Chromatophore patterns, packing, and scaling on a growing squid

Many biological patterns are formed during growth, and various modeling approaches have repeatedly shown that growth can substantially impact pattern formation. However, experimental testing of these ideas has been limited, largely due to the difficulty in precisely measuring organism growth while simultaneously tracking the dynamics of pattern formation. To address this, we turned to the skin of the oval squid. The oval squid grows rapidly, hatching with a length of approximately 16mm and reaching 90mm within 3 months. Throughout development, its skin is populated by pigment-filled cells called chromatophores. Following insertion into the skin, chromatophores do not move. This means that squid chromatophores, besides being the constitutive elements of a point pattern, can also function as reference points to precisely determine skin growth. For the more biologically-minded, I will explain how the chromatophore pattern emerges through the interplay of growth and decreasing chromatophore growth rates. For those who lean physics, I will talk about how due to the combination of volume exclusion and growth, chromatophores exhibit a scaling in which relative density fluctuations grow with spatial scale, akin to a critical system.