Didemnum psammatodes (Sluiter, 1895)
Didemnum psammatodes is a colonial tunicate. Colonies form thin encrusting sheets that sometimes produce fleshy lobes or irregular twig-like branches up to 8 cm long and 1 cm diameter. It is sometimes called the chocolate tunicate and appears smooth and glossy, but is gritty to the touch (Smithsonian Marine Station 2011). The color of colonies varies with the substrate, ranging from cream-colored in coral reef habitats to brown in muddy habitats (Kott 2001). The brown color of the colony derives from dense packing of fecal pellets throughout the tunic (Gretchen Lambert, personal communication 2012). Chlorophyte microalgae are often embedded in the colony surface and spicules (small crystalline structures embedded in the tunic) are scattered throughout. Some spicules have 11-13 relatively short conical pointed rays with bases separated on the central mass. Other spicules are burr-like with cylindrical or paddle-shaped rays. The spicules are very small, sparse and dissolve easily, making them difficult to find or absent in preserved speciems. Colonies have characteristically constricted common cloacal cavities that are shallow with brown fecal pellets embedded throughout (Kott 2001). Zooids are very small, being less than 1mm long, with a wide atrial aperture, which exposes much of the branchial sac to the cloacal cavity. The branchial sac has eight stigmata in the first row, but numbers of stigmata in other rows could not be determined accurately (Kott 2001). The gut loop is bent at right angles to the long axis of the thorax (Kott 2001). Larvae have three adhesive papillae and four pairs of ampullae (da Rocha and Monniot 1995).