Abstract [Related Publication]: There is increasing evidence that projected near-future carbon dioxide (CO₂) levels can alter predator avoidance behaviour in marine invertebrates, yet little is known about the possible effects on predatory behaviours. Here we tested the effects of elevated CO₂ on the predatory behaviours of two ecologically distinct cephalopod species, the pygmy squid, Idiosepius pygmaeus, and the bigfin reef squid, Sepioteuthis lessoniana. Both species exhibited an increased latency to attack and altered body pattern choice during the attack sequence at elevated CO₂. I. pygmaeus also exhibited a 20% decrease in predation rate, an increased striking distance, and reduced preference for attacking the posterior end of prey at elevated CO₂. Elevated CO₂ increased activity levels of S. lessoniana comparable to those previously shown in I. pygmaeus, which could adversely affect their energy budget and increase their potential to be preyed upon. The effects of elevated CO₂ on predatory behaviours, predation strategies and activity levels of cephalopods reported here could have far- reaching consequences in marine ecosystems due to the ecological importance of cephalopods in the marine food web.
Methodology Overview: Here, we tested the effects of current-day control (438µatm) and two projected near-future CO₂ levels (737 and 934µatm) on the predatory interactions of pygmy squid with a common prey item, the glass shrimp, Acetes sibogae australis (Jackson, 1992). The CO₂ treatments were selected to match projected CO₂ levels in the atmosphere and ocean by the end of this century for a moderate and high emissions trajectory based on representative concentration pathways (RCP) 6.0 and 8.5 respectively (Collins et al., 2013).
We exposed bigfin reef squid to current-day control (435µatm) and projected future CO₂ levels (935µatm) following RCP8.5 (Collins et al., 2013), and tested their predatory interactions with a common reef fish, the spiny chromis damselfish, Acanthochromis polyacanthus. Additionally, the activity levels of bigfin reef squid were also compared between CO₂ treatments to determine if elevated CO₂ increases activity as observed in pygmy squid (Spady et al., 2014), or decreases activity, as seen in paralarval D. pealeii (Zakroff et al. 2017).
The following information was extracted from the videos:
• proportion of animals that attacked prey
• latency to attack – from when prey is introduced to when attack pose begins
• time holding attack pose – squid aligns anteroposteriorly with prey and streamlines mantle and arms, concluded when strike with tentacles is initiated
• striking distance – the distance from the tentacles to the prey when arms are splayed open to expose striking tentacles
• attack direction – the angle of the squid in relation to the prey at time of attack in which the posterior end of the of the prey is 0˚ and the anterior end is 180˚; Attacks were defined as ‘posterior’ if less than 45˚, ‘lateral’ if between 45˚ and 135˚, and ‘anterior’ if greater than 135˚
• body pattern – choice of body pattern during the attack pose, categorised as ‘dark mottle’ or a more transparent ‘uniform blanch’ (Hanlon and Messenger, 1988)
• proportion of animals that captured prey on first strike attempt
For bigfin reef squid, LoliTrack software (Loligo Systems v4.2) was used to extract the total distance moved, proportion of time the squid was active, and average speed from videos.
The full methodology is available in the publication shown in the Related Publications link below.