Light and electron microscopy studies of siphon and siphonal sheath formation in the infaunal bivalve Tresus keenae (Bivalvia: Mactridae)

Abstract

To understand the habitat ecology of Tresus keenae, an infaunal bivalve, the microanatomical structure of the siphon and the method of siphonal sheath formation were described. The diameter of the incurrent siphon was approximately 1.2 times greater than that of the excurrent siphon. Several irregular tentacles developed inside the distal end of the siphon. The tentacles in the incurrent siphon were approximately twice as long as those in the excurrent siphon. The siphon consisted of six tissue layers, which, from the outside inward, were the siphonal sheath, matrix, outer epithelial layer, connective tissue layer, muscular layer, and inner epithelial layer. The siphonal sheath was composed of an outer cuticle and dense microfilament layer and had vertical ducts. The matrix showed a loose microfilament layer. The outer epithelial layer was simple consisting of ciliated columnar epithelia and secretory cells. There were two types of secretory cells: arenophilic cells and proteinous granular cells. These were all unicellular glands, with cytoplasmic projections developing on the free surface and microstructural features of the cytoplasm showing secretory activity. Histochemical analysis indicated that the secretory granules of the secretory cells, the dense microfilament layer, and the matrix were composed of neutral carboxylated mucopolysaccharides. From these characteristics, it was concluded that the siphonal sheath was formed via the transportation of substances secreted by secretory cells of the outer epithelial layer to the outside through the duct. The hemolymph sinus developed in the connective tissue layer. The muscular layer had alternating longitudinal and circular muscle layers. The inner epithelial layer was simple and consisted of ciliated columnar epithelial cells and secretory cells. Secretory cells are goblet-like cells and contain acidic carboxylated substances. The siphonal sheath was identified starting at approximately 3.5 mm in shell length before the infaunal stage; as it grew, the siphonal sheath thickened, reflecting the infaunal habitat.

Keywords: Infaunal habitat; Microanatomy; Siphon; Siphonal sheath; Tresus keenae.