Polypterus senegalus

Meunier, François J. & Gayet, Mireille, 2020, Comparative morphology of the finlet spines of the extant Polypteridae (Osteichthyes; Cladistia). Systematic interest, Cybium 44 (1), pp. 19-37 : 25-27

Comparative morphology of finlet spines in Polypterus senegalus ( Figs 8-11 )

This description was done with a complete series of 10 finlets for an adult male Polypterus senegalus of 260 mm TL and with additional comparative observations of other specimens.

We observe an anterior-posterior gradient of spine height in Polypterus senegalus as for all the Polypterus species, though with a difference in largest to smallest spine height ratio ( Fig. 8 ). Even if finlet differentiation is posterior-anterior oriented, the shape of the posterior spine, generated first, is more similar to the first lepidotrichium at the second part of the dorsal fin than to the anterior spines, i.e. more bent at its lower third versus larger for the most anterior spines ( Fig. 8 : S10). In transverse section, the shaft of the first spine shows a nearly straight anterior margin ( Fig. 7B ), even though it is more or less rounded for distal sections ( Fig. 7 C-I) as it is for the posterior spines.

The anterior face of the finlet shaft is covered by a thin continuous ganoine layer ( Fig. 3 B-E). It runs over the articular head downward to about its mid-height, ending at the superior transversal pad ( Fig. 3D, E ). The transversal pad forms two anterior upper processes directly connected to the lateral processes ( Figs 3C , 4C ; and see “posterior face description”). It is responsible for the lock of the spine inbetween the paired scale at a 45° angle when the finlet erects. The pad is not formed from the ganoine layer; rather it is continually present even though the ganoine surface decreases from the first to the last spine ( Fig. 3 B-E). The web is attached to the base of the following finlets for the most anterior (before the 6 th or 7 th finlet) and to the first quarter for the most posterior ( Fig. 3 C-E).

On its posterior face, the articular head is roughly square, due to the presence of two lateral and two basal processes ( Fig. 4D ). The four processes allow the ligament connection to the scales and articulation of the finlet. The lateral processes are located on both sides, approximately at the level of the basal foramen (basal opening of the medullar canal of the finlet shaft; Fig. 4D ). They are well developed on the first spine and are almost absent on the last ( Fig. 8 : S10). The inclinator muscles present in Actinopterygians ( François, 1959) and attached to the analogue of the lateral processes are missing in polypterids ( Daget, 1950; Bartsch and Gemballa, 1992; Gemballa, 1995). The basal processes, which are the largest and the roundest of the processes, frame the glenoid cavity (articulation point with the pterygiophore; Figs 2E , 4F ), and are tied to the anterior processes of the two basal scales by two strong ligaments. Two posterior processes, significantly smaller and still individualized ( Fig. 4D ), reach distally to the basal processes and lean against the anterior median margin of the basal scales while the finlet is relaxed on the back of the animal; they allow the attachment of the anterior section of the depressor muscles. The glenoid cavity of the first spine is small or not visible at all in a posterior view, due to its lower position ( Fig. 9 ). In contrast, it becomes more and more visible in the following spines as its position rises ( Fig. 9 ; see basal face).

The joint between the two half spines is composed of a basal median bony bridge, which isolates the basal foramen from the median groove ( Fig. 4B, D ). In larva, the finlets are formed in a posterior-anterior sequence ( Sewertzoff, 1924; Bartsch and Gemballa, 1992). The ratio between the total height of the spine and the height of the basal bridge is nearly the same for all specimens (sex and size): it varies from 1.70 for a 216 mm TL specimen to 1.81 for a 380 mm TL specimen (maximal size observed so far): the spine growth rate is mostly constant.

The finlet spines of Polypterus senegalus are most nearly straight with a slight posterior-oriented curve at their distal extremity ( Fig. 10 ). The ventral roundness of the articular head is a consequence of the development of the basal processes. Correlated to the pinnulae position and/or the specimen age, this roundness is posteriorly interrupted by the increase of a concavity separating the posterior processes from the basal processes. Indeed, in contrast to the posterior-anterior formation of the finlet shaft, the formation of the articular head is anterior-posterior oriented. Thus, the developmental stage of the articular head is more advanced at the first spine. The head of the ultimate spine is almost identical to the head of the closest following lepidotrichium: the posterior processes are well pronounced and the lateral processes are weakly distinguishable ( Figs 8-10 ). Moreover, the shape of the last spine in adults is similar to the shape of the most anterior spine in juveniles with only a size difference: the adult’s last spine is larger than the juvenile’s first spine ( Figs 8 , 10 ).

The basal surface of the articular head is more or less quadrangular shaped ( Figs 4F , 9 ) due to the basal processes and the nearly undifferentiated anterior upper and lateral processes. The first spine appears rectangular (anterior-posterior compression), but the last one looks more like a square ( Fig. 9 ). This shape difference between the first and last spines can be observed in juveniles as well as in adults. This is quite unexpected because of the similarity of the processes of the juvenile first spines and of the adult last spines. It seems that only the spine position (relative to the posterior margin of the dorsal fin) is able to influence this shape more than the developmental stage.

At the first spine, the glenoid cavity is perfectly rounded ( Fig. 9 ). As one moves posteriorly, its boundaries tend to disappear and its size enlarges. The ultimate spine shows a narrow anterior-posterior oriented slot at the bottom of the glenoid cavity ( Fig. 9 : S10). It corresponds to an incomplete fusion of the two hemi-lepidotrichia at the origin of the spine. Interestingly, the two basal hemi-segments of the first lepidotrichium of the dorsal fin’s caudal section are not fused either.

