The common octopus, Octopus vulgaris Cuvier 1797, is an economically important resource throughout the Iberian Peninsula. Official average landings in Galicia (NW Spain) for the period 2010-2013 were 2984 t (www.pescadegalicia.com).

This mobile species is well adapted to living in different biotopes (sandy and muddy bottoms, rocks, coral reefs and seagrass) and it is an opportunistic carnivorous predator during adulthood, feeding mostly upon crabs, molluscs, polychaetes and bony fish (Mangold 1983). Like other octopus species, O. vulgaris modify their soft habitats by excavating dens and by accumulating midden piles of different materials, often prey remains (Mather 1991).

Focussing on assessment of preference at specific den location, Anderson et al. (2008) showed that while the population had a wide choice of prey items, the individual choices were much narrower, indicating that octopuses are specializing generalists. Furthermore, the ability of octopus, including O. vulgaris, to drill holes in mollusc shells is well known (Nixon and Boyle 1982, Nixon 1987, Nixon and Maconnachie 1988, Mather and Nixon 1990) and was found in juveniles O. vulgaris inhabiting Viños islet within the National Park of the Atlantic Islands of Galicia (NPAIG) (Guerra and Nixon 1987).

To avoid predation and competition, many octopuses select shelters where they remain most of the time, particularly during daylight (Mather 1988). In consequence, the characteristics of the bottom have been thought to be the main factor influencing their density and distribution patterns (Aronson 1991, Hanlon and Messenger 1996, Leite et al. 2009). Information about relative abundance, distribution pattern and biomass fluctuations in time and space are critical for understanding the ecology of any species (Kaiser et al. 2011) and for fishery management (Cochrane and Garcia 2009).

A number of physical and biotic factors may influence octopus spatial distribution and density of natural populations. Den availability and preference for edges were crucial in tropical reefs (Aronson 1986, Anderson 1997). Allocation of shells in which the octopus hid was found to be a key factor explaining the Octopus joubini’s aggregate distribution in a subtropical ecosystem (Mather 1982). Substrate, kelp cover and shallow waters had a positive correlation for Enteroctopus dofleini (Scheel 2002). Den availability was also a limiting characteristic for octopus distribution (Altman 1967, Hartwick and Thorarinsson 1978, Mather 1982, Hartwick et al. 1984). Octopuses may exercise substantial choice in habitat and shelter, and often modify shelters to their satisfaction (Mather 1994). This home choice was documented for O. vulgaris living on soft bottoms in the Mediterranean Sea (Katsanevakis and Verriopolus 2004a, b).

Other factors than habitat, such as seasonality (Hartwick et al. 1984, Iribarne 1991), reproduction (Iribarne, 1990), depth and sea water temperature (Katsanevakis and Verriopolus 2004a), and predator pressure and reproductive demand (Aronson 1986, Leite 2007, Huffard et al. 2008) were also essential, whereas prey availability was not essential to O. joubini and E. dofleini (Mather 1982, Scheel et al. 2007).

Some of these ecological and behavioural aspects of O. vulgaris were considered in the central-east Atlantic and the Mediterranean Sea from fishery databases and fishing surveys (Guerra 1981, Sánchez and Obarti 1993, Quetglas et al. 1998, Belcari et al. 2002, Tsangridis et al. 2002), and Moreno et al. (2014) found the essential habitats for pre-recruitment along the Portuguese coast. However, there are many features that cannot be investigated through fishing surveys requiring direct observations in the field (Mather 1991, Hanlon et al. 2008, Katsanevakis and Verriopolus 2004b). Moreover, the study of interactions of individuals and the use that mobile and intelligent benthic octopuses make of the natural environment can be drastically altered by the restrictions imposed by captivity conditions (Mather 1982).

The dynamics of habitat selection in octopus are complex and likely important to fisheries and marine communities (Mather 1993). Despite the commercial importance of O. vulgaris, studies on its distribution patterns, density and den ecology are practically non-existent in the northeast Atlantic.

In order to understand some ecological and behavioural traits of O. vulgaris living in dens on soft bottoms of Rodas inlet (Cíes archipelago within the National Park) visual censuses were carried out. The site was selected because it has an unusual abundance of octopuses, which are intensely fished, and because it has a high abundance of dens (cefaparques.blogspot.com). Direct visual observation was chosen because it had already been used as an important tool for the study of these aspects in octopods (e.g. Mather 1991, Oosthuizen and Smale 2003, Hanlon et al. 2008, Leite et al. 2009), and because its impact is not harmful to the species and the environment. The main objectives of this work were: 1) to estimate density of O. vulgaris and 2) to test whether it has a non-random distribution, related mainly to substrate and body size.

