Fig. 208.—Air-bladder and its muscles in Micropogon undulatus. a.b, Air-bladder; l.b, right lateral caecum; m, m, musculo-tendinous extensions from the muscles of the body-wall, which partially invest the surface of the air-bladder. (From Sörensen.)
Whatever the precise relation of the air-bladder to its muscles it is probable that the physiological effect is in most cases the same. By the rapid alternating contraction and relaxation of the muscles, some part of the wall of the bladder becomes alternately compressed and relaxed in such a way as to initiate a series of vibratory movements in the gases of that organ, and so produce definite sounds. In not a few of the Fishes the cavity of the bladder is subdivided by external constrictions or by internal septa, or is complicated by the development of lateral, tubular, caecal branches; and hence the vibratory movements of the gases will be greatly strengthened by their passage across the edges of the septa, or the apertures of the caeca, and the intensity of the resultant sounds also increased. It will be readily understood that the nature and quality of the sounds emitted by different Fishes will necessarily vary with the shape of the air-bladder, the number and arrangement of the internal septa and the caeca, and the strength and disposition of the contracting muscles. In a few Teleosts (Triglidae and Zeidae) sounds are said to be produced by the rapid vibration of an annular, or centrally-perforated, muscular diaphragm, which stretches across the cavity of the air-bladder.[[423]] Nevertheless, it must be strongly emphasised that, while in some Fishes the air-bladder and its muscles undoubtedly constitute a vocal organ, there are many others in which the bladder can only be inferred to be sound-producing from its general agreement in anatomical structure with the same organ in Fishes where its vocal function has been clearly proved.
By one or other of these various methods the air-bladder is either known to be sound-producing, or is believed with good reason to be such, in the following Teleosts,[[424]] and many others:—Certain species of the South-American genera of Siluridae, Pimelodus, Sorubim, Platystoma, Piratinga, Centromochlus, and Trachelyopterus; species of the South-American family Characinidae; Amblyopsis spelaea, the blind Fish from the Mammoth Cave of Kentucky (Amblyopsidae); among the Syngnathidae, the short-snouted Sea-Horse (Hippocampus brevirostris) of the British Coasts; certain Sclerodermi, such as the Trigger-Fishes, Batistes vetula, Triacanthus brevirostris, T. biaculeatus, and Monacanthus pardalis, and also some "Coffer Fishes" (e.g. species of Ostracion); some Gymnodontes (species of Diodon and Tetrodon); a few Serranidae (e.g. species of Therapon and Pristipoma); species of Holacanthus (Chaetodontidae) and in Holocentrum sogho (Berycidae); such Sciaenidae as the "Drum" (Pogonias chromis), the "Maigre" (Sciaena aquila), which has sometimes been taken in British waters, Umbrina cirrhosa, Otolithus regalis, and Micropogon undulatus, and, with more or less probability, many other species of the same family; one species of Zeidae, the John Dory (Zeus faber); Batrachus tau among the Batrachidae; several species of Gurnards (Triglidae) belonging to the genera Prionotus and Trigla; the so-called Flying Gurnard, Dactylopterus volitans (Dactylopteridae); the Indian species Ophiocephalus marulius and O. gachua (Ophiocephalidae); amongst the Gadidae, the Cod (Gadus morrhua) and the Haddock (G. aeglefinus); in such Zoarcidae as the blind Fish (Lucifuga subterranea) from the subterranean waters of the caves of Cuba, and also in some Ophidiidae (e.g. species of Ophidium).
In Fishes other than Teleosts, instances of normal sound-production by special vocal structures are rare. No recorded instances are known in the Cyclostomes or the Elasmobranchs,[[425]] but there is evidence that sounds are emitted by Polypterus among the Crossopterygii, and by the Dipnoids Neoceratodus,[[426]] Protopterus, and Lepidosiren, although it is not certainly known how they are produced, or that they may not be the accidental concomitants of the inspiratory or expiratory action of the lungs in breathing.
As to the nature of the sounds produced by the air-bladder and its muscles in different Teleosts, a few examples may be given.
