Structural and functional connectivity of squid chromatophores

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Skin-color-changes in coleoid cephalopods (squid, cuttlefish, octopus) were described by Aristotle more than 2,000 years ago (Borelli et al. 2006), but these chromogenic behaviors have been studied in relatively few taxa. Nearly all work in squid has focused on several species in the family Loliginidae that inhabit coastal shelf environments that include visually complex benthic features. Skin-color patterns in such species underlie crypsis, intra-specific signaling and deimatic displays (Hanlon & Messenger 1996).

Dosidicus gigas, the only species in the genus, is an open-ocean squid of the family Ommastrephidae and arguably in a different sub-order from loliginids (Jereb et al. 2010). This species typically inhabits mesopelagic depths of 200-400 m during daytime, a region with little ambient light and no static visual features (Gilly et al. 2012; Stewart et al. 2013). At least two ommastrephids (Dosidicus and Sthenoteuthis oualaniensis) show novel chromogenic behaviors not reported in loliginids (Rosen et al. 2015). “Flashing” is a rapid oscillation of red over the entire body, with amplitude, frequency (2-4 Hz) and phase being under active control. Flashing occurs during intra-specific encounters and is analogous to signaling displays in loliginids. “Flickering” consists of patches of low-amplitude, seemingly chaotic activity with variable low-frequency spectral components (1-8 Hz) that mimic features of downwelling sunlight in the upper water column (Darecki et al. 2011). This behavior may provide dynamic crypsis in an open-ocean habitat. Flickering can be globally and quickly “paused,” often just before a flashing bout, suggesting that it is under inhibitory control.

Flashing and flickering in ommastrephids primarily involve temporal patterning rather than spatial patterning that is more characteristic of loliginids. Albeit less commonly, Dosidicus also exhibits static displays (Trueblood 2010), as do some unrelated deep-sea species (Bush et al. 2009; Burford et al. 2014). Loliginids can also generate time-varying patterns and oscillations (2-6 Hz), at least within clusters of chromatophores (Suzuki et al. 2011). These observations suggest that both families of squid share basic mechanisms of chromatophore control. What differs is the extent to which individual species employ these patterning strategies in a given ecological or behavioral situation.

Many details of chromatophore function in squid are not well understood and cellular-level work has focused on loliginids. Descending “vertical” motor-control from the brain is well established and drives precise spatial patterning in loliginids and probably flashing in Dosidicus, but control of flickering in Dosidicus, and potentially of behaviors in other taxa, may differ. Specifically, vertical control cannot account for spontaneous waves of chromatophore activity in skin from Dosidicus with neuronal activity blocked by tetrodotoxin (TTX) (see Fig. 7) and in loliginids after chronic denervation of a chromatophore field (Packard 1995, 2006, 2011). In both cases, inputs from known motor pathways are not operational.

We propose that peripheral “horizontal” control in the skin of squid permits coordination between chromatophores in the absence of descending neural control, and that horizontal and vertical control normally operate in concert to enable complex temporal and spatial patterning. The structure and function of the elements underlying this putative horizontal transmission within the chromatophore network are unknown, and elucidating these features is a focus of this project.