The total area swept in the visual censuses was 13.735 ha (Table 1). The surveyed area during the closed and open fishing seasons was 5150 m² and 2850 m², respectively. The mean rate of occupancy during the octopus closed season in June (VC 1-3) was 85.6% (range 78.57%-90.91%), whereas it was 70.0% when the creel fishery targeting octopus was open in October (VC 4). The presence of O. vulgaris in the dens was not significantly different (p-value=0.692) between the samples taken during the closed and open fishing seasons. However, depth showed a significant negative relationship with occupancy (Fig. 3). The value obtained for the Morisita method (I_δ =1.77) indicated that animals occupying dens (Table 1) were not randomly dispersed but aggregated.

The mean number and the standard deviation of dens per 1000 m² was 3.84±0.84 in June, whereas it was 3.89 in October (Table 1). Consequently, there was a den each 260 m² (range 208-303). Den number estimation in the area surveyed varied between 1586 and 2050. Considering the mean den occupancy (ARDO=85.7%; Table 1), the number of specimens in the Rodas inlet (5 to 21 m depth) would be 1015, which represents 3.38 individuals per 1000 m².

The total number of dens found was 53 (Table 2). The largest number of dens (76.5%) was found at depths ranging from 5 to 10 m. The value of the Morisita index estimated considering the number of dens within the range 5-15 m depth (I_δ =1.44) showed that dens had a crumpled distribution.

Table 2 also shows the mean and standard deviation of the diameters of 33 dens found in VCs 1-3 for June by depth range and occupancy rate by depth. The same data are given to 20 dens from VC 4. No significant differences were found for octopus size in relation to den diameter (p=0.129) within the three size classes.

Associated dens were observed: two were coupled, one was composed of two crevices, and the third was composed of three holes. Scavengers (serpent and sea stars, sea urchins and nettle dog whelks) were found in all dens found at the deeper depth range (15-21 m).

Table 3 indicates the size classes in total number and percentage of octopuses by depth range. Forty-two individuals were found in total: 66.7% between 5 and 10 m depth, 28.6% from 10 to 15 m, and 4.7% between 15 and 21 m. Small octopuses (Size class 1) were found at all surveyed depths: 52% between 5 and 10 m, 40% between 10 and 15 m and 8% in deeper waters. Medium octopuses were only present in shallower waters, while large individuals were found from 5 to 15 m depth, being more abundant (35.7% of the total) in the shallowest depth range. No significant differences were found for octopus size in relation to depth (p=0.146).

Table 4 summarizes the most abundant bivalve species found in the dens during VCs 1-4. None of the bivalve shells examined was drilled. There were no significant differences in den diameter for the different shell types found (p=0.494). By contrast, in relation to the depth variable, significant differences were observed between the shell types (p<0.001), showing dens with Ensis arcuatus and Glycimeris glycimeris at lower depth.

Four characteristics of 55% of the observed dens in the soft bottom of the Rodas inlet suggested that these constructions could be “permanent”, that is, they could be used for several octopus generations. These characteristics were i) holes narrow and deep and their inner walls reinforced with shells; ii) rim of the dens very long-established, showing encrusted shell-boring polychaetes (Polydora spp.), bryozoans and honeycomb structures produced by the sponge Cliona celata (Fig. 4); iii) very large and well-built dens occupied by small octopuses; and iv) the rim of dens around three times the diameter of the hole.

The Rodas inlet has resulted to be a suitable area for O. vulgaris ranging from 200 to 2000 g, but especially for small individuals (76 and 73% in VCs 1-3 and VC 4, respectively; Table 3). The Rodas inlet is greatly influenced by a seasonal upwelling and is therefore a highly productive marine realm (Álvarez-Salgado et al. 2002). Species such as O. vulgaris may take advantage of these productive ecosystems by living there, as has occurred in other areas of the NW Atlantic Ocean (Moreno et al. 2014). Sea bottom temperature (SBT) and sea bottom salinity (SBS) were the main environmental variables limiting pre-recruit or sub-adult aggregation in the eastern Mediterranean (Katsanevakis and Verriopoulos 2004a), the Mediterranean Sea and the eastern Atlantic (Hermosilla et al. 2011) as well as in Portuguese waters (Moreno et al. 2014). O. vulgaris is most often found in water with a SBT warmer than 10°C and cooler than 30°C and a salinity ranging from 32 to 40 (Mangold 1983). Both parameters were within these ranges during the surveys in the Rodas inlet (Table 2).

