Digestive system functionality is a key process linked to larval recruitment and survival. However, little is known about organ development and enzyme maturation of the digestive system of North Sea Atlantic herring (Clupea harengus). In this study, herring larvae were reared at 13 °C from hatching to 69 day post hatch, covering four developmental stages: (1) yolk sac (8–9 mm), (2) pre-flexion (9–14 mm), (3) flexion (12–18 mm) and (4) post-flexion stages (15–30 mm). Combined histological (semi-quantitative scoring) and enzyme analyses (pancreatic and intestinal) showed that developmental stages are strongly linked to physiological changes. The larvae lack a functional stomach and use the intestine as the primary site of digestion which is mainly supported by pancreatic enzyme activity. The intestine acquired adult enzymatic digestive features with a functional brush border at the end of the flexion stage and pyloric ceca started to develop during the post-flexion stage. The transition from pre-flexion to flexion stage and the end of the post-flexion stage are energetically taxing periods as indicated by a reduced number and size of liver vacuoles. Based on these findings, we consider these moments as critical periods, where herring larvae could be dramatically affected by suboptimal feeding conditions in the field. This implies that pre-flexion stage larvae with low or no liver reserves may not be able to proceed to the next developmental stage. Hence, the level of energy storage in first-feeding larvae needs to be examined for its use as a field indicator of survival and development.

消化系统功能是与幼体募集和生存相关的关键过程。然而，关于北海大西洋鲱鱼（Clupea harengus）消化系统的器官发育和酶成熟的了解却很少。）。在这项研究中，从孵化到孵化后69天，将鲱鱼幼体在13°C下饲养，涉及四个发育阶段：（1）卵黄囊（8–9毫米），（2）屈曲前（9–14毫米）， （3）屈曲（12–18 mm）和（4）屈曲后阶段（15–30 mm）。组织学（半定量评分）和酶分析（胰腺和肠道）的结合显示，发育阶段与生理变化密切相关。幼虫缺乏功能性的胃，并以肠为主要消化部位，主要由胰腺酶活性支持。小肠在屈曲阶段结束时获得了具有功能性刷状边界的成人酶消化功能，并且在屈曲后阶段开始出现幽门盲肠。从屈曲前到屈曲阶段的过渡以及屈曲后阶段的结束是精力充沛的时期，如肝液泡数量和大小的减少所表明的。基于这些发现，我们将这些时刻视为关键时期，在该时期中，鲱鱼幼虫可能会受到田间次优饲喂条件的严重影响。这意味着屈曲前期的幼虫肝脏储备低或没有，可能无法进入下一个发育阶段。因此，需要检查初生幼虫的能量储存水平，以将其用作生存和发育的现场指标。田间次优饲喂条件可能会极大地影响鲱鱼幼虫的生长。这意味着屈曲前期的幼虫肝脏储备低或没有，可能无法进入下一个发育阶段。因此，需要检查初生幼虫的能量储存水平，以将其用作生存和发育的现场指标。田间次优饲喂条件可能会极大地影响鲱鱼幼虫的生长。这意味着屈曲前期的幼虫肝脏储备低或没有，可能无法进入下一个发育阶段。因此，需要检查初生幼虫的能量储存水平，以将其用作生存和发育的现场指标。