Coral Cohabiters: Time for a Status Update?
● Symbiosis derives from the terms sym for together, and biosis for life. The coral reef appears to be a poster child for a lifetime of togetherness. The soft tissues of coral polyps are embedded with hundreds of single-celled, free-swimming dinoflagellates, captured from nutrient poor, crystal clear tropical waters. Photosynthesis by dinoflagellates provides 95% of the organic food used by the polyps. In return, the dinoflagellates are housed in a safe environment where their hosts supply them with carbon dioxide and minerals needed for photosynthesis.
● Friends with Benefits: Like a Facebook status, the relationship of coral symbionts is complicated. Clearly, the coral benefits: oxygen and sugars produced by trapped dinoflagellates enable these corals to grow as much as three times faster as those without symbionts. But the converse is not true: in the symbiotic relationship, it takes ~70 days for the dinoflagellates to double, in contrast to a mere 3 days outside the coral. So symbiosis has a fitness cost for the algae. In reality, the coral host is more like an active farmer, who lures and engulfs the free-living dinoflagellates into captive domestication. When the coral is stressed, it loses control of the delicate energy balance in this relationship and expels its colorful guests en masse. Coral bleaching devastates the entire reef ecology and is a symptom of climate change which brings warmer, more acidic, nitrogen rich waters.
● All relationships lie along a continuum: from truly mutualistic, where both partners benefit and the success of one is tied to the success of the other, to commensalism, where one partner benefits but the other is neither harmed nor helped, and the extreme cases of parasitism, in which one organism exploits and harms the other. Isn’t there a parallel with human relationships as well?
● The more we learn about the diversity of life and the structure of genomes, the more it appears that much of the evolution of biodiversity is about the manipulation of other species—to gain resources and, in turn, to avoid being manipulated (John Thompson, 1999). True mutualism may be rare in nature. Evolutionary selection tends to maximize individual fitness and conflict of interests are inevitable!
REF: Is the coral-algae symbiosis really ‘mutually beneficial’ for the partners? S.A. Wooldridge (2010) Bioessays 32: 615-625
IMAGES: Check out more stunning coral photographs by Daniel Stoupin at http://www.microworldsphotography.com/