Potential energetic and oxygenic benefits to unstable photosymbiosis in the cladobranch slug, <i>Berghia stephanieae</i> (Nudibranchia, Aeolidiidae)

Some cladobranch sea slugs host algae (Symbiodiniaceae), forming stable or unstable photosymbiotic relationships. Although some benefits from retaining symbionts have been described in stable photosymbioses, unstable photosymbioses remain largely uninvestigated. We examined two potential benefits – nutrition and oxygen produced via photosynthesis – in the unstable cladobranch model species, <i>Berghia stephanieae</i>. To investigate potential nutritive benefits, we conducted transmission electron microscopy and observed both partially digested symbionts and lipid droplets, indicating that <i>B. stephanieae</i> benefits energetically from hosting zooxanthellae through digestion. Since increased temperatures can cause oxygen limitation, any oxygenic benefits <i>B. stephanieae</i> receives from photosynthesis could influence their thermal tolerance, allowing photosynthetic slugs to withstand higher temperatures than specimens where photosynthesis is limited or absent. To assess this, we measured the maximum temperature they can withstand before succumbing to heat-shock under three light intensities (0, 100 and 700 µmol m^-2s^-1). Oxygen uptake was measured before and after heat-shock to determine whether uptake was affected by thermal stress. Slugs exposed to high light displayed significantly lower thermal limits than those at zero or moderate light, indicating exposure to acute high light negatively impacts thermal tolerance. Lastly, we assessed if and how light affects juvenile development and survival. Juveniles exposed to moderate light survived longest, while both other light intensities reduced survival. These investigations demonstrate that unstable photosymbiosis provides <i>B. stephanieae</i> with nutritive benefits during different ontogenetic stages. Oxygenic benefits are less clear, as slugs exposed to thermal stress in dark and under moderate light did not display different thermal tolerances.部分枝鳃类海蛞蝓可与虫黄藻科（Symbiodiniaceae）藻类建立共生关系，形成稳定或不稳定的光合共生体系。尽管在稳定光合共生关系中，保留共生体所能带来的部分益处已有相关报道，但不稳定光合共生关系的研究仍相对匮乏。本研究以不稳定共生的枝鳃类模式物种贝氏海蛞蝓（Berghia stephanieae）为研究对象，探究了其通过光合共生可能获得的两项潜在益处：营养供给与光合产氧。 为探究潜在的营养益处，我们开展了透射电子显微镜（Transmission Electron Microscopy）观测，同时观察到部分被消化的共生体与脂滴，这表明贝氏海蛞蝓可通过消化虫黄藻获得能量收益。由于温度升高会引发氧气限制，贝氏海蛞蝓从光合作用中获得的产氧相关益处，可能会影响其热耐受性，使得具备光合共生能力的个体比光合作用受抑制或无共生的个体能耐受更高温度。为验证这一假设，我们在三种光照强度（0、100和700 µmol·m⁻²·s⁻¹）下，测定了贝氏海蛞蝓在热休克前所能耐受的最高温度；同时在热休克前后测定了其耗氧量，以明确热胁迫是否会对耗氧量产生影响。实验结果显示，相较于光照强度为0或中等光照组的个体，高光照组的海蛞蝓表现出显著更低的热耐受极限，这表明急性高光照暴露会对热耐受性产生负面影响。最后，我们评估了光照是否以及如何影响幼体的发育与存活情况：处于中等光照条件下的幼体存活率最高，而其余两种光照强度均会降低幼体存活率。本研究表明，不稳定光合共生关系在不同个体发育阶段均能为贝氏海蛞蝓提供营养益处；但产氧相关的益处尚不明确，因为处于黑暗与中等光照条件下、经受热胁迫的海蛞蝓并未表现出显著的热耐受性差异。