Thermal Responses in Japanese Pearl Oysters, Pinctada fucata martensii

Abstract

In order to examine the thermal responses in pearl oysters, both the mRNA expression levels of heatshock protein (Hsp) in their tissues and changes in the mineral levels of their hemolymph were analyzed using heattreated and heat-untreated pearl oysters. Pearl oysters that live in Tsukumo Bay on the Noto Peninsula were used as experimental materials. Pearl oysters kept at 25 C in artificial seawater were exposed to high temperatures (30 C or 35 C) for 1 h. After exposure, the pearl oysters were transferred into the seawater at 25 C and kept for 4 h. Thereafter, the hemolymph was collected by inserting a syringe into the adductor muscle of the heat-treated pearl oyster, and then hemocytes were separated from hemolymph by centrifugation. In addition to hemocytes, the mantles and gills were taken from heat-treated pearl oysters to analyze Hsp mRNA expression. The collected hemolymph was used to analyze its mineral concentration. The data of pearl oysters treated at a high temperature (30 C or 35 C) were compared with those of pearl oysters kept at the initial temperature (25 C) as a heat-untreated control. First, the mRNA expressions of several Hsps (Hsp20, Hsp40, Hsp70, and Hsp90) were analyzed using the hemocytes, mantles, and gills to examine the thermal responses of pearl oysters. With heat treatment, the mRNA expressions of all Hsps (Hsp20, Hsp40, Hsp70, and Hsp90) used in the present study were significantly increased. Especially, Hsp20 mRNA expression in hemocytes, mantles, and gills was remarkably increased by heat treatment. The responsiveness of Hsps mRNA expression was greatly higher at the rate of increase at 35 C than at the rate of increase at 30 C. Among the tissues used in the present study, the mRNA expression levels of Hsp20, Hsp40, Hsp70, and Hsp90 were higher in gills than in hemocytes and mantles. Second, we analyzed the mineral concentrations of hemolymph in the heat-treated pearl oysters. The calcium, magnesium, sodium, chlorine, and potassium ion levels in the hemolymph of pearl oysters kept at 35 C increased significantly as compared with those in the initial hemolymph of pearl oysters kept at 25 C. This suggests that heat treatment influences excretory organs such as gills and kidneys. The fact that the expression level of Hsps was highest in gills, which function as respiratory and International Journal of Zoological Investigations Contents available at Journals Home Page: www.ijzi.net Editor-in-Chief: Prof. Ajai Kumar Srivastav Published by: Saran Publications, Gorakhpur, India 372 excretory organs, supported the influences of the excretory system. This study is the first report to demonstrate changes in both the monovalent and divalent ions of hemolymph in response to heat treatment in Japanese pearl oysters. We plan to investigate the detailed mechanism of the above mineral regulation together with HSP expressions in the near future.