THE SUBSTRATUM 



159 



include many insects such as water striders, 

 whirligig beetles, springtails, and a variety 

 of spiders, all of which run about on the 

 surface without breaking through to the 

 water below. Many of the organisms of this 

 supraneuston, those above the surface film, 

 are also at home in the water itself. The 

 surface-dwelling habit has persisted for a 

 long time, long enough, apparently, for the 

 gyrinid beetles to have evolved divided 

 eyes, with the upper part suitable for vision 

 in air and the lower part adapted to vision 

 under water. One of the few marine insects 

 is a water strider that runs about on the 

 surface film. 



Grasshoppers and other heavy-bodied ter- 

 restrial insects may be partially or wholly 

 supported by the surface film for a time. 

 The presence of such land-dwelling insects, 

 helpless on the surface of the water, sup- 

 plies a source of food for predaceous aqua- 

 tic animals and may be related to the life 

 history of such parasites as Gordhis. 



A glance at the lower side of the surface 

 film in a well-stocked aquarium gives some 

 appreciation of the possible richness of life 

 that may hang there. Some soldier fly lar- 

 vae (Stratiomyidae) and mosquito larvae 

 and pupae, among others, are suspended 

 with their spiracles exposed to the outer air. 

 Even fairly large pulmonate snails may also 

 cling so while breathing and may anchor 

 the upper end of a mucus thread to the 

 surface film; by this thread they make pe- 

 riodic trips up and down in the water. The 

 common hvdra can move along the under 

 side of the film or hang suspended from it, 

 and planarian worms glide along its under 

 surface. 



In addition to pulmonate snails, animals 

 in varied assortment make periodic excur- 

 sions to the surface film to breathe. They 

 may attach momentarily while replenishing 

 their supply of stored oxygen, or the surface 

 film may be significant only as a phase 

 boundary between water and the air above. 

 Large animals, frogs, crocodiles, and hip- 

 popotami float just below the surface with 

 only their protuberant eyes and nostrils ex- 

 posed to the air. These animals obviously 

 are not members of the neuston, even 

 though they occur in ludicrously close 

 juxtaposition. Some ecological associations 

 may indeed seem ludicrous; different mech- 

 anisms mav keep different organisms in the 

 same general region, and many different 

 ecological relations may be illustrated by 



animals that live alongside each other. A 

 community of interests may usually be dis- 

 cerned even among widely different animals 

 thus brought into the same habitat. 



WATER-LITHOSPHERE INTERPHASE 



It is common usage to call the water-air 

 interphase the top or surface of the water 

 and to regard the zone of contact between 

 water and the underlying lithosphere as its 

 bottom. The water itself is a substratum for 

 many pelagic organisms that rarely make 

 physical contact with either surface or bot- 

 tom phase boundary and for bottom-dwell- 

 ing forms that may spend much of their 

 life swimming or floating freely in the 

 water. Honoring present interests, we pass 

 directly from top to bottom to discuss the 

 physical relations of bottom-dwelling forms. 

 These compose the benthos of the sea and 

 the pedon of fresh waters. 



Pedonic or benthic animals are closely 

 related to the physical character of the bot- 

 tom. They are roughly divided into (a) 

 those that live much of their lives in the 

 water and descend to the bottom for breed- 

 ing, feeding, or resting, as contrasted (b) 

 with animals that spend most of their lives 

 on the bottom; and finally, (c) the bur- 

 rowers. These categories are not mutually 

 exclusive. 



The solid bottom characteristic of rockv 

 shores makes one extreme type contrasting 

 with the soft mud or sand that lies at or 

 near the other extreme of physical consist- 

 ency. Hard rocks provide secure places for 

 attachment and are difficult to penetrate; 

 on the other hand, sand and mud furnish 

 insecure attachment and easy burrowing. 

 Each of the many variants supports a more 

 or less characteristic animal community, the 

 composition of which depends also on many 

 other factors, such as geographical location, 

 extent of the given habitat, depth, tempera- 

 ture, salinitv and other chemical conditions, 

 including biotic pollution, turbiditv, light- 

 ing, currents, and associated organisms. In 

 final analysis, even closely knit biocoenoses 

 —the oyster-bed, for example— depend on 

 the relations between key organisms and 

 their phvsical substrate. 



It is difficult to fudge the relative impor- 

 tance of the independent conditions of exist- 

 ence in a given habitat. In a studv of the 

 factors controllino; the distribution of com- 

 munities of littoral invertebrates in shal- 

 low water near Woods Hole, Massachusetts. 



