Metolachlor herbicides are derived from the chloroacetamide chemical family of which there are the S- and R-metolachlor isomers. S-metolachlor is a selective herbicide that inhibits cell division and mitosis via enzyme interference. The herbicide is used globally in agriculture and studies report adverse effects in aquatic organisms; however, there are no studies investigating sub-lethal effects of S-metolachlor on swim bladder formation, mitochondrial ATP production, nor light–dark preference behaviors in fish. These endpoints are relevant for larval locomotor activity and metabolism. To address these knowledge gaps, we exposed zebrafish embryos/larvae to various concentrations of S-metolachlor (0.5–50 µM) over early development. S-metolachlor affected survival, hatching percentage, and increased developmental deformities at concentrations of 50 µM and above. Exposure levels as high as 200 µM for 24 and 48 h did not alter oxygen consumption rates in zebrafish, and there were no changes detected in endpoints related to mitochondrial oxidative phosphorylation. We observed impairment of swim bladder inflation at 50 µM in 6 dpf larvae. To elucidate mechanisms related to this, we measured relative transcript abundance for genes associated with the swim bladder (smooth muscle alpha (α)-2 actin, annexin A5, pre-B-cell leukemia homeobox 1a). Smooth muscle alpha (α)-2 actin mRNA levels were reduced in fish exposed to 50 µM while annexin A5 mRNA levels were increased in abundance, corresponding to reduced swim bladder size in larvae. A visual motor response test revealed that larval zebrafish exhibited some hyperactivity in the light with exposure to the herbicide and only the highest dose tested (50 µM) resulted in hypoactivity in the dark cycle. Regression analysis indicated that there was a positive relationship between surface area of the swim bladder and distance traveled, and the size of the swim bladder explained ~10–14% in the variation for total distance moved. Lastly, we tested larvae in a light dark preference test, and we did not detect any altered behavioral response to any concentration tested. Here we present new data on sublethal endpoints associated with exposure to the herbicide S-metolachlor and demonstrate that this chemical may disrupt transcripts associated with swim bladder formation and morphology, which could ultimately affect larval zebrafish activity. These data are expected to contribute to further risk assessment guidelines for S-metolachlor in aquatic ecosystems.

甲草胺除草剂衍生自氯乙酰胺化学家族，其中有S-和R-甲草胺异构体。S-异丙甲草胺是一种选择性除草剂，可通过酶干扰抑制细胞分裂和有丝分裂。除草剂已在全球范围内用于农业，并且研究报告了其对水生生物的不利影响。但是，目前尚无任何研究研究灭草胺对游泳膀胱形成，线粒体ATP产生以及鱼的明暗偏好行为的亚致死作用。这些终点与幼虫的运动活性和代谢有关。为了解决这些知识空白，我们在早期开发过程中将斑马鱼的胚胎/幼虫暴露于各种浓度的异丙甲草胺（0.5–50 µM）。异丙甲草胺会影响存活率，孵化率，浓度大于等于50 µM时发育畸形增加。暴露水平高达200 µM的24和48 h不会改变斑马鱼的耗氧率，并且在与线粒体氧化磷酸化有关的终点均未检测到变化。我们在6 dpf幼虫中观察到50 µM的游泳膀胱充气受到损害。为了阐明与此相关的机制，我们测量了与游泳膀胱相关的基因的相对转录本丰度（平滑肌α（α）-2肌动蛋白，膜联蛋白A5，B细胞白血病前同源盒1a）。暴露于50 µM的鱼中的平滑肌α（α）-2肌动蛋白mRNA水平降低，而膜联蛋白A5mRNA水平大量增加，这对应于幼虫的游泳膀胱尺寸减小。视觉运动反应测试表明，幼虫斑马鱼在暴露于除草剂的光照下表现出某些过度活跃，只有最高测试剂量（50 µM）在黑暗周期中会导致过度活跃。回归分析表明，游泳囊的表面积与行进距离之间存在正相关关系，并且游泳囊的大小解释了总移动距离的变化约为10–14％。最后，我们在暗暗偏好测试中测试了幼虫，并且没有检测到对任何测试浓度的行为变化。在这里，我们介绍了与暴露于除草剂S-异丙甲草胺相关的亚致死终点的新数据，并证明了该化学物质可能破坏与游泳膀胱形成和形态有关的转录本，最终可能影响幼虫斑马鱼的活动。预计这些数据将有助于进一步提高水生生态系统中甲草胺的风险评估指南。