Bichuette, 2011, a highly troglomorphic species that inhabit caves in the Serra da Bodoquena region in Brazil, also exhibits a well-developed adipose dorsal fin-fold extending along the entire dorsal profile of the trunk and confluent with the caudal fin, the posterior part of which (i.e., posterior to the dorsal fin) is homologous to the adipose fin ( Rizzato et al., 2011: 480, fig. 2, 486, fig. 12). Rizzato et al. (2011) hypothesized that the presence of this character in T. dali is related to its subterranean habits, therefore, it probably also represents an independent acquisition within Trichomycterinae .
The monophyly of the genus Scleronema , therefore, can be supported by these two synapomorphies: (1) the presence of a skin flap on the posterior margin of the opercle, and (2) the presence of a distinct adipose fin posteriorly confluent with the dorsal fold of the caudal fin. The former is exclusive of Scleronema and has been traditionally used to diagnose the genus ( Eigenmann, 1917, 1918; de Pinna, 1989a, 1998; Arratia, 1990a; Wosiacki, 2002; Ferrer & Malabarba, 2020, Costa et al., 2022a), while the second represents a new synapomorphy herein proposed, homoplastically shared with only a few other trichomycterines. Based on the presence of these two characters, S. carijo is included in the genus Scleronema .
Monophyly of the subgenus Plesioscleronema
Costa et al. (2022a) postulated three synapomorphies to support the subgenus Plesioscleronema , composed of a single species, S. auromaculatum , namely: 1) the presence of an expansion on the posterodorsal portion of the quadrate directed towards a concavity on the anterior outgrowth of the hyomandibula ( Costa et al., 2022a: 91, fig. 5f); 2) the presence of a broad hemal spine on the preural centrum 2, its width about twice or slightly more than twice the width of the anteriorly adjacent hemal spine (i.e., of preural centrum 3; Costa et al., 2022a: 91, fig. 5l); and 3) the uroneural separated from the dorsal hypural plate by a small (i.e., narrow) interspace ( Costa et al., 2022a: 91, fig. 5l).
The posterodorsal margin of the quadrate of S. carijo ( Figs. 8–9 View FIGURE 8 View FIGURE 9 ) is mostly smooth, lacking any pronounced processes, running aligned to the correspondingly smooth anterior border of the hyomandibula ( Figs. 8–9 View FIGURE 8 View FIGURE 9 ). This condition, presumably plesiomorphic, is in fact present in all other species of Scleronema other than S. auromaculatum , such as illustrated for S. minutum ( Ferrer, 2016: 177, fig. 16) and S. operculatum ( Ferrer, 2016: 173, fig. 11; Katz et al., 2018: 562, fig. 5g). This character is also present in several other members of Trichomycterinae , e.g., in the genera Bullockia Arratia, Chang, Menu-Marque & Rojas, 1978 ( Fig. 21 View FIGURE 21 ; Arratia et al., 1978: 164, fig. 2b; Arratia, 1990b: 204, fig. 10c; Ferrer, 2016: 176, fig. 15), Cambeva (Bockmann et al., 2004: 232, fig. 6; Katz & Barbosa, 2014: 5, fig. 3; Katz et al., 2018: 562, fig. 5b, d, f; Reis et al., 2023: 6, fig. 3b), Eremophilus von Humboldt, 1805 ( Arratia, 1990b: 204, fig. 10b; Wosiacki, 2002: 306, fig. 63), Hatcheria Eigenmann, 1909 , Ituglanis Costa & Bockmann, 1993 ( Costa & Bockmann, 1993: 45, fig. 4; Datovo & Landim, 2005: 459, fig. 4; Campos-Paiva & Costa, 2007: 57, fig. 3; Rizzato & Bichuette, 2014: 585, fig. 9; Mendonça et al., 2018: 478, fig. 4), Silvinichthys Arratia, 1998 ( Arratia, 1998:360, fig. 8b), and Trichomycterus ( Bockmann & Sazima, 2004: 66, fig. 6; Rizzato et al., 2011: 482, fig. 6; Angulo et al., 2018: 536, fig. 4a–b; Costa et al., 2020b: 2912, fig. 2b, e, h; Costa, 2021: 170, fig. 3; Reis & de Pinna, 2022: fig. 21; Costa et al., 2022b: 498, fig. 4a–g). Even when the margin exhibits some irregularity, it is quite subtle, not comparable to the conspicuous backward-facing process of the quadrate of S. auromaculatum ( Costa et al., 2022a: 91, fig. 5f). This character is, therefore, considered autapomorphic of S. auromaculatum and diagnostic for Plesioscleronema .
Like most species of the subgenus Scleronema , the hemal spine of the preural centrum 2 of S. carijo is thicker than the neural spine of the preural centrum 1, but only slightly ( Fig. 16 View FIGURE 16 ), certainly not comparable to the condition of S. auromaculatum , in which the width of the hemal spine of preural centrum 2 is about twice or slightly more than twice the width of the anteriorly adjacent hemal spine (i.e., of preural centrum 3), as described and illustrated by Costa et al. (2022a: 91, fig. 5l). Thus, this characteristic, which is plesiomorphically absent in S. carijo and all other members of the subgenus Scleronema , is corroborated as being autapomorphic for S. auromaculatum and, therefore, diagnostic for the subgenus Plesioscleronema .
