Three-dimensional brain atlas of pygmy squid, Idiosepius paradoxus, revealing the largest relative vertical lobe system volume among the cephalopods

Embryonic development of a centralised brain in coleoid cephalopods

The last common ancestor of cephalopods and vertebrates lived about 580 million years ago, yet coleoid cephalopods, comprising squid, cuttlefish and octopus, have evolved an extraordinary behavioural repertoire that includes learned behaviour and tool utilization. These animals also developed innovative advanced defence mechanisms such as camouflage and ink release. They have evolved unique life cycles and possess the largest invertebrate nervous systems. Thus, studying coleoid cephalopods provides a unique opportunity to gain insights into the evolution and development of large centralised nervous systems. As non-model species, molecular and genetic tools are still limited. However, significant insights have already been gained to deconvolve embryonic brain development. Even though coleoid cephalopods possess a typical molluscan circumesophageal bauplan for their central nervous system, aspects of its development are reminiscent of processes observed in vertebrates as well, such as long-distance neuronal migration. This review provides an overview of embryonic coleoid cephalopod research focusing on the cellular and molecular aspects of neurogenesis, migration and patterning. Additionally, we summarize recent work on neural cell type diversity in embryonic and hatchling cephalopod brains. We conclude by highlighting gaps in our knowledge and routes for future research.

Diffusion MRI Connections in the Octopus Brain

Using high angle resolution diffusion magnetic resonance imaging (HARDI) with fiber tractography analysis we map out a meso-scale connectome of the Octopus bimaculoides brain. The brain of this cephalopod has a qualitatively different organization than that of vertebrates, yet it exhibits complex behavior, an elaborate sensory system and high cognitive abilities. Over the last 60 years wide ranging and detailed studies of octopus brain anatomy have been undertaken, including classical histological sectioning/staining, electron microscopy and neuronal tract tracing with injected dyes. These studies have elucidated many neuronal connections within and among anatomical structures. Diffusion MRI based tractography utilizes a qualitatively different method of tracing connections within the brain and offers facile three-dimensional images of anatomy and connections that can be quantitatively analyzed. Twenty-five separate lobes of the brain were segmented in the 3D MR images of each of three samples, including all five sub-structures in the vertical lobe. These parcellations were used to assay fiber tracings between lobes. The connectivity matrix constructed from diffusion MRI data was largely in agreement with that assembled from earlier studies. The one major difference was that connections between the vertical lobe and more basal supra-esophageal structures present in the literature were not found by MRI. In all, 92 connections between the 25 different lobes were noted by diffusion MRI: 53 between supra-esophageal lobes and 26 between the optic lobes and other structures. These represent the beginnings of a mesoscale connectome of the octopus brain.

Toward an MRI-Based Mesoscale Connectome of the Squid Brain

Using high-resolution diffusion magnetic resonance imaging (dMRI) and a suite of old and new staining techniques, the beginnings of a multi-scale connectome map of the squid brain is erected. The first of its kind for a cephalopod, this includes the confirmation of 281 known connections with the addition of 145 previously undescribed pathways. These and other features suggest a suite of functional attributes, including (1) retinotopic organization through the optic lobes and into other brain areas well beyond that previously recognized, (2) a level of complexity and sub-division in the basal lobe supporting ideas of convergence with the vertebrate basal ganglia, and (3) differential lobe-dependent growth rates that mirror complexity and transitions in ontogeny.

Development of a contrast-enhanced micro computed tomography protocol for the oval squid (Sepioteuthis lessoniana) brain

Coleoid cephalopods (squid, cuttlefish, and octopus) have a well-developed and complex central nervous system. Its absolute size is the largest among invertebrates, and the brain-to-body mass ratio is larger than that of fish and reptiles and equivalent to that of birds and mammals. Although a number of histological studies have been conducted on the brains of cephalopods, most of them used a light microscope or an electron microscope, which show the microstructure of the brain, but often cannot image the whole brain instantaneously. Of late, micro computed tomography (CT) has gained popularity for imaging animal brains because it allows for noninvasive three-dimensional (3D) reconstruction and preprocessing that are not cumbersome. To perform micro-CT on cephalopod brains, we first tested conditions suitable for preprocessing, paying special attention to staining conditions that would provide high contrast images. Four agents, iodine in 99.5% ethanol, iodine potassium iodide in water (IKI), phosphotungstic acid in 70% ethanol, and nonionic iodinated contrast agent in water, were tested at various concentrations and durations on brain of juvenile oval squid. To evaluate the quality of staining, we calculated the contrast ratio of the two-dimensional (2D) images and compared 3D segmentation of the best and worst 2D images. We concluded that 3% IKI staining for 7 days was the best combination to enhance the images contrast of the oval squid brain, in which each brain lobe was clearly detected and 3D segmentation of the whole brain was possible. The wider applicability of this preprocessing method for micro-CT of the brains of other cephalopods is discussed.

