Identifying the social structure on masts
To examine the territorial behaviour and social structure of Dyopedos bispinis on masts, 285 photos were obtained in July 2012 in various biotopes in the White Sea (Kandalaksha Gulf, Velikaya Salma Strait 66.55-66.56°N, 33.09-33.12°E, depth 8-15 m). Before the photographs were taken, a ruler with 1-mm divisions was placed on the bottom. From the photographs, the numbers of adult males, females and juveniles were counted on each inhabited mast that was in focus (790 masts), and the proportions of masts with one, two, or more than two adult amphipods were calculated. Adult amphipods were distinguished by their length (approximately 5 mm), specific body form (slender body and curved back) and brighter coloration, and adult males were identified by their strongly enlarged second gnathopods. In the cases when the maturity of the amphipods could not be accurately determined, the masts were not included in the analysis.
Over 10 dives, all masts with 3 and more adult residents were collected; the masts were plucked with forceps near the mast base and carefully placed in individual tubes that were immediately closed. In the lab, the specimens were fixed in 96% ethanol, and the amphipods on each mast were counted as described above. Collected such method adult females were distinguished by the presence of oostegites and often by the presence of eggs in the marsupium, males were identified by enlarged second gnathopods. A total of five masts were collected.