The Distribution and Ontogeny of Neuromast Receptor Organs and a Comparison of Methods for Chemical Albation of the Lateral Line System in Two Cichlid Fishes

The lateral line system is composed of a series of mechanoreceptors called neuromasts, which are found on the head and trunk in specific patterns. On the head, larger canal neuromasts (CNs) are enclosed in cranial canals and smaller superficial neuromasts (SNs) are embedded in the skin in lines or clusters. Among species the lateral line canals can be narrow, widened, branched, or reduced. Through the use of fluorescent stains, Scanning Electron Microscopy, and cleared and stained specimens the distribution and ontogenetic appearance of SNs and CNs were mapped in two species of Lake Malawi cichlids with divergent adult lateral line morphologies: Tramitichromis sp. (narrow canals) and A. stuartgranti stuartgranti (widened canals).This study provides: 1) the first description of cranial neuromast distributions in representatives of the genera Tramitichromis and A. stuartgranti , 2) evidence that CN patterning is the same in the 2 taxa despite differences in adult canal morphology, 3) evidence that SN patterns (e.g., 9 groups of NM) are the same, but SN numbers vary between the two taxa, and 4) evidence that the timing and appearance of some SN groups varies between the two species. Chemical and pharmacological ablation of neuromasts are methods frequently used to inactive the lateral line system. Fluorescent staining of neuromasts is also a common technique used to visually assess the effects of ablation on neuormasts. The two techniques, however, have only been used sequentially once before to verify that lateral line ablation occurred and that the behavior of the fish was a reflection of this. The interpretation of the use of these methods and how they might differentially affect SNs and CNs is very ambiguous. This study provides the first detailed description of the ablation effects of Cobalt (II) chloride heptahydrate and Gentamicin on superficial and canal neuromasts using fluorescence staining (4-Di-2-ASP). Two species of Lake Malawi cichlids, Tramitichromis sp. and A. stuartgranti stuartgranti, were used in this study. Following treatment, it was determined that: 1) CoCl2 in Ca++ free water and Gentamicin had comparable effects on SNs and CNs in both species, 2) Treatment with CoCl2 in Ca++ free water and Gentamicin resulted in full recovery of both superficial and canal neuromasts by Day 4 or Day 7, 3) Treatment with CoCl2 in tank water with Ca++ did not effectively ablate SNs and CNs on Day 0, when compared to CoCl2 in Ca++ free water and Gentamicin, 4) Gentamicin does, in fact, affect SNs, which refutes previous experiments. The stain 4-Di-2-ASP proved to be a reliable and effective means of visually documenting the effects of Cobalt (II) chloride heptahydrate and Gentamicin on ablation and recovery of superficial and canal neuromasts in two cichlid species