PELAGIC ANDPENTHIC FISHES, SWIMBLADDER, ECONOMY OF DEEP SEA-LIFE 113 

 well-formed, gas-filled swimbladders. They are muscular, active little fishes with large eyes and 

 brains and highly developed light-organs. Because of the extra propulsive power that a swimbladder 

 allows, they are able to make daily visists to the food-rich, surface-waters, which they must do to 

 maintain their highly organized bodies. Living with them are almost as many predatory fishes, 

 nearly all of which have lost an internal float. In spite of this, these fishes come close to neutral 

 buoyancy by reducing their muscular and skeletal tissues. All have well-formed eyes and brain, and 

 many have complex batteries of light organs. Some of these fishes also migrate to the surface-layers, 

 compensating for their reduced myotomes by increased size of body. 



Living below them in the deep mid-waters, are more than a hundred species of pelagic fishes with 

 no swimbladders and very reduced tissues. They are nearly all predatory fishes, many of which lure 

 their prey instead of pursuing it. As they are likely to be neutrally buoyant they can hover in the water 

 without undue effort. The energy balance sheet of these species seems closely fitted to the limited 

 supplies of potential energy around them. 



On the abyssal plain, about 250 species of benthic fishes have been taken below a depth of 2000 m. 

 Well over half have well-formed swimbladders and a number of species may range as deeply as 

 5000 m. Compared with the deeper bathypelagic fishes they are highly organized, but show some 

 reduction of bone and muscle to compensate for the reduced positive buoyancy of a swimbladder at 

 great depths. Most of the species without a swimbladder have an equally good organization, but the 

 food supply seems sufficient to satisfy their day to day requirements. 



These differences in form and organization of the tissues are reminiscent of Raunkiaer's (1934) 

 concept of life form in plants. 1 ' All over the world environments varying from place to place deter- 

 mine the existence of different life forms, because the demands of the plants, which are, at any rate, 

 partially expressed by their structure, must of necessity be in harmony with the environment if life 

 is to continue.' Raunkiaer saw that certain structural features were correlated with certain types of 

 environment and that these features were shared by diverse species of plants. Reference to this 

 section, particularly to the table on p. 105, will show that this is also true of the assemblages of fishes 

 that live at the upper and lower levels of the bathypelagic environment. As I hope to continue with 

 this problem, to look more closely at some of the knots in Bigelow's (1930) 'endless web of netting', 

 these few remarks will suffice to conclude this Report. 



SUMMARY 



Structural development of the swimbladder (pp. 6-50) 

 Dissection of about ninety species of bathypelagic teleosts has shown that a well-developed, gas-filled 

 swimbladder is present in numerous stomiatoid fishes (most Gonostomatidae, Sternoptychidae, 

 Astronesthes spp.); salmonoid fishes (Opisthoproctus, Winterid) Myctophidae (most species), Melam- 

 phaidae (most species) and Chiasmodon niger (Chiasmodontidae). The swimbladder regresses during 

 adult life in the Miripinnati, certain of the Stomiatidae and a few Myctophidae and Melamphaidae. 

 It is completely absent in numerous stomiatoids (Melanostomiatidae, Chauliodontidae, Idiacanthidae 

 and Malacosteidae), Alepocephalidae, certain Myctophidae, Scopelosauridae, Alepisauroidea, Gigan- 

 turoidea, Lyomeri and Ceratioidea. 



1 Certain continental zoologists have also developed the idea of 'Lebensformtypen' in animals (see Macfadyen, 1957), but 

 the concept is more familiar to botanists. 



15 



