150
Horst Bleckmann et al.
In contrast to the electrosensory system of fishes (review Bullock and Heiligenberg 1986) and the visual, auditory and somatosensory (mechanosensory) systems
of terrestrial vertebrates and invertebrates (review Delcomyn 1997), the mechanosensory lateral line is still poorly investigated. In consequence, our knowledge
regarding the behavioral relevance and physiology of the lateral line is far from
complete. This may have several reasons. First, compared to visual, acoustic, and
tactile stimuli, it is difficult to generate, manipulate, quantifY, and measure water
motions. Second, humans lack a hydrodynamic sense and thus cannot easily grasp
the biological importance and role of hydrodynamic sensory systems. This, no
doubt, makes it difficult to ask biologically relevant questions and consequently to
design lateral line experiments.
This chapter will not review the lateral line in detail. Instead, it will focus on the
information necessary to follow and understand our argument that we will fully
comprehend lateral line information processing only if we take into account the
ecology of fishes. This means that we have to consider the hydrodynamic
conditions that are prevailing in the habitat of a given species and determine
biologically relevant stimuli and background noise conditions. Using this
information, experiments have to be defmed in which more natural stimulus
conditions can be created, for instance experiments in which hydrodynamic signals
can be applied in background noise. Physiological data from such experiments can
then be correlated to the various morphological specializations of the lateral line
system that were already pointed out by Dijkgraaf (1952). For more general
information on lateral line systems the reader is referred to Coombs et al. (1988),
Coombs et al. (1992), Bleckmann (1994), Montgomery et al. (1995a), and
Schellart and Wubbels (1998). Because of limitations in available space we will,
whenever possible, cite only recent review articles.
2 Morphology of the Periphery
The smallest sensory unit of the lateral line is the neuromast which occurs
freestanding on the skin, in pits, on pedestals raised above the skin, and - if we
exclude amphibians - in fluid-filled canals. The sensory receptor cells within a
neuromast are hair cells. Hair cells carry a hair bundle at the apical surface which
is comprised of stereovilli and a single eccentrically placed kinocilium. The hair
bundle protrudes into a gelatinous cupula which connects the bundle with the
water surrounding the fish.
The design and number of lateral line canals as well as the design and number of
superficial and canal neuromasts can be very different in even closely related
species. Superficial neuromasts are smaller, contain fewer hair cells, and are
innervated by fewer nerve fibers than canal neuromasts (Munz 1989). Within a
given animal, but also between species, cupula length and shape may vary
(Bieckmann 1994). The variability of lateral line canals includes their number,
Horst Bleckmann et al.
In contrast to the electrosensory system of fishes (review Bullock and Heiligenberg 1986) and the visual, auditory and somatosensory (mechanosensory) systems
of terrestrial vertebrates and invertebrates (review Delcomyn 1997), the mechanosensory lateral line is still poorly investigated. In consequence, our knowledge
regarding the behavioral relevance and physiology of the lateral line is far from
complete. This may have several reasons. First, compared to visual, acoustic, and
tactile stimuli, it is difficult to generate, manipulate, quantifY, and measure water
motions. Second, humans lack a hydrodynamic sense and thus cannot easily grasp
the biological importance and role of hydrodynamic sensory systems. This, no
doubt, makes it difficult to ask biologically relevant questions and consequently to
design lateral line experiments.
This chapter will not review the lateral line in detail. Instead, it will focus on the
information necessary to follow and understand our argument that we will fully
comprehend lateral line information processing only if we take into account the
ecology of fishes. This means that we have to consider the hydrodynamic
conditions that are prevailing in the habitat of a given species and determine
biologically relevant stimuli and background noise conditions. Using this
information, experiments have to be defmed in which more natural stimulus
conditions can be created, for instance experiments in which hydrodynamic signals
can be applied in background noise. Physiological data from such experiments can
then be correlated to the various morphological specializations of the lateral line
system that were already pointed out by Dijkgraaf (1952). For more general
information on lateral line systems the reader is referred to Coombs et al. (1988),
Coombs et al. (1992), Bleckmann (1994), Montgomery et al. (1995a), and
Schellart and Wubbels (1998). Because of limitations in available space we will,
whenever possible, cite only recent review articles.
2 Morphology of the Periphery
The smallest sensory unit of the lateral line is the neuromast which occurs
freestanding on the skin, in pits, on pedestals raised above the skin, and - if we
exclude amphibians - in fluid-filled canals. The sensory receptor cells within a
neuromast are hair cells. Hair cells carry a hair bundle at the apical surface which
is comprised of stereovilli and a single eccentrically placed kinocilium. The hair
bundle protrudes into a gelatinous cupula which connects the bundle with the
water surrounding the fish.
The design and number of lateral line canals as well as the design and number of
superficial and canal neuromasts can be very different in even closely related
species. Superficial neuromasts are smaller, contain fewer hair cells, and are
innervated by fewer nerve fibers than canal neuromasts (Munz 1989). Within a
given animal, but also between species, cupula length and shape may vary
(Bieckmann 1994). The variability of lateral line canals includes their number,