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Exaiptasia pallida (Anthozoa, Cnidaria) controls the proliferation of its symbionts, Symbiodinium spp.

Hisham Shaikh 2018


Cnidarians harbor photosynthetic dinoflagellates belonging to the genus Symbiodinium in its gastrodermis. Anemones can feed heterotrophically by actively catching its prey using the tentacular nematocysts or autotrophically through dinoflagellate photosynthesis. This study aims at examining the symbiont proliferation with varying nutrition sources. The anemones were subjected to different light (hour light/dark per day 24/0, 12/12, and 0/24) and nutrition (fed and starved) regimes for 7 days and chlorophyll-a concentrations were determined spectrophotometrically. Mortality rate in 24/0 was extremely high, possibly due to photodamage. No statistical differences in chlorophyll-a concentrations were observed between the treatments.


Osha Gray Davidson, in his book ‘The Enchanted Braid: Coming to Terms with Nature on the Coral Reef’,calls the rainforests as the coral reefs of the land (Davidson, 1998), going against the more acclaimed converse notion. One can easily draw resemblance between the both in terms of species richness, supporting complex habitats, and high levels of productivity.Covering only 1% of total land area, coral reefs support about 830,000 species of multicellular organisms worldwide, excluding fungi (Fisher et al., 2015).  However, the secret to the humongous success of coral reefs lie within something very small and relatively unknown – The phenomena of ‘Symbiosis’.

Symbiosis, the interaction between individuals, in an ecosystem occurs as a continuum between the extremes of mutualism, where both the partners are benefited, and parasitism, where one benefits over the expense of other. One will find numerous examples of symbiosis in a coral reef ecosystem. The anemone fish and the anemone; the Gnathiid isopods and elasmobranch fishes (Smit & Davies, 2004); the remoras  and the sharks (Sass, 2002);but the most renowned of it all has to be the one between the corals and the ‘zooxanthellae’.

Photosynthetic dinoflagellates from the genus Symbiodinium, or commonly known as ‘zooxanthellae’, are shown to form association with various marine invertebrate taxa, including Porifera (Lee et al., 2001), Cnidaria (Banaszak et al., 1993; Stat et al., 2006), Platyhelminthes (Douglas, 1987; Parke & Manton, 1967), Mollusca (Belda-Baillie et al., 2002), and Ascidia (Lewin, 2012). A cnidarian host shelters the dinoflagellates in its gastrodermis where it is bound to ‘symbiosomes’, membrane complexes having both algal and host origins (Kazandjian et al., 2008;Wakefield et al., 2000), where it has access to the nutrients from coral’s waste, sufficient light and protection from grazers. The photosynthetic products (‘photosynthate’) provided by the dinoflagellates are of utmost importance to the cnidarians, especially in the nutrient-poor tropical and subtropical waters, for utilisation in metabolism, growth, reproduction and overall survival (L Muscatine, 1990; L Muscatine etal., 1984).90% of the gross photosynthetic productivity is translocated to the host (Trench, 1993).Interestingly, the coral-dinoflagellate symbiosis has thought to be the direct driver of appearance of coral reefs in Triassic (Leonard Muscatine et al., 2005). The symbiosis, though being magnificent, is very sensitive to changes in its environmental conditions. A temperature change as little as 1-20C over the summer average can lead to a failure in symbiosis. This would lead in expulsion of the symbiontsand will cause a phenomenon referred to as coral ‘bleaching’ (B. E. Brown, 1997). If remain unestablished, it may lead to death of the host (Hoegh-Guldberg, 1999). Temperature-induced coral bleaching occurs due to damage in photosystem II functions (Takahashi et al., 2009; Warner et al., 1999). Low levels of antioxidants in hospite as compared to in isolation,suggested suppression of antioxidant production in presence of host mechanism (Richier et al., 2005; Yakovleva et al., 2004). Increase in the heat shock proteins released by the host tissue is a clear indication of stress as demonstrated in Goniastera aspera by Brown et al. (2002).

Sea anemones are predatory in nature. Tentacular nematocysts are used to penetrate the prey and inject cytolytic venom (Lotan et al., 1995). As no morphological features for mastication and as the gastrovascular cavity is filled with sea water,enzymatic digestion is only possible if the enzymes are directly injected within its prey. The presence of internal nematocyst was hypothesized by Fautin& Mariscal (1991)and later proven by Schlesinger et al.(2009).

Exaiptasia pallida,formerly known as Aiptasia pallida,belongs to family Aiptasiidae and was originally described by Agassiz in Verill, in 1864 (Grajales & Rodríguez, 2014).It is found in tropical and subtropical marine waters in association with Symbiodinium spp. belonging to ITS2 typeB1, ITS2 type A4 and rarely, mixed populations of Symbiodinium B1 and C1 (Grajales et al., 2016; J. et al., 2013).The symbionts are brown because of the pigment fucoxanthin which masks the green coloration of chlorophylls. Commonly known as the brown glass anemone, it is considered as a pest and can reach to plague proportions. In recent years,along with Aiptasia pulchella, E. pallida has been developed as a model to study the coral-dinoflagellate symbiosis in details (Voolstra, 2013).

