Morphological measurements of arm loss for eight octopus species

Sublethal predation is prevalent in multi-limbed marine invertebrates and holistically quantifying their injuries has potential to provide important ecological and physiological insights including the rate of trophic energy transfer. We demonstrate the utility of an Injury Severity Index (ISI) to holistically describe the magnitude of appendage injury in invertebrates, using data collected from octopuses in museums and the field. ISI, when applied to octopus, quantifies the numbers of arms lost and the proportion of tissue lost on a scale of 1.01-16.00. ISI revealed that museum specimens commonly lost part of one arm, with a mode ISI value of less than 2.00, while wild Octopus bimaculatus had a mean ISI value of 3.13, representing two to three arm injuries. We then calculated tissue volume lost using two methods for estimating arm length: 1) using scaling relationships between intact arms and body size, referred to here as scaling-based volume, and 2) calculating a mean length using intact arms of an individual, which we call in situ volume. We show that while both methods are comparably effective at estimating biomass transferred to sublethal predation in octopus populations, in situ volume has strong potential for calculations in the field to enhance long-term survey data.

Methods
The museum measurement data were calculated from observations of arm lengths, body sizes, and arm truncation injuries in museum specimens of Octopus bimaculatus, O. bimaculoides, O. californicus, Paroctopus digueti, Enteroctopus dofleini, O. fitchi, O. hubbsorum, and O. rubescens.

The injury data (number missing, proportion of length missing, Injury Severity Index, two measures of volume estimates) were calculated based on the measurement data.

The field data were collected from live Octopus bimaculatus surveyed around the Two Harbors small boat area on Santa Catalina Island, California, USA.