Coral-Zooxanthellae Symbiosis - Costs

Despite the clear advantages obtained by the symbiosis, there are several costs induced from it. The coral sustains costs due to investment of resources in a system to expel algae if numbers exceed the optimal level or if environmental conditions are not favourable. Also the excretion of perialgal vesicles containing nutrients destined for the host cells uses resources. For the zooxanthellae, regulation of growth rate occurs as a result of the symbiosis as growth rate is slower when residing within coral than when in the coccoid stage. Expulsion from the host is also costly, as well as the costs involved in transport of carbon to the host, limiting its own usage. In order for the symbiosis to be a mutually beneficial one, the benefits must outweigh the costs for both members. If this is not the case, the symbiosis is likely to collapse, with the zooxanthellae being expelled or leaving the coral. A host of environmental factors act on members of the symbiosis, with extreme conditions potentially leading to an unbeneficial symbiosis and loss of zooxanthellae from the corals.