The Effects of Ocean Acidification in the California sea hare (Aplysia californica)

Abstract

Fish and some invertebrates have demonstrated to be strong acid‐base regulators following exposure to ocean acidification‐relevant carbon dioxide (CO2) levels through the accumulation of bicarbonate (HCO3−) in extracellular and intracellular fluids. This allows for pH compensation, however, internal pCO2 and HCO3− remain elevated. This compensation has been hypothesized to cause negative behavioral impairments, by altering ion gradients (HCO3−/Cl−) along neuronal cell membranes that affect the function of the GABAA receptor, an important inhibitory receptor in the central nervous system of vertebrates and invertebrates. Aplysia californica, known for their simple and well‐mapped nervous system, have the potential to be strong model organisms to examine the relationship between CO2 compensation and impaired behavior in invertebrates. We hypothesized that CO2‐exposed animals would compensate for an acidosis by accumulating HCO3−. In the second portion of the study, we hypothesized that two important behaviors, the righting reflex and tail‐withdrawal reflex, would be impaired following CO2 exposure, since this has been noted in many other species. Aplysia were exposed for 4–11 days to either control (400 μatm), 1,200 μatm (close to end of century predictions), or 3,000 μatm CO2. Hemolymph pH was measured using a custom gas‐tight chamber with a fiber‐optic pH microsensor, and HCO3− and pCO2 were calculated using pH and measurements of total CO2 using the Henderson‐Hasselbach equation. The amount of time it took the animal to right following release from the water column (righting reflex), and the amount of time it took the animal to relax its tail to 50% of the original length (tail‐withdrawal reflex) following a mild tail depression were recorded. We observed that animals were able to fully compensate pH at 1,200 μatm CO2, but experienced a significant but mild acidosis at the 3,000 μatm level. Hemolymph HCO3− and pCO2 increased at both CO2 levels, and were statistically significant when compared to controls. Increased CO2 exposure did not significantly affect the righting reflex, but tail‐withdrawal reflex demonstrated a significant 36–37% decrease in relaxation time at both CO2 levels, suggesting increased boldness and altered behavior at projected future CO2 levels. Given these findings, Aplysia are promising models to study CO2‐induced behavioral impairments since they are capable of regulating HCO3− and have a CO2‐impaired behavior linked to simple neural networks amenable to electrophysiological testing. Further research using this model may help illuminate physiological mechanisms underlying CO2‐induced behavioral impairments noted in many marine species that could threaten ecosystems in future oceans.Support or Funding InformationThis research was supported by the National Institute of Health Bridge to Baccalaureate Program (Award R25GM050083).