Trace element biomineralisation in the carapace in male and female Argulus japonicus.

Beric M Gilbert,Annemariè Avenant-Oldewage,Suzannah Rutherford

doi:10.1371/journal.pone.0197804

Abstract

Parasites of fishes have been shown to be effective bioindicators of the aquatic environment. Few investigations have been conducted on ectoparasite models and therefore little is known about the fate of trace elements and metals which they accumulate. In this study trace element sequestration was observed in the carapace of the fish louse, Argulus japonicus and found to relate to the sex of the parasite, as well as, the degree of sclerotization of the carapace. Adults of A. japonicus were collected from cyprinid hosts in the Vaal Dam, South Africa. Parasites were removed and flash frozen in liquid nitrogen before being sectioned with a cryomicrotome. Sections and whole mounts of parasites were prepared and treated with Phen–Green TM FL cell–permeant diacetate. Cryosections were assessed for trace elements and metals using a scanning electron microscope equipped with energy dispersive spectroscopy. Results indicated that in both male and female parasites, trace elements become bound to the carapace and produce more intense fluorescence than in soft tissues. Sexual dimorphic differences were further observed between male and female parasites. The intensity of the fluorescence signals was greater in the carapace of male parasites than in females, particularly when comparing the carapace of the ventral side of the thorax. In females, an amorphous layer of material surrounding the eggs was observed and produced an intense fluorescent signal. Levels of trace elements and metals detected were not significantly different between male and female parasites. Results observed serve as a demonstration for the first time of trace element sequestration in a freshwater crustacean parasite and possible mechanisms employed to reduce body burdens of trace elements and metals.

Highlights

Aquatic organisms are continually exposed to a highly dynamic environment which in many instances is loaded with extraneous chemicals, most of which are of anthropogenic origin
The use of fluorescence microscopy in the present study allowed for the discernment of trace element and metal accumulation in A. japonicus and serves to indicate the validity of the technique for studying a number of morphological characters of crustacean parasites
Milne and Avenant-Oldewage [29] and Wong et al [30] showed that fluorescence of the haptoral sclerites of monogenean parasites, Paradiplozoon ichthyoxanthon and Diplozoon paradoxum respectively could be used to study the morphology of these structures

Summary

Introduction

Aquatic organisms are continually exposed to a highly dynamic environment which in many instances is loaded with extraneous chemicals, most of which are of anthropogenic origin. Trace elements and metals are omnipresent components of aquatic ecosystems, generally occurring in low concentrations. Aquatic organisms are constantly exposed to and accumulate these elements in their bodies. Studies investigating accumulation of trace elements and metals in aquatic invertebrates have found this to be highly variable between. Trace elements in Argulus japonicus analytical facility at the University of Johannesburg is thanked for providing infrastructure for acquiring images of sections. The University of Johannesburg is thanked for providing GES postdoctoral funding (GES PDRF bursary) to BMG. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

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