Effects of ocean acidification on the settlement and metamorphosis of marine invertebrate and fish larvae: A review

Most marine organisms present an indirect lifecycle where a planktonic larval stage reaches competency before settling to the substrate and metamorphosing. Despite the critical importance of these early life history stages, little is known about how global change-related stressors, in particular ocean acidification (OA), affect marine larval settlement and metamorphosis.

To date, 48 studies have investigated the effects of OA on larval settlement, focussing mostly on tropical corals (16), echinoderms (11) and fish (8). Most studies show negative effects of OA during settlement and post-settlement processes. For instance, reduced settlement is typically seen along natural pH gradients and in experimentally lowered pH treatments.

This generally results in reduced settlement selectivity and metamorphosis and poorer post-settlement fitness. Carryover effects of OA exposure can also occur, with larval environmental history influencing early post-settlement performance. We conclude that OA may (1) alter larval supply for settlement by altering horizontal swimming behaviour or vertical migration; (2) directly influence settlement success through changes in the nature and distribution of suitable settlement substrates (e.g. biofilm, crustose coralline algae); and (3) mediate carryover effects at settlement by altering larval development or larval energy budgets.

In contrast to fish larvae, there is little evidence for most invertebrate larvae that their perception of settlement cues is directly influenced by reduced pH. A summation of how OA affects the settlement and metamorphosis of marine invertebrates is timely, since altered settlement rates will influence the future distributions, abundances and ecology of marine benthic communities.