Effects of ocean acidification on abalone (Haliotis spp.) reproduction, early development, and growth: a review

Ocean acidification changes the saturation of calcium carbonate in aquatic environments, thereby affecting marine shelled mollusks such as abalone, as well as other organisms that require calcium carbonate to form their skeletons and other similar anatomical structure (e.g., coral spicules). Alterations in pH also leads to acidosis, which hampers several physiological functions (Kavousi et al., 2022). Mollusk larvae are often more vulnerable to pH stress compared to adult mollusks due to their lack of specialized ion-regulatory epithelia necessary for maintaining acid-base balance (Parker et al., 2013). During the early stages of development, a lower pH can decrease fertilization rates, delay embryonic development, reduce hatching rate, larval length, and shell growth rates, and increase the malformation rate (Li et al., 2013). A summary of the impacts of ocean acidification on the early development of abalone (Haliotis spp.) is presented in Table 3.

Kimura et al. (2011) performed two experiments to examine the effects of ocean acidification on the fertilization success of Ezo abalone (Haliotis discus hannai). The initial experiment involved exposing seawater to CO₂ partial pressures (pCO₂) of 450, 650, 850, and 1,050 µatm, resulting in seawater pH values of 7.96, 7.83, 7.79, and 7.73, respectively. In this first experiment, the 1,050 µatm treatment resulted in the lowest rate of fertilization. The second experiment involved setting pCO₂ levels at 500, 1,100, 1,650, and 2,150 µatm, which resulted in pH values of 7.94, 7.68, 7.54, and 7.43, respectively. In this case, a pCO₂ level of < 1,100 µatm did not have any impact on the fertilization rate of abalone. However, a pCO₂ of 1,650 µatm (pH 7.54) resulted in the lowest fertilization rate, followed by treatment with pCO₂ of 2,150 µatm (pH 7.43). These data indicates that the optimal pCO₂ range for ezo abalone is between 1,000 and 1,600 μatm. This outcome is comparable to that of Red abalone (Haliotis rufescens), which exhibited a drop in fertilization rate when exposed to a pH ranging from 7.95 to 7.2. Particularly, a pH below 7.55 under 9°C and 13°C seawater temperatures resulted in an apparent reduction in fertilization rate (Boch et al., 2017). According to Byrne et al. (2010), the fertilization rate of Reddish-rayed abalone (Haliotis coccoradiata) did not vary significantly across a pH range of 8.2 to 7.6. Moreover, Boch et al. (2017) indicated that lower fertilization rates may be due to the potential impact of lower pH on sperm kinematic and/or swimming behavior. Shi et al. (2017) demonstrated that excessive CO₂ absorption notably impairs sperm motility in Blood clams (Tegillarca granosa). The ocean absorbs excess carbon dioxide in the atmosphere, increasing the quantity of hydrogen ions. These ions penetrate into the cells, causing intracellular acidosis and modifying intracellular pH. A decrease in intracellular pH can disrupt the sperm activation, sperm chemotactic behavior, and flagellar motility (Shi et al., 2017). Lower pH levels may diminish sperm mitochondrial membrane potential (Schlegel et al., 2015), limiting sperm flagellar motility (Shi et al., 2017), and impairing the activity of dynein ATPase, a protein involved in sperm movement (Boulais et al., 2018). Similar to acidic pH conditions, lower sperm concentrations have also been linked to reduced fertilization rates (Parker et al., 2013).

