Cephalopods show behavioral parallels to birds and mammals despite considerable evolutionary distance [1, 2]. Many cephalopods produce complex body patterns and visual signals, documented especially in cuttlefish and squid, where they are used both in camouflage and a range of interspecific interactions [1, 3-5]. Octopuses, in contrast, are usually seen as solitary and asocial [6, 7]; their body patterns and color changes have primarily been interpreted as camouflage and anti-predator tactics [8-12], though the familiar view of the solitary octopus faces a growing list of exceptions. Here, we show by field observation that in a shallow-water octopus, Octopus tetricus, a range of visible displays are produced during agonistic interactions, and these displays correlate with the outcome of those interactions. Interactions in which dark body color by an approaching octopus was matched by similar color in the reacting octopus were more likely to escalate to grappling. Darkness in an approaching octopus met by paler color in the reacting octopus accompanied retreat of the paler octopus. Octopuses also displayed on high ground and stood with spread web and elevated mantle, often producing these behaviors in combinations. This study is the first to document the systematic use of signals during agonistic interactions among octopuses. We show prima facie conformity of our results to an influential model of agonistic signaling [13]. These results suggest that interactions have a greater influence on octopus evolution than has been recognized and show the importance of convergent evolution in behavioral traits.

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Summary:The Sydney octopus (Octopus tetricus) occurs in unusual numbers on a shell bed of its prey remains that have accumulated as an extended midden where additional octopuses excavate dens. Here, O tetricus are ecosystem engineers, organisms that modulate availability of resources to other species and to their own species by causing physical state changes in materials. A community of invertebrate grazers and scavengers has developed on the shell bed. Fishes are attracted to the shell bed in numbers significantly greater than in nearby habitats. Large predators, including wobbegong sharks, were attracted to and fed on concentrations of fish, inhibiting the activities of the original engineers, the octopuses. Positive feedbacks included the accumulation of shell debris, increasing shelter availability for additional octopuses and aggregating fish. Negative feedbacks included reductions of nearby prey size and availability, aggression among octopuses, and predator limitation to octopus activity that would otherwise maintain the shell bed.Keywords: Jervis Bay; social; population density; denning; aggregation. Octopus tetricus (Mollusca: Cephalopoda) como ingeniero de ecosistemasResumen: El pulpo Sydney (Octopus tetricus) aparece en cantidades inusuales en un fondo de cascajo producido por los restos de sus presas que se han acumulado como un extenso estercolero donde otros pulpos excavan sus guaridas. Aquí, O. tetricus se comportan como ingenieros del ecosistema, organismos que modulan la disponibilidad de recursos a otras especies y su propia especie provocando cambios físicos en los materiales. Una comunidad de invertebrados herbívoros y carroñeros se desarrolló en el fondo de cascajo. Los peces son atraídos a dicho fondo en número significativamente mayor que en hábitats cercanos. Grandes depredadores, como los tiburones wobbegong son atraídos y se alimentan a partir de concentraciones de peces inhibiendo las actividades originales de los ingenieros de los pulpos. La reacción positiva a la acumulación de escombros incrementa la disponibilidad de refugio para los otros pulpos y la concentración de peces. Los efectos negativos incluyen la reducción de la disponibilidad y el tamaño de los peces, la agresión entre los pulpos, y la limitación de la actividad de los pulpos para mantener el fondo de cascajo como resultado de la presencia de depredadores.

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