Planktonic larval stages—an adaptation to lower predation rates in the pelagic environment?

A majority of marine benthic macroinvertebrate and fish species have planktonic larval stages that disperse in the water column from days to months. However, the adaptive significance of pelagic larvae, and whether predation is higher in the pelagic or the benthic environment, is still debated, partly due to a lack of studies assessing larval predation in offshore, pelagic habitats. Here we assess predation mortality on shore crab megalopae (Carcinus maenas) using tethering techniques in the eastern North Sea, comparing diel differences in predation rates in five different benthic and pelagic habitats, tethering close to 900 megalopae over a 2‐year period. Our results showed that relative predation rates on shore crab megalopae were up to 10 times higher in shallow benthic nursery habitats (0.5–4 m depth; 23–58% 6 h−1) compared to nearshore and offshore pelagic habitats (10–100 m depth; 5–7% 6 h−1). This pattern was consistent both during day and night conditions in both years, and also in a 24‐h experiment when the periods of dusk and dawn were included. The higher predation rates in shallow benthic areas were likely a direct effect of an estimated 3 to 4 orders of magnitude higher abundance of potential predators in this habitat compared to the pelagic environment. By demonstrating an order of magnitude lower predation rates in a pelagic offshore environment for the first time, the study provides new support that planktonic larvae may be an adaptation to use pelagic environments as a spatial refuge from high predation rates in shallow nursery habitats.