Acidification also negatively impacts the hatching rate and viability of fertilized eggs. In the Donkey ear abalone (Haliotis asinina), decreasing the pH level by 0.19 to 0.57 units relative to normal seawater levels not only decreased hatching rates but also impaired the growth of trocophore larvae. Hatching rates exhibited a significant decline at pH 7.6, which is 0.3 units lower than the normal pH level (Tahil & Dy, 2016). Kimura et al. (2011) also observed notable decrease in hatching rate compared to the control group in H. discus hannai at pH levels ranging from 7.43 to 7.54. This finding aligns with the study conducted by Li et al. (2013) on the same species, which found no significant difference in hatching rate at pH 7.6 to the control group. Substantial differences were observed in the pH 7.3 treatment. These findings suggest that a pH level of 7.54 or lower has a negative impact on the hatching rate of H. discus hannai. However, Pratiwi et al. (2024) discovered that the pH range of 7.0–8.2 had no significant effect on the hatching rate of H. discus hannai. In contrast, H. rufescens and Multicolored abalone (Haliotis diversicolor) exhibited notable variations in hatching rates at pH 7.6 (Gómez-Reyes et al., 2023) and 7.71 (Guo et al., 2015), respectively. Jiang et al. (2021) attributed these decrease in hatching rate to the weakening and loss of elasticity of the egg membrane caused by acidic conditions, resulting in the premature release of the embryo from the egg followed by death. García et al. (2024) suspect that low pH interferes with the ability to regulate acid-base homeostasis in embryos, thus slowing down embryo development and hatching time. Acidification can also induce a stress response in mollusks that enhances energy metabolism for survival, thereby suppressing growth activity (Li et al., 2013). Alternatively, acidic conditions could also decrease RNA concentrations, thereby decreasing the production of the key proteins during early development and lowering hatching rates (Franke & Clemmesen, 2011). Low pH could impact hatching enzyme secretion, decreasing the hatching rate (García et al., 2024).

In some abalone species, lower pH has been linked to deleterious effect on larval growth. Li et al. (2013) reported that H. discus hannai larvae grown at pH 7.6 and 7.3 developed more slowly and exhibited higher deformity rates, slower metamorphosis rates, and a shorter shell length. Pratiwi et al. (2024) also discovered that 30 hpf H. discus hannai larvae cultured at pH 7.0 were significantly shorter than controls. The malformation rate of 30 hpf larvae cultured at pH 7.9, 7.6, 7.3, and 7.0 increased compared to control but was only significant at pH 7.0 and 7.3 (Pratiwi et al., 2024). Similarly, European abalone (Haliotis tuberculata) larvae developed more slowly, had fewer shelled larvae, and were shorter when the pH was 7.7 or 7.6. These abalone larvae also exhibited lower survival and shell mineralization rate (Wessel et al., 2018). When exposed to pH 7.85 and 7.65, the shell length of H. asinina (Linnaeus 1758) shrank and the number of malformations rose compare to the control animals reared under ambient pH conditions (Santander & Sayno, 2018). This decrease in shell length and calcification can occur due to decreased calcium carbonate saturation in more acidic conditions. In turn, the larvae deposit less calcium carbonate and produce a thinner shell. The earliest instance of CaCO₃ deposition in abalone occurs during the early veliger stage. During this process, amorphous calcium carbonate (ACC) is deposited and quickly converted into crystalline aragonite (Auzoux-Bordenave et al., 2010). Decreased seawater pH can lead to a lack of saturation of aragonite and calcite, which hinders the generation of new shells or skeletons by larvae (Mostofa et al., 2016). Moreover, decrease in shell size are likely a result of the inhibition of metabolism caused by hypercapnia, leading to delayed development (Brennand et al., 2010). In H. coccoradiata, the impact of pH on the calcification and shell formation process in abalone is further exacerbated by increasing the temperature 2°C and 4°C above normal. Byrne et al. (2011) found that only a minority of eggs developed into healthy larvae under warm temperatures (+2°C to 4°C) and low-pH conditions (pH −0.4 to 0.6 U). Smaller larval mollusks with weaker and thinner shells may require more time in their the planktonic stage to accumulate sufficient energy for metamorphosis. A prolonged larval life span can potentially result in reduced survival rates and elevated mortality rates due to heightened susceptibility to predation and additional environmental stressors (Parker et al., 2013).

