Biomineralisation and metal sequestration in a crustacean ectoparasite infecting the gills of a freshwater fish

Abstract

It has been suggested that parasites are effective bioindicators as they are sensitive to environmental changes and, in some cases, accumulate trace elements in higher concentrations than their hosts. Accumulated elements sequester in different organs. In monogenean and crustacean ectoparasites, sclerotised structures and egg yolk appear to be the preferred site for element sequestration. In this study, the sequestration of trace elements; Mg, Al, Ca, Fe, Cu, and Zn in Lamproglena clariae was studied from two rivers. Adult L. clariae were collected from the gills of Clarias gariepinus from Lake Heritage in the Crocodile River and in the Vaal River below the Vaal Dam, South Africa. Collected parasites were flash frozen in liquid nitrogen and sectioned with a cryomicrotome. Sections were treated with Phen-Green to observe fluorescent signals. Trace elements in the parasite were analysed using a scanning electron microscope with an energy-dispersive spectroscope (SEM–EDS). Results showed more intense fluorescence signals in the exoskeleton compared to tissues, and in the egg yolk. Analysis by SEM–EDS confirmed the presence of elements in the parasite from both sites. Levels of Al were higher in L. clariae from the Vaal River than those from Lake Heritage, and Fe was higher in L. clariae from Lake Heritage. Element distribution patterns in the parasite matched those in the water from the sites. Unlike other crustaceans, regulation of metals in adult females of L. clariae does not occur through moulting, but high levels occurred in the yolk.