External Morphology

Herdmania momus exhibit a large degree of morphological differentiation between populations and even between individuals within populations. This polymorphism is somewhat responsible for past confusion regarding species classification. Specimens generally have an inflated, spherical body that is slightly longer along the antero-posterior (head-butt) axis. Two short, diverging, trumpet shaped siphons feature at the anterior end (see picture below). These siphons are red-ish, often slightly darker in colour than the body. The body is covered with a complex tunic (also known as a test) from which the term tunicate is derived. The tunic of H. momus varies in consistency from soft, thin and delicate to a leathery, thick and robust. Tiny calcareous spines, or spicules, are common within the lining of both internal and external structures such as the tunic, siphons and branchial basket (Lambert and Lambert 1987). These are thought to provide some form of protection and can be the cause of great irritation when handling specimens, so be sure to wear gloves! Other morphological characteristics vary, including the tendency for the test to be covered in small organisms; this is called epibiotic growth. Although not usually harmful to the ascidian, this type of growth can camouflage an individual and may further complicate identification.

Tropical populations inhabiting the Great Barrier Reef are generally up to 8cm long and are covered in a soft, translucent test. Even within these populations, individuals show variation in colour and test thickness. Size is thought to be indicative of age (Berril 1975) with mature individuals thought to be those greater than 5cm in length (Degnan 1996).