Hydrodynamic reception

Abstract: Marine mammals often forage in dark and turbid waters. While dolphins use echolocation under such conditions, pinnipeds seem to lack this sensory system. Instead, species of the families Phocidae (true seals) and Otariidae (eared seals) both possess richly innervated whiskers (synonymously “vibrissae”) representing highly sensitive hydrodynamic receptors that enable these animals to detect fish-generated water movements. The third family of pinnipeds, the Odobenidae (walruses), is less well studied. As water movements in the wake of fishes persist for several minutes, they constitute hydrodynamic trails that should be trackable by piscivorous predators. Hydrodynamic trail following has indeed been shown for the harbor seal (Phoca vitulina) and the California sea lion (Zalophus californianus). However, in experiments with a sea lion aging of the trails resulted in an earlier decrease in performance. This difference in tracking performance most likely is due to differences in the structure of the respective vibrissal hair shaft. In the harbor seal the high sensitivity and excellent tracking performance is ascribed to the specialized undulated structure of the whiskers that largely suppresses self-generated noise in the actively moving animal. In contrast, the whiskers of a swimming California sea lion, which are smooth in outline, are substantially affected by self-generated noise. However, in the sea lion such self-generated noise contains a characteristic carrier frequency that might allow hydrodynamic reception by being modulated in response to hydrodynamic stimuli impinging on the hair. Thus, in the course of pinniped evolution at least two types of whiskers evolved that realized different mechanisms for the reception of external hydrodynamic information.

Abstract: With the lateral line system, fish and aquatic amphibians detect minute water motions. The hydrodynamic information that is received by the lateral line sense organs, the neuromasts, is represented by the activity of afferent nerve fibers and is analyzed by the brain to determine identity and location of a source of hydrodynamic disturbance. This chapter presents our current knowledge on the processing of various hydrodynamic stimuli at different levels of the ascending lateral line pathway. Different stimuli have been used to study the function of central lateral line units, including dipole stimuli, moving objects, bulk water flow, and vortex streets. Compared to primary afferent nerve fibers, most central units are less sensitive to dipole stimuli and exhibit more complex spatial receptive fields and highly selective responses to moving objects. When exposed to bulk water flow, flow-sensitive central units may increase or decrease their ongoing discharge rate. As a consequence, their responses to dipole stimuli or moving objects may be masked. When stimulated with a vortex street, central units may represent the vortex shedding frequency in their activity. Anatomical studies have uncovered somatotopic representations of the lateral line periphery in various brain regions. Physiological data, however, that are indicative of a systematic representation of hydrodynamic information such as source location or bulk flow velocity in the form of a central map are scarce. More studies are needed to uncover the computational rules and the circuit diagrams implemented in the central lateral line.

Abstract: Beside their haptic function, vibrissae of harbour seals (Phocidae) and California sea lions (Otariidae) both represent highly sensitive hydrodynamic receptor systems, although their vibrissal hair shafts differ considerably in structure. To quantify the sensory performance of both hair types, isolated single whiskers were used to measure vortex shedding frequencies produced in the wake of a cylinder immersed in a rotational flow tank. These measurements revealed that both whisker types were able to detect the vortex shedding frequency but differed considerably with respect to the signal-to-noise ratio (SNR). While the signal detected by sea lion whiskers was substantially corrupted by noise, harbour seal whiskers showed a higher SNR with largely reduced noise. However, further analysis revealed that in sea lion whiskers, each noise signal contained a dominant frequency suggested to function as a characteristic carrier signal. While in harbour seal whiskers the unique surface structure explains its high sensitivity, this more or less steady fundamental frequency might represent the mechanism underlying hydrodynamic reception in the fast swimming sea lion by being modulated in response to hydrodynamic stimuli impinging on the hair.

Abstract: In the present study the time course and spectral-amplitude distribution of hydrodynamic flow fields caused by moving fish, frogs, and crustaceans were investigated with the aid of laser-Doppler-anemometry. In the vicinity of a hovering fish sinusoidal water movements can be recorded whose velocity spectra peak below 10 Hz (Fig. 2). Single strokes during startle responses or during steady swimming of fish, frogs, and crustaceans cause short-lasting, low-frequency (<10 Hz), transient water movements (Fig. 3). Low-frequency transients also occur if a frog approaches and passes a velocity-sensitive hydrodynamic sensor. In contrast, transient water movements caused by a rapidly struggling or startled fish or water motions measured in the wake of a slowly swimming (≤47 cm/s) trout can be broadbanded, i.e., these water movements can contain frequency components up to at least 100 Hz (Figs. 4, 5A, 6). High-frequency hydrodynamic events can also be measured behind obstacles submerged in running water (Fig. 5C). The possible biological advantage of the ability to detect high-frequency hydroynamic events is discussed with respect to the natural occurrence of high frequencies and its potential role in orientation and predator-prey interactions of aquatic animals.