Histology of Convergent Probing Appendages in Mormyridae

Abstract

Mormyridae is an early diverging family of Teleostean fishes that produce an electric field for navigation and communication using an electric organ. This clade has a diverse array of soft-tissue rostral appendages, such as the chin-swelling, the Schnauzenorgan, and the tubesnout combined with a Schnauzenorgan, that have evolved multiple times. Here we assess if macroscopically convergent, soft-tissue rostral appendages are also histologically convergent. Further, we investigate how the histology of these appendages can inform their function. We sampled independent gains of the chin-swelling and Schnauzenorgan to understand similarities and differences in their anatomies. We show that macroscopically convergent rostral appendages are also convergent at a histological level, and different types of rostral appendages share a similar anatomy; that said, minor differences likely relate to their specific functions. Based on a comparison of the skeletal muscle distribution and the differing attachment shapes of each appendage to the dentary, we conclude that the Schnauzenorgan is capable of a wider range of movements than the chin-swelling. Furthermore, the anatomy suggests that these soft-tissue rostral appendages likely function as electrosensory foveas (i.e., an appendage that focuses a sensory system). Lastly, these histological data support the hypothesis that the chin-swelling may be a precursor to the Schnauzenorgan.

A review of the reproductive biology of mormyroid fishes: An emerging model for biomedical research

“…Mormyroids also possess unique craniofacial morphologies, particularly in the mouth and jaw, exhibiting blunt‐nose snouts, tube snouts, chin‐swellings, and Schnauzenorgans (Figure 1c) (Peterson, Sullivan, Hopkins, et al, 2022). Recent work by Peterson et al (2023) demonstrates that some of these morphologies have convergently evolved within different mormyroid lineages at the macroscopic and histological scales. Remarkably, these unusual craniofacial morphologies have also convergently evolved between mormyrids and gymnotiforms (Ford et al, 2022).…”