Friday, May 14, 2010

Bryozoan coronate larvae

I study the cheilostomate bryozoan Schizoporella unicornis (Johnston), which encrusts hard substrates and looks like a bright orange patch, as large as a quarter coin or bigger. A recent paper by Tompsett et al. (2009) has suggested that S. unicornis on the west coast is in fact S. japonica. I have found colonies of this bryozoan on the California mussels (Mytilus californianus) growing on the floating docks of the inner boat basin in Charleston, OR. Bryozoans are colonial animals; each individual within a colony is called a zooid. S. unicornis broods its embryos in modified zooids called ovicells. When exposed to bright light for several hours, S. unicornis releases lecithotrophic (non-feeding) coronate larvae, which are approximately 300 μm in length.

The larvae swim towards the light, actively changing their shape with muscular contractions. The larva appears bright orange in reflected light (although it looks brownish in transmitted light, as you can see) with two dark reddish pigment spots one on each side of the apical organ. The entire body surface is covered with cilia (the outer ciliated epithelium of a coronate larva is called corona ciliata). The cilia of the corona ciliata beat in a clockwise direction when viewed from the apical pole. The internal sac is well defined and visible through the body wall at the broader posterior end of the larva. The thin tri-radial dark line (top picture) is the lumen of this thick-walled epidermal invagination. This invagination is everted during metamorphosis, helps the larva attach to the substratum, and makes up a significant portion of the epidermis of the founding zooid of the colony.

Between the two dark red pigment spots lies the sensory region called apical organ (it looks like a finely outlined oval). Ventral to the apical organ is a smaller lighter red pigment spot (middle and bottom pictures) marking the location of the ciliated cleft, which contains a bundle of longer stronger beating cilia, called the vibratile plume (see bottom picture, at about seven o-clock). The vibratile plume is a sensory structure which plays a role in selecting the appropriate substratum for larval settlement.

These larvae are fascinating, but ephemeral. If a suitable substrate is available, they will settle within hours of being released, and metamorphose (transform) into the founding zooid (ancestrula) of a new bryozoan colony!

Tompsett S, Porter JS, Taylor PD. 2009. Taxonomy of the fouling cheilostome bryozoans Schizoporella unicornis (Johnston) and Schizoporella errata (Waters). J Nat Hist. 43:2227-2243.

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