Spatial Distribution and Morphological Characteristics of the Trunk Lateral Line Neuromasts of the Sea Bass (Dicentrarchus labrax, L.; Teleostei, Serranidae)

Flow field perception based on the fish lateral line system

Fish are able to perceive the surrounding weak flow and pressure variations with their mechanosensory lateral line system, which consists of a superficial lateral line for flow velocity detection and a canal lateral line for flow pressure gradient perception. Achieving a better understanding of the flow field perception algorithms of the lateral line can contribute not only to the design of highly sensitive flow sensors, but also to the development of underwater smart skin with good hydrodynamic imaging properties. In this review, we discuss highly sensitive flow-sensing mechanisms for superficial and canal neuromasts and flow field perception algorithms. Artificial lateral line systems with different transduction mechanisms are then described with special emphasis on the recent innovations in the field of polymer-based artificial flow sensors. Finally, we discuss our perspective of the technological challenges faced while improving flow sensitivity, durability, and sensing fusion schemes.

A model of the lateral line of fish for vortex sensing

In this paper, the lateral line trunk canal (LLTC) of a fish is modeled to investigate how it is affected by an external flow field. Potential flow theory is adopted to model the flow field around a fish's body in the presence of a Karman vortex street. Karman and reverse Karman streets represent the flow patterns behind a bluff body and a traveling fish, respectively. An analytical solution is obtained for a flat body, while a fish-like body is modeled using a Joukowski transformation and the corresponding equations are solved numerically. The pressure distribution on the body surface is then computed employing Bernoulli's equation. For a known external flow, the flow inside the LLTC is driven by the pressure gradient between a pair of consecutive pores, which can be solved analytically. Governing dimensionless parameters are obtained from this analytical solution, and the effects of these numbers on the amplitude or features of the velocity distribution inside the canal are studied. The results show that the main characteristics of a vortex street including the magnitude of vortices, their translational speed, their spacing, their distance from the fish's body and the angle of the vortex street axis can all be recovered by measuring the velocity distribution along the canal and its changes with time. To this end, the proposed LLTC model could explain how a fish identifies the characteristics of a Karman vortex street shed by a nearby object or a traveling fish. It is also demonstrated that while this model captures the ac (alternating current) component of the external velocity signal, the dc (direct current) component of the signal is filtered out. Based on the results of our model, the role of the LLTC in a fish's schooling and its evolutionary impact on fish sensing are discussed.

Determination of object position, vortex shedding frequency and flow velocity using artificial lateral line canals

The lateral line system of fish consists of superficial neuromasts, and neuromasts embedded in lateral line canals. Lateral line neuromasts allow fish to sense both minute water motions and pressure gradients, thereby enabling them to detect predators and prey or to recognize and discriminate stationary objects while passing them. With the aid of the lateral line, fish can also sense vortices caused by an upstream object or by undulatory swimming movements of fish. We show here that artificial lateral line canals equipped with optical flow sensors can be used to detect the water motions generated by a stationary vibrating sphere, the vortices caused by an upstream cylinder or the water (air) movements caused by a passing object. The hydrodynamic information retrieved from optical flow sensors can be used to calculate bulk flow velocity and thus the size of the cylinder that shed the vortices. Even a bilateral sensor platform equipped with only one artificial lateral line canal on each side is sufficient to determine the position of an upstream cylinder.

Ultrastructure and distribution of superficial neuromasts of blind cavefish, Phreatichthys andruzzii, juveniles

Transmission and scanning electron microscopy (TEM, SEM) were used to study the ultrastructure of superficial neuromasts in 15 six-month old blind cavefish juveniles, Phreatichthys andruzzii (Cyprinidae). In five specimens examined with SEM, the number of superficial neuromasts over the fish body (480-538) was recorded. They were localized mainly on the head (362-410), including the dorsal surface, the mentomandibular region, and laterally from the mouth to the posterior edge of the operculum. Neuromasts were also present laterally on the trunk and near the caudal fin (116-140). A significantly higher number of neuromasts were present on the head compared to the trunk (t-test, P < 0.05). Superficial neuromasts of the head and those along the trunk were similar in ultrastructure. Each neuromast comprised sensory hair cells surrounded by nonsensory support cells (mantle cells and supporting basal cells) with the whole covered by a cupula. Each hair cell was pear-shaped, 15-21 microm high and 4-6 microm in diameter, with a single long kinocilium and several short stereocilia. Most support cells were elongated, with nuclei occupying a large portion of the cytoplasm. In the margin of the neuromast, mantle cells were particularly narrow. Both types of support cells had well-developed Golgi apparatus and rough endoplasmic reticulum. The number of hair cells and nonsensory support cells of the anterior lateral line (head) did not differ significantly from those of the posterior lateral line (trunk) (t-test, P > 0.05).

Impact of chronic cadmium exposure at environmental dose on escape behaviour in sea bass (Dicentrarchus labrax L.; Teleostei, Moronidae)

The effect of chronic exposure to a low concentration (0.5 microg l(-1)) of cadmium ions was investigated on escape behaviour of sea bass, Dicentrarchus labrax, using video analysis. Observations were also performed on the microanatomy of lateral system neuromasts. When fish were exposed for 4h per day over 8 days to the cadmium ions, most of both types of neuromasts observed remained intact. However, some of them presented damaged sensory maculae. Whereas before cadmium exposure, fish responded positively to nearly all the lateral system stimulations, after exposure they decreased by about 10% their positive responses to stimulations. From the 15th day after the beginning of cadmium exposure, neuromasts presented progressively less damage, cadmium accumulation in gills and scales decreased significantly and fish escape behaviour had recovered. This study presents a new concept in ecotoxicology: using behavioural change to reveal the effects of pollution levels, scarcely detectable by currently used techniques (physiological responses).

