Survey of survival rate and histological alterations of gills and hepatopancreas of the Litopenaeus vannamei juveniles caused by exposure of Margalefidinium / Cochlodinium polykrikoides isolated from the Persian Gulf

Abstract

Harmful algal blooms (HABs) have had harmful effects on the health of a wide range of organisms and are becoming a noteworthy risk to fisheries, human health and wildlife around the globe. Although the lethal influences of dinoflagellate Margalefidinium / Cochlodinium polykrikoides have been well-documented for finfish, the impacts on decapod crustacean species, especially, Litopenaeus vannamei are poorly understood. Here, we examined the exposure of L. vannamei juveniles with different M. polykrikoides densities (T1: 1 × 10³, T2: 2 × 10³, T3: 3 × 10³, T4: 9 × 10³ and T5: 30 × 10³ cells ml^-1), in order to observe possible histological alterations in gills and hepatopancreas. After 96 h exposure, shrimp exhibited increased mortality and survival rate of juveniles in control group were meaningfully (P < 0.05) higher than treatments exposed to 9 × 10³ and 30 × 10³ cells ml^-1 of M. polykrikoides. The lowest survival observed in treatments T4 (95.56%) and T5 (82.20 %), respectively. The juveniles exposed to high densities of M. polykrikoides had flaccid bodies and gills were yellow in color with some black spots (melanization) in pereiopods. At the end of the experiment, shrimps' hepatopancreas and gills exhibited various histological alterations. The irregular appearance of the tubular structure of the hepatopancreas, R- and B-cells reduction, separation of epithelium and myoepithelial layer, ruptured epithelial cells and degeneration of tubules were detected in the hepatopancreas of shrimps exposed to 30 × 10³ cells ml^-1. In gills of shrimp exposed to 30 × 10³ cells ml^-1, alterations in epithelial structure, increases in the number of intercellular hemocytes, and changes in tissue structure were observed. Finally, findings suggest that M. polykrikoides blooms can lead to significant impacts on survival and major histological alterations in gills and hepatopancreas organs when the density of M. polykrikoides was over 9 × 10³ cells ml^-1.

Keywords: Dinoflagellate bloom; Harmful algae; Hepatopancreas; Histology; Shrimp; Survival rate.