Shell disease dynamics in the non-indigenous Atlantic rock crab (Cancer irroratus) in Icelandic waters

Crustacean shell disease, characterized by progressive degradation of the chitinous exoskeleton due to secretion of chitinases and other enzymes by microorganisms, has been reported in crustaceans worldwide. Diseases affect all natural populations to some extent. However, under normal circumstances, they are generally not in high prevalence. Disease epidemics are regularly experienced in various marine animals. Data on their causes is often scarce, at least partly due to limited or nonexistent baseline data on the prevalence and intensity of even the most common agents, under normal circumstances. This study presents the first description of shell disease in the non-indigenous Atlantic rock crab Cancer irroratus in Icelandic waters, a finding of significant concern. Between June and August from 2017 to 2023, a total of 5,818 C. irroratus were sampled in Hvalfjörður, SW-Iceland. The shell disease was assessed in relation to sex, and the pattern of infection on exoskeleton surfaces was described. The prevalence and severity of shell disease showed a concerning upward trend, with the infection rate increasing from 47% in 2017 to 85% in 2023. Samples taken from diseased crabs and subjected to histological, bacteriological and molecular analyses, revealed severe bacterial infections in lesions, composed of a mix of nine species of chitinolytic bacteria of five genera. Furthermore, a known pathogenic oomycete (fungal-like organism) was observed in all crabs examined. As in similar crustacean shell disease studies, the condition in C. irroratus from Icelandic waters appears to result from synergistic effects of various bacterial species, possibly exacerbated by the observed oomycete pathogen. However, the cause of an epidemic like this is unknown and can only be speculated on with current knowledge.