Cd. Mcquaid et Tl. Lindsay, Effect of wave exposure on growth and mortality rates of the mussel Perna perna: bottom-up regulation of intertidal populations, MAR ECOL-PR, 206, 2000, pp. 147-154
The effect of wave exposure on the growth of low-shore populations of Perna
perna (Linnaeus) was determined using 3 independent approaches at 6 sites
on the south coast of South Africa: (1) Individual marking; notches were fi
led on the growing edges of mussels in situ. After 111 d, growth from the m
ark to the new growing edge was measured. (2) Internal shell-growth bands;
mussels from the mark-recapture experiment were used to establish that grow
th bands are laid down tidally. Unmarked individuals were then used to meas
ure growth over the last 60 tides. (3) Shepherd's length-composition analys
is (SLCA); growth rates were determined from length-frequency distributions
in 11 samples collected at 6 wk intervals over 15 mo. All 3 approaches sho
wed that growth rate was approximately twice as fast on exposed as on shelt
ered shores (ANCOVA, p < 0.05 in all cases). Mean length for the first year
of growth (averaged for the 3 methods) was 47 and 22 mm, respectively. The
re were, however, considerable differences among methods. Marking provided
the lowest estimates of growth rates, and SLCA the highest. The mean mortal
ity index (Z yr(-1)) was significantly higher and annual survival rate lowe
r for exposed shores, indicating faster turnover under exposed conditions.
Mean longevity on exposed shores (2.6 yr) was only one-third of estimated l
ongevity on sheltered shores (6.7 yr). These findings show that degree of e
xposure has important interactions with growth and mortality of mussels on
the low shore, and that these effects are independent of the effects of exp
osure on density. The findings also indicate strong bottom-up regulation of
mussel populations.