A RARE ABYSSAL BAIKAL AMPHIPOD, POLYACANTHISCA CALCEOLATA (CRUSTACEA, AMPHIPODA) AT THE ST. PETERSBURG COLD METHANE SEEP, CENTRAL BAIKAL

摘要

In 2009-2010 during the diving of the deep-water manned submersibles "Mir-1" and "Mir-2" at the St. Petersburg cold methane seep, central part of Lake Baikal, aggregations of the rarest endemic abyssal amphipod, Polyacanthisca calceolata Bazikalova 1937 were found at the depth of 1000-1402 m. Only 29 specimens of P. calceolata had been collected earlier since their description. Samples collected during the diving from the seep were rich in P. calceolata specimens, amounting to several hundred. Females prevailed in all catches, mainly ovigerous females, from 87 to 100% of the total number of specimens in the samples. The ratio of males to females was higher in the near-bottom zone (1 : 7) than at the bottom (1 : 20). The sizes of females and males were 17.0-24.5 mm and 18.0-22.5 mm, respectively. The number of eggs in females with a size of over 20 mm and less than 20 mm averaged 83 and 42, respectively. Powerful bundles of very long setae located at the base of pereopod dactyls served as a device for living on soft substrates. Large calceoli in P calceolata, specialized mechanoreceptor structures, were located both in males and females on both pairs of antennae and on all flagellum's articles except the last one. Aesthetascs, chemoreceptor structures, were single on the flagellum's articles of antenna 1. Microtrichs II, functioning as mechano- and chemoreceptors, were located in the dorsal part of all body segments. No lateral units have been found. Polyacanthisca calceolata mandibles of the basic gammaridean type were non-specialized. During the inter-moult period, the mandibular teeth did not wear out, this being characteristic of species feeding on soft food. According to the data obtained, P. calceolata feeds on amphipods (soft tissues and probably eggs) and entomostracans instead of dead fish as it was suggested earlier. Undoubtedly, this is a carnivorous species, a predator, but not a scavenger. It is likely that this species can hunt not only at the bottom, but also in the near-bottom zone. The main habitat zones for P. calceolata are large depths (over 1000 m) in the central and southern basins of Lake Baikal. This species is likely to prefer the areas close to methane discharge and gas hydrates deposits. The findings of aggregations of the rare Baikal species at the St. Petersburg methane seep can be attributed to the relatively poor prospection at large depths of the lake, the presence of special habitats with rich food sources, and a complicated structure of the bottom with plenty refuges from predators.