Epibiosis is a widely distributed phenomenon in the marine environment. It is defined as a facultative association of two organisms: the basibiont (or host) and the epibiont. The epibiont is attached to the surface of a living substratum, while the host lodges and constitutes support for its epibiont . The exoskeleton and gills of P. camtschaticus are colonized by several epibionts/symbionts including hydrozoans, nemerteans, polychaete worms, fish leeches, bivalve and gastropod mollusks, amphipods, cirripedians, copepods, and bryozoans [11,12,13,14]. The echinoderm epibiosis on king crabs has been poorly documented, but predator-prey interactions between P. camtschaticus and echinoderms are well documented [15,16].
Coastal sites of the Barents Sea play a crucial role in the growth and recruitment of the local red king crab population . For this reason, since 2003, monitoring of the P. camtschaticus population has been conducted by specialists from the Murmansk Marine Biological Institute (MMBI) in coastal waters of the Kola Peninsula [4,7,13,17,18]. Regular studies of symbiotic associations of non-indigenous species are important because they allow the tracking of the establishment and adaptation processes of such alien host species in new places of their distribution and could help evaluate the impacts of these crabs on local communities [19,20,21]. In addition, such studies could provide new information on the biodiversity of targeted areas and detect changes in local communities associated with human activities and climate shifts [20,22].
Crabs were examined for associated species in the laboratory in Dalnezelenetskaya Bay by eye, according to previous studies . The body of red king crabs was divided into five sections: carapace, limbs (walking legs and claws), abdomen, mouthparts, and gills. All epibionts were fixed in 4% formalin and then identified in the laboratory in Murmansk using a stereomicroscope MBS-10.
The starfish was removed from the host carapace. Weight of this epibiont was 42 g, arm length (AL) 29 mm. The second starfish specimen (weight 55 g, AL 34 mm) was registered on a male red king crab (new shell, CL 126 mm, weight 1194 g) collected in the area at 13 m on July 08, 2018, and the final finding was recorded on July 09, 2019, when our divers captured an adult male crab (new shell, CL 136.4 mm, weight 2115 g) colonized by a single sea star weighing 41 g. Both epibionts had the same localization as we recorded in 2010. Thus, the prevalences of A. rubens were 0.7, 0.8, and 1.1% in 2010, 2018, and 2019, respectively.
During our previous studies conducted in the same area and in other coastal areas of the Barents Sea [12,13] only one echinoderm species, i.e., Ophiura robusta (Ayres, 1852) was found on red king crabs in the Barents Sea, thus the present records expand a list of echinoderm epibionts that could settle on P. camtschaticus.
Although there are no prior reports that suggest epibiotic interactions between P. camtschaticus and A. rubens, O. sarsii, and C. frondosa, predator-prey relationships between these species have been well documented in the Barents Sea both by Russian and Norwegian scientists [6,15]. While in Dalnezelenetskaya Bay, bivalves and gastropods were the most important prey items for red king crabs (with a frequency of occurrence over 60%), sea urchins, sea stars, and brittle stars were also frequently consumed by red king crabs (>50% of all adults) .
We found higher prevalences of echinoderms on male red king crabs. This pattern is more likely associated with differences in the behavior of males and females. Males tend to migrate more frequently and cover longer distances than females  and, therefore, they have a higher chance to be colonized by epibionts.
Echinoderm colonizers may derive a variety of other benefits from red king crabs. It is known that adult red king crabs show a defense behavior, and they protect themselves against attacks from their conspecifics and other predators . In addition, during their migrations, red king crabs can cover distances as long as 69 km per 90 days . For these reasons, living in association with P. camtschaticus may be advantageous for sea stars, brittle stars, sea urchins, and sea cucumbers because it may provide them protection from predators and increases their mobility. Male crabs migrate more actively than females [1,2] and they have a higher chance to be colonized by epibionts. Our data support this pattern because only one epibiont was found on a female red king crab. Additionally, red king crabs could provide sea stars and brittle stars with food items such as mussels Mytilus edulis, polychaetes Harmothoe imbricata (Linnaeus, 1767) and Chone sp., copepods Tisbe spp. or/and amphipods Ischyrocerus commensalis (Chevreux, 1900) and Ischyrocerus anguipes (Krǿyer, 1838) which can colonize the carapaces and other parts of red king crab in large amounts [11,12,13,47]. Cucumaria frondosa could ingest detritus concentrated on the crab body. 2b1af7f3a8