Abstract
To date the study of ocean acidification on fish otolith formation has been mainly focused on larval and juvenile stages. In the present pilot study, wild-captured adult Atlantic cod (Gadus morhua) were exposed to two different levels of pCO2, 422µatm (ambient, low pCO2) or 1091µatm (high pCO2), for a period of 30 weeks (from mid-October to early April 2014–2015) in order to study the effects on otolith size, shape and CaCO3 crystallization amongst other biological parameters. We found that otoliths from cod exposed to high pCO2 were slightly smaller (− 3.4% in length; − 3.3% in perimeter), rounder (− 2.9% circularity and + 4% roundness) but heavier (+ 5%) than the low pCO2 group. Interestingly, there were different effects in males and females; for instance, male cods exposed to high pCO2 exhibited significant changes in circularity (− 3%) and roundness (+ 4%) compared to the low pCO2 males, but without significant changes on otolith dimensions, while females exposed to high pCO2 had smaller otoliths as shown for length (− 5.6%), width (− 2%), perimeter (− 3.5%) and area (− 4.8%). Furthermore, while the majority of the otoliths analysed showed normal aragonite deposition, 10% of fish exposed to 1091µatm of pCO2 had an abnormal accretion of calcite, suggesting a shift on calcium carbonate polymorph crystallization in some individuals under high pCO2 conditions. Our preliminary results indicate that high levels of pCO2 in adult Atlantic cod might affect otolith growth in a gender-specific way. Our findings reveal that otoliths from adult cod are affected by ocean acidification, and we believe that the present study will prompt further research into this currently under-explored area.
Highlights
Rising levels of atmospheric CO2 are leading to an increase in the average ocean CO2 partial pressure, which translates to a decrease in environmental pH, a phenomenon known as ocean acidification (Caldeira 2005)
We found that females from the high pCO2 treatment had smaller (OL, otolith perimeter (OP) and otolith area (OA) were − 5.6, − 3.5 and − 4.8% smaller; P = 0.03, 0.03 and 0.04, respectively) and heavier (OW and otolith density (OD) increased + 6% and + 4%; P = 0.03 and P = 0.02, respectively) otoliths than females of the low pCO2 group (Fig. 2; Table 3, Supplementary File 4)
Otoliths from females exposed to high pCO2 treatment (High pCO2) were relatively smaller compared to low pCO2 females, while otoliths from males exposed to the high pCO2 treatment appeared slightly rounder (− 3% in Cicl and + 4% in Rd) when compared to those from the low pCO2 group (Supplementary File 6)
Summary
Rising levels of atmospheric CO2 are leading to an increase in the average ocean CO2 partial pressure (pCO2), which translates to a decrease in environmental pH, a phenomenon known as ocean acidification (Caldeira 2005). Elevated pCO2 levels in sea water modify the saturation states of the different calcium carbonate (CaCO3) polymorphs (Jones et al 2017) and alter the formation/dissolution rates of CaCO3-based structures (Hofmann et al 2010). It is known that organisms with external calcium carbonate structures, such as mollusc shells, arthropod exoskeletons or coral “skeletons”, will be sensitive to ocean acidification owing to the increase in carbonate dissolution rates that will weaken such structures (Hofmann et al 2010). Otolith formation is a very dynamic process and varies significantly in response to environmental conditions (such as temperature or pH), fish ontology (such as age, gender, size or sexual maturation) and physiological status (nutrition, spawning or stress) (MoralesNin 2000; Bestgen and Bundy 1998; Radtke and Fey 1996)
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