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Fig. 4. Five different types of cellular mechanisms of symbiont loss from Cnidarian
host tissues. The different types are discussed in the text from top to
bottom. Symbionts lost by in situ degradation die or are killed in
the host cell and are either digested or expelled (not shown). Symbionts lost
via exocytosis are expelled free within the gastrovascular cavity. In
host cell detachment, symbionts are lost when whole host cells with their
symbionts still resident within, become detached from the mesoglea and
surrounding cells and are released into the gastrovascular cavity. Host cells
undergoing apoptosis, shrink and form multiple apoptotic bodies and in the
process release viable or degrading symbionts into the gastrovascular cavity.
Host cells dying by necrosis, swell and burst, releasing their contents,
including symbionts (either viable or degrading) into the gastrovascular
cavity. Normal host cells (H) are anchored to the acellular mesoglea (M).
Symbionts (S) are contained with host vacuoles or symbiosomes (Sy). Figure
adapted from Gates and colleagues (Gates
et al., 1992).
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