Spatio-temporal variability of larval abundance and settlement of Perna perna: differential delivery of mussels

Abstract

We examined larval availability and settlement of the intertidal mussel Perna perna simultaneously at different spatial and temporal scales using a nested design at 2 sites, 3 km apart on the south coast of South Africa. Each site had 3 locations (300 m apart) where 5 artificial settler collectors were placed about 20 cm apart. Collectors were replaced on temporal scales varying from fortnightly (for 16 mo) to daily (2 series of 15 to 20 d). Each intertidal location was paired with an inshore location (these too were 300 m apart) within 500 m of the shore, where larval availability was measured by 3 vertical plankton hauls collected on the same dates as for settler sampling. There was strong temporal variation in abundances of larvae and settlers, and no correlation (r always < 0.14) was found between the two. Larvae were abundant only at the start of sampling and rare for the rest of the study, while distinct peaks in settler numbers occurred later. No spatial effect was detected for larval availability, while there was strong spatial variation in settlement at the location level. These results indicate that, on scales of 100s of m to km, delivery of larvae from the nearshore water column onto the shore is strongly differential, with some locations consistently receiving more settlers than others. We conclude that, at these sites, the patchiness in settlement observed on scales of 100s of m depends on differential delivery, rather than differential offshore distribution of larvae. We suggest that differential delivery is due to the effect of nearshore bottom topography on local hydrodynamics.

Causes of decoupling between larval supply and settlement and consequences for understanding recruitment and population connectivity

“…While in some studies settlement and larval supply appear to be correlated 12 (Minchinton and Scheibling, 1991;Yund et al, 1991;Hurlbut, 1992;Jeffery and Underwood, 13 2000; Ross, 2001;Ma, 2005), other studies find no correlation (Miron et al, 1995;Olivier et al, 14 2000;Porri et al, 2006;Rilov et al, 2008), suggesting caution in interpreting settlement as proxy 15 for larval supply. 16…”

“…(3) The small 9 size of many planktonic invertebrate larvae, which makes them possibly more susceptible than 10 fish larvae to be transported by purely physically processes transport (McQuaid and Phillips, 11 2000). (4) The consistency of larval delivery to sites separated by hundreds of meters (Pineda, 12 1994a;Porri et al, 2006;Rilov et al, 2008). Systems characterized by "coupled settlement" 13 mechanisms would generally experience density-dependent mortality of prey (White, 2007) and 14 are more likely to be strongly regulated by predation on early stages (at settlement, White, 2008).…”

“…Operationally, researchers 5 have measured larval supply as the number of planktonic larvae available near settlement sites 6 (e.g., Minchinton and Scheibling, 1991;Bertness et al, 1992;Miron et al, 1995;Sponaugle and 7 Cowen, 1996;Noda et al, 1998;Jeffery and Underwood, 2000;Doherty et al, 2004;Jonsson et 8 al., 2004;Todd et al, 2006;Beaulieu et al, 2009), and we use this definition here. Researchers 9 have used other terminology to refer to these available larvae (e.g., Porri et al, 2006). 10…”

“…As larvae are transported from offshore waters to within 10 -100's meters of the shore, delivery 12 of larvae to the bottom will be influenced by hydrodynamic forces such as surface gravity waves, 13 through oscillatory flows, shoaling and breaking, and wave induced along-shore and cross-shore 14 flows (e.g., Miller and Emlet, 1997;Porri et al, 2006;Rilov et al, 2008;Dudas et al, 2009). A 15 relationship between larval supply and settlement is most likely to be detected when larval 16 supply is measured within a few meters or less of the settlement substrate (e.g., Otaíza, 1989;17 Jeffery and Underwood, 2000;Todd et al, 2006;Tremblay et al, 2007).…”

“…In 9 many species, (e.g., mussels, Rilov et al, 2008) competency must be assessed by testing whether 10 larvae settle in the laboratory, although size has also been used as proxy (e.g., Porri et al, 2006). 11…”