Miniaturization in Alestidae (Ostariophysi: Alestidae)

Abstract: Alestidae (African tetras) is one of the most species-rich families of the order Characiformes and exhibits a pronounced diversity of body size. Two independent miniaturization events (a drastic phylogenetic decrease in body size) have been identified in Alestidae. Miniaturization is frequently accompanied by morphological novelties, structural reductions and losses, hyperossifications, and increased intraspecific variation, which can create difficulties when establishing natural classifications. Traditionally, all “small” species of Alestidae with reduced multicuspid teeth were grouped in the tribe Petersiini. This tribe has since been recognized as polyphyletic, but a limited understanding of the intrarelationships of Alestidae persists. Few osteological descriptions for small-bodied alestids are available but are needed to assess the effects of miniaturization on osteological features of these fish. I conducted morphological studies of continuous measurements and meristic characters within a sample of a small-bodied alestid, Hemigrammopetersius barnardi (Herre) (n = 161), and a petersiin with a comparatively larger body size, Petersius conserialis Hilgendorf (n = 17). Specimens of H. barnardi, P. conserialis, and juveniles of an alestid that attains a larger adult size, Alestes dentex (Linnaeus), were cleared and counterstained for bone and cartilage for comparative osteological analyses and to identify potential convergent features resulting from a decrease in body size. Measurements in the examined sample of H. barnardi was normally distributed; all specimens had a relatively small body size (19.2 – 34.9 mm standard length) compared with the reported maximum standard length of 70 mm for H. barnardi. Potential sexual dimorphism was identified in the anal fin of cleared and stained specimens of H. barnardi, supporting previous observations in the literature. I compared osteological descriptions for H. barnardi and P. conserialis, which represent some of the few osteological descriptions for small-bodied characiforms and will serve as a foundation for future comparisons. I describe a size series for P. conserialis that addresses previous discrepancies in the literature regarding the presence or absence of a parietal fontanelle and reports the presence of a parietal fontanelle in all examined specimens. I document negative allometric growth between standard length and the length and width of the parietal fontanelle. Finally, I compared specimens of H. barnardi and P. conserialis to juvenile specimens of A. dentex. This allowed the identification of features that are convergent due to a decrease in body size. I found that certain bones, including the supraorbital and postcleithrum 3, are absent in H. barnardi but are present in A. dentex and P. conserialis. In addition, I found reductions to the lateral line system, such as the loss of the anterodorsal branch of the circumorbital sensory canal on the dermosphenotic and an incomplete lateral line, in H. barnardi in comparison with other examined species. These features arise towards the end of development and have been reported as lost or reduced in other small and miniature characiforms. Caution should be used when including features that are susceptible to developmental truncation for phylogenetic analyses as they are likely to independently occur with body size decreases and may not be indicative of close phylogenetic relationships.