In the Piti Mouth, a typical biotope of Sousa plumbea and Neophocaena phocaenoides in the Indus delta, the sounds produced by Johnius belangerii (Boro) and the rather weaker crustacean sounds constitute the main component of the ambient noise. The first ever analysis of the Boro sounds was carried out.
The individual burst of Johnius has a duration of 140-260 ms and consists of 4-14 pulse sequences. The first pulse is always separated from the others by 40 ms. The following pulses follow at regular intervals. The repetition rate averages 50 ips. The bursts produced by an animal follow each other at intervals of a few seconds. Apart from these intraspecific characteristics, there are certain individual differences with regard to pulse shape and frequency. The sound pressure level of the bursts reaches +21 to +30 dB re 1 μbar at 1 m. The chorus consists of connected bursts. An initial burst separated from the others by 40 ms no longer occurs here. The sound pressure level of several synchronized sound groups exceeds that of the individual bursts by 10 dB. Both in the individual burst and in chorus, the frequency range is 0.08 to 10 kHz, with the energy concentrated in the 0.5-4 kHz portion. For the short single sound, the dominant frequency falls in the 0.5-1 kHz range, whereas for long single sounds and the chorus, the dominant frequency is around 0.75-1.25 kHz.
Crab sounds consist of numerous short noise pulses. The cracking sound covers the entire frequency range from 0 to 20 kHz. The acoustic pressure level of these sounds is around +16to +19dB re 1 μbar at 1m. In addition to Johnius and crustaceans, water and shipping noises are the other sources of ambient noise. The frequency ranges from 0 to 10 kHz. The acoustic pressure level is generally lower than that of the fish and crab noises.
Owing to the high-frequency component and the likewise high acoustic pressure level, the sonar sound of Sousa and the HF click of Neophocaena are outside the range of other animal sounds and the ambient noise. The frequency range of the screams and whistles which extends from 3 to 30 kHz and 3-15 kHz partly overlaps that of the Boro sounds. Owing to the higher acoustic pressure level, however, they are relatively easily distinguishable from the crab sounds and the water and ship noises. The LF component of Neophocaena clicks covers a frequency range of 1.6-2.2 kHz with only a low acoustic pressure level of +5 to +8 dB re 1 μbar. This portion of the sound is thus completely obscured by the water and ship noises and the crab and fish sounds.
The acoustic pressure level of the ambient noise is raised about 30 dB by the Johnius belangerii chorus. Similar values were observed in other Sciaenidae species from Eastern Indian Ocean, Western Pacific Ocean and Australian waters. The ambient noise level is generally attenuated with rising frequency.
The sound apparatus of Johnius belangerii is very well developed. The hammer-shaped swim bladder and the 13 paired, 5-6 fold dichotomously ramified appendages are in intraspecific feature. The cephalic pair of appendices extends on one side to the ear capsule and on the other to the region of the gills and the spine. The appendages are embedded in a thick fatty body. The swim bladder is firmly fixed rostrodorsally to the apophyses of the second, fourth and fifth vertebrae. The widening of the ventral apophyses of the fourth vertebra to a uniform U-shaped plate is a specific feature of the sciaenidae. The innervation of the swim bladder is effected by one occipital nerve pair and the first spinal nerve pair.
The tadpole pattern on the inner surface of the sagitta of Johnius belangerii is of the johniine type. In relation to body length, the otolith of the Boro sacculus is very large; compared with 9 other families, the representatives of which belong to the sound-producing species, the Sciaenidae have the largest sagitta, together with the Scorpaenidae.
The brain of Johnius belangerii is likewise highly developed. With regard to the morphological development of the cerebrum and cerebellum, it is on a par with the Sparidae, and in other respects – for example, the valvula cerebelli, the shape and size of the infundibular gland, the uncovered rhomboid fossa and the lobi posteriores joined by the commissure – it resembles the Percidae. Compared with Coregonus (60 %) and Lates (70 %), the lobi inferiores are strikingly large, attaining – as in Gobius niger – about 80 % of the midbrain width. In addition to the I, II, V and X brain nerves, the nervus stato-acousticus (VIII) is also very thick, which in conjunction with the large sagitta suggests a good hearing ability. The relative brain weight of Johnius belangerii amounts to 0.3 % of the body weight, or 10 times that of the relative brain weight of certain silurides from the same region.