22 



Fishery Bulletin 102(1) 



10 Sep 1997; mode: 0.30mm, 

 scale: 0.04, n=341 



13 May 1998; mode: 0.41mm, 

 scale: 0.06, n=38 



11 Jun 1998; mode: 0.51 mm, 

 scale: 0.12, n=292 



10 Jul 1998; mode: 0.57mm. 

 scale: 0.10, n=610 



13 Aug 1998; mode: 0.64mm, 

 scale: 0.12. n=456 



1 1 Sep 1998; mode: 0.70mm. 

 scale: 0.17, n=202 



14 Oct 1998; mode: 0.76mm. 

 scale: 0.17. n=162 



0.0 



22 Apr 1999; mode: 0.74mm. 

 scale: 0.15, n=265 



+ + 



13 May 1999; mode: 0.81mm, 

 scale: 0.20. n=241 



0.2 t 



t 



H 



0.1 



00 



28 Jul 1999; mode: 2.14mm, 

 scale: 1.06, n=63 



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3456 0123456 



Shell length (mm) 



Figure 4 



Shell-length compositions of a single cohort of Corbicula japonica spawned in 1997. The raw data 

 (shell lengths) are shown by +. The largest extreme value distribution estimated by the best model 

 ( model 4. 1 in Table 1 1 is shown by a solid line. The largest extreme value distribution independently 

 fitted by the maximum likelihood method is shown by a dashed line. The sampling date and values 

 of location parameter I mode) and scale parameter independently fitted by the maximum likelihood 

 method are shown in each panel. 



variation should be validated by reciprocal transplanta- 

 tions or laboratory experiments (or both) in future inves- 

 tigations. Prolonged phases of meiobenthic development 

 have been reported in some marine bivalves (Nakaoka, 

 1992; Harvey and Gage, 1995). However, a prolonged phase 

 of meiobenthic development as a geographical variation is 

 rarely reported. 



In many species of bivalve, populations from higher lati- 

 tudes have a slower initial growth rate; but longevity and ul- 

 timate size in these populations are frequently greater than 

 at lower latitudes (Newell, 1964; Seed, 1980). The extremely 

 slow growth of C. japonica juveniles in Lake Abashiri may be 

 an extreme example of this phenomenon. In Lake Abashiri, 

 C. japonica failed to spawn in ten out of 21 years for which 



