I know my neighbour: individual recognition in Octopus vulgaris

Abstract

Background: Little is known about individual recognition (IR) in octopuses, although they have been abundantly studied for their sophisticated behaviour and learning capacities. Indeed, the ability of octopuses to recognise conspecifics is suggested by a number of clues emerging from both laboratory studies (where they appear to form and maintain dominance hierarchies) and field observations (octopuses of neighbouring dens display little agonism between each other). To fill this gap in knowledge, we investigated the behaviour of 24 size-matched pairs of Octopus vulgaris in laboratory conditions.

Methodology/principal findings: The experimental design was composed of 3 phases: Phase 1 (acclimatization): 12 "sight-allowed" (and 12 "isolated") pairs were maintained for 3 days in contiguous tanks separated by a transparent (and opaque) partition to allow (and block) the vision of the conspecific; Phase 2 (cohabitation): members of each pair (both sight-allowed and isolated) were transferred into an experimental tank and were allowed to interact for 15 min every day for 3 consecutive days; Phase 3 (test): each pair (both sight-allowed and isolated) was subject to a switch of an octopus to form pairs composed of either familiar ("sham switches") or unfamiliar conspecifics ("real switches"). Longer latencies (i.e. the time elapsed from the first interaction) and fewer physical contacts in the familiar pairs as opposed to the unfamiliar pairs were used as proxies for recognition.

Conclusions: Octopuses appear able to recognise conspecifics and to remember the individual previously met for at least one day. To the best of our knowledge, this is the first experimental study showing the occurrence of a form of IR in cephalopods. Future studies should clarify whether this is a "true" IR.

Figure 1. Percentages of avoidance in sight-allowed and isolated pairs.

Mean (± SE) percentage of interactions with no physical contacts (avoidance) in sight-allowed (n = 12) and isolated (n = 12) pairs for each of the three days of cohabitation (Phase 2). *: P<0.05.

Figure 2. Percentage of physical contacts in sight-allowed and isolated pairs.

Mean (± SE) percentage of physical contacts in sight-allowed (n = 12) and isolated (n = 12) pairs for each of the three days of cohabitation (Phase 2). *: P<0.05.

Figure 3. Latency of the first interaction in sight-allowed and isolated pairs.

Mean (± SE) latency of first interaction in sight-allowed (n = 12) and isolated (n = 12) pairs for each of the three days of cohabitation (Phase 2). *: P<0.05.

Figure 4. Scheme of the experimental design.

Phase 1 (Days 1–3, acclimatization): sight-allowed (and isolated) pairs were maintained in contiguous tanks for three consecutive days separated by a transparent (and opaque) partition that allowed (and blocked) the vision of the conspecific. Phase 2 (Days 4–6, cohabitation): individuals of each pair (both sight-allowed and isolated) were transferred into an experimental tank and were allowed to interact with each other for 15 min every day and for three subsequent days. Phase 3 (Day 7, test): each pair (both sight-allowed and isolated) was subject to either a sham or a real switch; thus, the dominant octopus within the pair encountered either a familiar (in the case of sham switches) or an unfamiliar conspecific (in the case of real switches). Both types of switch were followed by a cohabitation of 15-min.