According to Costa et al. (2022a), the uroneural of Scleronema auromaculatum is separated from the dorsal hypural plate by a small (i.e., narrow) space ( Costa et al., 2022a: 91, fig. 5l), whereas in the remaining species of the genus, the uroneural and the dorsal hypural plate contact each other ( Costa et al., 2022a: 91, fig. 5m). In S. carijo , the space between the uroneural and the dorsal hypural plate is also narrow ( Fig. 16 View FIGURE 16 ), being, in some specimens, negligible, a condition widespread among other members of the subgenus Scleronema . Therefore, the uroneural state of S. carijo appears to be polymorphic.
In addition to these three synapomorphies, the two following osteological characters were listed in the diagnosis of Pseudoscleronema, but their phylogenetic polarities had not been evaluated by the authors ( Costa et al., 2022a): 1) the presence of a well-developed lateral process in the vomer ( Costa et al., 2022a: 91, fig. 5c); and 2) the shape of the metapterygoid, described as robust and longer than deep ( Costa et al., 2022a: 91, fig. 5f). Such structures of
ANATOMY, ECOLOGY, & BEHAVIOR OF A NEW SPECIES OF SCLERONEMA Zootaxa 5297 (1) © 2023 Magnolia Press · 33
Scleronema (Pseudoscleronema) auromaculatum are compared here with those of S. carijo and other Scleronema species, to assess their phylogenetic signals for the subgenera Pseudoscleronema and Scleronema , as well as their diagnostic value for the former subgenus.
The presence of a well-developed lateral process in the vomer presumably distinguishes Scleronema auromaculatum from all other species of the genus belonging to the subgenus Scleronema , which would bear, instead, a rudimentary lateral process in the vomer ( Costa et al., 2022a: 91, fig. 5c). Indeed, the illustration of the skull of S. auromaculatum provided by Costa et al. (2022a: 91, fig. 5c) shows a vomer with well-marked (longer and narrower), backwards-directed lateral processes, giving the bone a winged aspect.According to the authors ( Costa et al., 2022a: 91, fig. 5d–e), the lateral process of the vomer of other Scleronema species would be, differently, shorter and wider so that the bone displays a mace-like appearance. However, the state of this character in S. carijo cannot be determined with certainty because both conditions may be present in the same specimen ( Fig. 4C View FIGURE 4 ). In addition, this polymorphism is also observed in other species of the subgenus Scleronema , such as S. minutum ( Ferrer, 2016: 174, fig. 12). Furthermore, a well-developed and pointed lateral process in the vomer in close resemblance with the character state reported for S. auromaculatum by Costa et al. (2022a) is present in S. macanuda and S. operculatum according to Ferrer (2016).
Costa et al. (2022a: 91, fig. 5f) described the metapterygoid of S. auromaculatum as robust, longer than deep, contrasting with the presumably slender, deeper than longer metapterygoid of the members of the subgenus Scleronema ( Costa et al., 2022a: 91, fig. 5g –h). However, the metapterygoid configuration shown by S. auromaculatum appears to have a wider distribution in the genus than suggested, being present at least in S. carijo ( Figs. 8–9 View FIGURE 8 View FIGURE 9 ), S. minutum ( Ferrer, 2016: 172, fig. 10; cited as S. angustirostre ), and S. operculatum ( Ferrer, 2016: 173, fig. 11). In a quick analysis, we noticed that there is a relevant degree of variation in the morphology of the metapterygoid in some species of Scleronema .
Thus, among the three synapomorphies proposed for Plesioscleronema ( Costa et al., 2022a), S. carijo definitely has the corresponding plesiomorphic states of two of them: 1) the posterodorsal margin of the quadrate mostly smooth, not forming any noticeable process; and 2) the hemal spine of the preural centrum 2 only slightly more robust than the neural spine of the preural centrum 1. Regarding the third putative synapomorphy for Plesioscleronema , the uroneural apart from the dorsal hypural plate by a narrow interspace, S. carijo exhibits the condition considered plesiomorphic, supposedly present in species of the subgenus Scleronema , as wel as the apomorphic one, presumably found only in Plesioscleronema . Therefore, until a reassessment is made to determine its states in Scleronema and a phylogenetic investigation among Scleronema species, this character should be excluded as a synapomorphy of Plesioscleronema . Despite the indecision on this feature, it is evident that S. carijo cannot be assigned to the subgenus Plesioscleronema .
As a result of this review, two of those characters were corroborated as diagnostic of the subgenus Plesioscleronema : 1) a distinctive expansion on the posterodorsal portion of the quadrate directed towards a concavity on the anterior outgrowth of the hyomandibula; and 2) a broad hemal spine on the preural centrum 2, its width about twice or slightly more than twice the width of the anteriorly adjacent hemal spine (i.e., of preural centrum 1). Regarding those two other osteological features mentioned in the diagnosis of Plesioscleronema only ( Costa et al., 2022a), referring to the morphology of the vomer and the metapterygoid, it was revealed that they are intraspecifically polymorphic or that they exhibit both states among the species of the subgenus Scleronema , being, therefore, devoid of diagnostic value for the genus.
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