Microsynodontis Boulenger

Heok Hee Ng, 2004, The Microsynodontis (Teleostei: Siluriformes: Mochokidae) of the lower Guinea region, west central Africa, with the description of eight new species., Zootaxa 531, pp. 1-52 : 43-50

[[ Genus Microsynodontis Boulenger ]]

Discussion

The identification of Microsynodontis species is often difficult, primarily because of the large amounts of purported variation observed in biometrics and color (Poll & Gosse, 1963; Matthes, 1964); one outcome of this observation was the synonymy of M. christyi with M. batesii by Matthes (1964). These conclusions were drawn without the examination of large series from a single locality (or at least multiple closely situated localities within the same drainage), and the high degree of variation reported is due to confusion between intraspecific and interspecific differences. However, after examining a large series of M. batesii in this study, the limits of intraspecific variation within Microsynodontis can be better understood, and M. christyi is found to be a valid species distinguished from M. batesii by coloration and other morphological characters (see below).

Examination of large series of specimens (identifiable as M. batesii ) from within several closely situated localities within the Ntem River drainage in northern Gabon and southern Cameroon leads me to conclude that although intraspecific variation in color (and certainly in biometrics) exists, color patterns are useful diagnostic characters once the degree of intraspecific variation is understood. Furthermore, diagnostic characters not previously identified, some of which have been used in other mochokid genera, were found to be useful for distinguishing species of Microsynodontis . These characters consist of the snout, fin spine (both dorsal and pectoral) and tubercle morphology, all of which have been previously used to diagnose species in other mochokid genera, but not in Microsynodontis . Although a combination of these characters is the easiest way to distinguish among the Microsynodontis of the lower Guinea region, some biometric characters can also be used to diagnose the species (diagnostic biometric characters for the other species of Microsynodontis not found in the lower Guinea region are given in Table 10). However, the numbers of oral teeth, a character considered diagnostic in other mochokid genera, are not useful in diagnosing species of Microsynodontis , as they are apparently highly conservative in the genus.

The variation in color is largely of an ontogenetic nature, and its use as a more reliable diagnostic character is possible once the limits of variation are understood. The ontogenetic change in color pattern is most marked for M. batesii , and consists of changes in both the pigmentation pattern of the abdomen and the shape of the light-colored markings on the body. In the former case, many juvenile specimens have a dense aggregation of melanophores concentrated in a series of evenly distributed large spots on the abdomen that fade with age (Fig. 1c), and in the latter case, the dorsal and ventral light-colored markings may coalesce to form complete bands encircling the body, especially in the region of the caudal peduncle (Fig. 1c). The light-colored markings on the ventral third of the body in juvenile specimens are also more vermiform (Fig. 1c). In any case, M. batesii (and all other species from the lower Guinea region) of all sizes always possess a light-colored band encircling the nape, which is absent in M. christyi of all sizes examined, even in preserved material. Therefore, the absence of this band is a useful diagnostic character for distinguishing M. christyi from M. batesii (and all other species of Microsynodontis from the lower Guinea region).

Microsynodontis christyi can also be distinguished from M. batesii in having a deeper caudal peduncle (10.0-11.8% SL vs. 5.8-9.2). The distributions of the two species also suggest that they are different: M. christyi is only known from the middle Congo River drainage while M. batesii is only known from the lower Guinea region (in the Campo, Ivindo, Ntem, Nyong and Sanaga River drainages). Very little material of Microsynodontis from the middle Congo River drainage was available for study, but the examination of all material available suggests that there are no species in common between the middle Congo River drainage and those of the lower Guinea region. This is so even when the tributaries of the Congo and the smaller coastal drainages of the lower Guinea region are immediately adjacent, as in the case of the material identified as M. batesii from the Dja River (a tributary of the Congo River flowing approximately southwest in southern Cameroon and located adjacent to the Ntem River drainage), which is not conspecific with M. batesii and represents a distinct, undescribed species.

