Paracanthopoma, Giltay, 1935

Pinna, Mário de & Dagosta, Fernando Cesar Paiva, 2022, A taxonomic review of the vampire catfish genus Paracanthopoma Giltay, 1935 (Siluriformes, Trichomycteridae), with descriptions of nine new species and a revised diagnosis of the genus, Papéis Avulsos de Zoologia 62, pp. 1-90 : 75-78

Monophyly of Paracanthopoma

Monophyly of Paracanthopoma has not been a relevant issue previously, because the genus has been monotypic for most of its history. The situation has changed drastically with the 13 species now documented. Below we provide evidence for the monophyly of Paracanthopoma as herein circumscribed. The evidence is straightforward, since the genus is diagnosed by numerous distinctive morphological characters unique across a wide phylogenetic spam of the family Trichomycteridae and often of Siluriformes . Suggestions of Paracanthopoma synapomorphies have been recently discussed in Dagosta & de Pinna (2021) and Henschel et al. (2021a, b). However, the most complete assessment yet done on the subject was in a phylogenetic analysis by DoNascimiento (2012), and that evidence is incorporated and discussed in this section. Some of the characters in DoNascimiento (2012) have not been included below because their significance has changed in face of the increased diversity of Paracanthopoma . For example, the fusion of the epioccipital with the Weberian complex (his character 96) indeed subsumes an interesting set of modification present in some species of the genus ( Pc. ahriman , Pc. cangussu , Pc. irritans , Pc. saci , Pc. vampyra ), but not others ( Pc. alleynei , Pc. capeta , Pc. carrapata , Pc. daemon , Pc. malevola , Pc. parva , Pc. satanica , Pc. truculenta ). Also, the anatomical situation across the various species seems to be more complex than simple fusion. At least in some cases (e.g., Pc. irritans ), what happens is a loss of the epioccipital and its topological replacement with an anterodorsal laminar expansion of the Weberian capsule. One specimen of Pc. irritans (INPA 20529) is particularly elucidative in that regard, having on one side a tiny independent epioccipital, and on the other side no epioccipital and instead an expansion of the swimbladder ossification invading the corresponding position of that bone. Proper understanding of the anatomical modifications involved in this complex will require more detailed investigation and examination of developmental series of different species.

Anothersynapomorphyproposedfor Paracanthopoma by DoNascimiento (2012) but not included below is the truncated proximal ends of the dorsal-and anal-fin basal radials (his characters 387 and 398). This is an interesting condition resulting from an incomplete ossification of the basal radials, which maintain a blunt cartilaginous proximal tip. In other trichomycterids and normally in catfishes and most other teleosts, the ossification of the basal radials progresses to their proximal tip, which thus end in a fine fully ossified extremity lacking any remaining cartilage. The derived, incompletely-ossified, condition applies to most species in the genus, except Pc. alleynei and Pc.vampyra , which maintain the plesiomorphic fine-pointed proximal tip of the basal radials.The apomorphic state seems to be a result of pedomorphosis, since the development of the basal radials starts as an entirely cartilaginous rod which gradually becomes ossified. The condition where there is a remaining cartilaginous tip proximally is a result of truncation in the ossification process of the structure. Curiously, this is not correlated with size, because the largest-bodied species of Paracanthopoma , such as Pc. parva and Pc. truculenta , have a pedomorphic truncated condition identical to that in some of the smallest, such as Pc. irritans and Pc. cangussu . Meanwhile, the two species with the fully ossified plesiomorphic state are relatively small-sized in the genus.This situation suggests that body size and pedomorphosis are decoupled within Paracanthopoma .

Overall, the correspondence of a monophyletic group with Paracanthopoma is simple. Previously described species Pc. cangussu and Pc. saci were originally correctly assigned to the genus and require no change. Paracanthopoma alleynei ( Henschel et al., 2021b) , on the other hand, was originally included in Paravandellia and is here transferred to Paracanthopoma (see taxonomic section above). The latter species is clearly more closely related to the other species in Paracanthopoma , including the type species Pc.parva , than to any other vandelliines, including those in Paravandellia .