The development of the basal surface of the spines varies in an anterior-posterior gradient. The last spine does not match this pattern and seems to be influenced by the proximity of the first non-modified lepidotrichium. This is probably the result of a particular development pathway: its articular head is the last of a set of spine series to differentiate, while its shaft is the first; this explains the large height of the shaft and the juvenile aspect of the articular head. Indeed, in spite of the fact that the ultimate spine shaft is formed first, the dichotomy of the lepidotrichium occurs only after the formation of all the caudal lepidotrichia of the dorsal fin, from the first to the last spine ( Sewertzoff, 1924).

The finlet spines of female and male Polypterus senegalus show several minor differences that do not affect the species description ( Fig. 11 ). The lower-anterior region of the shaft spine (just above the articular head) is larger in females than in males. In posterior view, the shaft of the female spine is wide and regularly rounded, whereas the male’s is narrow and concave on the edges ( Fig. 11A ). The lateral processes of all female spines are more individualized and are located at a more anterior position. In lateral view, the posterior processes appear more pronounced for females than for males, clearly breaking the roundness of the articular head base ( Fig. 11B, C ). Finally, in basal view, the basal surface of the male spines appears from the first spines to be more squared than for females due to its narrower shaft and articular head.

The spine of each pinnule is composed of a set of odontodes of variable size, fitted together and resting on a bony base ( Fig. 6B, C ) (see Meunier, 1980; Meinke, 1982). The central medular cavity opens at the base of the ray at the level of the basal foramen ( Figs 7C , 11A ), and becomes first a tube, the medular canal, ( Fig. 7C, D ) that quickly turns into a median groove on the posterior-distal section of the spine ( Fig. 7 E-I). Each odontode is organized around this cavity and is composed of dentine covered on the anterior face by a unique stratified layer of ganoine. The ganoine layers of several odontodes can overlap locally, forming a multiple strata pattern ( Figs 6B , 7B ) ( Meunier, 1980). The spine growth essentially acts at the distal extremity by addition of neo-formed odontodes ( Meunier, 1980; Meinke, 1982). The spine thickens through the overlay of one or several older units by the base of new odontodes. This particular organization can be observed in transverse section: several levels of odontodes more or less aligned ( Fig. 7B ). However, the histological structure of the spines is quite different from that of the scales ( Meunier, 1980). In the case of the finlet spines, the neo-formed odontodes are not totally fused, in contrast to the true so-called odonto-complexes of scales sensu Ørvig (1977).

Figures

Figure 2. – A-B: Polypterus senegalus and Erpetoichthys calabaricus, respectively; body scales in the dorsal region of the two first finlets. C: P. senegalus; outer face of the first left scale just behind the articulation of the spine. D: P. endlicheri; inner face of the last right predorsal scale just before the articulation of the spine. E: E. calabaricus; the articulation of the finlet spine with its pterygi- ophore. The arrows indicate the head direction of the fish. sp1, sp2: first and second spine articulation. Scale bars:A, B, C, D = 1 mm; E = 0.5 mm.

Figure 3. – A: Polypterus ansorgei; the anterior face of the spine shows the basal articular head (asterisk), and the ganoine patches on the shaft, and the two distal tips (arrows). B-E: P. senegalus; anterior view of the articular head of the 1st (B), 5th (C), 8th (D) and 10th (E) finlet spines; the ganoine layer is lacking where the web membrane fits on the spine. Scale bar = 1 mm.

Figure 4. – A-B: Polypterus bichir katangae; A: The anterior face of the spine is covered by an irregular ganoine layer, and shows the basal articular head (AH) and the shaft (SH); B: The posterior face of the spine shows, the median groove and the basal foramen on the shaft; the two arrow-heads indicate the two lateral pads. C-F: P. senegalus; detail of the anterior (C), the posterior (D), the left lateral (E), and the basal face (F) of the articular head, respectively, to show the various significant processes; the arrowhead points to the median groove (F). Scale bars:A, B = 1 mm; C-F = 0.5 mm.

Figure 6. – Polypterus senegalus (male, 260 mm TL). A: Posterior side of the shaft showing, in the median groove, three branches of the lepidotrichium: the primary branch and two secondary branches going out of the median grove. B-C: Respectively lateral and sagittal axial sections of a finlet spine showing the vascular network (B: in black) and the insertion (in C) of the branches of the lepidotrichium (modified from Meunier, 1980). Scale bars = 1 mm.

Figure 7. – Polypterus senegalus (male, 260 mm TL). A-I: Nine successive transversal sections (from the base to the tip) of a finlet spine: section F crosses the glenoid cavity and is associated to a lateral scale; section B crosses the foramen; the seven other sections (C-I) are regularly dispatched along the shaft, the two last ones crossing four branches of the lepidotrichium: the primary branch at the bottom of the median groove and three secondary branches. The asterisks point to the medullar canal. Scale bar = 0.5 mm.

Figure 8. – Polypterus senegalus. Posterior view of the ten finlet spines (S1-S10) and the first lepidotrichium of the posterior part of the dorsal fin. Scale bar = 1 mm.

Figure 9. – Polypterus senegalus. Basal view of the articular head of the ten finlet spines (S1-S10) of the anterior dorsal fin. Scale bar = 1 mm.

Figure 10. – Polypterus senegalus. Lateral view of the ten finlet spines (sp1-sp10) and the first lepidotrichium (lep) of the posterior part of the dorsal fin. The anterior and posterior sides are respec- tively on the left and on the right. Scale bar = 1 mm.

Figure 11. – Polypterus senegalus. A: The posterior face of 6th finlet spine from a female on the left and a male on the right, and showing the basal foramen on the articular head, the median groove and a lepidotrichium on the shaft. The symmetrical distal points of the spines correspond to apical odontodes. B-C: The anterior (on the left) and right (on the right) lateral views of the articular head of the spine respectively of a female and a male. Scale bars = 1 mm.