Structure of the andropodium of the viviparous halfbeak genus Nomorhamphus (Atherinomorpha: Beloniformes: Zenarchopteridae), endemic to Sulawesi, Indonesia

In halfbeaks (Zenarchopteridae), viviparity is known in three of the five genera, including Nomorhamphus. During the extremely short copulation, the transfer of spermatozeugmata from the male genital papilla to the female urogenital opening is apparently facilitated by the andropodium, an organ composed of the strongly modified male anterior anal-fin rays. Substructure of the andropodium varies among species, and traits of the modified anal-fin rays have been used as taxonomic characters for species delimitation. The present study examines the microanatomy of the andropodium across 11 of the 12 Nomorhamphus species, which are endemic to Sulawesi. Methods applied include contrast-enhanced µCT-imaging and clearing and staining approaches. Similarity in andropodial fin ray traits correlates with general morphology and spatial proximity. Species occurring in sympatry possess similar andropodia; the copulatory organ of Nomorhamphus rex is most distinct. In general, andropodial traits allow clear discrimination in most of the species examined, but require careful examination. The supposed incomplete calcification of the modified rays and their resulting flexibility provide arguments against the hypothesis of the andropodium as a true intromittent organ. The structure might rather help to orient the genital papilla in direction of the female genital opening during mating.