The ecology of the horned octopus, Eledone cirrhosa (Lamarck, 1798) in Atlantic Iberian Waters | Ecologia do polvo-do-alto Eledone cirrhosa (Lamarck, 1798) no Atlántico Ibérico | Ecología del pulpo blanco Eledone cirrhosa (Lamarck, 1798) en el Atlántico Ibérico

204 pages

This Ph.D. Dissertation deals on the study of several aspects of the ecology of the horned octopus Eledone cirrhosa (Lamark, 1798) in the NW Iberian Peninsula, from western gulf of Biscay to north Portugal. The horned octopus is a benthic octopod distributed along the continental shelf and slope throughout the Mediterranean and northeast Atlantic from Morocco to Norway, including the British Isles and Iceland. In the NW Iberian Peninsula this species is considered as a by-catch of trawling fishery, mainly targeting hake (Merluccius merluccius), monkfish (Lophius sp.) and Norway lobster (Nephrops norvegicus). Horned octopus landings show a cyclical pattern consistent with its life cycle, and its commercial interest considerably varies throughout its geographic range. According to ICES reports, landings of E. cirrhosa in VIIIc and IXa north subareas reached 1145 t in 2013. In terms of both body size and main reproductive parameters (such as maturation, fecundity, and eggs/spermatophores size), specimens of E. cirrhosa from northwest Iberian Peninsula have intermediate characteristics to those exhibited by individuals from the north-eastern Atlantic Ocean and those from the Mediterranean Sea, suggesting some gradation on environmental conditions. This gradient, in turn, was also found in our study area. During the sampling, carried out over two years, a significant bias was observed in the relative abundance between sexes, always favourable to females. The spawning season of the species also varies throughout its range. Specifically, in the northwest Iberian it concentrates in May and June. Similarly to the maximum size, the size-at-maturity increased with latitude. This population parameter ranged from 100 to 134 mm mantle length (ML) for females and between 91 and 108 mm ML for males, depending on the fishing ground

The analysis of catches per unit effort (CPUE) obtained from scientific surveys and carried out at different times of the reproductive cycle of the species shows the existence of spatio-temporal variations on its distribution and abundance in the northwest Iberian sector. The geostatistical modelling of catches depicts the seasonal migration performed by mature individuals to shallower waters during the spawning season. The diet of the horned octopus in the north-western Iberian Peninsula was studied for the first time by combining visual and molecular analysis of stomach contents. The diet of E. cirrhosa in the area was mainly composed by crustaceans (74.64%), followed by fish (16.84%) and, to a lesser extent echinoderms (5.51%), mollusks (4.92%) and polychaetes (2.09%). Multinomial logistic regression (MLR) was used for modelling feeding trends of the species. According to it, the main factors affecting feeding patterns were maturity stage, fishing ground and season. The obtained results, in combination with the previous literature on the distribution of prey, suggest that E. cirrhosa is not a totally opportunistic hunter, but rather a selective generalist. The age of E. cirrhosa was estimated for the first time by stylet increment analysis. A total of 122 individuals, ranging from 67 to 950 g, were selected from a total sample of 2220 specimens using a modal progression analysis (MPA), in order to represent all size classes. The estimated age ranged from 106-516 days and females were significantly more long-lived (17 months) than males (14 months). Based on the estimated ages, hatching date was back-calculated, concluding that hatchings, although occur throughout the year, are mostly concentrated during winter. The instantaneous relative growth rate, i.e. the increase in % body weight per day, estimated for age classes of 90 days, ranged between 0.03% and 2.17%, decreasing throughout the development of individuals

New behavioural information on this species is provided based on the observation of live specimens under confined conditions. E. cirrhosa spontaneously spawned within the shelters arranged inside the tanks. The species do nesting and care their eggs in a similar way to Octopus vulgaris. During this breeding period the female remains inside the shelter, taking care of the eggs. The species easily adapted to aquarium conditions, suggesting its suitability for rearing with either experimental or commercial purposes. Despite its importance for the fishing sector, the scarcity of studies focused on this species makes the information offered in the present work an important milestone in understanding the life cycle of E. cirrhosa in NW Iberian Peninsula, and, therefore, it represents a meaningful contribution to a comprehensive knowledge of the Iberian teuthofaun

Peer reviewed