Stepwise changes in morphology during the settlement process in a merobenthic octopus, Octopus sinensis, raised in the laboratory

Cephalopoda have no true larval stage because they lack any abrupt morphological changes during growth. However, in shallow-water merobenthic octopuses, changes occur in the relative proportions of their body parts during the planktonic paralarval phase and the transitional phase from plankton to benthos. How this lifestyle transition is achieved is unclear because the morphological changes during their early life have yet to be fully elucidated. To address this issue, using laboratory-raised East Asian common octopus, Octopus sinensis, aged 0–100 days after hatching, measurements were taken of total length (TL), mantle length (ML), arm length (AL), the number of suckers on an arm (ASC), and body dry weight (DW). These values were used to explore break points (BPs) in the log-transformed allometric growth equation: lny = lna + blnx, where x is TL (a reference body dimension) and y is one of the other measurements. In all measurements, a three-segmented allometric growth model with two BPs was statistically evident, and each segment had a different relative growth coefficient b along with growth, indicating that there are distinct stages of ontogenetic morphological change. Taking into account the times of behavioral changes reported so far, four stepwise phases were detected in the progress of O. sinensis towards settlement: (1) a true planktonic paralarval phase with rapid increase of ASC; (2) a transitional phase between plankton and benthos, characterized by rapid AL increase; (3) a provisional initial benthic juvenile phase, attaining longer AL than ML and approaching adult-like proportion; and (4) a true benthic juvenile phase, reaching almost completion of the change in body proportion. The stepwise morphological changes seem to be a compromise measure to achieve changes in functional morphology and behavior necessary to lifestyle transition from plankton to benthos for an organism that goes through morphological change without metamorphosis.