LOUGH: LARVAL DYNAMICS OF DUNGENESS CRAB 



specific kind of food organism encountered may 

 not be as important as its size. The size of food 

 organisms available for each larval stage must be 

 within a restricted range in order for a larva to 

 successfully capture and ingest. The progression 

 of larval size with development would indicate 

 that the different larval stages can utilize in- 

 creasingly greater sizes of food organisms. Reed 

 (1969) found in laboratory culture that the larvae 

 of C magister survived well feeding on Artemia 

 salina (0.475-0.752 mm length) and Balanus 

 gladula nauplii (0.370-0.420 mm length), but 

 would only survive for a limited period on smaller 

 size veliger larvae of Mytilus edulis (0.100-0.300 

 mm length?). He also reported that unfed C. 

 magister zoea larvae would only survive for 14 

 days. This implies that under natural conditions 

 larvae will not survive if a suitable food organism 

 is delayed in its appearance by more than 2 wk, 

 and that certain kinds of food organisms selected 

 by the larvae are nutritionally inadequate for 

 their long-term metabolic needs. 



Chamberlain (1961, 1962) reared the larvae of 

 two xanthid crabs, Neopanope texana sayi and 

 Rhithropanopeus harrisii, on a variety of foods 

 and found that development was retarded when 

 larvae were fed on a mixture of nauplii and algae. 

 Larvae fed algae alone would not molt and only 

 lived 6-10 days in culture. Algae appeared to be 

 nutritionally inadequate for successful develop- 

 ment and restricts the intake of more suitable 

 food by indiscriminate larval feeding. Costlow 

 and Sastry (1966) suggested that high mortality 

 of Callinectes sapidus larvae at the time of the 

 third zoeal stage in nutritionally inadequate cul- 

 ture may be due to the initial availability of a 

 large pool of free amino acids within the eggs 

 through the first and second zoeal stages. They 

 also pointed out that the variability in tolerance 

 to suboptimal conditions may be related to the 

 size of such a free amino acid pool. 



Although the gut-fullness analysis in the pre- 

 sent study did not provide insight into the differ- 

 ence in larval abundance between the 2 yr, it did 

 suggest the existence of an optimum zone for 

 adequate feeding between 3 and 20 miles offshore 

 where suitable kinds and densities of food or- 

 ganisms occur. Zooplankton volumes along the 

 Washington coast decrease to a minimum level 

 during the winter and increase to maximum 

 levels during the spring (Frolander 1962). During 

 the winter, the volume of zooplankton and abun- 

 dance of copepods were greater inshore than 



offshore as a consequence of the onshore trans- 

 port of surface waters (Frolander 1962; Anderson 

 1964; Peterson 1972). Anomalous weather condi- 

 tions such as occurred during the winter of 1971 

 may have been ultimately responsible for altera- 

 tions in the usual types and availability of food 

 organisms encountered during the first few weeks 

 of larval feeding. 



Hypothesis 3: Predators and Competitors 



The importance of the combined or separate ef- 

 fects of predation and competition on larval popu- 

 lations is difficult to assess. Predation has gener- 

 ally been regarded as the major cause of larval 

 mortality (Thorson 1946, 1950). Lebour (1919a, b, 

 1920, 1921, 1922, 1923) observed many species of 

 young fish and medusae to prey upon crab larvae 

 as well as most other small organisms in the 

 plankton. Cannibalism is well known in labora- 

 tory culture. Knudsen (1960) observed in the 

 laboratory that xanthid first stage zoea were 

 eaten by older zoea and megalops as well as by 

 copepods. Other predators known to feed on 

 marine larvae, such as ctenophores, chaeto- 

 gnaths, euphausiids, and shrimps, appear sea- 

 sonally in high densities along the North Pacific 

 coast. Their effect on larval populations has not 

 been fully ascertained. Peterson (1972) compared 

 the ratios of copepod nauplii to total copepods off 

 the Washington coast and found that more naup- 

 lii were hatched inshore than offshore throughout 

 the year, but fewer developed to adults suggest- 

 ing greater predation in the inshore area. Preda- 

 tion was reduced during the winter compared to 

 other seasons within the inshore area. These 

 findings might similarly apply to relative preda- 

 tion rates on C. magister larvae along the North 

 Pacific coast. 



Factors in the environment such as abnormally 

 cold temperatures or lack of food that extend the 

 pelagic life of the larval phase have been consid- 

 ered detrimental due to predation. It has been 

 assumed that the longer the larvae remain in the 

 plankton the more they will be preyed upon, al- 

 though predation pressure upon their recruit- 

 ment to the benthic habitat may be just as great, 

 or greater (Thorson 1966). Larvae genetically 

 feeble or weakened by some environmental fac- 

 tors may be more subject to predation so that 

 under usual circumstances, the importance of 

 predation may be secondary in mortality proces- 

 ses. The effect of predation on larval populations 



369 



