APPENDIX: OYSTER BIOENERGETICS 



Oysters, like all heterotrophic orga- 

 nisms, use energy in proportion to their 

 growth rate, their reproductive invest- 

 ment, and their efforts to obtain food, 

 remove waste, defend themselves against 

 parasites and predators, and maintain a 

 favorable osmotic balance. This section 

 discusses the rates and partitioning of 

 energy expenditures for individual inter- 

 tidal oysters and the oyster population as 

 a whole. The energy requirements of the 

 entire reef, a prerequisite for under- 

 standing the dynamics of the oyster reef 

 community, are estimated in Chapter 3. 



Ecologists and environmental managers 

 are beginning to realize the value of 

 information regarding the rates and path- 

 ways of energy flow in communities of 

 organisms and entire ecosystems. Energy 

 units are interconvertible, and, there- 

 fore, the energy "cost" of totally dif- 

 ferent processes is the common denominator 

 by which these processes can be compared 

 objectively and ranked in terms of their 

 ecological importance. The first ecologist 

 formally to apply this principle to the 

 study of ecosystems was Raymond Lindemann, 

 who in 1942 published a landmark treatise 

 on the partitioning of energy flow through 

 an ecosystem (Lindeman 1942). Since then, 

 it has become common practice to include 

 energetics in ecological research. Good 

 review sources on bioenergetics include 

 Phillipson (1966) and Wiegert (1968). 



The extant oyster literature includes 

 several compilations of energy budgets for 

 various species of oysters in different 

 areas. Extrapolations from some of these 

 studies are necessary to fill in energy 

 budget data gaps for intertidal oysters in 

 the study area. 



The calculation of an energy budget 

 for a population of organisms involves the 

 use of one or another equation of the 

 general form: 



P(net) = I-E-R-W 



(1) 



''(net) ~ "^^ secondary production 

 rate, or growth of the pop- 

 ulation in a given time 

 (including somatic growth, 

 gamete production, and mor- 

 tality losses) 

 I = ingestion rate 

 E = egestion and excretion rates 

 R = respiration or metabolic rate 

 W = the rate at which external work 

 is performed by the organisms 



The term W is usually ignored (Wie- 

 gert 1968), but for some animals (such as 

 mound-building termites and reef oysters), 

 work may be substantial because these or- 

 ganisms build vertical structures against 

 gravity. 



In mature populations, the production 

 equation may attain a steady state, in 

 which no net growth can be measured and 

 annual energy inputs equal losses. Oyster 

 reefs appear to attain this steady-state 

 maturity when they achieve a critical ver- 

 tical elevation relative to tidal stage or 

 when oyster growth is equal to maintenance 

 costs. 



Before a rough energy budget for in- 

 tertidal oysters is presented, the prob- 

 lems involved in compiling such a budget 

 must be discussed. The terms in the 

 energy budget Equation (1) are measured 

 for an oyster population in the following 

 ways. Net production P(net) ""s sometimes 

 calculated by measuring the increase in 

 size of experimental animals over a unit 

 of time. This technique requires measur- 

 ing the individual oysters. Another tech- 

 nique calculates time elapsed between age 

 classes in the size-frequency distribution 

 of a natural population. The latter tech- 

 nique is tedious since age classes quickly 

 become indistinct because of continuous 

 waves of spawning over the warm season. 



Total production P(nross) includes 

 gamete production (and release) as well as 

 mortality and predation (and harvesting) 

 between sampling periods. The growth rate 



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