Determination of age and growth of the horned octopus Eledone cirrhosa (Cephalopoda: Octopoda) using stylet increment analysis

Abstract

Age and growth of the horned octopus Eledone cirrhosa are estimated for the first time by stylet increment analysis. Specimens were collected between March 2009 and March 2010 from the commercial trawl fishery operating off northwestern Iberian coasts. A pool of stylets representing all size classes were defined by means of modal progression analysis performed separately for each sex in northern and western fishing grounds over a total of 2220 individuals (306 males and 595 females from the northern fishing ground; and 379 males and 940 females from the western fishing ground). In total, 122 stylets belonging to individuals with a total weight ranging from 67 to 950 g were successfully analysed. Age of the specimens was estimated assuming a daily deposition rate of growth increments within the stylet. Estimated age of the specimens varied from 106 to 516 days. Differential longevity between sexes was observed, with females reaching a life span of 17 months while males attained reached only 14 months, although the possibility that this difference might be even greater is discussed. Based on estimated age, hatching date was back-calculated and it was concluded that hatching occurs throughout the year, with a peak in winter. Relative instantaneous growth rates estimated for 90-day age classes, ranged from 0.03% to 2.17% of body weight per day, depending on age. The results obtained are compared with the information available on this species.