406 THE BIOLOGY OF MARINE ANIMALS 



against the under-surface of the telson. The stridulating apparatus may be 

 confined to the males, or occur in both sexes. 



In some crustaceans the sounds appear to have a warning significance. 

 Palinurus makes a creaking noise with its antennae when it is disturbed. 

 The snapping shrimp shoots its pistols for stunning small prey or dis- 

 couraging too inquisitive visitors. Ocypode, a littoral crab, stridulates to 

 warn other crabs that its burrow is occupied. And male fiddler crabs, 

 Uca, living on sandy shores, drum on the ground with their large chelipeds 

 as part of the breeding display (11, 26, 29, 35, 86a). 



Fish. Some species offish are surprisingly vocal. Noises are produced by 

 stridulation, i.e. by moving parts of the skeleton against one another or by 

 using the swim-bladder as a resonator. Horse-mackerel, trigger-fishes, 

 etc., emit harsh sounds by grinding the pharyngeal teeth. In boar-fishes, 

 file-fishes and others the spines of the fins produce harsh noises when 

 moved against one another or against neighbouring parts of the skeleton. 



The swim-bladder of teleosts is commonly involved in sound production. 

 In this role it acts in one of two ways : 



1. As a resonator, by picking up and amplifying noises made by ad- 

 jacent structures. In trigger-fishes (Balis tes) the swim-bladder functions 

 as a resonator for sounds produced by stridulation of the post-clavicles 

 against the cleithra, and in Haemulon for grinding noises produced by the 

 pharyngeal teeth. 



2. As an intrinsic sound-producer. In this role the swim-bladder is set 

 into resonant vibration by contractions of adjacent muscles, as in gurnards 

 (Triglidae), toad-fish (Opsanus), midshipman (Porichthys), etc. (Fig. 9.21). 



The muscles which throw the swim-bladder into resonant vibration are 

 attached to it in various ways. In some species the external muscles extend 

 from the air-bladder to adjacent structures, to the anterior ribs in the cod 

 Gadus callarias and to the lateral body wall in sciaenids (Cynosciori). 

 In others the external muscles are attached solely to the air-bladder, as in 

 the pollack (Gadus pol/achius), gurnards, midshipman, toad-fish, etc. 



By suitable electrical stimulation of nerves and muscles supplying the 

 air-bladder, sounds have been produced which are characteristic of the 

 fish under investigation. The air-bladder in the gurnard (Trig/a) contains 

 a perforated transverse partition which is set into vibration by air forced 

 from one compartment of the bladder into the other. When the anterior 

 spinal nerves supplying the external swim-bladder muscles are stimulated, 

 the transverse diaphragm is set into vibration and grunting noises are 

 produced. Kymograph recordings made from the squeteague reveal that 

 the abdominal wall vibrates at a frequency of 24/sec. These movements are 

 induced by the external swim-bladder muscles which contract at the same 

 rate. 



The frequency and intensity of noises made by various fish have been 

 measured in recent years. In the majority of species tested the principal 

 frequencies measured lie between 75 and 300 c/s. The croaker (Micropogon 

 undulatus) produces a rapid burst of drumming noises, each pulse lasting 



