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The bathypelatic zone, the largest environment on earth, is cold and dark and the most deserted life zone in the ocean, both in numbers of organisms and of species.

Apart from their ontogenetic migrations, the deep set ways of bathypelagic animals would seem to give them little or no direct contact with the more productive waters above their habitat. There may well be overlap between populations of lower mesopelagic and upper bathypelagic animals.

Thus, dark-coloured lantern-fishes, most of which live at lower mesopelagic levels and undertake daily vertical migrations, are sometimes eaten by female angler- fishes.

But virtually nothing is known of the exchange of organic material in the overlapping zones. Certainly the zooplankton biomass near 1,000 metres is considerably greater than that at lower levels, but the main factor may simply be depth of residence; those highest in the bathypelagic wate column are the first recipients of organic crumbs from above and thus have the means of highest productivity.

Copepods, which must be the main metazoan users of organic particles, are well represented at bathypelagic levels. At all depths copepods, including their copepodid stages, make up the bulk of the zooplankton. Indeed, off Bermuda, Deevey and Brooks found that the relative proportions of copepods in the total zooplankton catch increased with depth in the upper waters to 91 per cent between 1,500 and 2,000 metres.

The decrease in numbers and quantities of zooplankton is greater between the upper waters and a depth of about 500 metres the vertical gradient for volumes or numbers of zooplankton falls exponentially, the rate of decline being greater between 503 and 2,003 metres than that below this depth range. For the biomass of total zooplankton, as measured by displacement volumes, Deevey and Brooks found that the slope of the gradient between 500 and 2,000 metres was – 5.66 x 104 log units per metres, which they compare with Vinogradov’s figure of – 3 to -2 x 104 log unit6 per metre for the gradient between 2,000 and 4,000 metres in the Indian Ocean.

There is also a decline with depth in the numbers of species. For cope pods, Grice and Hulsemann recorded 153 bathypelagic species in the western Indian Ocean, of which 122 occurred between 1,000 and 2,000 metres, 73 between 2,000 and 3,000 metres and 13 between 3,000 and 4,000 metres. Of the 326 species they identified from catches made between the surface and 2,000 metres off Bermuda, Deevey and Brooks recorded 128 species between 0 and 500 metres, 204 between 500 and 1,000 metres, 172 between 1,000 and 1,500 metres and 119 between 1,500 and 2,000 metres.

At upper levels and in an area of high productivity it may well be, as Harding (1974) found, that protists are an important food source for bathypelagic copepods. But in the poorly productive Sargasso Sea at deeper levels his analyses show that the most frequent gut contents of copepods were mineral particles, detrital remains and detrital balls. Deevey and Brooks conclude that phytoplankton production in the Sargasso Sea is insufficient to support resident populations of filter- feeding copepods.

Throughout the water column, they suggest that most copepods are omnivorous opportunists, consuming whatever particulate material, micro-organisms and nauplius larvae are available. They contrast their findings with those of Vinogradov from the Kurile- Kamchatka region, where carnivorous species increase in importance between 1,500 and 3,000 metres while omnivores prevail at lower levels. But, as they say much trophic stratification is probably typical of regions of high primary productivity.

Clearly, there is much we do not know of bathypelagic food pyramids, but recent investigations are suggestive, Light scattering measurements of the standing crops of suspended particles at bathypelagic levels in the Atlantic Ocean reveal a quantitative distribution that matches the pattern of surface productivity which is also reflected in the standing crops of bathypelagic and benthic organisms.

Even more significant, we shall see later that the faecal products of copepods and other zooplankters, especially from the epipelagic zone, are a major source of food for benth.c, deep-sea animal? Many such waste packages must be intercepted and used by bathypelagic zooplankton.

The greater the surface productivity the more will be the snowfall’ of organic particles and the ‘hail’ of faecal materials, and both together, particularly the latter, may well go for to determine levels of productivity in bathypelagic and benthic environments. At mesopelagic levels, as we have seen, there is also the powerful influence of vertical migrations.

The coepods, which are so predominant numerically at bathypelagic levels, must go for to nourish resident populations of arrow-worms, euphausiids prawns and diverse fishes. Apart from the small Cyclothone pygmaea in the Mediterranean and C. livida off Westren Africa, black species of Cyclothone range over the three oceans from lower * mesopelagic to bathypelagic levels.

The open ocean species are not only larger than the transparent mesopelagic species, but they have a relatively larger head and mouth.

Their prey ranges widely in size from other Cyclothone through euphausiids, amphipods and arrow worms to octracods and copepods, As stressed already, where food is scarce it is fitting to have the means for securing such a ranging regimen.

Thinking particularly of this desideratum, we turn to the deep-sea angler-fishes, which are the most diverse group of bathypelagic fishes: indeed, the 100-odd species make up at least two-third of this bathypelagic fish fauna. Between adolescence and when they are ready to seek their partners the free-living dwarf males have relatively small jaws and a more or less fusiform body. Unlike the females, they have no angling system.

In some families the males, which eventually become parasitic on the females, are not known to take food after metamorphosis. Males of other families have jaws that are not only suitable for taking prey but also, presumably, for hanging on to the skin of a female during the breeding season. Such males grow after metamorphosis and those of Melanocetus at least, feed on zooplankton.

