Haskin (1971) related this phenomenon to a 

 water temperature increase during flood 

 tides over intertidal mudflats. The slack 

 water areas of eddy currents also seem to 

 favor heavier than average spatfall pat- 

 terns (Roughley 1933). Spatfall will be 

 discussed again in Chapter 4 when the dis- 

 tribution of reefs in an estuary is con- 

 sidered. 



The biological cues to oyster larval 

 settling are related to the fact that oys- 

 ter larvae are gregarious and apparently 

 respond to a waterborne pheromone or me- 

 tabolite released by oysters that have al- 

 ready metamorphosed (Hidu and Haskin 

 1971). Larvae also seem to respond posi- 

 tively to a protein on the surface of oys- 

 ter shells. This gregarious tendency is 

 important to a reef-building (colonial) 

 organism such as the oyster, which re- 

 quires settlement in proximity for suc- 

 cessful fertilization (Crisp and Meadows 

 1962, 1963). See Chapter 3 for additional 

 details of gregarious behavior. 



2.3 OYSTER FEEDING, DIGESTION, 

 AND ASSIMILATION 



The feeding organs of oysters are (1) 

 the ciliated gills that provide the water 

 currents (with the assistance of the man- 

 tle) and sort particles; (2) the palps 

 surrounding the mouth that also play a 

 role in the particle-sorting process; (3) 

 the crystalline style, a semirigid clear 

 rod composed of digestive enzymes that 

 function in the mechanical breakdown of 

 food particles; (4) the gastric shield 

 against which the style rotates to grind 

 food particles; (5) the stomach, in which 

 food and digestive enzymes are mixed; and 

 (6) the digestive diverticula surrounding 

 the stomach, a group of blind-ending tu- 

 bules with ducts leading to the stomach. 

 The latter function in intracellular di- 

 gestion. 



The feeding of all filter-feeding bi- 

 valves (including oysters) had been as- 

 sumed to be a continuous process in those 

 organisms that are always submerged. The 

 ciliary feeding currents and the produc- 

 tion and erosion (dissolution) of the re- 

 volving crystalline style have been 

 thought to occur continuously in undis- 

 turbed animals. This view was challenged 



by Morton (1973, 1977), who presented per- 

 suasive evidence that even in many sub- 

 tidal bivalves, the feeding process is 

 cyclic and discontinuous, affected by 

 tidal and seasonal factors. 



It is obvious that an intertidal oys- 

 ter cannot feed when exposed during ebb 

 tides, but an interesting aspect of Mor- 

 ton's hypothesis is that the feeding pro- 

 cess is necessarily cyclic in subtidal as 

 well as in intertidal bivalves. The impli- 

 cation of discontinuous ciliary suspension 

 feeding with a tidal rhythym is that tidal 

 and seasonal cycles were incorporated by 

 ancestral bivalves in the evolution of 

 their feeding process. 



According to Morton (1977), the feed- 

 ing of intertidal oysters occurs in three 

 cyclic stages: (1) a feeding stage during 

 which the oyster pumps water with ciliary 

 currents produced by the gills; (2) an ex- 

 tracellular digestive stage, during which 

 the crystalline style acts on ingested 

 food that has been rolled into mucous 

 strings; and (3) an intracellular diges- 

 tive stage, during which small particles 

 of food are further digested, absorbed, 

 and assimilated within the digestive 

 diverticula of the stomach. The three 

 stages are illustrated in Figure 9. Note 

 that the production of pseudofeces (con- 

 solidated particulate matter that is 

 expelled without undergoing the digestive 

 process) occurs during the active feeding 

 cycle when rejected particles accumulate 

 in the inhalent chambers. Fecal produc- 

 tion results from the extracellular diges- 

 tive and intracellular digestive process- 

 es, but feces and pseudofeces cannot be 

 released except during inundation. Morton 

 concluded that the three feeding cycles 

 occur during two alternate phases: (1) 

 food is collected, filtered, selected, and 

 passed to the stomach; (2) food collection 

 ceases and the accumulated material is 

 digested. 



The specific diet of intertidal oys- 

 ters, like that of most estuarine consum- 

 ers, is not clearly understood. The gills 

 of the adult oyster have been reported to 

 retain diatoms, dinoflagellates, and 

 graphite particles from 2\i to 3p but to 

 pass 70% to 90% of Escherichia coli and 

 80% of graphite particles from Ip to 2p . 

 On the other hand, Loosanoff and Engle 



24 



