Neurally underdeveloped cuttlefish newborns exhibit social learning

Oxygen loss compromises growth and cognition of cuttlefish newborns

Ocean deoxygenation and standing levels of hypoxia are shrinking fundamental niches, particularly in coastal areas, yet documented repercussions on species development and behaviour are limited. Here, we tackled the impacts of deoxygenation (7 mg O2 l-1), mild hypoxia (nocturnal 5 mg O2 l-1) and severe hypoxia (nocturnal 2 mg O2 l-1) on cuttlefish (Sepia officinalis) development (hatching success, development time, mantle length), cognition (ability to learn individually and socially) and behaviour (ability to camouflage and to explore its surroundings spatially). We found that hypoxia yielded lower survival rates, smaller body sizes and inhibited predatory (increased latency to attack the prey) and anti-predator (camouflage) behaviours. Acute and chronic exposure to low oxygen produced similar effects on cognition (inability to socially learn, increased open-field activity levels, no changes in thigmotaxis). It is thus expected that, although cuttlefish can withstand oxygen limitation to a certain degree, expanding hypoxic zones will diminish current habitat suitability.

Allometry for Eyes and Optic Lobes in Oval Squid (Sepioteuthis lessoniana) with Special Reference to Their Ontogenetic Asymmetry

Eyes develop in relation to body size and brain area for visual processing in some vertebrates. Meanwhile, it is well known that many animals exhibit left–right asymmetry in both morphology and behavior, namely, lateralization. However, it remains unclear whether the eyes and visual processing brain areas synchronously develop for their asymmetry. Oval squid (Sepioteuthis lessoniana) exhibits lateralization of optic lobe volume and left or right eye usage toward specific targets during their ontogeny. We address the question of how left–right asymmetry of the eyes and optic lobes exhibit an allometric pattern. To examine this question, we estimated the left and right volumes of eyes and optic lobes using microcomputed tomography. We found that, for the optic lobe volume, the right enlargement that appeared at ages 45 and 80 days then shifted to the left at age 120 days. In contrast, the volume of eyes did not show any left–right asymmetries from hatching to age 120 days. We also found that the volume of the eyes and optic lobes showed a slower increase than that of the whole-body size. Within these two visually related organs, the eyes grew faster than the optic lobes until age 120 days. These results are discussed in the context of the survival strategy of oval squid that form schools, two months post-hatching.

Potential evidence of peripheral learning and memory in the arms of dwarf cuttlefish, Sepia bandensis

CREB (cAMP response element-binding) transcription factors are conserved markers of memory formation in the brain and peripheral circuits. We provide immunohistochemical evidence of CREB phosphorylation in the dwarf cuttlefish, Sepia bandensis, following the inaccessible prey (IP) memory experiment. During the IP experiment, cuttlefish are shown prey enclosed in a transparent tube, and tentacle strikes against the tube decrease over time as the cuttlefish learns the prey is inaccessible. The cues driving IP learning are unclear but may include sensory inputs from arms touching the tube. The neural activity marker, anti-phospho-CREB (anti-pCREB) was used to determine whether IP training stimulated cuttlefish arm sensory neurons. pCREB immunoreactivity occurred along the oral surface of the arms, including the suckers and epithelial folds surrounding the suckers. pCREB increased in the epithelial folds and suckers of trained cuttlefish. We found differential pCREB immunoreactivity along the distal-proximal axis of trained arms, with pCREB concentrated distally. Unequal CREB phosphorylation occurred among the 4 trained arm pairs, with arm pairs 1 and 2 containing more pCREB. The resulting patterns of pCREB in trained arms suggest that the arms obtain cues that may be salient for learning and memory of the IP experiment.