FISHERY BULLETIN: VOL. 74, NO. 2 



optimal conditions have been observed to survive 

 for considerable periods of time, apparently un- 

 able to molt successfully. These same larvae 

 eventually die, but laboratory experiments often 

 are terminated before full mortality can be ob- 

 served. Low salinity during the winter of 1971 

 may have been an important factor resulting in 

 the demise of C. magister larvae that year. Subtle 

 changes in the flux and composition of the inter- 

 nal ionic constituents can alter the molting pro- 

 cess; larvae which appear normal in early de- 

 velopment may mask deficiencies that express 

 themselves later in development. Nevertheless, 

 short-term exposure to extreme conditions may 

 be just as detrimental as slightly suboptimal con- 

 ditions over a long period of time (cf Lough and 

 Gonor 1973a, b). Although the nearshore surf 

 salinities on a monthly average are in the range 

 of tolerance by the larvae, daily measurements 

 occasionally drop below 2Q"L (Gonor et al. 1970). 

 No larvae survived below 201. salinity in Reed's 

 (1969) laboratory study. 



The effect of low salinity in conjunction with 

 wider than normal temperatures may play an 

 important role in larval survival as indicated 

 from the analyses. Low and high temperatures 

 greatly accentuated the effects of marginally di- 

 lute salinities on C. magister larval survival. But 

 again, the ecological significance of a synergistic 

 effect has not been fully established in this 

 study. More detailed, short-term studies of 

 salinity-temperature variability and larval 

 monitoring are needed in the nearshore area. 

 Sastry and McCarthy (1973) observed distinct dif- 

 ferences in temperature-salinity tolerances and 

 metabolic responses of the larvae of two species of 

 Cancer sympatrically distributed along the east 

 coast of North America. Complete development 

 for C. irroratus larvae occurred over a wide range 

 of temperatures, whereas C borealis larvae was 

 restricted to a narrow range. The metabolic- 

 temperature pattern of C. irroratus larvae indi- 

 cated a progessive narrowing in temperature sen- 

 sitivity. In contrast, the early stages of C borealis 

 initially were sensitive to warmer temperatures 

 but in the later stages sensitivity shifted to colder 

 temperatures. Hatching of the two Cancer species 

 is separated in time so that the diverse metabolic 

 responses observed are believed to be adaptations 

 by larvae of the two species to the different tem- 

 perature conditions encountered. 



The combined effects of salinity and tempera- 

 ture have been studied under controlled labora- 



tory conditions on other species of brachyuran lar- 

 vae by Costlow et al. (1960, 1962, 1966), Costlow 

 and Bookhout (1962), and Costlow (1967). Al- 

 though the adults inhabit euryhaline waters, 

 specific larval stages have been shown to require 

 restricted ranges of salinity and temperature to 

 varying degrees for complete development. In 

 many cases, both temperature and salinity and 

 the interaction of various combinations of the two 

 environmental variables were observed to affect 

 larval survival and retard development. Salinity 

 generally has an immediate effect on survival 

 while temperature appears to play a modifying 

 role within the extremes of tolerance. Most of 

 their work indicates that mortality was highest 

 during the early zoeal stages and that the 

 megalops stage was the least subject to environ- 

 mental stress, although exceptions are reported. 

 Recently, Costlow and Bookhout (1971) investi- 

 gated the effects of cyclic temperatures compared 

 to constant temperatures on the larvae of the es- 

 tuarine mud crab, Rhithropanopeus harrisii. 

 Duration of larval life and survival were about 

 the same but survival was enhanced under cyclic 

 temperatures at the higher end of the experimen- 

 tal range. Short-term fluctuations in temperature 

 or other environmental variables throughout the 

 water column have not been adequately moni- 

 tored along the North Pacific coast. Their effect 

 on C. magister larvae is not knowTi and should be 

 investigated. 



Hypothesis 2: Food Quality and Quantity 



May (1974) reviewed Hjort's (1914) critical 

 period concept for fish larvae since Marr's (1956) 

 evaluation and concluded from recent work that 

 starvation may be an important cause of mortal- 

 ity, especially during the period immediately fol- 

 lowing the yolk sac stage. Although crab larvae 

 do not have a strictly comparable yolk sac stage 

 in their planktonic life, adequate food densities 

 for efficient feeding may be of critical importance 

 during a brief period following hatching. There is 

 limited knowledge concerning the types of food 

 organisms normally available and selected by C. 

 magister larvae and concerning the densities of 

 these food organisms sufficient for development. 

 Most crab larvae are omnivorous, requiring sub- 

 stantial protein in their diet (Costlow and Sastry 

 1966; and others). Attempts to distinguish gut 

 contents of field-caught C. magister larvae were 

 unsuccessful in the present study. However, the 



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