An anemone uptakes nutrient both autotrophically, using the photosynthetic dinoflagellates, and heterotrophically by actively hunting. This study aims at understanding the host control on symbiont proliferation. E. pallida was studied for the effect onchlorophyll-a concentration with changes in the day/light cycle along with the nutrition. The null hypothesis was that there is no difference in the chlorophyll-a concentration with changes in the light cycles and nutritional treatment. The hypothesis was that the anemone provided with more access tolight should have more chlorophyll-a and vice versa. The anemones having more access to heterotrophic nutrients should have lower chlorophyll-a concentrations.

Materials and Methods

Collection of samples

A total of 72 E.pallida were collected around 1300 hours from the School of Biological Sciences Aquarium, The University of Queensland, Australia in May 2018. The anemones were in a 12-hour light/12-hour dark light cycle in filtered sea water(FSW) and were fed 1:1:1:1 mixture of Nanno 3600TM: Isochrysis 1800TM:TW 1200TM: Pavlova 1800TM by Instant Algae®. Ascalpel was used to detach them from the glass surface by scraping under thepedal disk (Lam et al., 2017). The sample population varied in size as the anemones were not cultured separately.


Transverse (n=10) and longitudinal (n=10) sections of E. pallida were obtained. Half of them were stained using H&E stain (Fischer et al., 2008) and the rest were observed under microscope without staining.


Anemones were randomly placed in the wells of 6-well culture plates filled with FSW. A total of 12 culture plates were obtained from the 72 anemones which were numbered and randomly assigned nutritional and light cycle treatments.‘Fed’ treatment included feeding of fish pellets until saturation and the ‘starved’anemone were not fed anything. The FSW still contained nutrients from the aquarium but the water wasn’t cycled continuously, therefore there were no fresh input of nutrients. Light cycle treatment included light/dark cycles of 12/12, 24/0 and 0/24 hours. 12/12 was performed in the aquarium. 0/24 and 24/0 were performed in dark chambers. A light source of Philips Halogen Classic 30 bulb (70 W, 240 V, Frosted) was used for the 24/0. All the treatments were submerged in FSW in a container of 50x20x10 cm for 7 days. 


Percentage mortality for each 6-well culture plates were measured by counting the number of ‘live’ anemones. Anemones with expanded tentacles were considered ‘live’. Shrunken anemones were considered as stressed but alive, therefore, were put under ‘live’. The wells without anemone and only black residual remains were considered as ‘dead’.


Four representative samples were chosen from every treatment at random for spectrophotometric estimation of chlorophyll-a content. The samples were introduced in 1% Trypsin to relax the anemones of their cnidocytes (Oppegard et al., 2009). The samples were then driedbetween six layers of tissue paper for 60 seconds before introducing in an Eppendorf microcentrifuge tube filled with FSW up to 1.0 mL mark. Micropipettes were used to withdraw water till the lower meniscus was back at the original level. The amount of liquid withdrawn was used as a proxy for size of the samples which was used to standardize the chlorophyll-a concentration readings.

Spectrophotometric Analysis

After measurement of size, the anemones were macerated using a mortar and pestle in 0.4 mL of 100% acetone. The mixture was left until dryness and was extracted using 1.0 mL of 100% acetone. 0.2 mL aliquot of the concentrate was centrifuged at 6,200 rpm for 60 seconds. 0.1 mL of the supernatant was diluted to 1.0 mL with 100% acetone and absorbance were measured at 630 and 663nm using a UV-VIS spectrophotometer. Chlorophyll-a was calculated suing the formula provided by Jeffrey and Humphrey (1975).

mg of chlorophyll-a per mL of tissue=[11.43*A663 - 0.64*A630]*V/1000*S


A663 and A630 -absorbance (Absorbance units) at 663 and 630 nm, respectively.

V - Final volume (mL)of the solution.

S – Size (mL) of thesamples.

Data Analyses

A Two-Way ANOVA test was performed to compare the effects of type of light cycle and nutritional treatment and the interaction effect between them on the concentration of chlorophyll-a in E. pallida using RStudio (Team, 2015).




Qualitative examination of tentatcular and gut section suggested a higher number of symbionts in the tentacles (Fig. 1& 2).


The initial water temperature was 240C measured using a Mercury-in-glass thermometer. Temperatures measured for 0/24, 12/12/and 24/0 were 200C, 240C, and 280C,respectively.


100% mortality was observed in the Fed-24/0 treatment whereas 50% of the anemones in Starved-24/0 treatment survived. Mortality rates of 33.33% and 8.33% were observed for Fed-0/24 and Starved-0/24 treatments.12/12 treatment showed 0% mortality (Fig. 3).


The size measured from the liquid displacement technique had a mean of 0.075 ± 0.082 mL (Fig. 4).

