Burukovsky R.N.1, Marin I.N.2,3 2018. The food composition of the symbiotic crab Pinnixa rathbunae Sakai, 1934 (Brachyura: Pinnotheridae) from burrows of the spoon worm Urechis unicinctus (von Drasche, 1881) (Echiurida: Urechidae) in Vostok Bay of the Sea of Japan // Arthropoda Selecta. Vol.27. No.4: 319–324 [in English].
1 Kaliningrad State Technical University, Sovetsky prosp., 11, Kaliningrad, 236022, Russia. E-mail: email@example.com
3 Biological Department, Altai State University, prospect Lenina, 61, Barnaul, 656049, Russia.
doi: 10.15298/arthsel. 27.4.06
ABSTRACT. The food composition of the symbiotic crab Pinnixa rathbunae Sakai, 1934 inhabiting burrows of the spoon worm Urechis unicinctus (von Drasche, 1881) in Vostok Bay (part of the Peter the Great Bay) of the Sea of Japan is studied. Totally, 167 stomachs of crabs with carapace width from 5.9 to 11.8 mm were examined. Their stomach content includes inorganic and organic components. The inorganic component is represented by a mineral suspension of formless particles about 0.01–0.07 mm in size, i.e. pelites (aleurite-clay and fine silt) (the frequency — 100%). Their further cementing (lithogenesis) inside the crab stomach is supposed: turning first into spheroid or pseudo-rhomboid formations of crystalline nature, with a stepped surface, forming bizarre splices (0.07–0.15 mm) that can be attributed to large aleurites and then converted into a “particle aggregates” with the dimension up to 0.7 ´ 0.7 mm — a large psammophyte, i.e. large sand (37.1%). Organic components consists of a suspension of unicellular organisms (97%), lumps of filament identified as hyphae of fungi (6.0%) and formless scraps (95.8–97%) remaining fragments of connective tissue about 0.35–2.0 mm in size including ovoid bodies from 0.003–0.005 mm to 0.03 in length. Two latter components possibly develop on walls of the host burrow and crabs probably clean them off as a food object. At the same time, crabs can also feed on mucusous host’s feeding net accumulating small phyto- and zooplankton. In this case, crabs probably perform the function of an attendant or “nurse of the burrow” and the relationship between P. rathbunae and Urechis can be considered as mutual, i.e. protocooperation.
KEY WORDS: Crustacea, Decapoda, Pinnixa rathbunae, Urechis, symbiosis, feeding, cooperation, Sea of Japan, Russian Federation.