The number of specimens in the Rodas inlet from 5 to 21 m depth (1015) represents 3.38 individuals per 1000 m². This density is 6.8 times higher than the highest densities estimated by Guerra (1981), Fonseca and Campos (2002), Belcari et al. (2002) and Katsanevakis and Verriopoulus (2004a) in different geographic areas. Moreover, additional observations during these VCs also showed that there were animals dwelling in deeper waters (down to 40 m depth) in the zone. This was also the case in several Mediterranean areas, where the highest abundances of O. vulgaris of comparable sizes were found within the depth range of 10-50 m (Belcari et al. 2002). Similarly, octopus densities were higher at depths of 20-40 m than further offshore on the NW African coasts (Faraj and Bez 2007).

O. vulgaris densities found in the Rodas inlet were within the range found for O. insularis in Fernando de Noronha Archipelago, Brazil. However, they were nearly three times the highest densities found for Enteroctopus dofleini in Prince William Sound, Alaska. However, these comparisons should be taken with caution because very different substrates and seasons were considered in the case of O. insularis and E. dofleini, whereas for O. vulgaris these two variables were not considered.

Despite its restrictions, the methodology used in this study pointed to an aggregate distribution of O. vulgaris. Our evidence supports the same crumpled distribution found for the species in central-eastern Africa (Guerra 1981), southeastern South Africa (Oosthuizen and Smale 2003), and the eastern Mediterranean (Katsenavakis and Verriopolou 2004a). The individuals need hard structures on soft substrates or penetrable areas in hard substrates. In the Rodas inlet O. vulgaris find bivalve shells in abundance, and also relatively hard but penetrable substrates (maërl) among a general scenario mainly made up of open sand of different-sized grains, which could be the key factor explaining the aggregate distribution found. The influence of small but significant differences in the hardness of the substrate is an aspect that should be studied more seriously. A crumpled distribution was also found in O. joubini in St Joseph Bay, Florida, which is shallow and has a mixture of open sand and Thalassa grass beds. In that case, the factors that may have influenced octopus distribution were the substrate of sand and grass, and shells in which to hide because O. joubini does not burrow in sand (Mather 1982). Scheel (2002) observed a positive association between E. dofleini density and substrate and shallow waters (up to 5 m depth) in Alaska. Grass beds cannot explain the O. vulgaris aggregate distribution found at the Rodas inlet because it is composed exclusively of sandy bottoms and there are no shells in which to hide. In our long experience diving in the Cíes archipelago we have only observed this behaviour in juveniles octopus larger than 200 grams.

The den type was also an important factor for O. insularis, especially for the smaller ones (Leite et al. 2009). Similarly to O. insularis, juveniles of O. vulgaris generally occupied holes sunk perpendicular into the substrate, while large-sized specimens excavated dens beneath rocks and ledges. However, this behaviour was not observed in the Rodas inlet, and both juveniles and large octopuses were found in dens of different configuration built away from rocks in open sand substrate or maërl at different depths. What we found were den modifications by octopuses, such as removing sand, shingles and bivalve shells and placing items to block the aperture, as observed by Mather (1994).

We indicate herein, for the first time in O. vulgaris in the wild, that some old and well-built dens on a soft bottom of the Rodas inlet can be used successively by several generations of octopuses, which is why we call them “permanent”. Their main characteristics are addressed in the results section. An old study by Hartwick and Thorarinssson (1978) on E. dofleini and a new one by Godfrey-Smith and Lawrence (2012) on O. tetricus suggest that this is very likely in a specific type of habitat. Den physical modifications by octopuses during long-term occupation, called “ecosystem engineering” by the last two authors, can be quite impressive, mostly involving handling of shells brought in during foraging. And it can be hypothesized that they could in turn result in higher densities being viable at the site.

