Vocal communication is important for the survival and reproductive success of all animals. This is especially true for the nocturnally breeding teleost fish, the plainfin midshipman – fondly nicknamed the `Californian singing fish' because their humming mating calls are familiar nocturnes to Californian locals. Reproductive females must recognize the high-frequency components of those `hums', which is absent in other types of vocal signal, to locate the love nests carefully prepared by the calling males. One question that has been puzzling fish biologists is how auditory function in fish changes during development. Joseph Sisneros of the University of Washington and Andrew Bass of Cornell University studied auditory encoding in the plainfin midshipman, and report significant differences in the auditory function of adults and juveniles of this species(p. 3121).
Sisneros and Bass conducted an analysis of age- and size-related changes in the encoding properties of individual auditory neurons in the plainfin midshipman. Recording individual neurons in very small juvenile fish is no mean feat. These are very delicate creatures, but the midshipman has now proven to be a useful model system for fish physiologists that want to study changes in hearing as an animal transitions from young life history stages to an older, reproductively mature state. Sisneros and Bass opened midshipman's skulls and inserted electrodes into auditory neurons in the inner ear's sacculus – the main hearing organ in midshipman. They then lowered the fish just below the water surface, ∼10 cm above an underwater loudspeaker in a tank that was housed inside an acoustic isolation chamber on a vibration isolation table.
In this sophisticated recording setting, Sisneros and Bass were able to test whether juveniles hear as well as adults, and whether they can discern the vocal signals that adults can hear. The pair first studied the basal firing rate of auditory neurons in adult fish and in small and large juveniles– 130–160 and 160–370 days post-fertilisation age,respectively – in the absence of acoustic stimulation. They found that basal neuronal activity, a reflection of the neuron's sampling rate and excitability, increases with fish age and body length. The researchers then determined the lowest intensity of a fixed-frequency acoustic stimulus that the fish could detect, as judged by the presence of neuronal firing. Interestingly, they discovered that large juveniles and adults hear five times better than small juveniles.
But can juvenile fish differentiate various types of vocal signal with different frequency composition? Animals preserve a sound's frequency information by adjusting neuronal firing patterns to the time-varying structure of the sound wave – a process known as synchronisation or`phase-locking'. Sisneros and Bass found that, like their non-reproductive adult counterparts, both small and large juveniles are best adapted to low frequency `grunts', which are important for agonistic encounters. But juveniles do not phase-lock to the higher frequency components in `growls' and`hums' used as mating calls by midshipman males, presumably because these signals are not yet relevant to them. `It isn't clear if the juveniles are sonic. They may make only `grunts' – to warn and to avoid predators may be the most important thing for them at this stage', Sisneros concludes.