The larval settlement process is an essential part of the life cycle of various benthic marine species, including abalone. Recent studies have indicated that future populations of abalone might be adversely affected by a decline in settlement (Espinel-Velasco et al., 2021). Studies investigating the impact of pH on settlement rate have been conducted on many species of H. genus, including Blackfoot paua (Haliotis iris) (Espinel-Velasco et al., 2021), Northern abalone (Haliotis kamtschatkana) (Crim et al., 2011), H. asinina (Tahil & Dy, 2015), European abalone (Haliotis tuberculata) (Kavousi et al., 2022), and H. rufescens (Gómez-Reyes et al., 2023). In H. kamtschatkana, a decrease in pH due to the administration of CO₂ did not have a significant impact on the settlement rate (Crim et al., 2011). Similarly, the settlement rate of H. tuberculata at pH 7.7 was not substantially different from the control. Moreover, decreasing the pH to 7.65 did not significantly affect the settlement rate of H. iris (Espinel-Velasco et al., 2021). These result were likely because the examined pH conditions were still within an acceptable range for the species to settle. However, abalones capable of settling under low pH conditions are likely to experience shell damage. In the case of H. kamtschatkana, nearly all larvae that undergo settlement and metamorphosis at a concentration of 1,800 ppm exhibit anomalous shell development or lack a shell altogether. This condition could render them especially susceptible to predation by bottom-dwelling organisms in their natural habitat (Crim et al., 2011). Studies conducted on different abalone species have demonstrated that lower pH levels can negatively impact larval settlement rates. For example, a reduction in pH to 7.76, 7.57, and 7.41 significantly affects the settlement rate of H. asinina (Tahil & Dy, 2015). Gómez-Reyes et al. (2023) found that short-term reduction of pH had a noticeable effect on the settling rates of H. rufescens larvae after being incubated under pH levels of 7.8 and 7.6. The effect of pH on settlement is believed to be caused by the disturbance of abalone chemosensory signals, which hinders their ability to detect appropriate settlement sites under low pH conditions (Crim et al., 2011). Acidic conditions can also disrupt the capacity of crustose coralline algae to cover settlement plates. Tahil & Dy (2015) reported a decrease in the coverage of crustose coralline algae after 15 days of exposure to pH levels of 7.57 and 7.41 compared to the normal pH levels (Tahil & Dy, 2015).

During their early life stages, abalone are more susceptible to the negative impacts of ocean acidification. Therefore, numerous studies have been conducted to characterize the effect of pH on the development of abalone larvae. However, very few studies have analyzed the impact of pH on juvenile and adult abalone. A summary of the impacts of ocean acidification on the juvenile and adult of abalone (Haliotis spp.) is presented in Table 4.

Abalone shells consist of two different types of calcium carbonate polymorphs. The outer prismatic layer may be calcite, aragonite, or a combination of both (Cummings et al., 2019), whereas the inner shell (nacre) is made of aragonite (Thompson et al., 2000). Aragonite and calcite saturation may decrease in response to an increase in atmospheric CO₂ (Feely et al., 2012), which can influence the development of abalone shells during all life phases from early development to the juvenile and adult phases. Abalone shells provide protection from predators, noxious substances, and adverse environmental circumstances. Therefore, inhibiting abalone shell growth or reducing their mechanical strength may reduce their survival rate (Guo et al., 2022).