The sound produced by the elastic-spring-apparatus of a recently caught Doras maculatus, has been described as a "deep growling tone," which may be distinctly heard at a distance of 100 feet when the Fish is out of the water. Under like conditions the air-bladder and its muscles, in a species of Platystoma, emit a similar sound. On the other hand, the sound produced by the elastic springs of the Electric Siluroid (Malopterurus electricus) has been compared to the hissing of a cat. The Sea-Horse (Hippocampus brevirostris) utters a monotonous sound analogous to that of a tambour, which is characteristic of both sexes, but is more intense and frequent in the breeding season. The "Coffer Fish" (Ostracion trigonus) emits a growling sound, as also does the "Globe Fish" (Tetrodon honckenii) when taken out of the water.[[427]] The air-bladder and its muscles in the "Drum" (Pogonias chromis), constitute the most powerful sound-producing organ yet found in any Fish. The sounds emitted by the "Drum" are better expressed by the word drumming than by any other, and have frequently been heard by persons in vessels lying at anchor on the coasts of the United States, where these Fishes abound.[[428]] The "Drum" begins its drumming noise in the spawning season in April, but is rarely heard afterwards. The "Maigre" (Sciaena aquila), whose musical performances are perhaps responsible for the Homeric fable of the song of the Sirens, is remarkable among Fishes for the variety of its sounds, which have been compared to bellowing, purring, buzzing, and whistling.[[429]] The sound is often so intense that it may be heard when the Fish is at a depth of 18 metres, and the ear of the observer two metres above the water; and it has been recorded that by listening for these sounds, shoals of Maigres have been successfully netted. They rarely emit sounds when isolated; but in shoals, during the breeding season, they do not cease to make sounds with a vigour and a persistency which apparently must soon wear out their strength. One of the Indian Horse-Mackerels (Caranx hippos) grunts like a young Pig when captured, and the sound is repeated whenever it is moved, as long as vitality remains. A West Indian species of the same family (Argyriosus vomer) has been observed to produce a like sound, while an Egyptian Caranx (C. rhonchus) is known to the Arabs as the "Chakoura" or "Snorter."[[430]] The sounds produced by the different British Gurnards, such as the Grey Gurnard (Trigla gurnardus), the Piper (T. lyra), the Elleck or Cuckoo Gurnard (T. cuculus), and the Tub-Fish (T. hirundo), have been compared to snoring, a sonorous and prolonged grunting, crooning (whence, perhaps, the term "crooner," by which the Grey Gurnard is known in Ireland), and croaking. The John Dory (Zeus faber)[[431]] also utters sounds analogous to those of the Gurnards. Among the Dipnoi Lepidosiren is said to make a growling sound, and Neoceratodus a grunting noise which may be heard at night for some distance.
Whatever the nature of the vocal mechanism, it is highly probable that the sounds produced by Fishes travel to considerable distances in the water, inasmuch as the latter medium is a far better conductor of sound than air, and, moreover, the transmission of sound-vibrations from the air-bladder to the water is facilitated in many Fishes by the fact that, for a portion of its extent on each side the bladder is in direct contact with the superficial skin behind the pectoral girdle.
From the by no means exhaustive list of examples given above, it is obvious that in some form or other vocal organs are present in a considerable number of Fishes, both freshwater and marine, belonging to widely different groups; and further, that even in the same species (e.g. Doras maculatus and other Siluridae), both stridulation and the action of extrinsic muscles on the air-bladder may be utilised as a means of sound-production. Certain Teleostean families like the Siluridae, the Sciaenidae, and the Triglidae, seem to be distinguished above all others by the prevalence of some form of vocal organ. According to Sörensen, the first mentioned of the three families includes no less than 68 species, which utilise the air-bladder alone as a sound-producing organ. Nevertheless, there still remain many Teleostean families, rich in genera and species, and with an almost world-wide geographical distribution, in which such organs have not yet been found.
The advantages which Fishes derive from the possession of sound-producing organs are sufficiently obvious.