Abstract
Future Ocean acidification (OA) has the potential to negatively affect marine ecosystems and the organisms they support, with the early life-history stages particularly sensitive to reduced seawater pH and carbonate saturation states. Most marine organisms reproduce through an indirect lifecycle, which includes a free-swimming larval stage. In benthic or sessile taxa, the lifecycle is marked by the larval settlement and metamorphosis processes. Here, at the end of the free-living (generally planktonic) stage, larvae selectively search for a preferred settlement substrate for attachment, with metamorphosis occurring before or after it. Larval settlement and metamorphosis are arguably the most important processes in the life cycle of marine invertebrates, since they determine and optimize the final location of the organisms. Altered larval settlement rates will therefore influence the ecology, abundances and distributions of future coastal communities.
The aim of this thesis was to investigate whether OA could affect the larval settlement success of marine invertebrates, and whether these impacts would be mediated through direct, indirect or carry-over mechanisms. Three key New Zealand coastal marine invertebrates were used as model organisms: the sea urchin Evechinus chloroticus, the black-footed abalone (Haliotis iris) or pāua and the serpulid polychaete Galeolaria hystrix.
Direct effects of OA occur through altered larval behaviour or physiology at the time of settlement in such a way that settlement success is affected. Direct effects of OA were investigated in E. chloroticus and H. iris, by exposing competent larvae to reduced seawater pH during settlement (Chapter 2). Settlement success in E. chloroticus was unaffected under a range of pH treatments (pHT 7.0 to ambient). Similarly, no direct effects were observed in the abalone H. iris when left to settle in reduced seawater pHT (7.65).
Exposure to OA could affect the settlement substrates or cues in such a way that it could indirectly alter the settlement success. Indirect effects of OA were tested in E. chloroticus, H. iris and G. hystrix (Chapters 2, 3 and 4, respectively). Settlement success in E. chloroticus was unaffected in presence of crustose coralline algae (CCA) pre-conditioned under reduced seawater pHT 7.7 for 28 days. Similarly, settlement success in H. iris was unaffected when presented with CCA-covered settlement tiles incubated at reduced seawater pHT 7.65 for 171 days. However, settlement success in competent G. hystrix was altered in presence of marine biofilms incubated in a range of six pH treatments (pHT 7.0 to ambient) for 7, 10 and 14 months, although the mechanisms behind it could not be discerned. This study revealed clear shifts in the marine biofilm community composition, at both eukaryote and prokaryote levels.
Carry-over effects due to exposure to OA during larval life were investigated and found in H. iris (Chapter 3). Larvae reared under reduced pHT (7,80) showed lower settlement success than their counterparts reared under ambient levels of pH.
Given the importance of marine invertebrates in coastal ecosystems, it is important to understand how ocean acidification, alone or in combination with other stressors, could affect early-life stages of coastal species. This will help us predict the effects on settlement and recruitment rates of coastal marine invertebrates, which in turn regulate their abundances and distributions in coastal environments. This could cause important changes on the population biology and life-histories of keystone species, which could have negative top-down or bottom- up effects on the fragile balance of coastal ecosystems. Similarly, altered settlement rates of commercially important species could impact the recruitment success and therefore the economic revenue and production outcomes the aquaculture industry and wild fisheries rely on.
Overall, this thesis contributes to a better understanding of the potential effects of OA on the larval settlement of marine invertebrates. Furthermore, the main observations of this thesis support the notion that the effects of OA will most likely be taxa-specific and be associated with the settlement selectivity in a particular species or group.