Changes in behavioural patterns from swimming to clinging, shelter utilization and prey preference of East Asian common octopus Octopus sinensis during the settlement process under laboratory conditions

Abstract

Settlement is the process whereby benthic octopuses with a planktonic paralarval phase make a behavioural change from being active swimmers to assume adult life on the seabed. This process is poorly understood, mainly because octopus juveniles are highly cryptic in the natural habitat and they lack any distinct morphological changes (i.e. metamorphosis) in concert with settlement. To investigate behaviour during the settlement process, ontogenetic changes in swimming, clinging and shelter-utilizing behaviours were examined during both day- and night-time using laboratory-raised East Asian common octopus, Octopus sinensis, aged from 10 to 28 days after hatching (ranging from 1.7 to 16.4 mg mean dry weight, DW). The accompanying change in feeding habit from planktivorous to benthivorous was assessed by comparing rates of consumption of planktonic zoea (of the swimming crab Portunus trituberculatus) and benthic juveniles of the Manila clam, Ruditapes philippinarum. During the daytime, octopus behaviour changed rapidly from swimming to clinging onto the surface of the wall and bottom of the test tank during growth from 3 to 9 mg DW. At night, the frequency of clinging behaviour increased along with growth but the behavioural change progressed more slowly, indicating a tendency to continue swimming during darkness. Shelter utilization during the day commenced mainly after reaching 8 mg DW, after which shelter use gradually increased with growth. In contrast, shelter utilization at night was rarely observed throughout the experiment. Regarding octopus prey preference, consumption of clam juveniles increased with growth while zoeae consumption remained constant. The increase in clam consumption, however, was gradual, indicating that the shift of feeding habit from planktivorous to benthivorous is by no means an abrupt transition. These results indicate that O. sinensis requires a considerable period of time to complete the transition to benthic life. The alternating behaviours of diurnal clinging and nocturnal swimming in the water column may facilitate safe migration of the octopuses from the location to which they have been passively advected towards a more suitable benthic habitat while minimizing their exposure to visual predators. Since shelter utilization and feeding on bivalves are likely to require the use of a certain degree of force and complex coordination of the arms and suckers, the extended, gradual increase in shelter utilization frequency and clam consumption may be attributable to the degree of arm and sucker development.