LOUGH: LARVAL DYNAMICS OF DUNGENESS CRAB 



'Gut-fullness is expressed as a reconverted arcsinVpercentage transformed mean followed by its standard error and the number of observations in 

 parentheses. 

 ^Tfie station samples in this table are from the 0.7-m bongo net sampler exclusively. 

 '5% level significant difference between successive station means based on two-sample /-tests. 

 **1% level significant difference between successive station means based on two-sample f-tests. 



curred between stations NH05 and NH20 (Table 

 9). Any zoeal stage caught within NH03 and 

 farther ofTshore than NH20 showed a marked de- 

 crease in gut-fullness. 



DISCUSSION 



The initial appearance of C magister larvae in 

 the plankton off the central Oregon coast in late 

 January and early February occurs at a time 

 when sea surface temperatures are generally 

 warming after the yearly mean low in January 

 (Gonor et al. 1970). High densities of early stage 

 zoea caught within 3-10 miles of shore are in 

 agreement with the knowni distribution of the 

 adults at this time. Relatively few occurrences of 

 early stage larvae were found beyond 10 miles of 

 shore during the sampling period as the north- 

 ward flowing Davidson Current tends to retain 

 the early developing larvae in the nearshore 

 area. A very strong onshore component of the 

 current has been observed within 5 miles of shore 

 (Keene 1971; Wyatt et al. 1972; Holton and Elliot 

 1973). During the March and April transition 

 period when the northward Davidson Current is 

 replaced by currents flowing to the south and 

 southwest, the larvae have developed to late 

 stage zoea and megalopae. The bulk of the C. 

 magister megalopae settle out of the water and 

 metamorphose to juveniles by April and May be- 

 fore the onset of intense coastal upwelling in 

 June and July, thus reducing the chance of being 

 carried offshore by the resulting Ekman Current. 

 During all seasons along the coast, larvae which 

 occur increasingly closer to shore would be sub- 

 ject to decreasing current transport either along- 

 shore or offshore. 



1970 Season 



It was observed during the 1970 larval season 

 of C magister that the late zoeal stages "disap- 

 peared" or were greatly reduced in numbers in 

 the inshore sampling area, whereupon the 

 megalopae reappeared after the proper time in- 

 terval in densities comparable to those of the pre- 

 viously sampled zoea. Hypotheses to explain their 

 disappearance and reappearance are as follows: 

 1) the late zoea were misidentified, 2) some stages 

 are skipped in development, 3) the sampling in- 

 terval missed those stages, 4) avoidance of the 

 samplers increases with zoeal stages of develop- 

 ment but decreases at megalopal stage, 5) the 

 late zoeal larvae are carried offshore or 

 alongshore but upon molting to the megalopal 

 stages are transported onshore or back to their 

 original release point, 6) the larvae were very 

 dispersed at late zoeal development so that the 

 volume of water filtered was not adequate, or 

 7) late stage zoea are resting on the bottom or 

 below the depth sampled. 



The late stages of C. magister larvae were not 

 misidentified as they are morphologically distinct 

 by this time and are nearly twice the size of any 

 other local cancrid species. Apparently, the late 

 larval stages of C. magister were not skipped in 

 their development since zoea 4 and 5 stages were 

 collected on the offshore stations in late March 

 and early April. It is not beheved that the late 

 zoeal stages have greater swimming ability com- 

 pared to the early zoea and megalopa which 

 would permit them to avoid the samplers to a 

 greater degree. On the contrary, personal obser- 

 vations of the late stage larvae in laboratory cul- 

 ture show them to be sluggish swimmers that 



365 



