Heterotrophy is so time-consuming: find prey, stalk prey, catch prey, consume prey… Preying all of the day and all of the night in some cases. How much more straightforward if you could just synthesise your own food and avoid all of that rushing about. Well, this particular calorific conundrum was solved by plants many hundreds of millions of years ago. But some animals have also cottoned on to this idea of a ‘free lunch’. Arguably, the most spectacular example of such an alliance is that between heterotrophic polyps and autotrophic zooxanthellae in coral reefs. Another – but entirely unsuspected – symbiosis has recently been discovered between the Spotted Salamander (Ambystoma maculatum) and a green alga (Oophila amblystomatis), within the adult reproductive tracts of the former, by Ryan Kerney and colleagues (PNAS 108: 6497–6502, 2011). An association between the alga and eggs of amphibians – and not just the spotted salamander – has been known for some time where it been suggested that the autotroph supplies oxygen to an otherwise hypoxic egg mass (e.g. Pinder and Friet, Journal of Experimental Biology 197: 17–30, 1994] and may in return benefit from amphibian nitrogenous waste. But identification of algal cells within the amphibian’s tissues was unexpected. This ‘association’ – it is too early to say what sort of symbiosis it might be or if the salamander obtains food from the alga, but which is viewed as a unique relationship between a vertebrate and a eukaryotic alga – poses many questions relating to cell–cell recognition and possible exchange of metabolites or DNA. And, like other symbioses, this new one raises further questions concerning the role(s) of ‘helping hands’ in the course of evolution.
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While I can understand the association with eggs, the association of algal cells with adult salamanders is extremely odd, especially for almost fossorial species such as A. maculatum which almost never expose themselves to light (outside of spawning time). This sort of possible symbiosis might make sense in a few amphibia such as bufonids which do occasionally sunbathe, but makes no sense to me in Ambystomids.