strongly indicated there was also no occurrence along the north ridge ter- 

 race. The only exception was the polychaete, Nephtys califomiensis , which 

 lives on the intertidal and very shallow subtidal beach. A few individuals 

 were found during the winter and early spring only at the shallow canyon 

 station P-1. The large amount of sediment slumping around the pier and the 

 proximity of the canyon head to the beach probably explain this seasonal 

 occurrence. Generally, populations appeared local and immobile within the 

 confines of the sampling stations. and periodicity. 



In summary, benthic macroinvertebrate assemblages in the canyon head 

 changed, as predicted, in response to changes in sediment movement or sub- 

 strate stability. A naturally controlled experiment indicated that sedi- 

 ment movement was also the critical factor determining the distribution 

 of animals along a gently sloping sand bottom. 



IV. REPRODUCTIVE ACTIVITY OF SELECTED INVERTEBRATES 



Knowledge of the reproductive activity of the local fauna is essential 

 to understand natural variations in benthic assemblages, to interpret the 

 sequence rate and end state of recolonizations , and to increase predictive 

 abilities . 



Reproductive data are from larval settling jars and benthic sampling. 

 The larval settling jars were similar to those used by Thorson (1946) and 

 Reish (1961a). Widemouthed plastic jars were positioned vertically in a 

 rack 1.5 meters above the bottom at the 20-meter control station (Fig. 1). 

 Exposure intervals of 8, 16, and 32 days were tested; jars exposed for 16 

 days produced more species and individuals. A collection was made every 

 8 days and the jars with and without sediment added were compared. The 

 added sediment did not appear to stimulate larval settling. The contents 

 of each jar were washed through a screen with 0.25 -millimeter- square open- 

 ings, stained with rose bengale, preserved in 10-percent Formalin, and 

 sorted under a dissecting microscope. Animals were identified, enumerated, 

 and transferred to 70-percent ethanol and 5-percent glycerin. A thin layer 

 of particulate material accumulated in the jars in 1 day and was 1 to 4 

 centimeters thick by the end of 16 days. 



Jars were first tested in March and April 1972. Regular sampling be- 

 gan on 12 September 1972 and ended on 1 June 1973. The three summer months 

 were not included; however, some information is available on the macro-^ 

 invertebrates in the benthic samples. 



The jars must be considered as selective sampling devices. Animals 

 may brood their young (many Crustacea (amphipods and ostracods) and some 

 polychaetes (S. arminger)) , have a very short or suppressed pelagic larval 

 stage, or do not settle in jars. The degree of selection is unknown. 

 Wilson (1951) made numerous laboratory studies of substrate selection by 

 invertebrate larvae. However, very little is known about selection under 

 natural conditions. 



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