The availability of shelters can be also a limiting factor for octopus distribution, as several studies of different species have described home choice and suggested characteristics used in selection of hiding places (Mather 1982, Altman 1967, Katsenevakis and Verriopoulos 2004b). However, dens do not appear to be limiting for E. dofleini in the eastern Pacific (Hartwick et al. 1984). The failure of octopus size to vary with den size, in contrast with the findings of Mather (1993), does suggest a limitation of den availability to O. vulgaris in the Rodas inlet. However, this aspect should be investigated more carefully because four surveys that do not target this specific facet are insufficient.

Environmental factors also influenced octopus density and distribution. A significant relationship of O. insularis size and depth was found by Leite et al. (2009). However, this was not the case for O. vulgaris in the Rodas inlet.

Temperature can also be a factor influencing octopus density and distribution. Smaller O. insularis prefer warmer temperatures than larger ones (Leite et al. 2009), perhaps in order to promote faster growth and thus shorten the period during which they are more vulnerable to predation, as observed by Katsanevakis and Verriopoulus (2004a) in O. vulgaris from the Mediterranean. This association between temperature and density and/or distribution was not investigated in the present study. However, estuaries like the Ría de Vigo are complex and very dynamic systems; their physical, chemical and biological properties show a sharp distributional gradient and large temporal variability of the meteorological factors that operate through an increase in the estuarine residual circulation. For this reason, SBT can change quite drastically in short time periods (Nogueira et al. 1997). It does not seem a very efficient strategy to subordinate octopus distribution to a factor (temperature) that can be very variable from week to week, provided that this factor does not go beyond the tolerance limits of the species, which it does not in any of the oceanographic situations that the ecosystem goes through in an annual cycle (Nogueria et al. 1997).

Another possibility is that predation pressure limits the distribution of very small octopuses (Aronson 1986). The main potential predators of O. vulgaris at Cíes archipelago are dolphins (Delphinus delphis and Turpsios truncatus) (López et al. 2004). Murray eel (Muraena helena), conger (Conger conger), sea bass (Dicentrarchus labrax) and some shark species (Rodríguez-Solorzano et al. 1983). Dolphins visit the inlet very infrequently because a marina for sports boats has been built (López et al. 2004). Murray eel, conger and shark species that can potentially prey upon octopus (Mustelus mustelus and/or Galeorhinus galeus) are not usually found on the sandy bottoms and shallow waters of the Rodas inlet (Garci and Hernández-Urcera, data not shown.). Adult sea bass in the northeastern Atlantic primarily target small pelagic fish, most notably mackerel (Scomber scombrus), scads (Trachurus spp.), anchovy (Engraulis encrasicolus), and sardine (Sardina pilchardus) (Spitz et al. 2013). Therefore, we consider that predation pressure by natural octopus predators at that site is not a main factor limiting their density and distribution. Nevertheless, fishing pressure can be a key factor in both the density and the size of the octopus we found in our surveys, for at least three reasons: 1) there was a relatively high discrepancy between the number of available dens (1586-2050) in the area and the number of individuals (1115), which were mainly of small size; 2) during the time when fishing is allowed (all year except in the quarter from June to September) the exploitation rate is very high around the Cíes archipelago (Ourens et al. 2010); and 3) the current law does not allow marketing of octopus under one kilogram (DOGA 2012), which, supposedly, are returned still alive to the sea if captured. Does that mean that O. vulgaris in the Rodas inlet is overexploited? This is an essential aspect of the management of the species in the whole Cíes archipelago that needs to be studied with more information. Oosthuizen and Smale (2003) suggested that the combined exploitation of inner and sub-tidal areas could be detrimental to the O. vulgaris stock in the temperate southeastern South Africa, as overexploitation of one area will impact on the other.

