First Description of Typhlosole in Ascidian Styela clava: Structure and Putative Function.

The ascidian Styela clava is an invasive species that easily colonizes new water areas and has a wide distribution. A previously undescribed typhlosole was discovered in S. clava and studied using histological techniques, transmission electron microscopy and computer microtomography. The typhlosole is a large fold of the dorsal intestinal wall that starts in the lower quarter of the stomach and continues to the rectum. Its shape varies greatly: from a rounded protrusion to a spirally twisted fold. The typhlosole is formed by the intestinal epithelium and a thick connective tissue layer, in which pyloric tubules and blood vessels are extended. The epithelium consists of glandular, ciliated, and nonciliated cells demonstrating apocrine secretion. The typhlosole intensifies the food digestion and absorption of nutrients. The features observed in the typhlosole may contribute to a high digestion efficiency even in low-productive waters, determining the biological success of this species.