Table 21. Number of Infaunal species collected at Imperial Beach 

 during predlsposal surveys 1 and II at the 3.7-iiieter 

 depth. 









A 





STATION 

 B 





C 









I 





II 



Surveys 

 I 11 



I 





II 





No. 



of species 



A9 





46 



37 37 



38 





23 



Station A is adjacent to a Dendraeter bed; stations B and C 

 are not near a Dendraeter bed. 



Although the sand dollar, D. excentrious , was not always 

 numerically abundant within the core samples (Fig. 30), underwater 

 observation showed this species was gregarious and formed extensive 

 beds at times. This aggregating behavior has been attributed to their 

 reproductive habits, as fertilization is external (Weihe and Gray, 

 1968), but the problem is probably more complex and further study is 

 needed. In Dexter's (1977) study of Imperial Beach, D . exaentriaus had 

 maximum densities of more than 1,200 per square meter. Diving observa- 

 tions found extensive beds with hundreds of individuals per square meter. 

 Densities calculated from core samples varied from to 350 per square 

 meter. Large variations in abundance patterns are expected considering 

 the biological and physical factors controlling distribution. 



Shoreward, D. excentriaus occurrence is limited by wave action 

 and a strong bottom surge. On the open coast, they are typically not 

 found within the surf zone (Ricketts and Calvin, 1952), but occur in 

 numbers just seaward of the breaker line. According to Merrill and 

 Hobson (1970), a size gradation of D. excentriaus is found from the 

 surf to the outer limit of approximately 4- to 12-meter depths. They 

 reported that juvenile D. exaentricus (<10 millimeter) were more abun- 

 dant near the shore, and the adults more abundant offshore. The ability 

 of D. exaentricus to migrate on and offshore is an important aspect in 

 understanding its distribution. At Imperial Beach, D. exentriaus popu- 

 lations moved more than 100 meters offshore and then back during the 

 study. Dexter (1977) found a seasonal pattern to these migrations. In 

 late spring and summer, D. exaentricus moved shoreward and there was a 

 general offshore movement into deeper, calmer water during winter months 

 and storm activity. 



84 



