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.