Thursday, April 26, 2012

Coronate larva of Crisia sp.

This picture shows a fragment of a bryozoan colony Crisia sp. I collected it at South Cove just south of Charleston, OR during low tide in April 2012. The colonies of Crisia sp. look like upright branching whitish tufts about 1.5 cm tall on the underside of rocks and under rock ledges. One can see openings of many individual feeding zooids (autozooids) and one zooid specialized for reproduction (gonozooid). Gonozooid is larger than the autozooids and looks like a yellow “pouch”. Crisia is interesting because it exhibits an unusual reproductive strategy called polyembryony. A single zygote is initially deposited within the gonozooid, it receives nutrients from the mother zooid via a kind of placenta, grows, and buds off secondary embryos, which, in turn, can bud off tertiary embryos. In this way each gonozooid ends up  filled with many genetically identical small embryos - a kind of embryonic cloning.  The calcareous wall,  which protects the gonozooid (much like the other zooids in the colony), is clearly visible.

The second picture shows a gonozooid in which I cracked and removed part of the wall with a pair of sharp forceps to expose the embryos. Each of the small yellow spheres is a brooded embryo. These embryos are tightly packed inside the gonozooid and readily spill out when it is opened. These brooded embryos emerge as ciliated coronate larvae.

The bottom image is a side view of a coronate larva I removed from a gonozooid of Crisia. Most of the larval surface is covered by a ciliated epithelium called corona ciliata. Corona ciliata is used for locomotion. Coronate larvae do not have a shell (unlike the other kinds of bryozoan larvae (cyphonautes and pseudocyphonautes), or a gut (so they do not feed). They spend only a few hours in the plankton before settling. Coronate larvae of Crisia are anatomically very simple. Aside from the corona ciliata they have an internal sac (not visible here) and an aboral non-ciliated region clearly visible here (top). A portion of this non-ciliated region is invaginated in a circular groove, which corresponds to pallial epithelium.   The coronate larva of Crisia sp. lacks the pyriform organ and vibratile plume found in coronate larvae of some other bryozoan species (e.g. Bugula and Schizoporella).

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