1 Presence of a branchiostegal velum – As proposed in Dagosta & de Pinna (2021: 15-16), the structure previously referred to as "free branchiostegal membrane″ in Paracanthopoma is actually a neomorph, unique to the genus. A branchiostegal membrane (or gill membrane, or branchial membrane) is defined as the membrane lying between the opercular bones and the isthmus, supported by branchiostegal rays ( McAllister, 1968: 4). Among trichomycterids, the branchiostegal membrane can be narrowly attached to the isthmus, a situation where they are united to the isthmus anteriorly at the midline only, leaving wide branchial openings. This is the condition in the entire previously considered phylogenetically basal portions of the family, including Copionodontinae and Trichogeninae , Trichomycterinae , Sarcoglanidinae , Glanapteryginae and most Tridentinae ( DoNascimiento, 2012) (the exception is Miuroglanis , which has an almost totally fused branchiostegal membrane). At the other extreme, the branchiostegal membranes are broadly united with the isthmus, forming a continuous integument cover over the entire gular region and leaving small branchial openings (usually between the opercle and interopercle). This is the condition in all Stegophilinae and Vandelliinae . A modification of the broadly united condition is one where the branchiostegal membranes form a free fold across the isthmus posterior to their fusion with the isthmus. This is the situation in some stegophilines ( Acanthopoma , Apomatoceros and Schultzichthys ), in which the free flap is clearly a portion of the membrane posterior to the fusion, because it contains embedded branchiostegal rays. In Paracanthopoma , the situation looks superficially similar (e.g., Figs. 5 , 6 and all other illustrations of ventral views of the head for species in this work, both in alcohol and SEM) but is in fact very different. The actual branchiostegal membrane in Paracanthopoma is nearly entirely fused to the isthmus, leaving the branchial opening reduced to a small passage limited to the region between the opercular and interopercular odontodophores as in all other vandelliines. The integument fold across the isthmus in Paracanthopoma is actually a different integumentary outgrowth that overlays the whole isthmal region ( Dagosta & de Pinna, 2021). It contains no branchiostegal rays, which are located in the soft tissue anterior to the fusion with the isthmus. The fold forms a broad and deep integument flounce extending continuously across the isthmus, with no inbedded rays. The Paracanthopoma fold is probably a derivative of the branchiostegal membrane, but constitutes a set of specialization exclusive to the genus. It is therefore an especialized condition and not the widespread plesiomorphic one among lower trichomycterids. The name branchiostegal velum ( Dagosta & de Pinna, 2021) has been employed to underscore its neomorphic nature, thus avoiding confusion with the actual branchiostegal membrane fold. The branchiostegal velum in Paracanthopoma is an easily-observable character that occurs nowhere else in Vandelliines or trichomycterids in general and has been part of the diagnosis of the genus since its establishment ( Giltay, 1935). An abnormal condition of the velum has been reported in the paratype of Pc. parva by Henschel et al. (2021b: 11, fig. 3), where the membrane is narrowly fused to the midline of the gular region.This is a low-frequency variant occasionally seen in available samples and does not change the diagnostic or phylogenetic significance of the character.

2 Median premaxilla with dorsal bilateral flanges bracing lateral margins of mesethmoid neck: The median premaxilla is a structure exclusive to vandelliines and a majority of stegophilines. The morphology of the median premaxilla in Paracanthopoma is unique, and present in all species of the genus, regardless of their size and degree of development.Two traits compose the typical Paracanthopoma median premaxilla. The first is the presence of dorsal bilateral flanges on the dorsal surface of the bone which wrap the neck of the mesethmoid ventrolaterally (e.g., Figs. 15 , 17 ) (the second is detailed in the next character). Mechanically, the flanges partly constrain the lateral movement of the bone, guiding the slide of the median premaxilla along an anteroposterior axis. While the shape of the median premaxilla can vary widely in the genus, the flanges are always present, from those species with large hypertrophied median premaxillae (e.g., Pc. truculenta , Fig. 41B ) to those with small and very delicate median premaxillae ( Pc. ahriman and Pc. saci , Figs. 7B , 34B ). Paravandellia phaneronema has gentle elevations in the equivalent position of its median premaxilla ( Fig. 46B ) which are perhaps an incipient homologous state of the flanges in Paracanthopoma .

