SYNOPSIS. Ascidian sperm are simplified by omission of the midpiece and proximal centriole with the single mitochondrion located next to the nucleus in the head. Small acrosome-like vesicles appear in some species but their distribution is unclear. Sperm structure correlates well with the location of fertilization; members of the orders Phlebobranchiata and Stolidobranchiata with external fertilization have sperm with short heads and long tails, while sperm from the exclusively internally

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... izing Aplousobranchiata have relatively longer heads and shorter tails. The mitochondrion swells when the sperm contacts the chorion, then translocates along the tail as it enters the chorion to be discarded when the tail disappears inside. Structural changes also occur in the anterior sperm head that have been interpreted as an acrosome reaction. Proteolytic enzymes are involved in penetration of the chorion. The mitochondrial transformation is under control of intracellular pH and Ca 2+ levels with the sperm releasing H + and taking up Ca 2+ during the sperm reaction. Acid release is from inactivation of a Na + requiring acidification system and triggering of a Cl" releasing HCO 3 requiring acid release system. An increase in intracellular pH increases the permeability to Ca 2+ , resulting in increased intracellular Ca 2+ , the proximal trigger to the mitochondrial