Responses of anterior lateral line afferent neurones to water flow and odour-conditioned rheotaxis in Anguilla spp

Abstract

Various aspects of the sensory biology of the New Zealand Shortfinned eel Anguilla australis Richardson and New Zealand Longfinned eel Anguilla dieffenbachia Gray, were investigated using morphological, neuro-physiological and behavioural techniques. Scanning electron microscopy and standard histological techniques were used to describe the basic anatomy and post-embryonic development of the lateral line system of Anguilla australis. Leptocephalus larvae had no visible lateral line receptors on either the head or body, although visual and olfactory senses were well developed. Glass eels that had recently migrated into the freshwater environment possessed a fully developed lateral line system, resembling that seen in juvenile (elver) and adult (yellow eel) stages. Lateral line development is most likely to occur during the phase II metamorphic period and prior to migration into the freshwater environment. Canal neuromasts in sexually mature migrants (silver eel) were considerably larger than canal neuromasts of immature resident eels, an effect that was independent of fish size. The response characteristics of anterior lateral line afferent fibres of Anguilla dieffenbachii were investigated using two types of stimuli: a vibrating bead, and unidirectional water flows (streaming water). The frequency-response characteristics of lateral line afferent fibres could be divided into: those that had a best response to low-frequencies (i.e. the response curve taking the form of a low-pass filter), and those that had a best response to high-frequencies, (i.e. the response curve taking the form of a high-pass filter). Responses of anterior lateral line afferent fibres to unidirectional water flows between 0.5 - 4.0 cm s-1 could also be divided into two populations: those that were flow-sensitive, and those that were flow-insensitive. Eighty percent (72/90), of all anterior afferent fibres were flow-sensitive to varying degrees, with the response magnitude (spk.s-1), increasing with flow rate. Flow-sensitive fibres gave non-adapting tonic responses, meaning that these fibres innervate lateral line receptors that detect absolute flow velocity. This is the first time tonic response properties have been demonstrated in lateral line afferent fibres. Further studies are needed to determine whether flow-sensitive and flow-insensitive fibres innervate superficial and canal neuromasts respectively. The olfactory search strategies of freshwater eels were investigated at night, in a semi-natural field setting. To localise the source of a turbulent odour plume eels used an odour-conditioned rheotaxis, relying on water flow as an orientating stimulus. In the presence of an odour plume eels swam directly upstream, toward the odour source. Moving beyond the lateral margins or losing contact with the odour plume resulted in the cessation of upstream movement and a switch to cross-stream casting, a strategy that facilitated re-contacting the odour plume.