COMMUNITY ORGANIZATION: STRATIFICATION 



457 



gregarious, indirectly herbivorous, and 

 diurnal when mature. This profound change 

 in the ecology of the adult population is 

 attributable (Keeble, 1910) to their in- 

 corporation with symbiotic organisms (zoo- 

 chlorellae). These turbellarian worms are 

 sand inhabitants and migrate landward and 

 seaward with spring and neap tides, as well 

 as vertically over the day-night period. 

 Horizontally, roscoffensis occupies a narrow 

 zone limited towards the sea by maximal 

 light intensity commensurate with sufficient 

 water to partially cover the worms, and 

 limited towards the land by the high-water 

 mark of neap tides. 



Further seaward, in shallow water not 

 usually withdrawn, even at low tide, a 

 second species, Convoluta paradoxa, lives 

 among the rock-attached seaweeds. 



Another special littoral habitat is aflForded 

 by wharf piles. From a broad view the 

 vertical, relatively dense, piling surface is 

 in the category of the rocky littoral, and 

 is produced by man and set in place some- 

 what as coral rock is biologically produced 

 by polyps, in distinction to the natural rock 

 shore. Wharf piles bear a well-known in- 

 vertebrate fauna. Among the more recent 

 treatment, that of McDougall (1943) sum- 

 marizes the vertical zonation on piles off 

 the North Carolina coast. 



McDougall thinks that the vertical strati- 

 fication is determined by three general proc- 

 esses: (1) Larvae settle abundantly at a 

 particular pile level and grow to maturity 

 (the cirripedians Balanus amphitrite and 

 Chthamalus fragilis, and the mussel Modio- 

 lus demissus); or (2) larvae settle at all 

 pile levels, but adverse conditions destroy 

 them above and below certain limits 

 (hydroids and the brvozoan Bugtila neri- 

 tina); while others (3) are motile adult 

 forms and tend to move into and re- 

 main at particular pile levels (such 

 as the echinoderm Arbacia and the oyster- 

 drill Urosalpinx). Light intensity and 

 gravity responses appear to control the level 

 at which larvae tend to settle, while the 

 stratified food-animals influence other or- 

 ganisms, such as the oyster-drill. 



It is unnecessary to discuss further the 

 numerous parallels existing between the 

 rock littoral, the coral reef, and the wharf 

 pile habitat. The essential point would seem 

 to be that the initial environment is 

 physically stratified, and this condition im- 



poses a parallel stratification on the inhabit- 

 ing organisms. 



The second littoral type, the sandy shore, 

 when characteristically developed, is in- 

 habited by a wholly different assemblage of 

 organisms. The more labile substrate oflFers 

 little attachment surface; hence seaweeds 

 are typically sparse or absent. This reduces 

 the species populations of encrusting 

 animals, such as sponges, hydroids, and 

 bryozoans. The sandy littoral, lacking a 

 resident photosynthetic industry, has no 

 normal base in its food web, and its 

 residents are primarily scavengers and 

 carnivores, with the exception of species 

 capable of feeding upon the littoral phyto- 

 plankton. 



Sand-dwelling marine animals are in 

 great part burrowers. The sandy littoral has 

 been studied by Pearse, Humm, and 

 Wharton (1942) at Beaufort, North Caro- 

 lina. They report in detail on this portion 

 of the Atlantic littoral, and their description 

 applies in general to similar shores else- 

 where; their bibliography may be consulted 

 for this literature. 



At this point it should be remembered 

 that there are all degrees of mixing of sand 

 with mud, producing a modified littoral 

 habitat. This sandy-mud substrate is an ex- 

 tensive habitat and forms an intergrade be- 

 tween that of the sand shore and the mud 

 shore. Probably the most typical among 

 many inhabitants are the bivalves, such as 

 the cockles (Cardiiim), the clams (Venus, 

 Tellina, and Donax), and the razor clams 

 (Solen) . Another group, the worms, are 

 nearly, if not quite, as typical of the sand 

 floor, including the suspension-feeders 

 (terebellids in general, Amphitrite), car- 

 nivores, such as sand- worms (Nereis), and 

 sand-swallowers (Arenicola) . Here, too, are 

 found the elongated holothurian, St/napta, 

 burrowing sea urchins (Echinocardium) , 

 shrimps (Crangon), and flatfishes of many 

 kinds, all adjusted to an existence on or 

 beneath the sand or sandy-mud. 



Species populations inhabiting this sub- 

 strate must adjust to both sand and mud 

 particles. This is evident in respiratory 

 adaptations, but is also discernible in 

 locomotor and food-obtaining behavior. 



The faima of the sandy-mud is more ex- 

 tensive than that of either the pure sand 

 littoral or the mud littoral. This suggests the 

 principle that there are more species, and 



