Bacterial Septicemia in Cancer pagarus

Vibriosis is a serious condition that affects many species in the marine environment, it is though that susceptibility in marine species changes depending on the life stage of the organism. Crustaceans have seen to be susceptible to many vibrio-induced infections and are prone to attack from a wide range of species including V. alginolyticus, V. fluvialis, V. harveyi and V. vulnificus. However, our knowledge of the importance of vibrios in Crustacea is limited to few studies only using a small number of species, including shrimp and the blue crab. This study looks at the importance of vibrios in infection of juvenile stages of the edible crab, Cancer pagurus. It has been previously suggested that the juvenile life stage is a ‘bottleneck’ for survival to adulthood, meaning the importance of susceptibility to virbriosis is highest at this point, allowing for optimal infection and bacterial septicaemia to persist.

Juvenile C. pagarus were collected from Mumbles Head and Gower Penisula, Swansea, UK. Their haemolymph was extracted and aseptically plated onto saline tryptone soya agar (TSA) and thiosulfate citrate bile salts sucrose agar (TCBS). Colonies were left to form at 25ᵒC for 2-5 days and a baseline of 2000 colony forming units (CFU) mL^-1 was used as an indication of the extent of bacterial septicaemia, below this level it was assumed to be low severity infection.

After incubation, the morphological characteristics of the bacteria were recorded, this showed the bacterial colonies from the haemolymph to be highly variable. 33% of the samples had vibrio-like bacteria culturable on TSA, however only 1% were of high severity (>2000 CFU mL-1). This bacterial susceptibility was shown to have no influence by carapace damage or limb loss which contrasts to previous studies considering the blue crab, which was shown to be more prone to bacterial infection if damaged or missing limbs. The extent of low level infection was shown to be influenced by the stage of moult, post-moult crabs with softer, more vulnerable cuticles were shown to have significantly higher levels of bacterial haemolymph infection. Infection was also shown to be of higher persistence in the summer to autumn months when sea surface temperatures were highest. This has been shown to be typical of Virbio species as it has been shown that higher water temperatures often favour their replication.

Gill damage was measured using histological studies, it showed correlation between cases of high level of haemolymph infection, this is suggested to be either the route of entry of the infecting bacteria or a resulting defect of septicaemia. Previous studies have shown nodule formation in Carcinus maenus to be a defence mechanism for dealing with bacterial septicaemia.

This study also looked at the effect of the dinoflagellate, Hematodinium, on bacterial infection. The presence of Hematodinium in the crabs hepatopancreas correlated with reduced bacterial infection, it is therefore thought that a competitive interaction exists between this dinoflagellate and bacterial infection as a possible result of phagocytic action.

Molecular analysis of 16s rRNA was used to identify the infecting agents colonies with the most potential for vibrio species from 3 separate samples. The colonies were amplified with PCR, sequenced by MWG eurofins operon and searched using BLAST and the Ribosomal Database project II using an identity threshold of 99%. All three of the tested colonies showed highest similarity to Virbio spp, with two out of the three being most similar to Vibrio pectenicida. These V.pectenicida colonies were only cultureable on the TSA agar, suggesting that this study may have had limitations by using TCBS agar which may have led to an underestimation of vibrio species in the samples.

This study showed Vibrio species to be present in juvenile C. pagarus, however there was little identification of the species contributing to infection in the blood. To further analyse the crab septicaemia, identification of the more prevalent colonies and correlation between the damage to the crabs and the bacteria present would be needed. However, they did show that bacterial infection is significantly reduced with the presence of the dinoflagellate Hematodinium spp., showing there is a potential of competitive and perhaps phagocytic activity between these two communities, further study into the relationship between them would be recommended.

Smith, A., Whitten, M., Hirschle, L., Pope, E., Wootton, E., Vogan, C. and Rowley, A. (2013). Bacterial septicaemia in prerecruit edible crabs, Cancer pagurus L. Journal of Fish Disease, 37, 8, 729-737.