Squid iridescent displays are mesmerising – and mysterious. ‘Squids are the only invertebrates known to exert neural control over their iridescence,’ explains Trevor Wardill, ‘but we still don't know for certain why they use iridescence, and which specific neural pathway is responsible for iridescence control.’ To shed light on these questions, Wardill teamed up with Paloma Gonzalez-Bellido and other colleagues at the Marine Biological Laboratory (MBL) in Woods Hole, Massachusetts, to take a closer look at how and why squid create their stunning visual displays (p.850).
Squid have two colour-changing systems in their skin: small pigment-filled sacs called chromatophores, which can be reshaped by muscles to produce rapid colour changes, and cells called iridocytes, which contain stacked thin platelets that interfere with different light wavelengths to generate iridescence. Both types of cell are known to be controlled by nerve fibres. All of the nerve fibres coming from a squid's brain are packed into a single bundle called the pallial nerve, which splits into two branches. One branch, the fin nerve, goes straight to the fin, but the other, the stellate connective, travels into the stellate ganglion – a small bundle of neurons in the periphery, similar to the ganglia found outside each vertebra along a human's spinal column. ‘We wondered if we could identify the specific neural pathway controlling iridescence by blocking information from each of these branches’, explains Gonzalez-Bellido.
First, the MBL marine resources section collected Atlantic longfin squid outside Woods Hole. Then Gonzalez-Bellido and Wardill cut the squids' pallial nerve using micro-scissors. As they had expected, fin movement stopped and the squids' skin blanched, indicating that both chromatophores and iridocytes had stopped working. When they cut the fin nerve instead, fin movement and chromatophore activity stopped, but iridescence remained bright. In contrast, when they severed the stellate connective, the squid were still able to move their fins and control their chromatophores, but their iridescence was extinguished. ‘This shows that iridescence signals are routed through the stellate ganglion, while chromatophore signals travel through the fin nerve’, says Gonzalez-Bellido. ‘The fact that the neural pathway controlling iridescence differs from the pathway controlling the chromatophores suggests that the iridescence pathway evolved at a different time point, perhaps for a different purpose altogether’, she adds.
Gonzalez-Bellido and Wardill realised that their discovery would allow them to measure iridescence changes in a live squid for the first time. Measuring iridescence is tricky, explains Wardill, because active chromatophores can mask iridescence changes. Also, fin movements change the angle of light hitting the skin, making it hard to assess iridescence accurately. So, to stop fin movements and chromatophore activity, the team severed the squids' fin nerves. Then they placed the squid in a blackened tent, and after 1 or 2hours took a flash photo to capture the iridescence at that instant. The dark-acclimated animals showed a decline in iridescence compared with the iridescence seen during an earlier exposure to normal light levels. This is useful, says Wardill: ‘being able to reduce iridescence levels in deeper, darker waters could help squid hide from predators.’ When they exposed the squid to light again, the animals responded by increasing their iridescence.
The team is now one step closer to elucidating the different functions of squid iridescence. ‘Now that we know which specific neural pathway to look for, we can map out on an evolutionary tree which squid species have it and which don't, which may help us discover its function’, says Gonzalez-Bellido.