Bathydemersal types. — The brink of the slope leading to the abyss of the ocean has a 

 depth of about 110 fathoms, a depth also marking the end of the continental shelf and the 

 " mud-line " as Murray called it. Mud, the linal product of marine erosion, is found at great 

 depths over a vast area. It covers sands and gravels and, as it forms, rocky ledges are worn 

 away. Mud gives to the vast deep-sea landscape its monotonous uniformity. Certain of the 

 bottom-dwelling fishes of the shelf, such as pleuronectids, rays, angler-fishes and conger-eels 

 are also to be found on the slope. Large-eyed Beryx fishes, more typical of the neritic zone, 

 shelter among the coral thickets. But other fishes appear, such as chimaeras and rat-tails 

 (Macrouridae), fishes that have a rather similar appearance although belonging to quite different 

 groups. The robust body tapers off to end in a long rat-like tail and a pointed snout enables 

 them to root in the mud. The mouth is set on the underside of the head, and behind the head 

 comes a strong spine marking the origin of the dorsal fin. The enormous eyes reflect a strange 

 greenish light, while the general body-colour is such as to suggest the name of " purple-blue 

 fishes " for these slope dwellers. 



The Ixilhypekuiic typr. - Whereas the blue fishes sport in the shimmering sunlit waters, 

 other fishes swim in dark abyssal layers. They may be called " black-lishes ", for their dark 

 colours match those of their surroundings. Down to about 1.100 or 1.600 fathoms their habits 

 are much the same as those of the surface fishes. The laiilern-fishes live in schools like herrings 

 and sardines, sometimes in impressive concentrations. But their movements iruist be more 

 restricted, for temperature and salinity conditions are less variable in the deeper waters than 

 in those at the surface. Seeking darkness, I hey move down during (he daytime when the sun's 

 rays are brightest and most penetrating, but during the night they climb so high in the water 

 that they are preyed on by tunny-fishes. The commonly held belief that deep-sea fishes explode 

 on being brought to the surface is without fouiuiation. The lieep-living pelagic fishes are quite 

 insensitive to changes in pressure, just as are the invertebrates. Fishes brought uji in a vertical 

 net from 1,600 fathoms can be kept for days on end by putting them in shaded containers filled 

 with water from the open sea. .Such conditions are quite umisual but they wit (island them even 

 better than herring or sardines taken along the shore. 



In this darkness which is so essential for them, the black fishes are guided by their lumin- 

 escent organs. These are set over the flanks like the lighted port-holes of a ship, the arrange- 

 ment varying with the species, rndoubtedly such differences rmist mean that the lights are 

 used as recognition signs for shoal formations. Other non-gregarious forms are provided with 

 an individual lighting, such as, for instance, the ceratioid anglers which dangle a veritable lamp 

 at the end of a long tle.xible ray placed on top of the head. In certain blind forms the loss of 

 sight is made up for by a very marked development of long filamentous tactile organs formed 

 from the fins. 



All these abyssal animals are very voracious, some having an enormous mouth armed with 

 formidable teeth and being perfectly capable of swallowing prey as large as themselves. The 

 stomach is often distensible and when full bulges out enormously to double the volume of the 

 predator. 



The bathypelagic fishes have floating eggs and are entirely lacking in [)arental instinct. 

 The skeleton is fibrous, due to the absence of calcium carbonate in solution in deep waters. 

 The skin is naked and quite without scales. 



This very rapid survey of the main ecological types gives but a cursory idea of the consider- 

 able changes that variations of medium and environment can bring about in the forms of 

 fishes. In the following chapters we shall see how these types have become adapted under 

 the influence of geographical conditions in different latitudes. 



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