An elaborate behavioural sequence reinforces the decoy effect of ink during predatory attacks on squid

Yuto Hikidi,Noriyosi Sato,Takashi Kasugai,Noritaka Hirohashi

doi:10.1007/s10164-020-00640-8

Yuto Hikidi, Noriyosi Sato + Show 2 more

Open Access

https://doi.org/10.1007/s10164-020-00640-8

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Abstract

The characteristic inking behaviour of cephalopods is a secondary defence mechanism that helps them to escape predation. However, although it has been postulated that ink creates a decoy by disrupting the visual information received by the predator, the underlying mechanisms by which ink helps squid to escape from predators remain unknown. Therefore, we observed the inking behaviour of the Japanese pygmy squid (Idiosepius paradoxus). Field observations showed the squid intermittently and linearly ejected ink while rapidly swimming backwards in response to a predator and also changed their bodies to a light colour. This behaviour was then followed by a sudden and sharp change in swimming direction or sometimes a sudden break from swimming with a concurrent change in body colour to black. We also recorded the escape behaviour with inking under captive conditions in response to predatory sculpins, which allowed the successful escape from a predatory attack. Furthermore, the greater the number of ink ejections, the higher the probability that the sculpins would initiate a predatory attack on the ink rather than the squid. Together, these results suggest that squid inking behaviour exhibits a substantial decoy effect on predators and is associated with a series of complex, spatio-temporally regulated behaviours. Digital video images related to the article are available at http://www.momo-p.com/showdetail-e.php?movieid=momo200107ip01a, http://www.momo-p.com/showdetail-e.php?movieid=momo200107ip03a, and http://www.momo-p.com/showdetail-e.php?movieid=momo200107ip04a.

Highlights

Cephalopods exhibit two types of characteristic defence mechanisms: camouflaging their body by changing its colour and texture to match the background and ejecting ink to confuse a predator (Hanlon and Messenger 2018)
The use of ink as a defence mechanism has been proposed based on field observations (Moynihan and Rodaniche 1982; Hanlon and Messenger 1988; Caldwell 2005); only few studies have experimentally investigated how ink can confuse the predator and whether it acts as a pseudomorph or a smokescreen (Wood et al 2010; Derby et al 2013)
We found that a change in body colour occurred more frequently in individuals whose initial body colour was darker (Table 1), and in 36 of 39 cases (92.3%), this darker body changed to a lighter colour, usually soon after the first ink secretion

Summary

Introduction

Cephalopods exhibit two types of characteristic defence mechanisms: camouflaging their body by changing its colour and texture to match the background and ejecting ink to confuse a predator (Hanlon and Messenger 2018). A recent experimental study by Wood et al (2010) showed that the ink pseudomorph produced by the Caribbean reef squid (Sepioteuthis sepioidea) significantly delays food capture by the predatory fish Haemulon flavolineatum by preventing it from receiving visual and chemical cues; Derby et al (2013) observed a similar effect in the longfin inshore squid (Doryteuthis pealeii). Both these studies were conducted under experimental conditions and did not use live squid; it is still unclear whether these effects can occur in field conditions. It is still unknown if inking has a decoy effect in cephalopods

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