Several studies have examined the consequences of ocean acidification on the survival and shell growth of juvenile and adult abalone. Cunningham et al. (2016) investigated the survival and shell growth of both large and small juvenile H. iris. During the experiment, which lasted 100 days in autumn/winter and 100 days in spring/summer, the juveniles were grown at three pH levels (8.1, 7.8, and 7.6). Only large abalone exhibited significant variation in survival rate during the autumn/winter period between pH treatments. However, shell growth exhibited a notable deceleration at low pH levels in both small and larger abalone, regardless of the season. These findings indicate that the impact of pH on the survival rate depend on the size of the juvenile and the season, with the latter parameter being related to the sea water temperature. Particularly, the negative effects of acidification on abalone survical may be exacerbated by higher temperatures (Parker et al., 2013). However, a study conducted by Cummings et al. (2019) demonstrated that exposing juvenile H. iris to a reduced pH of 7.6 for a 4 months did not have any impact on their survival and growth rate. These discrepancies were likely due to difference between the carbonate saturation levels in the aforementioned studies, with the study by Cunningham et al. (2016), reporting a lower saturation, as well as disparities in feed and ambient conditions (Cummings et al., 2019).

The shell growth of juvenile H. tuberculata cultured for three months at four different pH levels (8.1, 7.8, 7.7, and 7.6) was also significantly slower at a lower pH (pH 7.6) compared to the control group. However, survival rates were not markedly different between pH treatments (Auzoux-Bordenave et al., 2020). Similarly, low pH conditions significantly affected juvenile H. discus hannai growth but had no effect on survival rate (Li et al., 2018). In contrast, exposing adult H. tuberculata to three pH levels for 15 days had no significant effect on survival rates and shell growth (Auzoux-Bordenave et al., 2021). These differences were likely attributable to species-specific variations, as well as the duration of the pH treatment. According to Gazeau et al. (2013), the reaction of shelled mollusks to low seawater pH and high pCO₂ appears to be species-specific and even varied within species.

The calcification process of abalone shells is known to be negatively impacted by ocean acidification. For example, juvenile H. discus hannai cultivated at pH 7.7 for three months exhibited shell surface deteriorated, with large regions of shell lacking periostracum. Additionally, their brown-colored shell apex was hard to recognize. SEM analysis further confirmed the occurrence of large-scale deterioration of the nacreous layer and periostracum layer in abalone raised at pH 7.7. Calcite dissolution was also noted as exposed nacreous layer surfaces that were uneven and rough (Li et al., 2018). Similarly, adult H. discus hannai and H. diversicolor exposed to high pCO₂ conditions for a year exhibited shell periostracum deterioration and reduced resistance to crushing force (Guo et al., 2023). Periostracum alterations were also observed in juvenile H. tuberculata that were cultivated under a pH 7.6. Furthermore, the shell mineral layers exhibited altered texture and porosity (Auzoux-Bordenave et al., 2020). Ocean acidification profoundly affects the thinner prismatic calcite layer, leading to a more noticeable erosion of the outer shell surface of juvenile H. iris at a lower pH of 7.6 (Cummings et al., 2019).

Ocean acidification has also been linked to the erosion and whitening of the periostracum layer, which is likely due to a decrease in the production of sulfated polysaccharides. These polysaccharides play a crucial role in preventing shell degradation (Cummings et al., 2019). Reduced calcification can occur due to either a direct impact on the dissolution of calcium carbonate (CaCO₃) or by indirect metabolic influences, such as the regulation of physiological and molecular processes involved in the formation of shell biominerals, including the expression of carbonic anhydrase, chitinase, tyrosinase, and other related factors. The effects of ocean acidification can also vary depending on whether aragonite or calcite are present in the shell, as aragonite is more sensitive to dissolution (Auzoux-Bordenave et al., 2020).

A further impact of thinning the abalone shell is that it can increase the vulnerability of the abalone to predation and pathogens (Barclay et al., 2020). Survival and health are also dependent on its shell because the shell is an external defense system (Muznebin, 2022). Some parasites or pathogens that attack abalone include shell-boring polychaete (Polydora hoplura) (Winkler et al., 2024), shell-boring sponge (Cliona sp.) (Huchette et al., 2006), herpes-like virus (Chang et al., 2005), and spherical viruses (Wang et al., 2004). Chang et al. (2005) first identified the herpes-like virus in 2005, and it was associated with the mass mortality of H. diversicolor supertexta in Taiwan in 2003. Spherical viruses can also cause high mortality and be linked to crack-shell disease (Wang et al., 2004).