Impact of acute cadmium exposure on the trunk lateral line neuromasts and consequences on the "C-start" response behaviour of the sea bass (Dicentrarchus labrax L.; Teleostei, Moronidae)

Behavioural responses of sea bass Dicentrarchus labrax were investigated after exposure to cadmium ions in laboratory-controlled conditions. The aim of this study was to discover whether environmental exposure to cadmium ions inactivates fish lateral line system neuromasts, and to determine the behavioural consequences of such a sensory blockage. For this, fish escape behaviour in response to an artificial water jet was recorded using a 25-frames s(-1) analog video camera before and after cadmium exposure. Experimental set up was tested with fish whose lateral line system was artificially inactivated by antibiotics (gentamicin and streptomycin). Histological analyses with scanning electron microscopy showed antibiotic treatment destroyed lateral line system neuromasts. In addition, these fish did not respond to stimulations provoked by the water jet after antibiotic treatment. Fish escape behaviour was then recorded before and after cadmium exposure at two different concentrations. When fish were exposed to the first concentration of cadmium tested (0.5 microg l(-1), which represents the maximal cadmium concentration encountered in contaminated estuaries), no alteration in neuromast tissue was observed. In addition, before cadmium exposure, fish responded positively in 98.41 +/- 4.95% of lateral line system stimulations (escape behaviour in response to the water jet). After cadmium exposure, no behavioural modification could be detected: the fish responded positively in 95.16 +/- 9.79% of stimulations (chi(2) = 2.464, p = 0.116). In contrast, the high cadmium concentration used (5 microg l(-1), which represents 10 times the concentration occurring in highly polluted estuarine areas) involved severe neuromast tissue damage. Just after such cadmium exposure, fish showed only 41.67 +/- 35.36% of positive responses to their lateral line system stimulations, while they responded positively in 95.93 +/- 9.10% of stimulations under control conditions (chi(2) = 24.562, p < 0.0001). Their lateral line system neuromasts seemed to regenerate about 1 month after cadmium exposure. Associated with this regeneration, from the 21st day after cadmium exposure, their escape behaviour had recovered and was not significantly different from that recorded under control conditions (86.74 +/- 20.82%, chi(2) = 2.876, p = 0.090). This study shows that although 5 microg l(-1) cadmium is able to damage lateral line system neuromasts and causes fish behavioural alterations, fish exposed to 0.5 microg l(-1) cadmium displayed neither tissue neuromast nor behavioural modification.

Quantitative Aspects of the Spatial Distribution and Morphological Characteristics of the Sea Bass (Dicentrarchus labrax L.; Teleostei, Serranidae) Trunk Lateral Line Neuromasts

The results presented herein report quantitative data relative to the distribution and morphological characteristics of both types of neuromasts encountered on the trunk lateral line of the sea bass (Dicentrarchus labrax, L.). These data were obtained from scanning electron micrographs. They indicate that, as expected, each modified scale of the sea bass possessed a single canal neuromast with long axis oriented parallel to the fish's long axis. In contrast to several fish species, two thirds of superficial neuromasts observed herein were oriented perpendicular to the fish's long axis. However, whatever the main orientation of superficial neuromasts, two thirds of their hair bundles were oriented parallel to the long axis of the animal with approximately half of them in the direction of the head. Similar ratios were observed for canal neuromasts whatever the area of the maculae: central or peripheral. For both types of neuromasts it was not possible to clearly distinguish a paired organization of hair bundles with opposing polarities. Superficial neuromasts on each trunk canal scale were located on either the dorsal or ventral side of the canal and appeared to be distributed along the trunk lateral line with a higher probability to be encountered closer to the operculum. The frequency of presence and the average number of superficial neuromasts per scale increased with fish size. We observed a size gradient for canal neuromasts between the operculum and caudal peduncle. This gradation was correlated with a reduction of the width of the central area of the canal segment. Canal neuromasts were always localized in the larger portions of the canal segments. Taken together, these results point out some specific features associated with the sea bass trunk lateral line. With the previous report, they establish the first full description of the trunk lateral line of sea bass and will be useful for upcoming experiments regarding the function of the two types of neuromasts.

Non-neural ectoderm is really neural: evolution of developmental patterning mechanisms in the non-neural ectoderm of chordates and the problem of sensory cell homologies

In chordates, the ectoderm is divided into the neuroectoderm and the so-called non-neural ectoderm. In spite of its name, however, the non-neural ectoderm contains numerous sensory cells. Therefore, the term "non-neural" ectoderm should be replaced by "general ectoderm." At least in amphioxus and tunicates and possibly in vertebrates as well, both the neuroectoderm and the general ectoderm are patterned anterior/posteriorly by mechanisms involving retinoic acid and Hox genes. In amphioxus and tunicates the ectodermal sensory cells, which have a wide range of ciliary and microvillar configurations, are mostly primary neurons sending axons to the CNS, although a minority lack axons. In contrast, vertebrate mechanosensory cells, called hair cells, are all secondary neurons that lack axons and have a characteristic eccentric cilium adjacent to a group of microvilli of graded lengths. It has been highly controversial whether the ectodermal sensory cells in the oral siphons of adult tunicates are homologous to vertebrate hair cells. In some species of tunicates, these cells appear to be secondary neurons, and microvillar and ciliary configurations of some of these cells approach those of vertebrate hair cells. However, none of the tunicate cells has all the characteristics of a hair cell, and there is a high degree of variation among ectodermal sensory cells within and between different species. Thus, similarities between the ectodermal sensory cells of any one species of tunicate and craniate hair cells may well represent convergent evolution rather than homology.