GROWTH AND SURVIVAL IN NEWLY SETTLED SPAT OF 

 THE MANILA CLAM, TAPES JAPONICA 



John G. Wiluams' 



ABSTRACT 



Substrate abxindances of adult Manila clam, Tapes japonica. were manipulated in July 1976 on a 

 portion of beach dug commercially in the southern region of Puget Sound, Washington. Differences in 

 clam spat growth and survival were measured between samples taken from substrates having vary- 

 ing levelsof adult clam abundance. 



The clam spat settled at 0.206 mm long. Initial growth of clams settling in the fall was much slower 

 than for clams settling in the summer, 9 months versus 2 months, respectively, to reach approxi- 

 mately 2.5 mm long. Summer settling clams form a visible growth checkmark by October of their first 

 year at approximately 5-8 mm. Fall settling clams form their first visible checkmark during their 

 second October at approximately 14-16 mm. The length of clams was found to be significantly less for 

 clams growing in substrates with natural or high adult clam abundances versus those from sub- 

 strates with no adult clams. 



Only 1.2S^ of the initial population of clams that settled in September 1976 survived until June 

 1977. The most likely cause of this mortality was by meiofaunal predators, particularly nematodes. 

 By June, no difference in survival rates was detectable between clams from substrates that contained 

 no adult clams versus those from substrates with natural or high adult clam abundance. 



Clam spat movement occurred along the beach and may have contributed to the high spat mortal- 



ity. 



The Japanese little-neck or Manila clam. Tapes 

 japonica Deshayes, is a native to Japanese and 

 Korean waters, but was introduced into Puget 

 Sound, Wash., along with the Pacific oyster, 

 Crassostrea gigas. in the 1930's (Quayle 1964). It 

 has since become an important part of the com- 

 mercial Puget Sound hard-shell clam fishery with 

 approximately 1 million lb harvested annually. It 

 has also been so heavily utilized by the sport 

 fishery that the populations on some Puget Sound 

 beaches have been almost eliminated. 



The majority of research on the Manila clam 

 has been performed in Japan (see Tamura 1966 

 for a review of the Japanese literature). Because 

 the water temperatures and climate in Puget 

 Sound are cooler than those in most of the 

 Japanese study areas, the results published by 

 the Japanese with respect to spawning times, 

 growth, and population numbers can be mislead- 

 ing when applied to clam populations in this area. 

 Studies conducted on the west coast of the United 

 States and Canada dealt with gonad development 

 (Holland and Chew 1974), planktonic larval 

 stages (Quayle and Bourne 1972), and growth 



and/or survival after settling and after some ar- 

 bitrary body size, usually based on sieve reten- 

 tion, had been reached (Jones 1974; Glock 1978; 

 Lukas^). Nosho and Chew (1972) made the only 

 attempt to investigate early settling stages of the 

 Manila clam. However, due to the sieve size they 

 used, they were unable to detect newly settled 

 spat. The lack of early life history information is 

 probably related to the difficulty in sorting out 

 newly settled spat from gravel samples, and in 

 specific indentification. Loosanoff et al. (1966) 

 found it difficult to identify pelagic larvae to 

 species and cited this as a reason for incomplete 

 life histories of many pelecypods. Quayle (1952) 

 found that identification of spat was even more 

 difficult than the identification of planktonic lar- 

 vae. 



I began a study in the summer of 1976 to de- 

 scribe the growth and survival of Manila clams 

 from settling size to formation of the first growth 

 ring. In addition, since the study was located in 

 an area with large numbers of adult Manila 

 clams, I investigated the possibility that the pres- 



'Washington Cooperative Fishery Research Unit, College of 

 Fisheries, University of Washington, Seattle, WA 98195. 



^Lucas, G. 1973- Clam-abalone spawning and rearing. 

 Commer, Fish. Res. Dev. Act. July 1, 1970 to June 30. 

 1973. Fish. Comm. Greg. Proc. Rep., 19 p. 



Manuscript accepted Mav 1979 



FISHERY BULLETIN: VOL. 77, NO 4. 1980. 



891 



