Grazing under experimental hypercapnia and elevated temperature does not affect the radula of a chiton (Mollusca, Polyplacophora, Lepidopleurida)

Chitons (class Polyplacophora) are benthic grazing molluscs with an eight-part aragonitic shell armature. The radula, a serial tooth ribbon that extends internally more than half the length of the body, is mineralised on the active feeding teeth with iron magnetite apparently as an adaptation to constant grazing on rocky substrates. As the anterior feeding teeth are eroded they are shed and replaced with a new row. The efficient mineralisation and function of the radula could hypothetically be affected by changing oceans in two ways: changes in seawater chemistry (pH and pCO2) may impact the biomineralisation pathway, potentially leading to a weaker or altered density of the feeding teeth; rising temperatures could increase activity levels in these ectothermic animals, and higher feeding rates could increase wear on the feeding teeth beyond the animals' ability to synthesise, mineralise, and replace radular rows. We therefore examined the effects of pH and temperature on growth and integrity in the radula of the chiton Leptochiton asellus. Our experiment implemented three temperature (10, 15, 20 °C) and two pCO2 treatments (400 µatm, pH 8.0; 2000 µatm, pH 7.5) for six treatment groups. Animals (n = 50) were acclimated to the treatment conditions for a period of 4 weeks. This is sufficient time for growth of ca. 7-9 new tooth rows or 20% turnover of the mineralised portion. There was no significant difference in the number of new (non-mineralised) teeth or total tooth row count in any treatment. Examination of the radulae via SEM revealed no differences in microwear or breakage on the feeding cusps correlating to treatment groups. The shell valves also showed no signs of dissolution. As a lineage, chitons have survived repeated shifts in Earth's climate through geological time, and at least their radulae may be robust to future perturbations.石鳖（Chitons，分类学上属于多板纲Polyplacophora）是底栖牧食性软体动物，拥有由八片文石质壳片组成的护甲。齿舌（radula）是一条贯穿体内、长度超过体长一半的带状序列齿结构，其活跃摄食齿段会以磁铁矿（iron magnetite）进行矿化，这显然是对长期在岩石基质上牧食的适应性演化。当前端的摄食齿发生磨损后，便会脱落并由新的齿列替代。 齿舌高效的矿化过程与功能，理论上会通过两种途径受到海洋环境变化的影响：其一，海水化学性质（pH与pCO₂）的改变可能干扰生物矿化通路，进而导致摄食齿的强度下降或密度改变；其二，温度升高会提升这类变温动物的活动水平，更高的摄食速率会加剧摄食齿的磨损程度，超出动物合成、矿化并替换齿舌齿列的能力上限。因此，本研究针对细鳞石鳖（Leptochiton asellus）的齿舌生长与结构完整性，探究了pH与温度的影响。 本实验设置了3个温度梯度（10、15、20℃）与2个pCO₂处理组（400 µatm，pH 8.0；2000 µatm，pH 7.5），共计6个实验组。每组合计50只个体，在对应处理条件下驯化4周。该驯化时长足够生成约7~9条新齿列，或使矿化部分完成20%的更新。各处理组中新生成的（未矿化）齿数量与总齿列数均无显著差异。通过扫描电子显微镜（SEM）对齿舌进行观察发现，各处理组的摄食齿尖的微磨损与断裂情况均无显著差异。其壳瓣也未出现溶蚀迹象。作为一个演化支系，石鳖在地质历史中经历了多次地球气候变迁，至少其齿舌结构对未来的环境扰动具备较强的耐受性。