CLUTTER and THEILACKER: PELAGIC MYSID SHRIMP 



for each of three mortality rates, are shown in 

 Table 8. The percentages for females and males 

 combined are not quite the same as the means 

 of the separate pei-centages for females and for 

 males. At the minimum death rate 55 S^ of the 

 energy loss would pass through the female half 

 of the population (58 ^'r if fertile eggs are in- 

 cluded). At the median death rate 52 Sr would 

 pass through the females, and at the maximum 

 death rate, 50 %. 



Table 8. — Relative amount (%) of energy lost by 

 Metamysidopsis populations in respiration, production 

 of infertile eggs, molting, and mortality; at minimum, 

 median and maximum mortality rates. 



Sex 



Death 

 rate 



Respira- 

 tion 



Infertile 

 eggs 



Molting 



Mortality 



If we assume that all the mortality is yield 

 to predators (Odum and Smalley, 1959; Engel- 

 mann, 1961), our mortality fractions are an 

 estimate of net ecological efficiency (energy 

 yield/energy assimilated). Apparently some 

 Crustacea regularly die from natural causes 

 other than mortality (e.g. Daphnia, Slobodkin, 

 1959). Many mysids of all ages died in our 

 laboratory cultures, but we do not attribute this 

 to senescence. In the field and in the laboratory 

 we observed Metamysidopsis much older than 

 the oldest animals that are involved significantly 

 in our energy calculations. Our best estimate 

 of the net ecological efficiency of the mysid pop- 

 ulation, for transfer of energy to a higher troph- 

 ic level, such as fishes, is about 32 'Jr. The net 

 efficiency of transfer to all trophic levels is 

 1 — respiration fraction = 43 S^- 



ASSIMILATION AND 

 GROSS ECOLOGICAL EFFICIENCY 



Assimilation Efficiency 



Gross ecological efficiency (energy yield/en- 



ergy ingested) is the product of net ecological 

 efficiency (energy yield/energy assimilated) X 

 assimilation efficiency (energy assimilated/en- 

 ergy ingested). Therefore, an estimate of as- 

 similation efficiency is required to estimate gross 

 ecological efficiency for the mysid population. 



We attempted to estimate the assimilation ef- 

 ficiency of Metamysidopsis directly by a carbon- 

 14 method described by Lasker (1960). This 

 failed because .the mysids did not filter sufficient 

 amounts of radioactive phytoplankton. An ex- 

 periment with another member of the family 

 Mysidae, taken from the same area, was suc- 

 cessful. This gave an estimate of 90 Tr assim- 

 ilation efficiency. 



Lasker (1966) obtained a similar high value 

 (84 Cf ) for the morphologically similar Eiiphau- 

 siapacifica; and Marshall and Orr (1955) found 

 values greater than 90 % for the copepod Cal- 

 ami^ finmarchicm. In his detailed reviews of 

 assimilation in zooplankton, Conover (1964, 

 1966) suggests that these values probably are 

 too high. The very large number of observa- 

 tions, many of them his own, that are cited by 

 Conover seem to be evidence that, although var- 

 iable, the mean assimilation efficiency for crus- 

 tacean zooplankton is at least 60 % and perhaps 

 greater. 



Gross Ecological Efficiency 



From the information presently available we 

 consider that the assimilation efficiency of the 

 mysids is between 60 9f and 90 ^r. Our best 

 estimate of net ecological efficiency (yield/as- 

 similated) is 32 Sf. Therefore, the minimum 

 estimate of gross ecological efficiency (yield/in- 

 gested) is 19 9r and the maximum estimate is 

 29 ^c. 



These estimates are well within the broad 

 range of available estimates of gross ecological 

 efficiency (see reviews by Patten, 1959; Slobod- 

 kin, 1961; Phillipson, 1966; Reeve, 1966), and 

 within the range of 8 '^'r to 30 'c that Engel- 

 mann (1961) considers to be acceptable. They 

 are about 2 to 3 times as high as the median 

 value of 10 % that is suggested by Slobodkin 

 (1961, 1962) , but lower than the values of 30 % 

 to 50 % suggested for marine zooplankton by 



113 



