456 



THE COMMUNITY 



live Collembola among insects, and a 

 variety of crustaceans {Ligia, Orchestia, 

 Gamniarus) . Beneath such stones, lower on 

 the shore where the tides always flood 

 them, are nemertean worms (Linens) and 

 annehds [Eulalia, Cirratulus). Still nearer 

 to low-water mark are numbers of certain 

 species of Nereis, shore crabs (Carcinus) , 

 hermit crabs, starfishes {Asterias), sea 

 urchins (Echinus), and a variety of shore 

 fishes. 



Holes and cracks in the rocks are oc- 

 cupied by various anneUd worms, crus- 

 taceans, small sea cucumbers (Cucumaria) 

 and rock-boring bivalves (Pholas). 



The fourth rocky shore habitat, the rock 

 pools, is distinctive. Here the enclosed 

 organisms escape direct wave action and 

 are exposed to high water temperatures and 

 increased salinity in summer. The walls of 

 such pools are covered by seaweeds, 

 sponges, hydroid coelenterates, bryozoans, 

 tunicates, and sea anemones and shelter a 

 variety of active benthic animals, such as 

 sea slugs (Aeolis, Doris) and prawns (Hip- 

 polyte). 



The reefs built by corals flourish only 

 in relatively shallow waters of tropical seas, 

 apparently usually originating on rock coast, 

 and the resulting reef habitat is more com- 

 parable to the rocky littoral than to the 

 other types of seashore. Such coral reefs 

 are developed in water seldom deeper than 

 60 meters, and their building activities 

 progress where surface sea temperatures do 

 not fall much below 20.5° C. This restricts 

 their distribution to a broad band between 

 30 degrees north and 30 degrees south 

 latitudes (Darwin, 1842). Within this 

 region barrier reefs, fringing reefs, and 

 atolls form through the accumulation of the 

 calcareous thecae of the cooperating polyps, 

 are buflFeted by waves and tidal action, and 

 support one of the most distinctive littoral 

 assemblages of organisms. 



The chief structural difference lies in the 

 substrate, which in the coral habitat is con- 

 structed by the dominant species of plants 

 and animals, especially the calcareous algae 

 and the corals, while the true rocky littoral 

 erodes away instead of growing upward 

 against the surf. 



Given a sufficiently high water tem- 

 perature, corals and other lime-secreting 

 organisms manufacture limestone at variable 

 rates, depending upon such influences as 



the depth of their submergence, tne 

 amount of water motion, the presence or 

 absence of sediment, and lack of dilution 

 of the sea water. Coral reef habitats 

 also tend to be both physically and bio- 

 logically stratified vertically and horizon- 

 tally. Lastly, such a habitat contains organ- 

 isms that Uve on the exposed reef, under 

 loosened coralHne boulders, in holes or 

 cracks in the reef, and in coral reef pools. 



The coral reef and the rocky Httoral offer 

 many striking parallels. Whether acting 

 upon organismally or geologically produced 

 substrata, the primary environmental gra- 

 dient sets up biological stratification. On 

 rocky shores seaweeds attach to rocks and 

 in turn form attachment surfaces for numer- 

 ous animals; on coral reefs, especially in the 

 Indian Ocean, calcareous seaweeds or nulH- 

 pores attach to the coral rock. Paradoxically, 

 on rocky shores seaweeds have relatively 

 Uttle effect other than being attached to the 

 rocks and forming attachment surfaces for 

 animals, while on coral reefs calcareous 

 algae aid in the consoHdation of the reef- 

 building process. 



The coral reef habitat has been studied 

 both above and below the sea surface, and 

 descriptions of its exotic fauna can be ex- 

 amined in Davis (1928), Beebe (1928), 

 Hesse, Allee, and Schmidt (1937, pp. 207- 

 221), and in the reports of the British Mu- 

 seum's Great Barrier Reef Expedition 

 (1930), andYonge (1930). 



The coral polyps are suspension feeders, 

 drawing in microplankters by their corona 

 of tentacles, and many species have their 

 gastrovascular cavities nearly obUterated 

 by symbiotic organisms (zoochlorellae and 

 zooxanthellae), which carry on photosyn- 

 thesis eventually to the mutual benefit of 

 both polyps and algae. These symbiont- 

 bearing polyps are sufficiently near the 

 sea surface to permit light utilization 

 by their symbionts, so that in a very 

 real sense the physical and biological 

 gradients overlap and are interdependent. 



There is an almost world-wide littoral 

 intergrade of rocks and sand, the biota of 

 which includes constituents of both the 

 rocky and the sandy littoral habitat. On 

 the northern French beaches of Brittany 

 and Normandy Hve multitudes of the platy- 

 helminth worm (Convoluta roscoffensis) . 

 The worms, although solitary, carnivorous, 

 and nocturnal in their youth, become 