Spectrophotometric Analysis                                                                                                      

A Two-way ANOWA test was conducted on the effects of the two independent variables light cycle and nutrition) on the chlorophyll-a content of the samples. Nutritional treatments included two levels (Fed and Starved)and light cycle consisted of three levels (0/24, 12/12, and 24/0). There was no significant difference observed between the treatments and the chlorophyll content. Th effect of nutritional type yielded an F ratio of F(1,18) = 0.047,P>0.05, indicating no significant difference between the Fed (0.541 ± 0.680 mgChl-a/mL tissue) and Starved ( 0.590 ± 0.574 mg Chl-a/mL tissue). The effect for light cycle yielded an F ratio of F(2,18) =2.811, p>0.05, indicating no significant difference between the 0/24 (0.935 ± 0.630 mg Chl-a/mL tissue),12/12 (0458 ± 0.607 mg Chl-a/mL tissue), and 24/0 (0.303 ± 0.332 mg Chl-a/mLtissue) light cycles. The interaction effect was not significant, F(2,18)=2.310, p>0.05 (Fig. 5).

Figure 1
Figure 2
Figure 3
Figure 4
Figure 5


The light cycle and nutritional treatments had no significant effect on the chlorophyll-a content in the anemones after 7 days of treatment.The concentrations in the ‘fed’ and the ‘starved’ treatment were similar, and so was the case with the light treatments – 0/24, 12/12, and 24/0.

The chlorophyll-a concentrations were higher in the fed nubbins of Stylophora pistillata than the unfed ones at the same light levels (Hoogenboom et al., 2012). This trend was only observed in the 0/24 treatment for E. pallida but was statistically not significant. There was an increase in the chlorophyll per cell concentration with increase in the irradiance in both fed and starved cultures of A. pulchella when treated for 5 weeks in the study carried out by Muller-Parker (1985).

Besides driving photosynthesis, light has detrimental effects on the photosystem II and can cause photodamage (Takahashi et al., 2004;Takahashi et al., 2009; Warner et al., 1999),if not checked by the protection machinery, including heat shock proteins,antioxidants, fluorescent and non-fluorescent proteins (Baird et al., 2009; Salih et al., 1998; Salih et al., 2000). The samples exposed to high photoperiod had more mortality. The high exposure to light could have led to unrepairable photodamage to the photosystem and would have left the anemone with only heterotrophic mode of nutrition. Wijgerdeet al. (2014)showed the increase in the mortality of S.pistillata in the presence of red light. The pellets fed to the anemones in the fed treatment were red in colour and could have enhanced the detrimental effects of the light by mere reflection and scattering of the red light. The effect of the colour of the pellets should be further studied or at least controlled. Fransolet et al. (2014)showed increase in the epidermal growth with impairment in photosynthetic ability of the symbionts in E. pallida. This suggests a shift to heterotrophy with loss of autotrophy. Epidermal growth rate could have been considered as another variable for the study. Beside all, thepresence of endogenous circadian rhythm was studied by Oren et al. (2015)and Hendricks et al. (2012)in Nematostella vectensis. Low mortality in 12/12 light treatment and extremely high mortality of 24/0 may possibly be due to disruption of the circadian rhythm.

More symbionts were present in the tentacles as compared to the gut were observed on careful examination of the histology slides under a light microscope. This is to increase the exposure of the symbionts to the light. Anemones are also shown to retract their tentacles to prevent the symbionts from photodamage. A quantitative analysis is must to concluded any significant results (Bates et al., 2010).

Polystyrene cuvettes were used for this study and the interaction of 100% acetone with the cuvettes was not foreseen. Therefore, the absorbance readings were corrected by subtracting them by 1. This was on the observation that the samples which had a darker green hue took longer to fog. This correction helped in salvaging the trend if not the accurate measurements. The use of glass cuvettes is strongly recommended for this study. The aim of the study was to dissect the preference between autotrophic and heterotrophic type of nutrition, and the shift in preference. Changes in temperature plays interfering role and therefore must be maintained (E. et al., 1996; Glynn &D'croz, 1990; Hoogenboom et al.,2012).O2 and CO2 levels must also be maintained to remove stress from depletion of nutrients (Towanda & Thuesen, 2012). Culturing of the anemones before the experiment would give a homogenous size and reduce the error associated with size standardisation. Use of a sonicator over a mortar and pestle would decrease the error associated with physical exertion in manual maceration of the tissue. This study must be repeated with an increased sample size and the readings must be recorded till an extended period of 5 weeks with careful considerations mentioned above.

In conclusion, the effect of photodamage was clearly seen in the mortality rates between the different light cycles with the 24/0 showing the highest mortality and 12/12 showing the least but the preference of nutritional source could not be established in E.pallida.


This work was done as a part of Independent research project in BIOL3211. I would like to thank BIOL3211 instructors, Dr Sandie Degnan and Dr Bernard Degnan along with the tutor Davide Poli for their guidance in development of this study. Special thanks to Eunice Wong for her assistance during the whole course of the study. I would also like to thank the School of Biological Sciences, Faculty of Science for permission to utilize the labs, aquarium and for provision of samples; Mr Arnault Gautheir for the histology slides; and Ananya Agnihotri, Judith Evangeline, Olivia Hewitt, Rafaela Ribeiro and Tessa Broholm for their support.


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