A new method of analysis of the periostracum in pallets of Bankia Gray, 1842 (Bivalvia: Teredinidae)

Abstract

The wood-boring family Teredinidae (Bivalvia) comprises 15 genera and 71 species. The identification of species is mainly based on the pallets, structures which are exclusive to this family. Each pallet is made up of a stalk and a blade, which is particulary elaborate in the genus Bankia Gray, 1842. In this genus, many cones form the blade and each cone is composed of a calcareous portion, covered by the periostracum that can expand laterally to form awns. The free border of the periostracum may be smooth or serrate. The presence or absence of awns, the characteristics of the periostracal free border and the shape of the calcareous portion of the cones have been considered of great value in the identification of species. The pallets may be intensely affected by ecological conditions or by bad preservation of specimens in collections. In such situations, modifications of the calcareous portion of the cones and of the periostracum may occur, leading to possible errors of identification. Even in well-preserved specimens, the transparency of the periostracum and the absence of contrast between this structure and the calcareous portion of the cones, interfere in the visualization of details of the cone and the free border of its periostracum. The 23 species of Bankia are especially difficult to identify and a revision of the genus is needed. Here we introduce a new technique to render pallet structures clearly visible, which should represent a step forward in the correct identification of species of this and other teredinid genera. The method uses vital staining to clarify the differences between the periostracum and calcareous cones, and details of the periostracal border. Several kinds of dye were used on a range of different species of Bankia, but the best results were obtained by using Janus Green B (MERCK). After immersing the pallets for 1 min or less in a solution of Janus Green B (1 mg/mL in distilled water), the periostracum immediately takes on a bluish colour, whereas the calcareous portion of the cones remains white. As the serrated border and the awns of the periostracum take on a more intense colour, differences in ornamentation and structure of cones become clear (Fig. 1A, B). After observation, the stain may be totally removed by rinsing the pallets in alcohol (90% or higher). This ease of removal makes the technique suitable for preserved museum specimens. The authors wish to thank the Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP) and the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq) for the financial support and to Dr Breno Grisi for translating the manuscript into English.