Background
One of the means by which a coral host may survive periodic high temperature extremes is, in the words of the authors, "by replacing its dominant symbionts with others that are more suitable for the new conditions," and they say, in this regard, that there are "two modes of acquisition of the symbiotic algae by the sexually-produced offspring of host species: either by direct transmission to the egg or the brooded larvae [as in a vertical or "closed" system], or from the ambient environment [a horizontal or "open" system] by the post-larval stages." However, there has been a long-standing question about the ability of cnidarian hosts and their symbionts that operate in a closed mode to vary from that mode and become an open system when certain conditions might warrant it.

What was done
The four Israeli researchers examined the possible entry of stained heterologous clade C zooxanthellae obtained from polyps of the soft coral Heteroxenia fuscescens into primary polyps of the soft coral Litophyton crosslandi, which is known to host clade A symbionts and is believed to be a vertically-transmitting-only cnidarian.

What was learned
Zurel et al. report that "primary polyps of L. crosslandi allowed the entry of DAPI-stained heterologous motile clade C zooxanthellae into their tissues, a finding that contradicts the previously held notion about the closed nature of this symbiotic system."

What it means
The Israeli biologists say their finding demonstrates that "cnidarian hosts previously considered as true closed system symbioses, may in fact possess the ability for entry and possible residence of heterologous symbionts also from the environment," and that it further supports "earlier findings by van Oppen (2004), suggesting a certain degree of flexibility in the closed system symbiosis," which flexibility, in their words, "may be beneficial to these hosts under stressful conditions, including bleaching events."

Reference
Van Oppen, M.J.H. 2004. Mode of zooxanthella transmission does not affect zooxanthella diversity in acroporid corals. Marine Biology 144: 1-7.