WILU AMS: GROWTH AND SURVIVAL OF MANILA CLAM SPAT 



Clams from substrates with no adults (Treat- 

 ments 1, 2) averaged 2.70 mm (N = 55; SE = 

 0.08) long, and clams from substrates with mod- 

 erate (Treatment 3) and high (Treatment 4) adult 

 clam abundances averaged 2.17 mm (N = 39; SE 

 = 0.08). 



Survival 



The density of newly settled clams sampled 

 from 105 cores ranged from 19 to 93, with a mean 

 of 54.3. Only 1.2%^ of the clams (equivalent to 

 250-450/m2) survived until June 1977, 9 mo 

 after the fall 1976 settlement (Figure 3). The 

 largest loss in clams occurred during the first 2 

 mo after settling when the density decreased by 

 57%. During the third, fourth and next 2 (com- 

 bined) mo after settling, the average density de- 

 crease from the previous sampling period was 

 34%, 56%, and 35%, respectively. A Kruskal- 

 Wallis test on the June data detected no differen- 

 tial (P>0.50) survivorship between clams that 

 had settled into substrates with adult clams ver- 

 sus those that settled into substrates with no 

 adult clams. 



DISCUSSION 



Growth 



There has been a great disparity in the reported 

 size of 1-yr-old Manila clams. Three areas in 

 Japan have reported three different lengths: in 

 Hokkaido, 8 mm (Yamamoto and Iwata 1956); in 

 the Inland Sea, 18 mm (Ohba 1959); smd in Ariake 

 Bay (South Japan), 27 mm (Tanaka 1954). Rodde 

 et al. (1976) grew Manila clams to 34 mm in 1 yr 

 under hatchery conditions with high tempera- 

 tures and nutrient rich water. Nosho and Chew 

 (1972) estimated that Manila clams in Hood 

 Canal, Wash. , were 24 mm at the end of 1 yr. These 

 conflicting reports have created some difficulties 

 for scientists in attempting to determine the age of 

 Msmila clams found in Puget Souhd beaches. The 

 fact that I found two sizes of clams that resulted 

 from settlements only a few months apart makes 

 this understandable. In some cases, others have 

 based growth data on checks in the shell without 

 studying the early stages of growth. However, 

 without the knowledge about the time of year that 

 a clam cohort settled and the period of growth 

 until the formation of the first visible checkmark, 

 the determination of the age of a clam can be 

 difficult. 



Figure 3. — Survival of Manila clams at each sampling period 

 for Treatments 1-4 expressed as a percentage of the initial levels 

 at settlement. 



The results of my research showed that growth 

 rings were formed by the end of October. For 

 clams that settled in the summer the first visible 

 check was formed at 3-4 mo, but for those settling 

 in the fall (just 2-3 mo later), the first visible 

 mark was formed in 13 mo. The modal lengths, 

 that I found until the first visible checkmark was 

 formed, of 5-8 mm and 14-16 mm for the summer 

 and fall cohorts should give scientists a more con- 

 cise baseline upon which to judge growth data for 

 the Manila clam. 



Although there was a difference in the grovrth 

 rates between clams that settled in the summer 

 and those that settled in the fall, the length of 

 newly settled clams that I observed was the same 

 for both periods and the same as those found by 

 Loosanoff et al. (1966) at the Milford Laboratory 

 in Connecticut and by Yoshida (1953) in Japan. 

 The clams from the fall settlement more than 

 doubled in size in 2 mo, but then their growrth 

 almost ceased for a 3 mo period and slowly in- 

 creased until March. This slow winter growth 

 was very similar to Japanese findings on larger 

 Manila clams (Yoshida 1935; Yamamoto and 

 Iwata 1956; Ikematsu 1957; Ohba 1959). In 

 southern Puget Sound, Glock (1978) found no 

 growth during this same period for Manila clams 

 with lengths from 12 to 20 mm. Wilbur and Owen 

 (1964) stated that growth is generally negligible 

 for most molluscan species below 5-10 C, which 



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