Decreases in water pH can lead changes in the immune system (Kim et al., 2023), hemolymph, respiration, and excretion rates (Avignon et al., 2020). Low pH levels, individually and in combination with high temperature stress, lead to oxidative stress in abalone, which can induce an increase in the levels of malondialdehyde (MDA), H₂O₂, catalase (CAT), superoxide dismutase (SOD), caspase-3, and apoptotic rate in H. discus hannai (Kim et al., 2023). SOD, a main scavenger of O₂⁻ formed during stressful situations, inhibits the production of oxygen radicals following exposure to low-pH seawater, stimulating CAT activity (Liao et al., 2019). A decrease in pH, especially when combined with high temperatures, triggers reactive oxygen species (ROS) production, which in turn stimulates the expression of SOD and CAT to stabilize ROS (Kim et al., 2023). Under excessively high ROS conditions, the secreted antioxidant enzymes may not be sufficient to alleviate oxidative stress, leading to DNA damage, enzyme inactivation, and membrane lipid damage (Liao et al., 2019). In turn, increasing cell toxicity can also lead to caspase-3 upregulation, which sends apoptotic signals and leads to cell death (Kim et al., 2023). Another study reported that adult H. tuberculata cultured at pH 7.7 for 1 week, 2 months, and 4 months did not exhibit significant differences in phagocytosis efficiency compared to the control treatment. However, acid-base regulation, as measured by the pHT of hemolymph, was reduced at pH 7.7, leading to general internal acidosis.

Respiration and excretion rates in H. tuberculata also did not vary significantly between pH 8.0 and 7.7 after 4 months (Avignon et al., 2020). Presumably, the 0.3 pH unit drop in seawater pH might have posed a significant challenge for H. tuberculata during the first two months of exposure but the abalones began acclimate after 4 months. In this case, acidification had a greater impact on shell growth and structural integrity than on metabolism (Avignon et al., 2020). However, lowering the pH may also have a significant impact on metabolism in other abalone species. For example, Greenlip abalone (Haliotis laevigata) experienced a decrease in oxygen consumption rate that was significantly different from the control (pH 8.27) after being incubated in seawater with extreme high and extreme low pH (9.25, 6.72, 6.08) for 68 days (Harris et al., 1999). The abalone from this experiment most likely experienced alterations in oxygen-hemocyanin affinity, which is aggravated by anaerobic metabolic products, resulting in a decrease in oxygen consumption rate (Harris et al., 1999).

Reduced seawater pH can influence abalone food consumption; however, the effect varies according to the species and age. At pH 7.16, juvenile H. laevigata consumed considerably less food than at pH 7.76. Meanwhile, juvenile Blacklip abalone (Haliotis rubra) food consumption was significantly reduced at pH 7.16 when compared to pH 9.01 (Harris et al., 1999). Ocean acidification can diminish the ability of mollusks to detect food sources, resulting in a decrease in food consumption (Horwitz et al., 2020). However, there was no significant influence on food consumption of adult H. tuberculata between pH 8.01 and 7.7 after three months of treatment (Roussel et al., 2024).

However, cultivating abalone using formulated feed or seaweed could have negative environmental consequences. Animal waste, including feces and urine, as well as uneaten food, accounts for the majority of waste water from abalone cage farms. This increases the acidity of bottom sediment (Kang et al., 2016), alters benthos populations, and induces eutrophication (Lee et al., 2016). Integrated multi-trophic aquaculture (IMTA) offers an alternate approach to addressing this issue and combating ocean acidification. Hamilton et al. (2022) demonstrate that integrated multitrophic aquaculture has the ability to improve growth, reduce nutrient loads, elevate system pH, and alleviate environmental conditions when compared to standard single-species culture approaches.