Description of the microbiota in epidermal mucus and skin of sharks (Ginglymostoma cirratum and Negaprion brevirostris) and one stingray (Hypanus americanus)

Skin mucus in fish is the first barrier between the organism and the
environment but the role of skin mucus in protecting fish against
pathogens is not well understood. During copulation in sharks, the male
bites the female generating wounds, which are then highly likely to become
infected by opportunistic bacteria from the water or from the male shark’s
mouth. Describing the microbial component of epithelial mucus may allow
future understanding of this first line of defense in sharks. In this
study, we analyzed mucus and skin samples obtained from 19 individuals of
two shark species and a stingray: the nurse shark (Ginglymostoma
cirratum), the lemon shark (Negaprion brevirostris) and the southern
stingray (Hypanus americanus).   Total DNA was extracted
from all samples, and the bacterial 16S rRNA gene (region V3-V4) was
amplified and sequenced on the Ion Torrent Platform.  Bacterial
diversity (order) was higher in skin and mucus than in water. 
Order composition was more similar between the two shark
species.  Alpha-diversities (Shannon and Simpson) for OTUs
(clusters of sequences defined by a 97% identity threshold for the16S rRNA
gene) were high and there were non-significant differences between
elasmobranch species or types of samples. We found orders of potentially
pathogenic bacteria in water samples collected from the area where the
animals were found, such as Pasteurellales (i.e. genus Pasteurella spp.
and Haemophilus spp.) and Oceanospirillales (i.e. genus Halomonas spp.)
but these were not found in the skin or mucus samples from any
species.  Some bacterial orders, such as Flavobacteriales,
Vibrionales (i.e. genus Pseudoalteromonas), Lactobacillales and Bacillales
were found only in mucus and skin samples. However, in a co-occurrence
analyses, no significant relationship was found among these orders
(strength less than 0.6, p-value > 0.01) but significant
relationships were found among the order Trembayales, Fusobacteriales, and
some previously described marine environmental Bacteria and Archaea,
including Elusimicrobiales, Thermoproteales, Deinococcales and
Desulfarculales.  This is the first study focusing on
elasmobranch microbial communities. The functional role and the benefits
of these bacteria still needs understanding as well as the potential
changes to microbial communities as a result of changing environmental
conditions.