Title: The Impact of Ocean Acidification on the Development of Early Life Stages in the Northern Red Abalone (Haliotis Rufescens)
The oceans absorb approximately one-third of the carbon dioxide (CO2) emitted into the atmosphere from human activities such as fossil fuel combustion and cement production. As atmospheric CO2 has increased over the past 200 years, the oceans have become approximately 30% more acidic. This phenomenon, known as ocean acidification (OA), represents an emerging threat to marine life. Early life stages of many fish and shellfish species are particularly vulnerable to OA because of the vital role that carbonate ions play in their calcification and development processes. The red abalone (Haliotis rufescens) is an ecologically and economically important marine gastropod found off the west coast of North America. Their larvae undergo a complex three-week developmental cycle which includes trochophore, veliger and juvenile stages before settling to the sea floor as tiny juveniles. Previous studies have shown that abalone larvae are negatively impacted by OA conditions, including reduced growth and calcification rates. Few studies have examined the sub-lethal impacts of OA across the entire larval period.
The objective of this study was to investigate how environmentally relevant levels of OA may affect key developmental processes and physiological parameters throughout the larval development of northern red abalone. Gametes from adult northern red abalone collected from coastal waters near Bodega Bay, California were used to fertilize eggs and produce larvae under controlled laboratory conditions. Larvae were reared in four treatment levels spanning a range of pCO2 levels from present-day conditions (~400 μatm) to levels projected for the year 2100 (~1000 μatm) under a “business-as-usual” CO2 emissions scenario. Developmental timing, growth rates, and calcification were assessed at four time points across the larval period including trochophore, early veliger, late veliger and pediveliger/juvenile stages. In addition, cellular stress responses were evaluated by measuring whole organism expression of heat shock protein 70 (hsp70) and antioxidant enzymes such as catalase and glutathione reductase.
The results showed that red abalone larvae exhibited significant impacts across multiple developmental endpoints with increasing pCO2. Larvae reared under 1000 μatm pCO2 took on average 5 days longer to reach metamorphosis compared to the 400 μatm control group. Significant reductions in shell size were also observed at the 1000 μatm treatment level, with larvae exhibiting shells 8-15% smaller compared to controls across all sampling points. Ocean acidification also appeared to elicit cellular stress responses, with abalone larvae in the highest pCO2 treatment showing marked upregulation of hsp70 expression (>2-fold) and antioxidant enzyme activity beginning at the early veliger stage. Increased pCO2 did not significantly alter larval mortality rates during the experiment.
Overall, these results indicate that red abalone larvae are highly sensitive to end-of-century OA conditions, experiencing detrimental effects on growth, calcification and physiological stress responses across their entire development period. Delays to metamorphosis could increase vulnerability to predation for settling juveniles in the field. Reduced growth and shell development early in life may also have carry-over impacts on future size, fecundity and reproductive success. Furthermore, the induction of cellular stress responses suggest energetic trade-offs that could compromise other fitness-related traits. Given the ecological and economic importance of red abalone, these sub-lethal impacts of OA pose concerns for their long-term sustainability in the wild. Further research is still needed to improve predictive models and determine best practices for abalone aquaculture and fishery management under continuing ocean change. The present study highlights the need for mitigation of global CO2 emissions to limit additional stressors to sensitive early life history stages of this commercially valuable shellfish species.
This experiment provides compelling evidence that environmentally relevant increases in ocean acidification can disrupt key biological processes across the full larval development period in northern red abalone. The results advance our understanding of sub-lethal OA impacts and have implications for abalone recruitment, populations and fisheries under future acidified conditions.