Den ecology of <i>Octopus vulgaris</i> Cuvier, 1797, on soft sediment: availability and types of shelter

Abstract

SUMMARY: To avoid predation, octopuses select and actively modify shelters (also called dens) in the substratum, where they remain most of the time, especially during daylight hours. The main questions that this study deals with are: Is den availability a significant constraint for the distribution of Octopus vulgaris on soft sediment? What kind of dens does O. vulgaris use on soft sediment and what factors affect the selection of one type instead of another? With population density measurements by SCUBA diving and enrichment experiments with artificial dens, we concluded that the availability of solid materials necessary for den construction is a limiting factor for the distribution of O. vulgaris on soft sediment. O. vulgaris used four different types of den on soft sediment: "well" (a vertical hole in the sediment), "rock/stone" (the octopus uses a rock or a large stone to dig a cavity under it), "shell" (an empty shell is used), "human origin" (a solid material of human origin is used). The relative proportion of the four types of den in the areas studied was: 38.7% "human origin", 29.7% "well", 21.5% "rock/stone", 2.9% "shell". Also, 7.3% of the octopuses were found outside their den. The main types of den were found in different relative proportions in relation to the depth, the distance from shore, the octopus size and the granulometry of the sediment.Key words: cephalopod, den ecology, marine debris, Octopus vulgaris, visual census. RESUMEN: ECOLOGÍA DE REFUGIOS DE OCTOPUS VULGARIS, CUVIER 1797, EN SEDIMENTOS BLANDOS: DISPONIBILIDAD Y TIPOS DEREFUGIOS. -Para evitar la depredación, los pulpos seleccionan y modifican de forma activa sus refugios en el sustrato donde permanecen casi todo el tiempo, sobretodo durante las horas del día. Las principales preguntas de las que trata el presente estudio son: ¿Es la disponibilidad de los refugios una restricción importante para la distribución del Octopus vulgaris en el sedimento blando? ¿Qué clase de refugios utiliza O. vulgaris en el sedimento blando y qué factores afectan la selección de un tipo en lugar de otro? A partir de medidas de densidad de población hechas por submarinistas y de experimentos hechos con refugios artificiales, concluimos que la disponibilidad de los materiales sólidos necesarios para la construcción de refugios es un factor que limita la distribución del O. vulgaris en el sedimento blando. Se descubrió que O. vulgaris utiliza 4 tipos diferentes de refugios en el sedimento blando: "pozo" (un agujero vertical en el suelo blando), "roca/piedra" (el pulpo utiliza una roca o una piedra grande para cavar un agujero debajo de ellos), "concha" (utiliza una concha vacía), "origen humano" (utiliza un material sólido de origen humano). Las proporciones relativas de los 4 tipos de refugios en las zonas estudiadas son: 38.7% "origen humano", 29.7% "pozo", 21.5% "roca/piedra", 2.9% "concha" y 7.3% de los pulpos fueron encontrados fuera de sus refugios. Los principales tipos de refugios se encontraron en proporciones relativamente diferentes en relación ...

Abundance of <i>Octopus vulgaris</i> on soft sediment

“…Trawl surveys are generally regarded as an unreliable method for assessing Nephrops stocks (Bailey et al, 1993). A large percentage of O. vulgaris live in 'wells' in the soft sediment (Katsanevakis and Verriopoulos, 2004), which is quite a similar case to that of N. norvegicus, and the variability in trawling catching efficiency with respect to octopuses is probably a significant constraint in abundance estimations as well. Another reason for our high octopus density values could be a comparatively greater abundance of potential dens in the areas of our study.…”

“…Far from the shore, solid materials (stones, rocks, litter etc.) that are necessary for den construction are usually less abundant, and this has proven to be a limiting factor for O. vulgaris abundance (Katsanevakis and Verriopoulos, 2004). Furthermore, 'wells' used as dens tend to be more common in areas far from the shore and octopuses living in 'wells' are comparatively more difficult to capture by trawls.…”

“…The efficiency of a trawl depends on the type of dens that the octopuses use in the surveyed area. The relative proportion of the various den types varies in relation to depth, distance from shore, octopus size and sediment granulometry (Katsanevakis and Verriopoulos, 2004), so the catching efficiency of a trawl varies from area to area and reliable octopus density estimations are difficult.…”

“…Altman (1967) conducted a pioneering diving survey and studied the diurnal pattern and behavior of the common octopus, by visual observations in the wild; he too highlighted the problems of this type of investigation. Since then, the daytime activity (Kayes, 1974;Mather, 1988), 'home' choice and modification by juveniles (Mather, 1994) and den ecology (Katsanevakis and Verriopoulos, 2004) of O. vulgaris have been studied by visual census. Several ecological aspects of other octopus species have also been studied by visual census: foraging by Octopus cyanea (Forsythe and Hanlon, 1997), habitat selection and shelter use by Octopus tetricus (Anderson, 1997), den ecology of Octopus briareus in a marine lake (Aronson, 1986) and shelter use by Octopus tehuelchus (Iribarne, 1990).…”

“…Visual census is used extensively to measure the abundance of aquatic animals, mainly benthic organisms (Edgar and Barrett, 1997;Jorgensen and Gulliksen, 2001), but also for more mobile animals like neritic fish (Chabanet et al, 1995;De Girolamo and Mazzoldi, 2001;Guidetti et al, 2002;Pet-Soede et al, 2001). The different types of dens encountered on soft sediment (Katsanevakis and Verriopoulos, 2004) are very easily located visually and failure to spot octopuses is minimised. Altman (1967) conducted a pioneering diving survey and studied the diurnal pattern and behavior of the common octopus, by visual observations in the wild; he too highlighted the problems of this type of investigation.…”