lowest food level compared with the old culture. 

 The interaction of food level and food type was 

 probably in large part due to food type b (/. gal- 

 bana and T. pseudonana), which gave the best 

 grow^th at food level 1 but poor growth at food level 

 3. 



The effect of the food level was to produce higher 

 growth rates at the lower food concentrations. 

 This occurred for all fivefood types (Figure 1). 



Discussion 



Contrary to published reports on various 

 bivalve larvae (Guillard 1959; Bayne 1965; Wil- 

 son 1978), we have observed poor growth of M. 

 edulis larvae when fed only P. lutherii. This was 

 true at all three food levels tested and whether the 

 P. lutherii culture was young or old. There was no 

 apparent inhibitory effect by the P. lutherii on 

 larval growth when fed in combination with the 

 other two algal species. It would appear that the 

 suppression of growth of the larvae when fed only 

 P. lutherii was the result of a dietary deficiency. If 

 it were due to toxins in the algal cells, one would 

 expect to see a greater suppression of the growth 

 rate in the larvae at food level 3 when P. lutherii 

 was combined with the other algal species. 



If the inhibitory effect of P. lutherii were pri- 

 marily due to the accumulation of metabolites in 

 the medium, there should be a more consistent 

 difference between the P. lutherii cultures of dif- 

 ferent age. In fact, there was only a small differ- 

 ence at food level 1. This may indicate that there 

 is some effect of metabolites which were in low 

 enough concentration in the young culture to be 

 diluted at food level 1 but not at the other food 

 levels. Nevertheless, it appears that the main ef- 

 fect of P. lutherii is or is equivalent to a dietary 

 deficiency. This could be due to the biochemical 

 compostion of the algal cells such that they are not 

 digested, lack of some essential nutrient, or are 

 not even ingested. The cells are not much bigger 

 thanl.galbana, especially when fast growing, and 

 there was no evidence of clumping of the cells into 

 large aggregates. 



There is some evidence in the data presented by 

 Davis and Guillard (1958) and Bayne (1965) of a 

 suppression of larval growth at high concentration 

 ofP. lutherii. But to our knowledge there are no 

 reports of suppression of growth in bivalve larvae 

 at lower concentration of P. lutherii. This algae 

 has been reported as producing substances toxic to 

 four species of prosobranch larvae (Fretter and 



Montgomery 1968). Apparently, a toxic substance 

 is emitted by the algae, which accumulates in the 

 algal culture. 



The results of the different food levels are not 

 new (Davis and Guillard 1958; Bayne 1965; 

 Rhodes and Landers 1973). The purpose of using 

 different food levels in this experiment was to look 

 for interaction with food type. 



At this point we can only speculate as to the 

 reasons for the lack of growth of larvae fed P. 

 lutherii. We would not want to generalize and say 

 that all P. lutherii could produce the same results. 

 Obviously others have obtained good results with 

 their cultures. (All our algal cultures are grown in 

 the f/2 medium of Guillard (McLachlan 1973), 

 which is commonly used in growing algae for 

 shellfish culture.) One explanation would be that 

 we have inadvertently developed through genetic 

 change a strain of P. lutherii which is of inferior 

 quality. Fretter and Montgomery (1968) have 

 suggested that bacteria can metabolize the toxic 

 substance produced by P. lutherii and render the 

 algae culture harmless to bivalve larvae. Perhaps 

 the absence of bacteria in our P. lutherii cultures, 

 or at least the appropriate bacteria, would explain 

 the discrepancy between our results and others. 

 Unfortunately, we did not check the algal cultures 

 for the presence of bacteria. 



The importance of our observations with P. 

 lutherii need to be assessed by other workers. The 

 culture conditions of algae will vary from lab to lab 

 and could easily have an influence on the growth of 

 bivalve larvae. 



Literature Cited 



B.^YNE, B. L. 



1965. Growth and the delay of metamorphosis of the lar- 

 vae of Mytilus eduhs (L.). Ophelia 2:1-47. 



Davis, H. C, and r. r. guillard 



1958. Relative value of ten genera of micro-organisms as 

 food for oyster and clam larvae. U.S. Fish. Wildl. Serv., 

 Fish. Bull. 58:293-304. 



Fretter. v., and M. C. Mo.ntgo.mery. 



1968. The treatment of food by prosobranch veligers. J. 

 Mar. Biol. Assoc. U.K. 48:499-520. 



Guillard. R. R. L. 



1959. Further evidence of the destruction of bivalve larvae 

 by bacteria. Biol. Bull. (Woods Hole) 117:258-266. 



McLachlan, J. 



1973. Growth media — marine. In J. R. .Stein leditor), 

 Handbook of phycological methods, culture methods and 

 grovrth measurements, p. 24-51. Camb. Univ. Press, N.Y. 



Rhodes, E. W., and W. S. Landers. 



1973, Growth of oyster larvae, Crassostrea virginica, of 

 various sizes in different concentrations of the chry- 



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