Feeding intensity and molecular prey identification of the common long-armed octopus, Octopus minor (Mollusca: Octopodidae) in the wild

The common long-armed octopus, Octopus minor, is an important component of systems and supports the local fisheries in the coastal areas of northern China. For the fishery management and artificial breeding, especially for the management of exclusive conservation reserves, its role in the ecosystem requires assessment. Therefore, the feeding intensity of O. minor was studied from April to July 2014 when females reaching ovary maturation, and prey composition was identified from stomach contents using a DNA barcoding method. Of the 172 sampled octopuses, 66 had stomach contents that were nearly digested into pulp. On the whole, the feeding intensity of octopus remained more or less the same during the first three months and significantly decreased in July. The changes of feeding intensity were different between females and males; in females, the intensity of feeding decreased from April to July; in case of males, however, the feeding activity increased from April to June and decreased thereafter. The feeding intensity of the females was extremely greater than that of the males. O. minor was a generalist predator and based on homology searches and phylogenetic analysis, a total of 10 different taxa were identified in the stomach contents. In terms of percent composition by frequency of occurrences (%N), fishes accounted for the most of the octopuses diet (50%), followed by cephalopod (25%), crustaceans (21.7%), annelid (1.7%) and nematode (1.7%). The families of Gobiidae and Octopodidae appeared in all months and Protunidae appeared in three months. The results confirmed that Gobiidae family (45.8%, by frequency of occurrences) was an important source of food during the time when females reaching ovarian maturation. From April to July, the observed cannibalism showed an increasing trend. Controlling and reducing fishing production of Gobiidae fishes in conservation area are recommended from April to June when female octopuses are actively feeding.