Thermal stress, associated with tidal immersion and emersion, and its affects on the physiology of the anemone Anthopleura elegantissima
Introduction
The anemone Anthopleura elegantissima is a sessile, intertidal invertebrate and as such it is unable to move to avoid stressors, such as temperature changes, associated with immersion and emersion cycles of tides. There can be wide ranges in the extent of exposure time associated with these cycles from day to day or month to month, as well as extreme ranges in temperature associated with the cycles between seasons (ex. the range in temperatures experienced during a cycle is much greater in summer than in winter) (5). Therefore, these anemones' ability to survive in the intertidal environment, as well as their vertical placement within the intertidal environment, can depend upon their adaptive responses to such physical constraints (5). Since such regular, and often extreme, fluctuations in temperature can have harmful consequences for cellular structures and proteins (5), A. elegantissima needs to adapt at the cellular, molecular, and even the endocrine level in order to survive in the intertidal zone.Cellular Responses
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A. elegantissima hosts symbiotic zooxanthellae in vacuoles within its endoderm cells where they mediate the flux of carbon and nutrients between the anemone and the environment (1,3). It has been shown, though, that temperature stress will cause the detachment and release of intact anemone endoderm cells which contain zooxanthellae (3). Once the endoderm cells are released into the surrounding water they rapidly degrade leaving isolated algae in the water (3). It is speculated that the dissociation of anemone cells from the endoderm is caused by cell adhesion dysfunction resulting from disruption of cytoskeletal elements and denaturation of proteins involved in cell adhesion (3). Since repair of heat-damaged proteins is energetically costly (5) the anemones' response of releasing intact host cells, rather than repairing the damaged proteins involved in cell adhesion, is likely a mechanism to conserve energy for other necessary functions, such as the repair of other more important proteins which were damaged during thermal stress.
Molecular Responses
It has been shown that thermal stress causes increased heat shock protein (HSP) expression during tidal emersion in A. elegantissima which recover to lower levels within several hours of re-immersion (2,5). This increase in HSP synthesis is also accompanied by a decrease in the production of other non-HSP proteins (2), which is likely associated with the energetic costs needed to rapidly synthesize HSPs in order to repair heat-damaged proteins (5). The number and relative quantity of HSPs synthesized has been related to the severity of thermal stress (i.e. increase in number and synthesis is associated with increasing severity of thermal stress) (2). Snyder and Rossi (2004) found that HSP70 protein is especially expressed in tentacle tissue and less expressed in all other anemone tissues (oral disk, column, pedal disk). This may be associated with the necessity of the tentacles for feeding, and thus, the necessity to repair damaged proteins in the tentacles so that feeding can resume when re-immersed. There is also a seasonally-induced HSP expression with summer low tides provoking higher HSP expression than winter low tides (5). This is likely associated with the greater temperature range experienced between tides in summer than in winter. Other researchers have also shown that A. elegantissima decreases total adenylate levels (ATP, ADP, and AMP) following thermal stress (5).Endocrine Responses
It appears that relatively little research has been done on the endocrine responses of Cnidarians in relation to environmental stress. However, it has been shown that Cnidarians lack many of the nuclear hormone receptors traditionally studied in organismal stress (4) which may be the main reason why so little research has been done on them. Researchers have shown that A. elegantissima have the lowest rates of asexual reproduction in the higher intertidal zone, where organisms are most often emersed and exposed to thermal stress (5). It is possible that this could be related to hormonal changes associated with thermal stress, however, more investigation is needed to determine whether or not this is true.Long-Term Implications
As global temperatures rise the thermal stress on intertidal organisms associated with tidal immersion and emersion cycles will increase as well. This will have implications for A. elegantissima on both a day to day scale as well as a seasonal one. From day to day the anemones will be exposed to higher temperatures when emersed in both summer and winter. Seasonally, the anemones will be exposed to a greater range of temperature, particularly in summer, during each immersion and emersion. This likely means that they will release a higher proportion of their endoderm cells containing their zooxanthellae partners, meaning that they will lose more and more help in the regulation of carbon and nutrients as the temperatures rise. In addition, the anemones will probably express higher number and quantity of HSPs as temperatures rise which will likely cause a larger and larger decrease in non-HSP protein expression. This will likely also have higher and higher energy costs on the anemones meaning that they will have less energy available for other aspects of their life history, such as reproduction and growth. Finally, higher HSP expression associated with a higher temperature during emersion will likely cause the recovery time to lower levels of HSP expression after re-immersion to increase.
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References
1. Baldauf, B. and Muller-Parker, G. 2003. Does temperature restrict the latitudinal distribution of symbiotic zooxanthellae within the sea anemone Anthopleura elegantissima? Experiments with Symbiodinium californium in culture. Journal of Phycology, 39(S1): 2-3.
2. Black, N.A., Voellmy, R. and Szmant, A.M. 1995. Heat shock protein induction in Montastraea faveolata and Aiptasia pallida exposed to elevated temperatures. Biology Bulletin, 188: 234-240.
3. Gates, R.D., Baghdasarian, G. and Muscatine, L. 1992. Temperature stress causes host cell detachment in symbiotic cnidarians: implications for coral bleaching. Biology Bulletin, 182: 324-332.
4. Reitzel, A.M., Sullivan, J.C., Traylor-Knowles, N., and Finnerty, J.R. 2008. Genomic survey of candidate stress-response genes in the estuarine anemone Nematostella vectensis. Biology Bulletin, 214: 233-254.
5. Snyder, M.J. and Rossi, S. 2004. Stress protein (HSP70 family) expression in intertidal benthic organisms: the example of Anthopleura elegantissima (Cnidaria: Anthozoa). Scientia Marina, 68(suppl. 1): 155-162.
This page was developed as part of the course at the University of Washington: Integrative Environmental Physiology