Scaeurgus unicirrhus

Scaeurgus unicirrhus is a benthic deep-sea octopus belonging to the family Octopodidae. It is widely distributed throughout the Mediterranean Sea and the eastern Atlantic Ocean, living between 30 and 600 m of depth.^[1] This species is known for its cryptic lifestyle, benthic habits, and morphological adaptations to deep-sea environments. It plays an important ecological role in continental slope ecosystems and has also been recorded around oceanic islands such as the Azores and Madeira.^[2][3]

S. unicirrhus has an oval, muscular body with a mantle length reaching up to 12 cm and arms extending to about 40 cm.^[1] The skin shows irregular papillae that provide camouflage on sandy or rocky bottoms. A distinctive characteristic is the single cirrus above each eye-hence the name "unicirrhus". Members of this genus possess two rows of suckers, a skin keel around the body margin, and males have the hectocotylus on the third left arm, unlike most other octopus species where the modified arm is the third right.^[4] According to Norman (2000), the species is typically reddish-brown, with robust arms and a tendency to inhabit soft substrates.^[5]

The genus Scaeurgus was described by Alcide d'Orbigny in 1840, who designated S. unicirrhus as its type species.^[4] Subsequent revisions confirmed S. unicirrhus as the only Mediterranean representative of the genus. Three additional species: S. patagiatus, S. tuber and S. jumeau are known from the Pacific Ocean.^[1]

Scaeurgus unicirrhus occurs throughout the Mediterranean Sea and along the eastern Atlantic coast from Portugal and the Azores to North Africa and southward to Senegal.^[6] It inhabits sandy, muddy or rubble substrates between 30 and 600 m of depth, sometimes occurring on hard bottoms.^[1] The species is primarily nocturnal and shelters in crevices, empty shells, or under stones during the day.

In the Azores Archipelago, S. unicirrhus was first reported in a regional synopsis of the local Octopoda fauna.^[2] Specimens were collected on upper-slope habitats, mostly between 200 and 600 m. FAO (2016) also lists the species among confirmed Atlantic records around oceanic islands.^[1]

In the Madeira Archipelago, the species was documented through additional records expanding the cephalopod fauna of the region.^[3] The Madeiran material derives from upper-slope environments similar to those of the Azores, suggesting continuity across the Macaronesian islands.^[1]

Scaeurgus unicirrhus is a benthic predator feeding mainly on crustaceans, bivalves, gastropods and occasionally small fish.^[7] It may bury itself partially and ambush prey. Females produce small eggs, approximately 3 mm in diameter, attaching them within sheltered cavities. They brood the eggs until hatching, after which planktonic young emerge.^[5][8] The species likely has a biennial life cycle, with spawning occurring in spring and early summer.^[7]

S. unicirrhus is a relevant species for studies of cephalopod evolution and adaptation to deep-sea environments.^[4] Its wide distribution across the Mediterranean and eastern Atlantic makes it a useful biogeographic indicator. FAO (2016) provides detailed taxonomic keys, distribution data, and anatomical drawings for accurate identification.^[1]