Development of vestigial tail muscle acetylcholinesterase in embryos of an anural ascidian species.

1. The ascidian Molgula arenata produces an anural larva lacking a tail and other structural features of typical urodele larvae in the family Molglidae, yet its embryos developed a histochemically detectable acetylcholinesterase in the tail muscle rudiment. Development of the myoblasts seemed to fail during the neurula stage. 2. Larval enzyme activity occurred at a mean of 5--6% of the level found in the urodele species Molgula occidentalis and Molgula manhattensis, as measured by scanning integrating microdensitometry of the histochemical reaction product. Some anural larvae had as much as 20% of the enzyme activity in urodele larvae. 3. This example of vestigial expression in the absence of other urodele larval features further illustrates the autonomy of a histospecific enzyme development thought to be controlled by an egg cytoplasmic determinant. Partial suppression of the determinant might be the cause of this diminished expression. 4. Two other anural molgulid species, Molgula occulta and Bostrichobranchus pilularis, did not have vestigial larval enzyme and possibly have lost the determinant completely.