Description of the unusual digestive tract ofPlatax orbicularisand the potential impact ofTenacibaculum maritimuminfection

Abstract

Background Ephippidae fish are characterized by a discoid shape with a very small visceral cavity. Among them Platax orbicularis has a high economic potential due to its flesh quality and flesh to carcass ratio. Nonetheless, the development of its aquaculture is limited by high mortality rates, especially due to Tenacibaculum maritimum infection, occurring one to three weeks after the transfer of fishes from bio-secure land-based aquaculture system to the lagoon cages for growth. Among the lines of defense against this microbial infection, the gastrointestinal tract (GIT) is less studied. The knowledge about the morphofunctional anatomy of this organ in P. orbicularis is still scarce. Therefore, the aims of this study are to characterize the GIT in non-infected P. orbicularis juveniles to then investigate the impact of T. maritimum on this multifunctional organ. Methods In the first place, the morpho-anatomy of the GIT in non-infected individuals was characterized using various histological techniques. Then, infected individuals, experimentally challenged by T. maritimum were analysed and compared to the previously established GIT reference. Results The overlapped shape of the GIT of P. orbicularis is probably due to its constrained compaction in a narrow visceral cavity. Firstly, the GIT was divided into 10 sections, from the esophagus to the rectum. For each section, the structure of the walls was characterized, with a focus on mucus secretions and the presence of the Na+/K+ ATPase pump. An identification key allowing the characterization of the GIT sections using in toto histology is given. Secondly, individuals challenged with T. maritimum exhibited differences in mucus type and proportion and, modifications in the mucosal and muscle layers. These changes could induce an imbalance in the trade-off between the GIT functions which may be in favour of protection and immunity to the disadvantage of nutrition capacities.

First Isolation of Virulent Tenacibaculum maritimum Isolates from Diseased Orbicular Batfish (Platax orbicularis) Farmed in Tahiti Island

“…Skin fragments of approximately 1 cm 2 , as well as internal organs (anterior kidney, spleen, liver), were collected from four fish showing typical skin lesions of 'whitepatch' disease using a sterile surgical scalpel. Samples were fixed in Davidson's solution (25% formaldehyde, 37.5% ethanol, 12.5% acetic acid and 25% water) for 48 h at room temperature, then washed and kept in 70% ethanol [31]. Skin samples were progressively dehydrated in an ascending series of alcohol (70 to 100% ethanol) followed by a xylene bath, using dehydration automate (Leica, ASP 300S), then embedded in paraffin, cut into 3 µm sections using a rotary microtome (Microm HM 340E, Thermo Fisher Scientific, Waltham, MA, USA) and stained with haematoxylin-eosin (H-E) using a fully-automated integrated stainer (Leica, CV5030 autostainer XL).…”