Review of the Nothobranchius ocellatus group

Watters, B. R., B. Nagy, P. D. W. van der Merwe, F. P. D. Cotterill & D. U. Bellstedt. 2020. Redescription of the seasonal killifish species Nothobranchius ocellatus and description of a related new species Nothobranchius matanduensis, from eastern Tanzania (Teleostei: Nothobranchiidae). Ichthyological Exploration of Freshwaters, IEF-1149, pp. 1–28.

The description of Nothobranchius ocellatus was based on a unique holotype, comprising an immature male, and a photograph of a female. The holotype is apparently lost. The species is here redescribed in detail, and a neotype designated from a site near the original type locality. The range of distribution of the species is within the lower Rufiji and lower Ruvu river systems, contained within intersecting, tectonically-controlled and hydrologically-linked graben structures, the Rufiji Depression and Ruvu Valley Trough, and at relatively low elevations.

Disjunct populations in the Matandu River drainage, previously regarded as the same species, are described as Nothobranchius matanduensis Watters, Nagy & Bellstedt, new species. The latter species occurs at significantly higher elevations than those of N. ocellatus and appears confined to a part of coastal Tanzania that has been tectonically relatively stable.

The two species comprise the ‘Nothobranchius ocellatus species group’. Differences in diagnostic characters between N. matanduensis and N. ocellatus are presented, the most obvious of which are grey scale margins and black spots on the head of males of the former (vs. red-brown scale margins and red-brown spots on the head of the latter). Further differences in colour pattern and morphology of both males and females of the two species are described in detail, and differences in cephalic sensory systems are also noted.

Genetic divergence in partial sequences of the mitochondrial genes, ND2 and COI, and three nuclear genes, Glyt, MyH6 and SNX33, supports the genetic distinction of the two species within a well-defined N. ocellatus Clade, which is deeply rooted in Nothobranchius, thereby also confirming the subgenus status of Paranothobranchius. The ecology and biology of the species group is reviewed; both species are relatively large and piscivorous in nature, occupy seasonal habitats, and always occur syntopically with various other, smaller, Nothobranchius species.