Cephalopod biology and care, a COST FA1301 (CephsInAction) training school: anaesthesia and scientific procedures

Molecular and Morphological Circuitry of the Octopus Sucker Ganglion

The octopus sucker is a profoundly complex sensorimotor structure. Each of the hundreds of suckers that line the octopus arm can move independently or in concert with one another. These suckers also contain an intricate sensory epithelium, enriched with chemotactile receptors. Much of the massive nervous system embedded in the octopus arm mediates control of the suckers. Each arm houses a large axial nerve cord (ANC), which features local enlargements corresponding to each sucker. There is also a sucker ganglion, a peripheral nervous element, situated in the stalk of every sucker. The structure and function of the sucker ganglion remain obscure. We examined the cellular organization and molecular composition of the sucker ganglion in Octopus bimaculoides. The sucker ganglion has an ellipsoid shape and features an unusual organization: the neuropil of the ganglion is distributed as a cap aborally (away from the sucker) and a small pocket orally (toward the sucker), with neuronal cell bodies concentrated in the space between. Using in situ hybridization, we detected positive expression of sensory (PIEZO) and motor (LHX3 and MNX) neuron markers in the sucker ganglion cell bodies. Nerve fibers spread out from the sucker ganglion, targeting the surrounding sucker musculature and the oral roots extending to the ANC. Our results indicate that the sucker ganglion is composed of both sensory and motor elements and suggest that this ganglion is not a simple relay for the ANC, but facilitates local reflexes for each sucker.

Neuronal segmentation in cephalopod arms

Prehensile arms are among the most remarkable features of the octopus, but little is known about the neural circuitry controlling arm movements. Here, we report on the cellular and molecular organization of the arm nervous system, focusing on its massive axial nerve cords (ANCs). We found that the ANC is segmented. In transverse cross sections, the ANC cell body layer wraps around the neuropil with no apparent segregation of sensory and motor neurons. In longitudinal sections, however, ANC neurons form segments, setting up a modular organization to the adjoining neuropil. ANC nerves exit in the septa between segments, and for each sucker, form a spatial topographic map ("suckerotopy"). A strong link between ANC segmentation and flexible sucker-laden arms is confirmed by comparative study of squid arms and tentacles. These ANC modules offer a template for modeling the motor control of soft tissues and provide a compelling example of nervous system segmentation in molluscs.

Neuronal segmentation in cephalopod arms

The prehensile arms of the cephalopod are among these animals most remarkable features, but the neural circuitry governing arm and sucker movements remains largely unknown. We studied the neuronal organization of the adult axial nerve cord (ANC) of Octopus bimaculoides with molecular and cellular methods. The ANCs, which lie in the center of every arm, are the largest neuronal structures in the octopus, containing four times as many neurons as found in the central brain. In transverse cross section, the cell body layer (CBL) of the ANC wraps around its neuropil (NP) with little apparent segregation of sensory and motor neurons or nerve exits. Strikingly, when studied in longitudinal sections, the ANC is segmented. ANC neuronal cell bodies form columns separated by septa, with 15 segments overlying each pair of suckers. The segments underlie a modular organization to the ANC neuropil: neuronal cell bodies within each segment send the bulk of their processes directly into the adjoining neuropil, with some reaching the contralateral side. In addition, some nerve processes branch upon entering the NP, forming short-range projections to neighboring segments and mid-range projections to the ANC segments of adjoining suckers. The septa between the segments are employed as ANC nerve exits and as channels for ANC vasculature. Cellular analysis establishes that adjoining septa issue nerves with distinct fiber trajectories, which across two segments (or three septa) fully innervate the arm musculature. Sucker nerves also use the septa, setting up a nerve fiber "suckerotopy" in the sucker-side of the ANC. Comparative anatomy suggests a strong link between segmentation and flexible sucker-laden arms. In the squid Doryteuthis pealeii, the arms and the sucker-rich club of the tentacles have segments, but the sucker-poor stalk of the tentacles does not. The neural modules described here provide a new template for understanding the motor control of octopus soft tissues. In addition, this finding represents the first demonstration of nervous system segmentation in a mollusc.

Signaling Ligand Heterogeneities in the Peduncle Complex of the Cephalopod Mollusc Octopus bimaculoides

INTRODUCTION

The octopus peduncle complex is an agglomeration of neural structures with remarkably diverse functional roles. The complex rests on the optic tract, between the optic lobe and the central brain, and comprises the peduncle lobe proper, the olfactory lobe, and the optic gland. The peduncle lobe regulates visuomotor behaviors, the optic glands control sexual maturation and maternal death, and the olfactory lobe is thought to receive input from the olfactory organ. Recent transcriptomic and metabolomic studies have identified candidate peptide and steroid ligands in the Octopus bimaculoides optic gland.

METHODS

With gene expression for these ligands and their biosynthetic enzymes, we show that optic gland neurochemistry extends beyond the traditional optic gland secretory tissue and into lobular territories.