A Brain Atlas of the Long Arm Octopus, Octopus minor

Cephalopods have the most advanced nervous systems and intelligent behavior among all invertebrates. Their brains provide comparative insights for understanding the molecular and functional origins of the human brain. Although brain maps that contain information on the organization of each subregion are necessary for a study on the brain, no whole brain atlas for adult cephalopods has been constructed to date. Here, we obtained sagittal and coronal sections covering the entire brain of adult Octopus minor (Sasaki), which belongs to the genus with the most species in the class Cephalopoda and is commercially available in East Asia throughout the year. Sections were stained using Hematoxylin and Eosin (H&E) to visualize the cellular nuclei and subregions. H&E images of the serial sections were obtained at 30~70-µm intervals for the sagittal plain and at 40~80-µm intervals for the coronal plain. Setting the midline point of the posterior end as the fiducial point, we also established the distance coordinates of each image. We found that the brain had the typical brain structure of the Octopodiformes. A number of subregions were discriminated by a Hematoxylin-positive layer, the thickness and neuronal distribution pattern of which varied markedly depending upon the region. We identified more than 70 sub-regions based on delineations of representative H&E images. This is the first brain atlas, not only for an Octopodiformes species but also among adult cephalopods, and we anticipate that this atlas will provide a valuable resource for comparative neuroscience research.

Calcium imaging method to visualize the spatial patterns of neural responses in the pygmy squid, Idiosepius paradoxus, central nervous system

BACKGROUND

Cephalopods exhibit unique behaviors such as camouflage and tactile learning. The brain functions correlated to these behaviors have long been analyzed through behavioral observations of animals subject to surgical manipulation or electrical stimulation of brain lobes. However, physiological methods have rarely been introduced to investigate the functions of each individual lobe, though physiological work on giant axons and slices of the vertical lobe system of the cephalopods have provided deep insights into ion conductance of nerves and long-term synaptic plasticity. The lack of in vivo physiological work is partly due to difficulties in immobilizing the brain which is contained within the soft body and applying calcium indicators to the cephalopod central nervous system.

NEW METHOD

We here present a calcium imaging method to visualize neural responses in the central nervous system of the smallest squid, Idiosepius paradoxus.

RESULTS

We injected calcium indicator Cal-520 into the brachial lobes and revealed a spatiotemporal pattern of neural responses to the electrical stimulations of the axial nerve cord in the first arm.

COMPARISON WITH EXISTING METHODS

We established a method to immobilize the central nervous system which is contained within the soft body and record the calcium responses from the intact central nervous system.

CONCLUSIONS

Our method provides a novel approach to investigate the mechanisms of how the characteristic organization of the cephalopod brain functions to induce their unique behaviors.

Morphological changes of the optic lobe from late embryonic to adult stages in oval squids Sepioteuthis lessoniana

The optic lobe is the largest brain area within the central nervous system of cephalopods and it plays important roles in the processing of visual information, the regulation of body patterning, and locomotive behavior. The oval squid Sepioteuthis lessoniana has relatively large optic lobes that are responsible for visual communication via dynamic body patterning. It has been observed that the visual behaviors of oval squids change as the animals mature, yet little is known about how the structure of the optic lobes changes during development. The aim of the present study was to characterize the ontogenetic changes in neural organization of the optic lobes of S. lessoniana from late embryonic stage to adulthood. Magnetic resonance imaging and micro-CT scans were acquired to reconstruct the 3D-structure of the optic lobes and examine the external morphology at different developmental stages. In addition, optic lobe slices with nuclear staining were used to reveal changes in the internal morphology throughout development. As oval squids mature, the proportion of the brain making up the optic lobes increases continuously, and the optic lobes appear to have a prominent dent on the ventrolateral side. Inside the optic lobe, the cortex and the medulla expand steadily from the late embryonic stage to adulthood, but the cell islands in the tangential zone of the optic lobe decrease continuously in parallel. Interestingly, the size of the nuclei of cells within the medulla of the optic lobe increases throughout development. These findings suggest that the optic lobe undergoes continuous external morphological change and internal neural reorganization throughout the oval squid's development. These morphological changes in the optic lobe are likely to be responsible for changes in the visuomotor behavior of oval squids from hatching to adulthood.

Complex Visual Adaptations in Squid for Specific Tasks in Different Environments

In common with their major competitors, the fish, squid are fast moving visual predators that live over a great range of depths in the ocean. Both squid and fish show a variety of adaptations with respect to optical properties, receptors and their underlying neural circuits, and these adaptations are often linked to the light conditions of their specific niche. In contrast to the extensive investigations of adaptive strategies in fish, vision in response to the varying quantity and quality of available light, our knowledge of visual adaptations in squid remains sparse. This study therefore undertook a comparative study of visual adaptations and capabilities in a number of squid species collected between 0 and 1,200 m. Histology, magnetic resonance imagery (MRI), and depth distributions were used to compare brains, eyes, and visual capabilities, revealing that the squid eye designs reflect the lifestyle and the versatility of neural architecture in its visual system. Tubular eyes and two types of regional retinal deformation were identified and these eye modifications are strongly associated with specific directional visual tasks. In addition, a combination of conventional and immuno-histology demonstrated a new form of a complex retina possessing two inner segment layers in two mid-water squid species which they rhythmically move across a broad range of depths (50–1,000 m). In contrast to their relatives with the regular single-layered inner segment retina live in the upper mesopelagic layer (50–400 m), the new form of retinal interneuronal layers suggests that the visual sensitivity of these two long distance vertical migrants may increase in response to dimmer environments.