Data from: Mismatch between the eye and the optic lobe in the giant squid

Giant squids (Architeuthis) are a legendary species among the cephalopods. They live in the deep sea and are well known for their enormous body and giant eyes. It has been suggested that their giant eyes are not adapted for the detection of either mates or prey at distance, but rather are best suited for monitoring very large predators, such as sperm whales, at distances exceeding 120 m and at a depth below 600 m (Nilsson et al. 2012 Curr. Biol. 22, 683–688. (doi:10.1016/j.cub.2012.02.031)). However, it is not clear how the brain of giant squids processes visual information. In this study, the optic lobe of a giant squid (Architeuthis dux, male, mantle length 89 cm), which was caught by local fishermen off the northeastern coast of Taiwan, was scanned using high-resolution magnetic resonance imaging in order to examine its internal structure. It was evident that the volume ratio of the optic lobe to the eye in the giant squid is much smaller than that in the oval squid (Sepioteuthis lessoniana) and the cuttlefish (Sepia pharaonis). Furthermore, the cell density in the cortex of the optic lobe is significantly higher in the giant squid than in oval squids and cuttlefish, with the relative thickness of the cortex being much larger in Architeuthis optic lobe than in cuttlefish. This indicates that the relative size of the medulla of the optic lobe in the giant squid is disproportionally smaller compared with these two cephalopod species. This morphological study of the giant squid brain, though limited only to the optic lobe, provides the first evidence to support that the optic lobe cortex, the visual information processing area in cephalopods, is well developed in the giant squid. In comparison, the optic lobe medulla, the visuomotor integration centre in cephalopods, is much less developed in the giant squid than other species. This finding suggests that, despite the giant eye and a full-fledged cortex within the optic lobe, the brain of giant squids has not evolved proportionally in terms of performing complex tasks compared with shallow-water cephalopod species.大王乌贼（Architeuthis）是头足类（cephalopods）中极具传奇色彩的物种。它们栖息于深海，以庞大躯体与硕大眼球闻名于世。有研究提出，其巨眼并非适配远距离探测配偶或猎物，反而最适于在距离超过120米、水深低于600米的环境中，监测抹香鲸这类巨型捕食者（Nilsson等，2012 Curr. Biol. 22, 683–688. (doi:10.1016/j.cub.2012.02.031)）。然而目前学界仍未明确大王乌贼的大脑如何处理视觉信息。 本研究针对台湾东北海域当地渔民捕获的一头雄性大王乌贼（Architeuthis dux，胴长89厘米），采用高分辨率磁共振成像（high-resolution magnetic resonance imaging）对其视神经叶（optic lobe）进行扫描，以解析其内部结构。研究显示，大王乌贼视神经叶与眼球的体积比，远低于曼氏无针乌贼（Sepioteuthis lessoniana）与虎斑乌贼（Sepia pharaonis）。此外，大王乌贼视神经叶皮层的细胞密度显著高于曼氏无针乌贼与虎斑乌贼，其视神经叶皮层相对厚度也远大于虎斑乌贼。这表明，相较于这两种头足类动物，大王乌贼视神经叶髓质（medulla）的相对体积不成比例地偏小。 本项仅针对视神经叶的大王乌贼大脑形态学研究，首次提供证据支持：作为头足类的视觉信息处理区域，大王乌贼的视神经叶皮层发育完善。相比之下，作为头足类的视觉运动整合中枢，大王乌贼的视神经叶髓质发育程度远不及其他物种。 这一发现提示，尽管大王乌贼拥有巨眼与发育完备的视神经叶皮层，但其大脑相较于浅水头足类物种，并未在复杂任务处理能力上出现相应的演化适配。