The larger females, which in most species tend to a globular shape, bear large jaws armed with recurved, depressible teeth. The length of the jaws is usually more than half the length of the head. Except in Neoceratias, all females bear a rod and attached bait containing in nearly all species a luminous gland.

The illicium projects from the head between or just before the eyes and is articulated to a horizontal basal bone set in a groove on the cranium. There are muscles from the basal bone to the base of the illicium such that the latter can be moved between an upright and a horizontal position.

Moreover, in some angler- fishes the basal bone is so long and movable that the rod and bait can be extended well in fomt of the mouth and then retracted Williams rightly urges critical circumspection before deciding that features have an adaptive significance and he writes, ‘A frequently helpful but not infallible rule is to recognize adaptation in organic systems that show a clear analogy with human implements. The angling system of ceratioids is surely an outstanding example.

Female angler-fishes feed largely on crustaceans and fishes. They also take arrowworms and squid. As in Cyclothone an individual may contain organisms of a wide size range. Thus, in the stomach of an adolescent Himantolophus there were small copepods, two amphipods, five euphausiids, four Cyclothone one hatchet-fish one lantern-fish, one melamphipods fish and ‘sudid’ fish and one squid beak. Moreover, the tissues of the stomach and abdominal wall are distensible enough to accommodate very large meals which may consist largely or entirely of one prey species.

In two Melanocetus johnsoni were found a single large black lantern-fish each twice to three times the length of the swallower A Linophaiyne quinaueramosus contained a large deep-sea eel a hatchet-fish two Cyclothone and five ‘shrimps’ After presenting these and other findings, Bertelsen remarks that though female angler- fishes have diverse kinds of angling devices, there is no evidence of differences in their choice of food. Indeed, as he and Piestch have shown, the liminous complex and dermal decoration of the bait differs from species to species.

Acting on conjectures that different kinds of esca might attract different kinds of prey, Pietsch analysed the stomach contents of Oneirodes species but found no signs of prey selection. Again, it seems clear that there is no choice but the widest possible range of diet in food-poor surroundings.

The exponential decline in the food available for production between succeeding links in a food chain is believed to limit the number of possible links to five or six. If this is so, very productive environments should support more links than do impoverished ones.

There are certainly five or six links in the euphotic zone of oceanic regions, but how many are there at bathypelagic levels? If copepods feeding mainly on faecal material, detritus and micro-organisms, correspond to herbivorous epipelagic species, than one might say that first level carnivores, in that they may well be largely dependent on the ‘herbivores’, are other predatory copepods, amphipods, euphausiids and arrow-worms.

Second- order carnivores would be prawns and fishes, but are the fishes, for instance, largely sustained by first-order carnivores? We have seen that they feed on each other and at all levels of the food chain; Trophic rules seem to be overruled in bathypelagic deserts.

The predatory design of bathypelagic fishes is very impressive, but how efficient are they as predators? When on RRS Discovery I unwittingly staged a striking demonstration of the predatory powers of female angler fish which are described elsewhere.

Ceratioid angler-fishes may be kept alive for sometime after capture, particularly in cool surroundings. I wanted to test the escape responses of ceratioids, for my guess is that the crossed pair of Mauthnerian neurones and fibres, a system essential in the quick escape movements of fishes, might be better developed in the males.

‘When I gently held the tail-fin of a male ceratioid in a pair of forceps he wriggled hard as though to escape. Using thumb and forefinger, I did the same to several females but they promptly turned round and tried to bite me. In discussion, Mr Peter David said that the quick turn-round of female ceratioids reminded him of Mr Nubar Gulbenkian’s taxicab. Mr Gulbenkian once marked that this taxi could turn round on six pence, “whatever that may be”. I thus came to call the negative escape response of female deep-sea angler-fishes the “Gulbenkian reflex”.

Indeed, the females of most species have relatively short luminous lures, a globular deep-bodied form and fins nicely related to the centre of gravity, features that are admirably fitted to a quick roundding on their prey. After all, a female cannot always expect prey to swim both towards her flashing lure and straight for her large and gently smiling jaws. Prey may approach from behind, where, incidentally the body is studded with free-ending lateral line organs, able to detect near by disturances in the water.

Studies of the organization of bathypelagic fishes and the ‘Gulbenkian reflex’ of female ceratioids led me to Sir D Arcy Thompson’s views on deep-sea fishes. In this book Growth and Form, Sir D’ Arcy remarks, the great depths of the sea differ from habitations of the living, not least in their eternal quietude.

The fishes which dwell therein are quaint and strange; their large head, prodigious jaws and long tails and tentacles are, as it were, gross exaggerations of the common and conventional forms. We look in vain for any purposeful cause or physiological explanation of these enormities; and are left under a vague impression that life has been going on in the security of all but perfect equilibrium, and that the resulting forms, liberated from many ordinary constraints, have grown with unusual freedom.’

Contrary to the thought behind Sir D’ Arcy’s last sentence, the more one looks the more one comes to appreciate how cunningly and closely angler-fishes are designed for life in their deep and difficult environment. It is true that bathy pelagic animals do not face the constraints imposed by vertical migrations.

They live in virtually changeless physical surroundings, and they are thus freed from developing physiological and biochemical means to cope with fluctuating conditions. But they are not liberated from one ordinary constraint, the continual need to make a living in impoverished circumstances. Indeed, their life in the most deserted part of the ocean as will now be argued is ingrained in their whole organization.