Ivone Giffard-Mena

Digestive tract ontogeny of Dicentrarchus labrax: Implication in osmoregulation

The ontogeny of the digestive tract (DT) and of Na+/K+‐ATPase localization was investigated during the early postembryonic development (from yolk sac larva to juvenile) of the euryhaline teleost Dicentrarchus labrax reared at two salinities: seawater and diluted seawater. Histology, electron microscopy and immunocytochemistry were used to determine the presence and differentiation of ion transporting cells. At hatching, the DT is an undifferentiated straight tube over the yolk sac. At the mouth opening (day 5), it comprises six segments: buccopharynx, esophagus, stomach, anterior intestine, posterior intestine and rectum, well differentiated at the juvenile stage (day 72). The enterocytes displayed ultrastructural features similar to those of mitochondria‐rich cells known to be involved in active ion transport. At hatching, ion transporting cells lining the intestine and the rectum exhibited a Na+/K+‐ATPase activity which increased mainly after the larva/juvenile (20 mm) metamorphic transition. The immunofluorescence intensity was dependent upon the stage of development of the gut as well as on the histological configuration of the analyzed segment. The appearance and distribution of enteric ionocytes and the implication of the DT in osmoregulation are discussed.

Adaptation of the sea-bass (Dicentrarchus labrax) to fresh water: Role of aquaporins and Na+/K+-ATPases

Sea-bass (Dicentrarchus labrax) grow under different salinity regimes, from the open sea to lagoons and even rivers, but some mortality has been recorded in juvenile stages when exposed to low salinity water. Changes in water permeability of different osmoregulatory tissues could be the cause of reduction in blood osmotic pressure and death in some fish in fresh water (FW). In order to explore this condition, we have studied the changes of aquaporins (AQP1 and AQP3), alpha1 and alpha4 Na(+)/K(+)-ATPase transcript levels in the digestive tract, kidney and gills after a long-term exposure of juvenile sea-bass to sea water (SW) and FW fish able to survive in SW and FW are called SW-adapted fish (SWS), FW successfully-adapted fish (FWS) respectively, while fish that die in FW are called FW unsuccessfully-adapted fish (FWU). AQP1 was highly expressed in SWS digestive tract and kidney, suggesting its involvement in water absorption. In FWU, AQP1 transcript levels in the digestive tract were higher than in FWS, suggesting higher water absorption. AQP3 transcript levels in gills were higher in FWS compared to SWS, suggesting a role in FW adaptation. AQP3 transcript levels in gills were higher in FWU than in FWS, suggesting an increase in gill water permeability or other solutes. Transfer to FW was followed in gills by an increase in alpha1 and alpha4 Na(+)/K(+)-ATPase levels in FWS and FWU, supporting the current model of ion absorption through the gills.

White spot syndrome virus (WSSV) infection in shrimp (Litopenaeus vannamei) exposed to low and high salinity

White spot syndrome virus (WSSV) has a worldwide distribution and is considered one of the most pathogenic and devastating viruses to the shrimp industry. A few studies have explored the effect of WSSV on shrimp acclimated to low (5 practical salinity units [psu]) or high (>40 psu) salinity conditions. In this work, we analysed the physiological response of WSSV-infected Litopenaeus vannamei juveniles that were acclimated to different salinities (5, 15, 28, 34 and 54 psu). We evaluated the osmotic response and survival of the shrimp at different times after infection (0 to 48 hours), and we followed the expression levels of a viral gene (vp664) in shrimp haemolymph using real-time PCR. Our results indicate that the susceptibility of the shrimp to the virus increased at extreme salinities (5 and 54 psu), with higher survival rates at 15 and 28 psu, which were closer to the iso-osmotic point (24.7 psu, 727.5 mOsmol/kg). Acute exposure to the virus made the haemolymph less hyperosmotic at 5 and 15 psu and less hypo-osmotic at higher salinities (>28 psu). The capacity of white shrimp to osmoregulate, and thus survive, significantly decreased following WSSV infection. According to our results, extreme salinities (5 or 54 psu) are more harmful than seawater.

Expression and Localization of Aquaporin 1a in the Sea-Bass (Dicentrarchus labrax) during Ontogeny

The successful establishment of a species in a given habitat depends on the ability of each of its developing stages to adapt to the environment. In order to understand this process we have studied the adaptation of a euryhaline fish, the sea-bass Dicentrarchus labrax, to various salinities during its ontogeny. The expression and localization of Aquaporin 1a (AQP1a) mRNA and protein were determined in different osmoregulatory tissues. In larvae, the sites of AQP1a expression are variable and they shift according to age, implying functional changes. In juveniles after metamorphosis (D32–D48 post-hatch, 15–25 mm) and in pre-adults, an increase in AQP1a transcript abundance was noted in the digestive tract, and the AQP1a location was observed in the intestine. In juveniles (D87–D100 post-hatch, 38–48 mm), the transcript levels of AQP1a in the digestive tract and in the kidney were higher in sea water (SW) than at lower salinity. These observations, in agreement with existing models, suggest that in SW-acclimated fish, the imbibed water is absorbed via AQP1a through the digestive tract, particularly the intestine and the rectum. In addition, AQP1a may play a role in water reabsorption in the kidney. These mechanisms compensate dehydration in SW, and they contribute to the adaptation of juveniles to salinity changes during sea-lagoon migrations. These results contribute to the interpretation of the adaptation of populations to habitats where salinity varies.

Osmoregulation, growth, and survival during the larval development of bullseye puffer fish Sphoeroides annulatus (Jenyns, 1842, Pisces: Tetraodontidae)

We analysed the effects of salinity on survival, growth and the osmoregulatory capacity (OC) of puffer fish larvae (Sphoeroides annulatus). In experiment 1, we compared the growth of larvae maintained at a constant temperature (28 ± 0.5 °C) under seven salinity levels (5, 12, 19, 26, 33, 35 and 40 psu) over a 28-day period. No significant differences were found for larval growth. However, salinity significantly affected survival. The larvae hyper-osmoregulated at low salinities and hypo-osmoregulated at high salinities. The isosmotic point ranged from 338.9 to 355.9 mOsm kg−1, depending on salinity. In experiment 2, survival increased with increasing salinity following direct exposure to the same seven salinity levels over a 72-h period. The isosmotic point ranged from 256.5 to 466.7 mOsm kg−1 depending on the developmental stage. This species hyper-regulated at 5–12 psu, hypo-regulated at 19–40 psu and demonstrated a high euryhalinity at 7 DAH with 95% survival.