Stylet weight as a proxy for age in a merobenthic octopus population

An array of direct and indirect ageing methodologies have been trialled to rapidly, reliably and accurately determine the age of octopus species. The collective results of these efforts have largely led to compromising between time/cost and accuracy/precision. By combining direct and indirect methods, the suitability of stylet (internal shells) weight as a proxy for age was tested on Octopus (cf) tetricus, a merobenthic species (with paralarval stage) endemic to the west coast of Australia. Captive animals were injected with calcine to mark their stylets, confirming that stylet increments were formed daily. Stylet Increment Analysis (SIA) was applied to directly age 251 wild caught octopuses. Estimated age (days) and stylet weight (g) demonstrated a strong power curve relationship for both females and males (r2 = 0.94 and r2 = 0.88, respectively). Application of stylet weight as a proxy for age enabled a further 3280 octopuses to be aged. Maximum ages for females and males were 542 and 677 days, respectively. Age was found to be strongly related to mantle length for both sexes across all ages, indicating that wild merobenthic octopus populations may have stronger length-age relationships than captive growth studies suggest. Initial observations suggest that females may cease forming stylet increments at the onset of spawning.