RESULTS

A key finding is that the classically defined olfactory lobe is itself a heterogeneous territory and includes steroidogenic territories that overlap with cells expressing molluscan neuropeptides and the synthetic enzyme dopamine beta-hydroxylase.

CONCLUSION

Our study reveals the neurochemical landscape of the octopus peduncle complex, highlighting the unexpected overlap between traditionally defined regions.

Steroid hormones of the octopus self-destruct system

Among all invertebrates, soft-bodied cephalopods have the largest central nervous systems and the greatest brain-to-body mass ratios, yet unlike other big-brained animals, cephalopods are unusually short lived.^1-5 Primates and corvids survive for many decades, but shallow-water octopuses, such as the California two-spot octopus (Octopus bimaculoides), typically live for only 1 year.^6,7 Lifespan and reproduction are controlled by the principal neuroendocrine center of the octopus: the optic glands, which are functional analogs to the vertebrate pituitary gland.^8-10 After mating, females steadfastly brood their eggs, begin fasting, and undergo rapid physiological decline, featuring repeated self-injury and leading to death.¹¹ Removal of the optic glands completely reverses this life history trajectory,¹⁰ but the signaling factors underlying this major life transition are unknown. Here, we characterize the major secretions and steroidogenic pathways of the female optic gland using mass spectrometry techniques. We find that at least three pathways are mobilized to increase synthesis of select sterol hormones after reproduction. One pathway generates pregnane steroids, known in other animals to support reproduction.^12-16 Two other pathways produce 7-dehydrocholesterol and bile acid intermediates, neither of which were previously known to be involved in semelparity. Our results provide insight into invertebrate cholesterol pathways and confirm a remarkable unity of steroid hormone biology in life history processes across Bilateria.

Genome and transcriptome mechanisms driving cephalopod evolution

Cephalopods are known for their large nervous systems, complex behaviors and morphological innovations. To investigate the genomic underpinnings of these features, we assembled the chromosomes of the Boston market squid, Doryteuthis (Loligo) pealeii, and the California two-spot octopus, Octopus bimaculoides, and compared them with those of the Hawaiian bobtail squid, Euprymna scolopes. The genomes of the soft-bodied (coleoid) cephalopods are highly rearranged relative to other extant molluscs, indicating an intense, early burst of genome restructuring. The coleoid genomes feature multi-megabase, tandem arrays of genes associated with brain development and cephalopod-specific innovations. We find that a known coleoid hallmark, extensive A-to-I mRNA editing, displays two fundamentally distinct patterns: one exclusive to the nervous system and concentrated in genic sequences, the other widespread and directed toward repetitive elements. We conclude that coleoid novelty is mediated in part by substantial genome reorganization, gene family expansion, and tissue-dependent mRNA editing.

E Pluribus Octo - Building Consensus on Standards of Care and Experimentation in Cephalopod Research; a Historical Outlook

The Directive 2010/63/EU "on the protection of animals used for scientific purposes" originally induced some concern among cephalopod researchers, because of the inclusion of cephalopod mollusks as the only invertebrates among the protected species. Here we reflect on the challenges and issues raised by the Directive on cephalopod science, and discuss some of the arguments that elicited discussion within the scientific community, to facilitate the implementation of the Directive 2010/63/EU in the scientific research context. A short overview of the aims of the COST Action FA1301 "CephsInAction," serves as a paradigmatic instance of a pragmatic and progressive approach adopted to respond to novel legislative concerns through community-building and expansion of the historical horizon. Between 2013 and 2017, the COST Action FA1301 has functioned as a hub for consolidation of the cephalopod research community, including about 200 representatives from 21 countries (19 European). Among its aims, CephsInAction promoted the collection, rationalization, and diffusion of knowledge relevant to cephalopods. In the Supplementary Material to this work, we present the translation of the first-published systematic set of guidelines on the care, management and maintenance of cephalopods in captivity (Grimpe, 1928), as an example of the potential advantages deriving from the confluence of pressing scientific concerns and historical interests.

Multiple optic gland signaling pathways implicated in octopus maternal behaviors and death

Post-reproductive life in the female octopus is characterized by an extreme pattern of maternal care: the mother cares for her clutch of eggs without feeding until her death. These maternal behaviors are eradicated if the optic glands, the octopus analog of the vertebrate pituitary gland, are removed from brooding females. Despite the optic gland's importance in regulating maternal behavior, the molecular features underlying optic gland function are unknown. Here, we identify major signaling systems of the Octopus bimaculoides optic gland. Through behavioral analyses and transcriptome sequencing, we report that the optic gland undergoes remarkable molecular changes that coincide with transitions between behavioral stages. These include the dramatic upregulation and downregulation of catecholamine, steroid, insulin and feeding peptide pathways. Transcriptome analyses in other tissues demonstrate that these molecular changes are not generalized markers of senescence, but instead, specific features of the optic glands. Our study expands the classic optic gland-pituitary gland analogy and more specifically, it indicates that, rather than a single 'self-destruct' hormone, the maternal optic glands employ multiple pathways as systemic hormonal signals of behavioral regulation.