EFFECTOR MECHANISMS 405 



The ecology of the swim-bladder shows many interesting features. In 

 shallow waters over the continental shelf the majority of free-swimming 

 teleosts possess swim-bladders. Such fish are able to maintain position in 

 mid-water with little apparent locomotory effort. In contrast a bladder is 

 frequently absent in bottom-dwelling and littoral species of the neritic 

 zone. These fish usually rest on the bottom and only rarely swim upwards 

 towards the surface, and then with effort. Examples of the last group are 

 flat fish (Heterosomata) and gobies (Gobiidae). But young flat fish, in 

 their larval pelagic stages, possess a swim-bladder. In the open oceans a 

 swim-bladder is found in surface-dwelling species such as flying fishes, but 

 is often absent in large oceanic species, e.g. tunnies. Pelagic teleosts dwell- 

 ing between 100-500 m frequently possess a well-developed swim-bladder, 

 e.g. Mycotophidae. These animals frequently undertake long vertical 

 migrations and their swim-bladders have well-developed gas-glands and 

 retia mirabilia. The swim-bladder is absent in many bathypelagic species 

 and bathybenthic species (60a, 61, 85). 



During conditions of asphyxia, marine fish draw upon the oxygen in the 

 swim-bladder. This mechanism may be of some value to species with air- 

 bladders under conditions of temporary anoxia (109). 



SOUND PRODUCTION 



Although it has long been known that some marine animals produce 

 sounds, it is only in recent years that detailed information has been secured 

 by under-water recording. Sound production occurs in three groups of 

 marine animals, namely crustaceans, fish and toothed whales. These 

 animals make noises with snapping or stridulating devices (crustacea and 

 fish), with swim-bladders (teleosts) or with the larynx (cetacea). 



Crustacea. Many members of the higher crustacea make noises by 

 stridulation, i.e. by rubbing parts of the exoskeleton against one another 

 or by snapping their claws. In warm shallow waters the most consistent 

 sound-producers are populations of snapping shrimps (spp. of Crangon 

 and Synalpheus). The snapping shrimp has a giant claw and produces a 

 sharp snapping sound by suddenly moving the finger against the fixed 

 portion of the chela. The sound spectrum ranges from 0T to at least 

 24 kc/s, with a broad peak from 2-15 kc s. The peak sound pressure at a 

 distance of 1 m is 200 dyn/cm 2 . In natural populations of snapping shrimps 

 there is a two- or threefold increase in the noises they produce shortly 

 before sunrise and again after sunset. The changes in noise levels probably 

 represent increased activity of the shrimp at those times. 



Many other crustaceans have specially differentiated stridulating organs. 

 The spiny lobster Palinurus, for example, has a stridulating apparatus on 

 the basal joints of the large antennae. Sound-producing crabs often have 

 a ridged or striated area on one of the claws, which is rubbed against a 

 modified area on the edge of the carapace or on one of the walking legs, 

 e.g. Pseudozius, Ocypode. Squilla produces sounds by rubbing the uropods 



