Thursday, April 26, 2012

Brooded embryos of bryozoan Dendrobaenia lichenoides

The first image depicts a fragment of a colony of the bryozoan Dendrobaenia lichenoides with brooded embryos (pink).  The D. lichenoides specimens were gathered by hand in South Cove at Cape Arago State Park south of Charleston, Oregon on April 24, 2012.  Each globular ovicell (a calcified brood chamber attached to the maternal zooid) houses a single large embryo.  Ovicells tend to be in the center of the colony where the older zooids are located.  The embryos undergo cleavage and gastrulation within the ovicell, developing into a lecithotrophic coronate larva that is eventually released into plankton and swims briefly before settling and undergoing metamorphosis.  The seasonal timing of larval release is largely dependent upon whether the colony is over-wintering or nascent (Strathmann 1987).  

The second picture shows three ovicells with embryos that I dissected from the colony using a pair of sharp forceps.  With care and practice one can gently crack the ovicell and extract the embryo. 

The bottom picture shows an embryo I was able to dissect out of the ovicell intact.  The embryo is enclosed by a transparent egg envelope, or chorion.  This particular embryo is at a relatively early stage of development, and one can clearly see the outlines of individual cells (blastomeres).  This picture illustrates the biradial cleavage pattern characteristic of bryozoans in general.

Strathmann, Megumi F.  Reproduction and Development of Marine Invertebrates of the Northern Pacific Coast.  United States: University of Washington Press (1987): pp. 505.  Print.

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