Chemical Mediation of Sperm Activity and Longevity in the Solitary Ascidians Ciona intestinalis and Ascidiella aspersa

Abstract

Sperm chemotaxis to compounds originating from eggs is common in solitary ascidians. This chemotactic response is associated with an increase in the activity level of the sperm. Here we examine the hypothesis that the period of sperm viability is reduced by the stimulation of increased activity by compounds associated with the eggs of solitary ascidians. The activity level and longevity of sperm from two species of solitary ascidian, Ciona intestinalis and Ascidiella aspersa, were measured after incubation in water that had been stored with homospecific eggs. Our results showed that the proportion of active sperm increased in homospecific egg water. This increase corresponded to a sharp decline in sperm longevity, which suggests a causal link between sperm activity and longevity. We suggest that sperm energy reserves are conserved in the absence of compounds associated with homospecific eggs, and that this may have significant ecological consequences for these organisms. Cross-reactivity in the chemotactic response of sperm from one species to the chemoattractant of another is common in ascidians, and increased sperm activity is associated with this response. Therefore, we investigated cross-reactivity in the activation of C. intestinalis and A. aspersa sperm to heterospecific egg water. Our results showed that A. aspersa sperm were activated by the egg water of C. intestinalis, resulting in a decline in sperm viability, but that this response was not reciprocated between the sperm of A. aspersa and the egg water of C. intestinalis. This suggests either that sperm of A. aspersa respond to compounds associated with the eggs with a higher degree of specificity than C. intestinalis sperm, or that compounds associated with A. aspersa eggs have more generalized activity than those of C. intestinalis.