Fertilisation and the potential for dispersal of gametes and larvae in the Solitary AscidianAscidia MentulaMüller

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Abstract
Ascidia mentula is a typical “primitive” solitary ascidian with external fertilisation producing a pelagic embryo which hatches into a simple relatively undifferentiated tadpole larva. The ecology of fertilisation and subsequent embryological and larval development are described. Sperm viability was lost after approximately 48 h, whereas ova were capable of being fertilised for up to 144 h. Normal swimming larvae were, however, only obtained from ova which had been fertilised within 96 h of removal from the oviduct. Although percent fertility varied widely between crosses, no significant difference in fertility was found between self- and out-crossed ova. Both crosses developed normally. By combining these data with field measurements of ambient flow velocities, potential dispersal distances were estimated to be on the orderof30 m (vertically) and 1.5km (horizontally). The implications ofthis result are discussed with regard to the population genetics and larval biology of the species.