The patchy distribution of O. vulgaris in the Rodas inlet could be influenced by food. Vincent et al (1998) suggested that selection of habitats within the intertidal may be influenced by prey abundance in E. dofelini. As occurred in other octopus species (Anderson et al. 2008, Leite et al. 2009), O. vulgaris is able to prey upon a high variety of prey items (Nixon 1987). However, in some situations the common octopus feeds mostly on bivalve molluscs (Nixon 1987 and Boyle and Rodhouse 2005 for reviews). A similar diet was found in its twin species, O. mimus, in the northern Pacific (Cortez et al. 1995). This seems to be the case of the Rodas inlet, where bivalves are very abundant and available (Ourens et al. 2010). In addition to our direct observations, this finding is supported by the fact that a commercial fishery for razor fish (Ensis arcuatus) carried out by a restricted and low number of divers was developed there recently (64 t per year from 2010 to 2013; www.pescadegalicia.com). Moreover, scallops and clams, which were commercially exploited in specifics zones of the inlet several years ago, have significantly declined in recent years (www.pescadegalicia.com). The high abundance of bivalve shells collected in the extended middens of O. vulgaris is certainly a clue as to prey species. However, there are several challenges to using the remains of food items outside the dens, as pointed out by Anderson et al. (2008). Bivalve shells can be moved by currents or waves (Mather 1991), or biotic factors (Hartwick and Thorarissson 1978), and the octopus may push shells away from the den to reduce their visibility to possible predators, or carry them to protect and/or construct their middens (Ambrose 1982). We observed several foraging strategies and even a cannibalistic behaviour in the Cíes archipelago (Hernández-Urcera et al. 2014) but very few gastropod and crabs remains in the octopus middens in the Rodas inlet. The preference for bivalves shown by O. vulgaris at this site is probably related to two factors: 1) the highly efficient and well-established exploitation targeting crustaceans (www.pescadegalicia.com), and 2) the positive energetic balance obtained by predation of abundant and easily available bivalves, mainly razors (McQuaid 1994).

Techniques for penetrating hard-shelled prey are varied (Fiorito and Gherardib 1999, Anderson and Mather 2007). When the initial pulling method proves rapidly to be inefficient, octopus did not give up but started to drill. This procedure has been described in several octopus species, including O. vulgaris (Nixon and Boyle 1982, Guerra and Nixon 1987, Nixon and Maconnachie 1988, Mather and Nixon 1990). The interesting finding in the present study is that small octopuses (>500 g) examined by Guerra and Nixon (1987) in Viños islet (with the National Park) showed drilled gastropod bivalve mollusc shells in their dens. However, drilled bivalve shells were not found in the material collected in six dens from the Rodas inlet (VCs 1-4) 27 years later. In the old observation, drill holes were always in thick shells, while in the recent one remains of razor clams (Ensis arcuatus) and clams (Chamelea sp.), mainly found outside the middens, were not drilled. These findings suggest that the present abundance of thin and less resistant bivalve shells in the Viños islet could make a bivalve meal totally accessible by pulling apart shells instead of drilling them, as may be occurring in the Rodas inlet. Such “cultural” differences demonstrate that feeding programmes in octopods are flexible and reinforce the evidence demonstrating that the ability to learn, so characteristic of these animals, benefits the species and ensures survival (Scheel et al. 2007, Anderson et al. 2008, Leite et al 2009).

Visual censuses in the Rodas inlet also showed double and triple dens occupied simultaneously. This observation raises the question of whether O. vulgaris is totally intolerant to the nearby presence of conspecifics or, on the contrary, it is tolerant to a certain degree of crowding. Except Eledone moschata (Mather 1985), benthic octopuses are typically known as solitary animals (Guerra 1981, Boal 2006). Despite their solitary habit, octopuses have not been seen defending territories (Altman 1967, Kayes 1974, Mather and O’Dor 1991) and usually the area around a den is not defended (reviewed in Boal 2006). It seems, therefore, that octopuses in their natural environment do not hold territories and seem to be somewhat tolerant of crowding (Mather 1982), but it is equally likely that octopuses are fundamentally asocial except when ready for mating. However, studies by Huffard et al. (2010) on the intertidal octopus Abdopus aculeatus suggest that this is not always true. Further studies of social relationships in this species will help researchers to disentangle these possibilities, which again require behavioural evaluations to make behavioural assumptions.

The Rodas inlet must have more than 1000 octopus dens between 5 and 21 m in depth. In consequence, the Rodas inlet, which represents 1.12% of the marine realm of the Cíes archipelago, may be a preferential habitat for O. vulgaris individuals ranging between 200 and 2000 g, but especially for small individuals. This is the first evidence of preferential habitats for O. vulgaris in the wild and furthers the understanding of its population dynamics within the NPAIG. This study, together with another study in preparation on where mating and brooding take place, will contribute to an effective management of the species.

Although none-invasive methods, namely visual censuses, are expensive and very time-consuming, they are ideal for enhancing the knowledge of this type.