Reproduction
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  Brooding
  Topics relating to reproduction include brooding considered here, and SPAWNING, LARVAL LIFE, SETTLEMENT & METAMORPHOSIS, and EARLY JUVENILE considered in other sections.
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drawing of 2 zooids of a cluster in a colonial tunicate to show where the larvae aggregate before release

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

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Research study 1
 

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 20min, 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




photograph of tunicate Corella inflata
Egg of Corella inflata showing flotation cells. The egg is approximately 250 um in diameter Two individuals of C. inflata, the one on the Right shows a large brood chamber. The view on the Left is through the chamber to the branchial basket beyond 0.6X
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Research study 2
 

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 eatenYoung 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)

drawing of tunicate Corella inflata showing its normal orientation on a vertical surface drawing of tunicate Corella inflata showing potential risk to larvae if the tunicate were to be positioned atypically
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Research study 3
 
The colonial tunicate Botryllus schlosseri is hermaphroditic and protogynous, that is, eggs mature before the sperm. This helps to ensure that sperm from another individual will fertilise the eggs without the risk of sperm from the same individual perhaps doing the job. Boyd et al. 1990 Biol Bull 178: 239. The eggs are produced in ovaries and incubated in the atrial space between the pharynx and outer body wall. Note the presence of several eggs and a bud in this drawing of an individual zooid. Spawning occurs synchronously in all zooids in a colony. The larvae are yellow-while or pale orange in colour and are about 1.6mm long. Three adhesive papillae are positioned at the front of the larva and form an attachment device, while 8 ampullae are arrayed within the larva. On release from the parent, the larvae swim for 1-2h and then attach by the adhesive papillae.  The 8 ampullae are then extended to complete attachment and metamorphosis ensues.
drawing of zooid system in the colonial tunicate Botryllus schlosseri drawing of an individual zooid of the colonial tunicate Botryllus schlosseri showing dispositions of eggs and embryos
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