While solitary tunicates mostly spawn eggs that hatch into tadpole larvae in the plankton, colonial tunicates usually brood their eggs in the atrial region until they hatch to tadpole larvae.  Later, usually cued by light, the parent zooid releases the larvae. The tadpole larvae swim for a short time, sometimes for only a few moments, before settling, attaching, and metamorphosing. It is not clear how the sperm gain entrance to the atrium to fertilise the eggs. Perhaps they pass through the branchial basket via the branchial (intake) siphon without being eaten, or perhaps a zooid squirts to back-flush seawater into the atrium via the common atrial siphon.

Mass of tadpole larvae clustered in the atruim of
a zooid of a colonial tunicate. The common atrial
siphon for the zooid cluster is shown

As noted, solitary tunicates rarely brood their eggs and larvae.  However, two local species that do so are Corella inflata and C. willmeriana.  Their eggs have a unique collar of about 90 large flotation cells that contain ammonium ions, making the eggs buoyant.  After release from the female gonopore the eggs float upwards into a special chamber in the atrial region where they remain for about a day until they hatch into tadpole larvae.  Less than a day afterwards they swim out the atrial siphon and, within about 20 minutes, attach to the substratum and begin to metamorphose.  The authors remark that as seawater contains negligible amounts of ammonium ions, maintenance of an ammonium gradient across the cell membranes of the flotation cells must require much energy expenditure.  Lambert & Lambert 1978 Science 200: 64; Lambert et al. 1995 Can J Zool 73: 1666.

NOTE  the study is done at Friday Harbor Laboratories, Washington

NOTE  other invertebrates that utilise ammonium in this way are jellyfishes and certain pelagic cephalopods

Corella spp. often inhabit vertical surfaces with the atrial siphon positioned uppermost (see drawing near Right). When the larvae are mature they swim upwards out of the atrial brood chamber as a response to negative geotaxis (larvae are released at night, so light is not a factor). Why do they usually orientate with the brood chamber uppermost?  One idea is that if they adopted the opposite orientation, with the atrial siphon lowermost (see diagram far Right), they would increase the risk of their own larvae being sucked into their branchial siphon and possibly eaten.  Young 1988 J Exp Mar Biol Ecol 117: 9.

NOTE cannibalism would also be avoided in side-by-side orientation; the author's data, however, show that relatively few adults orientate in this way - by far the majority orientates with brood chamber uppermost (at least in laboratory settling experiments)