ORIGINAL PAPER J. Engelmann ® W. Hanke ® H. Bleckmann Lateral line reception in still- and running water Accepted: 30 May 2002 / Published online: 13 July 2002 Ó Springer-Verlag 2002 Abstract The lateral line of fish is composed of neuro- masts used to detect water motions. Neuromasts occur as superficial neuromasts on the skin and as canal neu- romasts in subepidermal canals. Fibres of the lateral line nerves innervate both. There have been extensive studies on the responses of lateral line nerve fibres to dipole stimuli applied in still water. However, despite the fact that many fish live in rivers and/or swim constantly, responses of lateral line nerve fibres to dipole stimuli presented in running water have never been recorded. We investigated how the peripheral lateral line of still water fish (Carassius auratus) and riverine fish (On- corhynchus mykiss) responds to minute sinusoidal water motions while exposed to unidirectional water flow. Both goldfish and trout have two types of posterior lateral line nerve fibres: Type I fibres, which most likely innervate superficial neuromasts, were stimulated by running water (10 cm s –1 ). The responses of type I fibres to water motions generated by a vibrating sphere were masked if the fish was exposed to running water. Type II fibres, which most likely innervate canal neuromasts, were not stimulated by running water. Consequently, responses of type II fibres to a vibrating sphere were not masked under flow conditions. Keywords Background noise ® Lateral line ® Mechanoreception ® PIV ® Teleost fish Abbreviations ALLN anterior lateral line nerve ® CN canal neuromast ® DASPEI 2-(4-dimethylaminostyryl)- N-ethyl pyridinium iodide ® PIV particle image velocimetry ® PLLN posterior lateral line nerve ® p-p peak-to-peak ® PSTH peri-stimulus time histogram ® SEM scanning electron microscopy ® SN superficial neuromast Introduction The mechanosensory lateral line of fishes consists of fluid filled canals containing canal neuromasts (CNs) and of lines or clusters of superficial neuromasts (SNs) on the skin of the head, trunk and tail (e.g. Coombs et al. 1988). Fish use the lateral line for prey detection (Schwartz 1965), rheotaxis (Montgomery et al. 1997), schooling (Pitcher and Parrish 1993), intraspecific com- munication (Satou et al. 1991) and for the identification and localisation of wave sources (e.g. Vogel and Bleck- mann 1997, 2000; Coombs et al. 2001). Both, SNs and CNs, are composed of hair cells whose kinocilia and stereocilia project into a gelatinous cupula (Flock 1971a). The hair cell resting potential is modu- lated by cupula displacements induced by water mo- tions. The directional response of a hair cell is defined by the anatomical polarisation of the ciliary bundle: bend- ing the stereocilia towards the kinocilium causes depo- larisation, bending in the opposite direction causes hyperpolarization of the membrane potential (Flock 1971b). Lateral line neuromasts exhibit hair cells ori- ented in two opposing directions parallel to the major axis of the neuromasts. Each fibre in the lateral line nerve innervates either a CN or one or several SNs (Mu¨nz 1985). Within a neuromast, single fibres may innervate more than one hair cell, provided they have the same orientation (Go¨ rner 1963). Fishes that live in running or turbulent water and/or are constant swimmers tend to have a well-developed canal system but only few SNs (e.g. Jakubowski 1967; Bleckmann and Mu¨ nz 1990; Vischer 1990). On the other hand, even closely related species that inhabit still water, are slow swimmers and/or have a sedentary life style often have many SNs but a less developed canal system (e.g. Dijkgraaf 1963; Marshall 1971; Merrilees and Crossman 1973). SNs of a stationary fish exposed to J Comp Physiol A (2002) 188: 513–526 DOI 10.1007/s00359-002-0326-6 J. Engelmann (&) ® W. Hanke ® H. Bleckmann Institut fu¨r Zoologie, Universita¨t Bonn, Poppelsdorfer Schloß, 53115 Bonn, Germany E-mail: Jacob.Engelmann@uni-bonn.de Tel.: +49-228-735476 Fax: +49-228-735458