Gonatus fabricii, the boreoatlantic armhook squid, is by far the most abundant squid in the Arctic and subarctic North Atlantic where it spends its entire life (in contrast to other species). It forms an important prey species for certain marine mammals in particular. While squid tissue is digested quickly in stomachs, their so-called beaks resist digestion and are used to identify squid as prey and infer their body size. Morphologically, this squid has a ‘sibling’ that looks very similar, and it is common that such similarities in appearance between species confound our knowledge on distributions and specific biological traits.