Date of Award

9-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biological Science

First Advisor

Jacqueline Webb

Abstract

The mechanosensory lateral line system is found in all fishes and mediates critical behaviors, including prey detection. Widened canals, one of the four patterns of cranial lateral line canals found among teleosts, tend to be found in benthic fishes and/or fishes that live in hydrodynamically quiet or light-limited environments, such as the deep sea. Little is known about the functional significance of widened canals because most fishes with this morphology are inaccessible for laboratory study. A representative of one genus of Lake Malawi cichlid fishes, Aulonocara, has widened canals and provides an opportunity to investigate the role of widened lateral line canals in prey detection. In addition, its behavior can be compared to that of Tramitichromis sp., another Lake Malawi cichlid that has narrow canals, since both feed on benthic invertebrates in sandy substrates. A behavioral assay was developed in which several pairs of “benthic” live and dead prey (tethered brine shrimp) were placed on the bottom of a large tank to determine if Aulonocara stuartgranti (Chapter 1) and Tramitichromis (Chapter 2) used the visual and hydrodynamic stimuli generated live, mobile prey (tethered brine shrimp) compared to immobile, dead prey, under both light and dark conditions. In addition, some fish were treated with cobalt chloride to temporarily inactive the lateral line system. Behavior was recorded using high definition video and statistically analyzed using generalized linear mixed models. The hydrodynamic stimuli generated by the brine shrimp were visualized and characterized using digital particle imaging velocimetery (DPIV). These two studies demonstrated that A. stuartgranti (widened canals) uses both vision and the lateral line system to feed under light conditions, but uses its lateral line system to feed in the dark, contributing to our appreciation of multimodal interactions and strongly suggesting that widened lateral line canals are an adaptation for prey detection. The later study showed that, in contrast, Tramitichromis (narrow canals) is a visual predator that does not use their narrow lateral line canals to find prey as illustrated by the fact that their behavior was unaffected by ablation of the lateral line system.

A similar set of experiments using the same behavioral assay was carried out to determine how a range of light intensities that mimicked intensities during sunrise/sunset and at depth affected the ability of Aulonocara and Tramitichromis to detect live and dead prey (Chapter 3). The results of this study showed that Tramitichromis does not feed in the dark, but can feed at surprisingly low light levels (1 lx), and demonstrated that Aulonocara feed at all light levels, including darkness, and thus at light intensities available at the range of depths that members of the genus occupy in Lake Malawi. These results all suggest that sensory biology of closely related species that exploit a common food resource may have important ecological implications, especially how sensory capabilities may contribute to trophic niche segregation.

Finally, a novel artificial stimulus delivery system (the “apparatus”) was designed to deliver two different water flow stimuli (Type I, Type II, visualized using DPIV) at different flow rates (in the range known to be generated by invertebrates) through tubes below the sandy substrate that mimicked benthic invertebrate prey, and were thus biologically relevant (Chapter 4). This apparatus was used to determine the role of the lateral line system in the absence of visual and olfactory cues presented by live and dead prey. Aulonocara were trained to respond to water flows generated by the apparatus using a food reward system. Fish responded to all flow rates and to both flow types but with a range of behaviors. The temporary inactivation of the lateral line system with cobalt chloride significantly reduced the ability of Aulonocara to detect flows, but flow sensing behavior returned to pretreatment levels within seven days. This work has established Lake Malawi cichlids as model system for the study of the sensory basis for prey detection behavior using both natural and artificial stimuli under different environmental conditions and has demonstrated the reversible effects of cobalt chloride on the lateral line system.

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