Dwellers in dens on sandy bottoms: Ecological and behavioural traits of <em>Octopus vulgaris</em>

Abstract

Summary: Four visual censuses targetingOctopus vulgaris living in dens on sandy bottoms were carried out from June to October 2013 in the National Park of the Atlantic Galician Islands (NW Spain). Censuses were undertaken by scuba diving between 5 and 21 m depth in daytime. The total area swept was 13.75 ha. There were no significant differences between octopus presence in dens during open and closed fishing seasons. Depth had a significant negative relationship with occupancy. The average number of dens per 1000 m 2 was 3.84±0.84 in June and 3.89 in October. The area per den was 260 m 2 . Den number estimations varied between 1586 and 2057. The largest number of dens (76.5%) was found between 5 and 10 m depth. Den distribution was clumped. No significant differences were found between octopus size classes (small, medium and large) and den diameter. Associate dens were observed. There were no significant differences in den diameter and shell types found around the middens. Many dens could be "permanent". Drilling bivalve shell behaviour is discussed. The surveyed area had around 1100 individuals, mainly small specimens. No significant differences were found between octopus size and depth. Substrate, den type and food abundance and availability (especially razors Ensis arcuatus) seem to be the main factors influencing dens and octopus density and distribution. Den availability does not appear to be a limiting factor in this case. Temperature, den availability, predators and fishing pressure influencing density and distribution are discussed. Rodas inlet may be a preferential habitat for O. vulgaris individuals ranging from 200 to 2000 g, but especially small specimens (≤1000 g).Keywords: den ecology; visual census; habitat selection; drilling behaviour; Octopus vulgaris; National Park of the Atlantic Galician Islands (NW Spain). Moradores de guaridas de fondos arenosos: rasgos ecológicos y de comportamiento de Octopus vulgarisResumen: Entre junio y octubre de 2013 se realizaron cuatro censos visuales enfocados a estudiar el pulpo común Octopus vulgaris que mora en guaridas en fondos arenosos. Los censos se realizaron en la ensenada de Rodas, ubicada en el archipiélago de Cíes dentro del Parque Nacional de las Islas Atlánticas de Galicia (NO España). Dichos censos se efectuaron con buceo autónomo entre 5 y 21 m de profundidad y durante el día. El área total barrida fue de 13.75 ha. No se encontraron diferencias significativas entre la presencia de pulpos en las guaridas durante la temporada de veda y la de pesca. La profundidad tuvo una significación negativa en relación con el grado de ocupación. El promedio de guaridas por cada 1000 m 2 fue de 3.84±0.84 en junio y 3.89 en octubre. El área por guarida fue de 260 m 2 . El número estimado de guaridas varió ente 1586 y 2057. El mayor número de madrigueras (76.5 %) se encontró entre 5 y 10 m de profundidad. La distribución de las guaridas era agregada. No se hallaron diferencias significativas entre el diámetro de los refugios y el tamaño de los pulpos. Tampoco hubo ...

Sepiolid paralarval diversity in a regional upwelling area of the NE Atlantic

“…The Marine Protected Area (Illas Atlánticas de Galicia National Park, PNIAG) studied here has already been identified as an important habitat for Octopus vulgaris juvenile settlement (Guerra et al, 2014), and this work suggests it is also a hotspot for sepiolids. Overall, seven sepiolid species have been identified in the planktonic communities around the PNIAG.…”

“…The Marine Protected Area located at the seaward end of the Ría de Vigo (Illas Atlánticas de Galicia National Park, PNIAG) has been recently identified as a preferred habitat for cephalopod spawning and juvenile recruitment (Guerra et al, 2014(Guerra et al, , 2015. A great effort has been made to study adult cephalopods-namely octopods, squids, and cuttlefishes-in this area, but research on sepiolids has been restricted to Sepiola atlantica (Rodrigues et al, 2011).…”

Octopus insularis (Cephalopoda: Octopodidae) on the tropical coast of Brazil: where it lives and what it eats

“…per 100 m -2 , and the clustered aggregation pattern are similar to those found in reef environments with high availability of dens such as in Pirangi. LEITE et al (2009c) in Fernando de Noronha, andfor BOUTH et al (2011) at Rocas Atoll, and for other species, such as Octopus joubini in sandy environments (MATHER, 1982), Octopus tetricus in reef environments (ANDERSON, 1997), and also for Octopus vulgaris in sandy environments (GUERRA et al, 2014). In this study, clustered distribution was observed at the edge of a reef at a depth of 0-2m, with lots of rocks, gravel and dens available, representing an attraction for octopuses.…”

“…This shift in food preference, of crustaceans to bivalves, appears to be strongly associated with the availability of prey in the habitat. A dominance of bivalves in the O. vulgaris diet in a sandy environment was also found in Portugal (GUERRA et al, 2014).…”

“…In Rodas Inlet, Portugal, in an environment similar to the Restinga with a depth between 5 and 21m, GUERRA et al (2014) found O. vulgaris dens built on sand, reinforced with bivalve shells, and suggested that more than half of the observed dens are permanent, being occupied by several generations of octopus. Furthermore, the density of 0.34 ind.…”