It is sometimes said that we know more about the dark side of the moon than we do about the residents of our deepest oceans, so when Hannah Rosen and her colleagues from Stanford University, USA, got the chance to hook up with Greg Marshall and his legendary Crittercam from National Geographic to study the giant Humboldt squid (Dosidicus gigas), the scientists leapt at the opportunity. Rosen explains that squid are covered in minute chromatophores that expand and contract to change colour, and adds that coastal species use their chromatophores to blend in with their surroundings. However, the open ocean is a relatively featureless environment, offering few opportunities for creative camouflage, leading Rosen, William Gilly and Lauren Bell to wonder how Humboldt squid use their colour-shifting abilities in the ocean's vast expanse (p.265).
Heading out into the Gulf of California, Gilly and Kyler Abernathy went fishing for giant squid. ‘Humboldt squid were very abundant in the Gulf when this study took place, so catching the squid using fishing rods and squid jigs was fairly simple,’ recalls Rosen. Although catching squid of the right size was less easy as the animals had to be over 80 cm in mantle length to carry the Crittercam. However, after capturing three animals and attaching a Crittercam to the back of each, the team then had to wait patiently for the equipment to bob back to the surface after its squid-back ride.
Back in the lab, Rosen admits that watching the footage of the animals' natural behaviour for the first time was exhilarating: ‘A view into this previously secret world was like a dream come true,’ she smiles. However, once the initial euphoria had worn off, she had to painstakingly analyse the squids' behaviour. Fortunately, Rosen was not restricted by the lack of colour in Crittercam's black and white recordings. ‘Dosidicus gigas contain only red chromatophores,’ explains Rosen, so she knew that dark zones on the animal's skin indicated when the chromatophores were expanded and red, while light regions indicated white colouring. And Rosen adds, ‘One of our biggest challenges was finding a way to ensure we were consistently viewing the same spot on a squid when looking for changes in chromatophore activity’, so Russell Williams designed software to track individual spots on the swimming squid's body as they changed colour while swimming.
Having catalogued several previously unobserved squid behaviours – including one that the team suspects may be mating – they focused on the animals' colour changes, identifying two specific patterns. The first, ‘flashing’ – when the animals rapidly switched colour over the entire body surface – was always associated with visual contact with other squid and Rosen suggests that the behaviour could represent a form of squid communication. ‘The frequency and phase relationships [synchronisation] between squid during flashing can be changed and this suggests that there is some information being conveyed that makes minute control over these details important to the squid’, she says.
The second behaviour that the team identified resembled the flickering patterns of light playing in water near the surface and only occurred when the animals were close enough to the surface for the patterns to be visible in natural light. ‘We propose that flickering provides dynamic crypsis [camouflage],’ says Rosen, and she suggests that flickering could be analogous to the camouflage adopted by squid that live in shallow waters.
Having discovered that Humboldt squid have tight control over their impressive performance and probably use flashing for communication and flickering for camouflage, Rosen and her colleagues are keen to extend their studies further afield to compare how squid use these colourful displays in other locations and circumstances.
- © 2015. Published by The Company of Biologists Ltd