2.4 STRESSES ON OYSTER POPULATIONS: 

 NATURAL AND CULTURAL 



Natural Stress 



Much oyster literature concerns the 

 variety of microscopic organisms that 

 cause oyster mortalities. These pathogens 

 have caused massive oyster die-offs in 

 local areas and sometimes in broad re- 

 gions, e.g., the infamous outbreak of the 

 bacterium "MSX" ( f^inchinia nelsoni ) in New 

 Jersey, Delaware, and Vircinia during the 

 late 1950's and early 1960's. "Disease 

 organisms" is an anthropomorphic and a 

 pejorative phrase typically applied to 

 organisms that appear to be harmful to 

 animals and plants valued by man, and it 

 often stands in the way of an objective 

 functional approach to ecosystems. Oys- 

 ters are ancient mollusks that undoubtedly 

 have been competitive with, preyed upon, 

 and parasitized by many species. Their 

 survival to the present attests to the 

 fact that they have maintained a comple- 

 mentary functional role within the estua- 

 rine ecosystem. As such, they have been 

 subject to various ecosystem feedback reg- 

 ulators, including so-called "disease 

 organisms" that maintain an oscillating 

 stability in oyster population density. In 

 the context of the present discussion, 

 protozoan, fungal, bacterial, and other 

 oyster parasites, comniensals, and preda- 

 tors, such as oyster drills and oyster 

 catchers, are considered oyster associ- 

 ates, or ecosystem regulators. These 

 function under natural conditions to con- 

 trol excessive populations and regulate 

 the distribution and density of oyster 

 reefs themselves. It appears, however, 

 that man-induced stresses on oysters miay 

 sometimes shift the balance in favor of 

 the oyster regulator by creating subtle 

 changes of temperature, oxygen, salinity, 

 or pollution levels (Galtsoff 1964). 



We are unaware of any studies at- 

 tempting to distinguish between oyster 

 vulnerability to "disease" in subtidal vs. 

 intertidal habitats. Since oyster disease 

 is often density-dependent, extremely 

 dense intertidal reef populations may be 

 more vulnerable than sparse communities. 

 Reefs, however, persist in some areas for 

 long periods (see Chapter 4), and oysters 

 apparently have adapted better to the 

 stress of intertidal existence than have 

 the pathogens. 



28 



Oyster-associated organisms, includ- 

 ing common oyster commensals, are dis- 

 cussed in Section 3.2. Usually, the oc- 

 currence and density of commiensals are 

 less in intertidal reef oyster populations 

 than in subtidal oysters. Common commen- 

 sals include the boring sponge ( Cliona ce- 

 lata ), the polychaete mud worm (PoTydora 

 websteri ) , and the pea crab (Pinnotheres 

 ostreum) . None of these organisms actual- 

 ly kills the oyster, but they do produce 

 a stress. The boring sponge and the mud 

 worm induce additional shell deposition; 

 the pea crab lives within the oyster's 

 mantle cavity and steals food and mucous 

 from the gills, and perhaps even feeds on 

 developing gametes (Galtsoff 1964). 



Other natural stresses include low 

 oxygen concentration, high temperature, 

 excessive turbidity (sedimentation), ei- 

 ther overabundance or shortage of appro- 

 priate food, crowding, and high wave ener- 

 gy or strong water currents. Oysters Are 

 remarkably tolerant of all these condi- 

 tions, however. For example, a subtidal 

 oyster population in the James River, 

 Virginia, was relatively resistant to a 

 severe freshet (flooding) associated with 

 the 1972 tropical storm Agnes (Larsen 

 1974). They close tightly and respire 

 anaerobical ly when exposed to the air or 

 during low oxygen conditions (Hochacka 

 and Mustafa 1972). Temperatures up to 

 40° C or more can be tolerated for short 

 periods (see Section 3.1). Reef growth 

 can accommodate slow, steady sedimentation 

 but not sudden pulses of sediment. Oysters 

 can withstand crowding, and as shown in 

 Chapter 3, population density is important 

 to their intertidal survival. Typically, 

 intertidal reef oysters are not robust and 

 fat, and do not contain high levels of 

 glycogen. The natural stresses of their 

 environment are reflected by the long nar- 

 row valves and watery tissue texture char- 

 acteristic of "coon" oysters. 



Man-related stress 



Man-induced perturbations on oysters 

 can conveniently be divided into eight 

 classes (Table 1) as follows: (1) physi- 

 cal disturbances, especially sedimentation 

 resulting from dredging and excessive boat 

 traffic; (2) salinity changes due to 

 freshwater diversion or local hydrologic 

 alteration; (3) eutrophication or over- 

 enrichment of water from organic matter, 



