Tidal pools 



Sea-water is often retained in depressions or 

 pools in the littoral zone and hence organisms here 

 are never completely exposed to the air. They are, 

 however, subject to high light intensity and increases 

 in the temperature between tides (Klugh 1924). 

 Tidal pools are usually rich in both plant and animal 

 life, and some species are largely restricted to them. 

 Red algae and kelps prefer the more shaded, cooler 

 pools : the green algae and some of the smaller brown 

 algae predominate in the well-insolated pools. Ani- 

 mals of both the sublittoral and littoral zones are 

 found here. 



Food chains 



The basic food elements in these rocky shore 

 communities are the free-floating plankton and de- 

 tritus in the water, the algae, and the organic debris 

 adhering to the rock surfaces (Dexter 1947). Many 

 organisms have straining mechanisms that automat- 

 ically collect food materials out of the large volume 

 of water with which they have contact. Snails crawl 

 over the rocks and seaweeds, scraping away at the 

 algae and plant tissue and eroding the rock surfaces. 

 Crabs and fish have a wealth of invertebrates upon 

 which to feed, and are generally at the top of food 

 chains along with birds that feed along the shore. 

 Marine shore animals have developed many de- 

 vices to protect them from predators (Flattely and 

 Walton 1922). Some crabs are concealed by sea- 

 weeds, hydroids, or other organisms growing on their 

 carapaces. Hermit crabs take refuge in the shells of 

 snails. Protective and warning coloration is com- 

 mon. Protective armor occurs in the form of shells, 

 chitinous exoskeletons, spicules, spines, setae, bristles, 

 and constructed tubes. Various forms of weapons 

 have evolved, some of them poisonous, as the nemato- 

 cysts of coelenterates, stylets of some gastropods, 

 spines of the king crab, and chelae of crustaceans. 

 Autotomy, or the ability to throw off an appendage 

 grasped by an enemy, is highly developed in crabs, 

 lobsters, and echinoderms. Well developed powers of 

 regeneration of lost parts occur in these forms, while 

 in worms, sponges, hydroids, and other groups, re- 

 generation of entire new bodies from small fragments 

 is often possible. 



Dominance and succession 



Dominance is exerted by those organisms that 

 compete most successfully for the space that is avail- 

 able. When they become established, they largely 

 control the presence of other species. This is true 



both with the seaweeds and with the more abundant 

 and successful animals. In describing competition 

 on wharf pilings at Beaufort, North Carolina, 

 McDougall (1943: 367) states: So many barnacle 

 larvae, for example, may settle on a small area of 

 dean surface that only a jraction of one per cent of 

 their number will ultimately find space to grozv to full 

 size. Incrusting bryosoans, such as Schizoporella, 

 spread over and smother barnacles and other low- 

 groiving species in their vicinity. Colonial hydroids, 

 sponges, and ascidians often form, densely matted 

 tangles which accumulate quantities of sediment and 

 effectively smother barnacles, oysters, hryozoans. and 

 other species less luxuriant than themselves. The 

 colonial hydroid Tubularia crocca dominates the pil- 

 ings in April and May but during June, with water 

 temperatures becoming higher, the animals die and 

 slough ofif, taking with them many associated species. 

 In the bare areas thus exposed, various other sessile 

 species become established. By the end of August, 

 two other colonial hydroids, Pennaria tiarella and 

 Eudendrium carneum, and a colonial bryozoan, 

 Bugula neritina. become dominant. In late October, 

 as water temperatures fall, these summer species die 

 and Tubularia again becomes active and reproduces 

 abundantly. Low winter temperatures temporarily 

 curtail its activities. It is apparent that true succes- 

 sion actually occurs but is passed through quickly 

 and may be obscured by seasonal changes in the spe- 

 cies composition of the biota (Redfield and Deevey 

 1952). 



PELECYPOD-ANNELID BIOME 



Habitat 



This biome develops on depositing sand and 

 mud bottoms in contrast to the biome just described 

 that occurs on eroding rocky shores. There is still a 

 good deal of wave action over sandy bottoms. Fine 

 sand particles shift about almost continuously, and 

 animals have difficulty in preventing their burrows 

 from collapsing. In general, the water over muddy 

 shores is shallower, quieter, and warmer. The mud 

 forms a soft, compact bottom, but is also easily moved 

 or shifted around by storms and wave action. Animal 

 burrows in mud are more permanent. Species tend 

 to segregate depending on the fineness of the soil 

 particles and on the amount of organic matter pres- 

 ent. Shores of high mud content may be low in oxy- 

 gen because of decaying organic matter, so animal pop- 

 ulations tend to be largest and most varied in a mix- 

 ture of mud and sand. Tidal currents are weaker, 

 and change in the level of water less pronounced on 

 sand and muddy shores than on rocky ones. 



364 Geographic distribution of communities 



