Paper: Hill, R., Fernance, C., Wilkinson, S., Davy, S., Scott A., (2014) Symbiont shuffling during thermal bleaching and recovery in the sea anemone Entacmaea quadricolor. Marine Biology. 161(12):2931–2937. DOI 10.1007/s00227-014-2557-9
Although they are animals, sea anemones are often hailed as flowers of the sea, which is quite fitting since they were named after a genus of flowering land plants. Some sea anemones, like Entacmaea quadricolor or the bubble-tip anemone, have reached the trifecta of symbiotic relationships (interaction between living organisms that live in close physical contact with each other). Not only are they sought out by bodyguards in the form of anemone fish or ‘Nemos’ as prize real estate–see image above, but just like corals, they play host to internal symbionts or endosymbionts. These endosymbionts are dinoflagellates (single-celled algae) of the Symbiodinium genus which crucially supplement the anenome diet by offering up fixed carbon via photosynthesis and are also the source of host anemone colour. Also like coral, sea anemones are not immune to thermal stress—even a small increase of 1°C above peak summer temperatures can cause bleaching. Bleaching occurs when hosts expel endosymbionts and can lead to mortality if prolonged. The bubble-tip anemone associates with six different types of endosymbionts and can harbour several types simultaneously. This raises the question: are there advantages to hosting different types of endosymbionts? Previous studies show that anemones rearrange or shuffle the composition of their endosymbionts during a short term bleaching event. Hill et al., followed up by investigating what happens to the cabinet during a prolonged bleaching event and post-bleaching event.
Four specimens of three distinct colour morphs of E. quadricolor were collected by North Solitary Island off the eastern coast of Australia and settled in aquariums. Bubble-tips were acclimated over 14 days, and everything from temperature (22.9°C—average summer temperature in their natural habitat), water flow, shade control and diet of prawn was strictly controlled. After acclimation, temperature of half the aquariums were maintained while the temperature of the other half was slowly increased over three days to 28.5°C. The first day of increased temperature was designated as Day 1. On Day 42, bleaching was apparent, and temperature was reduced back to 22.9°C and maintained for another 75 days. Experimental time totaled 117 days and on days 1, 47, 67, and 117, four tentacles were clipped from each bubble-tip for analysis. Two of the tentacles were analyzed for symbiont density, while the other two were submitted for symbiont genotyping. Fun fact, both types of analysis involved a lot of blending and centrifugation—kind of like the spin cycle of a washing machine—of tentacles. The result is a symbiont algal pellet and liquid of other ‘stuff’. Glass beads were also used to ‘mill’ Symbiodinium cells to access the DNA for genotyping. A slew of statistical analyses were then applied in order to determine changes in symbiont density and change in ratio of symbiont types.
Symbiont density in bubble-tips under the control treatment decreased 60% versus a 90% under the bleached conditions. The symbiont density decline under the control treatment was surprising, but may be explained by the higher exposure to light intensity during experimentation than the bubble-tips are used to in the field which has been known to elicit this type of response. There was no further significant change in symbiont density in both treatments for the remainder of the experiment suggesting that reducing the temperature of the bleached conditions, prevented additional symbiont loss–see figure to left.
All twelve of the anemone specimens had both Symbiodinium C25 and C3.25. The C25:C ratio was higher in the control specimens versus the bleached treatment on first day. At day 47, the ratio had increased 6.6% in the control and 13.2% in the bleached treatment and the ratio was equal in both treatments. At day 67, the ratios were significantly different, having decreased in the control and increased in the bleached treatment. However, by the end of the experiment, the ratio was similar indicating that the anemones had recovered (see figure on left).
The changes in the ratio of symbionts during the course of this experiment suggests that dynamic shuffling of symbionts occurs during periods of thermal stress. In this case, C25 may be more stress tolerant than other symbionts and be held preferentially by bubble-tips. During the recovery period, C3.25 started to make a comeback showing that it may be more beneficial in the absence of bleaching conditions. Perhaps it grows and divides faster than C25, making it more competitive. Ratios of symbionts are affected by multiple factors: temperature, light, period of thermal stress, and even host phenotype. Having an arsenal of different symbionts with different physiological performances may allow the bubble-tip anemone to internally strategize in the face changing environmental conditions.
Turns out, mom was right all along, it’s not about what’s on the outside, but what’s on the inside that counts.
I’m a past oceanbites writer, occasional editor and guest poster. I graduated with a Masters of Coastal & Marine Management from the University of Akureyri in Iceland, and am currently working in marine conservation. In particular, I’m supporting an Indigenous-led initiative to safeguard the largest inland sea in the world (Hudson Bay & James Bay, Canada). I love weird ocean critters and *sigh…I really do enjoy long walks on the beach.