This study reveals the importance of some biometric measurements as diagnostic characters. In particular, two of the species described here, M. nannoculus and M. nasutus , are distinguished from congeners chiefly by biometric measurements. Bivariate analyses (ANCOVA) of the regression lines of eye diameter (Fig. 17), snout length (Fig. 18), caudal peduncle depth (Fig. 19), adipose basal length (Fig. 20) and caudal-fin length (Fig. 21) on SL are significantly different (given the number of taxa used in the analysis, it was not possible to display all of them on the biplots without obscuring key patterns and only the key taxa for each biometric value are used in Figs. 17-21). The P values of the analyses are given in Table 11, and it can be seen that regression lines are all significantly different for the eye diameter of M. nannoculus and for the snout length of M. nasutus , when both are compared to the corresponding data for all congeners.

Sexual dimorphism in Microsynodontis is reported for the first time in this study. Males of Microsynodontis species can be distinguished from females by the presence of a conical genital papilla immediately posterior to the anus (females have a smaller papilla that is distally flattened) and (especially in mature adults of all lower Guinea species except for M. emarginatus ) by a much denser aggregation of tubercles on the dorsal and lateral surfaces of the head, especially in the region on the sides of the head from the snout to the preopercle. The presence of tubercles has been used as a diagnostic character in the Mochokidae , e.g. in Synodontis (see Boulenger, 1900) and Chiloglanis (see Roberts, 1989), but has not been previously used for Microsynodontis . The results of this study indicate that tubercle shape is useful in diagnosing at least one species, M. hirsutus , from its congeners. Although the number and density of tubercles differ both sexually and ontogenetically, the tubercles retain their characteristic shape in specimens of both sexes and all sizes in M. hirsutus , making it a useful diagnostic character for this species.

Despite the fact that Microsynodontis species are occasionally imported for the aquarium trade and are not considered rare, very little is known of their biology, from either field or aquarium observations. This is probably because these fishes, like many other small fishes in ichthyological expeditions, are often overlooked and are thus not particularly well represented in collections. With the number of species identified in this study, it is clear that this element of the ichthyofauna (the miniature species) is in need of further study.

Artificial key to the Microsynodontis of the lower Guinea region, west central Africa

1. Caudal fin emarginate (Fig. 9a) ( Ogooué River drainage) ..................... M. emarginatus

- Caudal fin rounded or truncate (Figs. 9b -c) ................................................................. 2

2. Anterior edge of pectoral spine smooth (Ivindo River drainage) .............. M. laevigatus

- Anterior edge of pectoral spine serrated ....................................................................... 3

3. Body always without numerous dark brown elongate spots; caudal peduncle slender (5.8-9.8% SL) ............................................................................................................... 4

- Body frequently with numerous dark brown elongate spots; caudal peduncle deep (9.6-11.9% SL) ( Ogooué River drainage) ..................................................... M. notatus

4. Adipose-fin base long (34.4-41.6% SL) ....................................................................... 5

- Adipose fin-base short (21.3-33.8% SL) ...................................................................... 6

5. Dorsal and lateral surfaces of head with long tubercles in both sexes (up to 0.3 mm long); dorsal spine straight (Ntem River drainage) ....................................... M. hirsutus

- Dorsal and lateral surfaces of head with small rounded tubercles in both sexes (not more than 0.1 mm long); dorsal spine gently curved (Campo, Ivindo, Ntem, Nyong, Ogooué and Sanaga River drainages)............................................................. M. batesii

6. Supracleithral process reaching to vertical through posteriormost tip of nuchal shield; eye large (19.3-25.0% HL) ( Ogooué River drainage)..................................... M. vigilis

- Supracleithral process not reaching to vertical through posteriormost tip of nuchal shield; eye small (10.6-19.6% HL)...............................................................................7

7. Snout long (50.0-53.3%HL) (Okano River drainage)................................... M. nasutus

- Snout short (43.1-48.5% HL).......................................................................................8

8. Body slender (13.7-15.0% SL); anterior edge of pectoral spine with retrorse (proximally directed) serrations along proximal half; eye larger (13.9-19.6% HL) (Ivindo River drainage).............................................................................................. M. armatus

- Body deep (17.6-19.9% SL); anterior edge of pectoral spine with anteriorly directed serrations along proximal half; eye smaller (10.6-12.2% HL) (Ntem River drainage) ................................................................................................................. M. nannoculus