At the James Cook University laboratory, pearl oyster spat are sorted into three size classes as early as 3.5 months:
– Large (>10 mm)
– Medium (5–10 mm)
– Runts (<5 mm), often culled because they are assumed to remain slow-growing.

This study evaluated the growth rates of Pinctada margaritifera and P. fucata in these three classes by suspending the spat in identical sea cages for six months.

For P. margaritifera, final size differences were significant (F2,87 = 167.67, P<0.01):
– <5 mm: 24.6 ± 0.4 mm
– 5–10 mm: 32.2 ± 0.4 mm
– >10 mm: 35.6 ± 0.4 mm
Several runts, however, managed to catch up with the other classes.

For P. fucata, the results were similar:
– <5 mm: 36.2 ± 0.3 mm
– 5–10 mm: 42.3 ± 0.4 mm
– >10 mm: 46.9 ± 0.4 mm
Growth rates were highest in the 5–10 mm class, but the smallest individuals sometimes achieved comparable growth.

Conclusion: Although runts generally remain smaller in the months following grading, some can catch up if growing conditions are optimal. Systematic culling at the first sorting would therefore be premature and could deprive the farm of part of its production potential.