Revision of the Octopus horridus species-group, including erection of a new subgenus and description of two member species from the Great Barrier Reef, Australia

Abstract

The tropical Indo-West Pacific region contains a distinctive group of small to moderate-sized octopuses referred to in the past as the Octopus horridus species-group. Member species are found primarily on intertidal reef flats. They possess small bodies, long arms and complex skin sculpture and body patterns enabling excellent camouflage. When attacked, these octopuses are capable of autotomising their arms at the base: the writhing severed arm acting as a decoy to predators and aiding escape. Lost arms regenerate within 2–3 months. Attributes of this group of octopuses are described and the subgenus Abdopus, subgen. nov. is here coined to define this group. Historically, many members of this subgenus have been incorrectly identified as Octopus horridus, a distinct large-egg species known only from the Red Sea and the northwest Indian Ocean. Seven species are here recognised as belonging in Abdopus, subgen. nov. Two member species from Great Barrier Reef waters (Octopus aculeatus d’Orbigny, 1834 and Octopus capricornicus, sp. nov.) are described. A number of additional, as yet undescribed, species occur throughout the Indo-West Pacific region. Certain earlier works have linked other octopus genera and species with members of this subgenus on the grounds of long arms and arm autotomy. Significant differences occur in the morphologies of these taxa and the nature of the arm autotomy processes. It is proposed that these groups have evolved independently and that arm autotomy has arisen more than once amongst the octopuses. The restriction of the subgenus Abdopus to the tropical Indo-West Pacific region suggests relatively recent origins and radiation.