3 Median premaxilla with well-defined median posterior recess: The shape and size of the median premaxilla varies widely in vandelliines and stegophilines. Its shape can be roundish, losenge-shaped or in a broad arc (as in species of Paravandellia ). In all cases, however, the bone is a relatively simple structure with a continuous profile.Species of Paracanthopoma have a unique shape, with a deep recess in the posterior margin forming a median slit which results in a bilateral structure of the median premaxilla (e.g., Figs. 7 , 15 , 19 , 23 , 41 ). The many variations of the median premaxilla make it difficult to ascertain their relative polarity,because more distant outgroups lack the bone entirely. It is plausible that the broad posterior concavity of the median premaxilla of Paravandellia may be a less extremely homologue of the condition in Paracanthopoma , and the situation in the latter genus is achieved by a narrowing of the concavity into a slit. In this case the general median premaxillary shape would be an additional synapomorphy for the two genera. Still, under such interpretation the situation in Paracanthopoma is a well-defined derivative state and as such also corroborating the monophyly of the genus.

4 Maxilla distally bifurcated: The maxilla in Paracanthopoma is distally bifurcated (e.g., Figs.10B,C , 15B,C , 19B, C , 30B,C , 43B, C ).Their relative lengths may vary, ranging from equal to one of the arms being three times longer than the other. Their relative width, however,is approximately the same. In all other vandelliines and remaining trichomycterids, despite much variation of shape and length, the maxilla is distally undivided. Paracanthopoma truculenta , ( Fig. 41B, C ) while clearly having a bifurcated maxilla, displays the least extreme condition, with the bifurcation restricted to the terminal portion of the maxilla. In Pc. daemon , the maxilla is reduced in overall size and modified into a rod-like small structure in two cleared and stained specimens available. In one such specimen,there is a slight distal expansion and incipient bifurcation, resembling a greatly attenuated form of the bifurcation seen in congeners. In CT images of the holotype, a bifurcation is clearly visible on the right-side maxilla ( Fig. 19B, C ).

5 Posterior articular process of palatine directed straight posteriorly, parallel to neurocranium: In vandelliines, the articulation between the palatine and the neurocranium is intermediated by a long process on the posterior region of the former. In Paravandellia , Plectrochilus , and Vandellia , this process is oblique relative to the longitudinal axis of the neurocranium and the actual articular surface is limited to its distal tip (cf., Fig. 46 ). The resulting morphology is that of a stalk-like connection of the palatine with the skull. Only in Paracanthopoma , the posterior process is directed straight posteriorly, its mesial margin flush with that of the palatine ( Figs. 7 , 10 , and equivalent images for other species). The articular surface of the process is thus long, extending for its entire mesial surface, which contacts an extended area of the anterior half of the lateral ethmoid.

6 Anterior margin of palatine with deep indentation on palatine for articulation with corresponding process of premaxilla. As first noticed by DoNascimiento (2012), the anterior margin of the palatine in species of Paracanthopoma has a deep indentation, framed by a spine-like process on each side, which accommodates the ascending process of the premaxilla (e.g., Figs. 10 , 17 , 23 ). The lateral spine-like process of the palatine is always larger than the mesial one, which can be attenuated in some taxa (e.g., Pc. vampyra , Fig. 43 ) but is invariably present. Such structure of the anterior margin of the palatine is unique among trichomycterids and other siluriforms. In species of Paravandellia , the ascending process of the premaxilla articulates ventrally with a horizontal platform on the anterior margin of the palatine. The specimen of Pv. phaneronema in Fig. 46 has the normal palatine condition on the left side (visible in Fig. 46B ), but an abnormal morphology on the right side.Curiously,the aberrant condition, which was not seen in any other examined specimen of any Paravandellia , resembles somewhat the normal situation in Paracanthopoma .

7 Coronoid process formed by well-developed dentary process, with anguloarticular portion reduced or absent: The coronoid process of the lower jaw is plesiomorphically formed by the dentary anteriorly and the anguloarticular posteriorly. This is the condition seen in most vandelliines and other trichomycterids, and is also widespread in catfishes in general, despite much variation of detail. In Paracanthopoma , the coronoid process is formed mostly or entirely by the dentary. The least extreme condition is seen in

Pc. alleynei , where a small but clearly formed anguloarticular process is adpressed to the posterior surface of the base of the larger dentary portion of the process, and is provided with a cartilage plug. Other species with a vestigial yet identifiable anguloarticular process include Pc. ahriman and Pc. satanica . Remaining species of Paracanthopoma have no trace of the anguloarticular portion of the coronoid process, which is formed exclusively by the dentary. An opposite situation occurs in Paravandellia , where the coronoid process is formed exclusively by the anguloarticular, a condition exclusive to the genus among vandelliines (see below).

