De Blainville, 1824
Ascidiacea are sessile tunicates, either living free in sand or mud, or attached to stones, rocks and other hard substrates. Ascidians can be solitary or colonial. The majority of species is found in the littoral waters. Most species are hermaphroditic with simple reproduction mechanisms: an ovary and a single testis. Generally, the external fertilisation of the egg is followed by the development of a free swimming "tadpole" larva (or appendicularia as it is less commonly called), which after a short planktonic life settles on a suitable substrate and metamorphoses to an oozooid. However, larval suppression occurs in various degrees, and some species display even a fully direct development.
Planktonic larva
The ascidian larva has a muscled tail with a notochord and a dorsal neural tube. The internal organs broadly display the same configuration as the adult Appendicularia, a class of pelagic tunicates, also known as the Larvacea. [Botrillus schlosseri-larva ].
Larvae of Ascidiacea are small, about 1 mm or less. In the anterior part of the larva is a pigmented cup: the statocyst. This is a distinctive structure of larvae when compared to Appendicularia, who lack a statocyst. Anteriorly is the mouth, which becomes later the buccal siphon in the adult stage. The larval mouth may be closed. The larvae seem to be non-feeding during this stage. The mouth gives entrance to the pharynx, which leads to a twisted digestive loop with a dorsally directed intestine. The pharynx contains a ventral endostyle, which secretes mucus. Anteriorly are three ectodermal protuberances, the fixation papillae. Again, this is a distinctive structure of the tadpole larva, as it is needed for attachment to the substratum before metamorphosis. These papillae are absent in the holoplanktonic Appendicularia.
Ascidian larvae of fully indirectly developing species are short lived, on average 12 hours to two days at the maximum.
Ascidian larvae are recognised in the key but not identified to species.