8 Absence of upper pharyngeal toothplate: The condition of the upper pharyngeal toothplates varies widely in parasitic catfishes and their close relatives. Taxa such as Ochmacanthus , Pareiodon , Potamoglanis , Stegophilus , and Tridentopsis have a well-developed toothplate, strongly ossified and bearing numerous functional teeth. The upper pharyngeal toothplate is reduced to a simple small toothless bone plate in Pv. phaneronema , Vandellia , Plectrochilus , and Tridens . Further reduction is seen in Pv. oxyptera where the plate is vestigial, represented by a small nodule of bone, sometimes asymmetrically present. Finally, total loss occurs in Paracanthopoma where the upper pharyngeal plate is entirely absent. Adult vandelliines always lack teeth on the upper pharyngeal toothplate. However, a juvenile of Vandellia beccarii (FMNH 97307) has a comparatively well-developed plate with seven or eight large conical teeth. Despite an obvious functional upper pharyngeal dentition in that specimen, the lower fifth ceratobranchial and corresponding lower dentition are entirely absent in that specimen, as in all adults of the subfamily.

9 Limitedarticulationbetweenneuralarchofcomplex vertebra and supraoccipital: In Paracanthopoma , the articular surface between the anterior margin of the complex centrum and the posterior margin of supraoccipital is reduced to a small cartilage-lined dorsomedian portion. This portion is differentiated as a small well-defined roughly squarish projection. In all other vandelliines and generally in trichomycterids, the articulation between the two structures comprises the entire anterior surface of the neural arch of the complex centrum. This character was first proposed as a synapomorphy for Paracanthopoma by DoNascimiento (2012) and is here confirmed in all currently-known species of the genus.

Figures

Figure 5. Paracanthopoma ahriman, FMNH 105525,holotype,19.9 mm SL,Venezuela,T.F.Amazonas,Río Autana.(A) Lateral view of body;(B) Dorsal view of head; (C)Ventral view of head.

Figure 6. Paracanthopomaahriman, FMNH 147290, SEM images of head.(A) Lateral;(B) Dorsal; (C) Ventral.Arrow indicates extended periodontodal fold between opercular and interopercular odontodophores.Scale bar = 500 μm.

Figure 7. Paracanthopoma ahriman, holotype FMNH 105525,CT scan images of head skeleton,(A) Lateral;(B) Dorsal;(C) Ventral.Specimen poorly calcified,some structures not properly shown.

Figure 10. Paracanthopoma alleynei, MZUSP 103052,CT scan images of head skeleton,(A) Lateral;(B) Dorsal;(C)Ventral.

Figure 15. Paracanthopoma capeta, holotype MZUSP 29154, CT scan images of head skeleton, (A) Lateral; (B) Dorsal; (C) Ventral. Specimen poorly calcified,some structures not properly shown.

Figure 17. Paracanthopoma carrapata, holotype MZUSP 100145, CTscan images of head skeleton,(A) Lateral;(B) Dorsal;(C) Ventral.

Figure 19. Paracanthopoma daemon, holotype,MZUSP 103047, CTscan images of head skeleton,(A) Lateral;(B) Dorsal;(C) Ventral.

Figure 23. Paracanthopoma irritans, holotype,MZUSP 126815,CT scan images of head skeleton,(A) Lateral;(B) Dorsal;(C) Ventral.Specimen poorly calcified,some structures not properly shown.

Figure 30. Paracanthopoma parva, CAS 118205,Brazil,Amazonas,upper Rio Catrimani.CT scan images of head skeleton,(A) Lateral;(B) Dorsal;(C) Ventral.

Figure 34. Paracanthopoma saci, holotype, MZUSP 125624, CT scan images of head skeleton, (A) Lateral; (B) Dorsal; (C) Ventral. Specimen poorly calcified, some structures not properly shown.

Figure 41. Paracanthopoma truculenta, holotype,MZUSP_30399,CT scan images of head skeleton,(A) Lateral;(B) Dorsal;(C) Ventral.

Figure 43. Paracanthopoma vampyra, holotype,MZUSP 86953,CT scan images of head skeleton,(A) Lateral;(B) Dorsal;(C)Ventral.

Figure 46. Paravandelliaphaneronema, MCZ 35874,CT scan images of head and anterior portion of body,(A) Lateral;(B) Dorsal;(C) Ventral.