Finn Hoebelheinrich
Senior Thesis | 2025
Relationships between biomass and symbiont density under different food and light treatments in temperate scleractinian
coral Astrangia poculata
Abstract
A fundamental question in coral biology is, “How much do endosymbiotic algae contribute to the energetics of the coral holobiont?” The relationship between biomass (a proxy for energy) and symbiont density was examined across different food and light levels in the temperate, facultatively symbiotic scleractinian coral Astrangia poculata. Little is known about this relationship, and this experiment sought to understand how the two factors interact with each other and how they are impacted by different ratios of autotrophy and heterotrophy. Using samples collected in Rhode Island, the biomass of each coral was calculated by correcting the ash-free dry weight (AFDW) to surface area. AFDW was measured by weighing the coral tissue before and after all of the organic matter was combusted, with the difference being the dry weight. The symbiotic state of each coral was assigned visually at the beginning of the experiment, and later the symbiont density was attained by analyzing an image of the sample in MATLAB, using the red channel value (RCV) of the image as a proxy for symbiont density. The corals were split into groups: fed and starved, and high, medium, low, and dark levels of light in experimental tanks. Measurements were taken after 90 days in these treatments. There was no significant relationship between biomass and symbiont density across the samples, but for aposymbiotic corals (corals without symbiotic zooxanthellae), a higher symbiont density was correlated with a lower biomass. There was little difference in biomass between treatments, possibly due to corals entering quiescence under low light and/or starvation, but symbiont density showed a greater influence from differing treatments.
Introduction
Most corals exist in a mutually beneficial symbiotic relationship with a group of single-celled dinoflagellate algae in the genus Symbiodinium, often called zooxanthellae, which grow on the coral’s polyps and supplement the coral with energy produced through photosynthesis. The relationship between coral and its symbiotic zooxanthellae is a matter of much interest. While most public knowledge of coral is limited to tropical, obligately symbiotic reef corals, the impact of symbiotic state and symbiont density on the health and physiology of the corals are challenging to study in these tropical corals since they are unable to survive without their zooxanthellae. Temperate, facultatively symbiotic scleractinian corals such as Astrangia poculata, commonly known as the northern star coral, can help to fill these gaps in research. A. poculata is an ideal organism for study because like most temperate corals, it is facultatively rather than obligately symbiotic (Dimond and Carrington 2007). Obligately symbiotic corals rely on the energy produced by their symbiotic zooxanthellae through photosynthesis for the bulk of their nutrition, and are more common in tropical, oligotrophic waters with few plankton. Facultatively symbiotic corals may also have this mutualistic relationship, but they can survive with or without a zooxanthellate symbiont. Facultatively symbiotic corals are more common at higher latitudes for various reasons, including decreased light availability for photosynthesis and greater macroalgae cover, which can block sun access (Dimond and Carrington 2007).
In the case of A. poculata, it exists naturally both in a mutualistic relationship with the algae Breviolum psygmophilum in a brown symbiotic state and without a symbiont in a white aposymbiotic state, as well as intermediary “mixed” states. Its range encompasses the area as far south as the Gulf of Mexico to as far north as Cape Cod, Massachusetts, and it is usually found in the shallow subtidal zone, where its main sources of competition
are other invertebrates and macroalgae (Aichelman et al. 2019, Grace 2017). It frequently survives entirely through heterotrophy in an aposymbiotic state without the added photosynthetic energy provided by the symbionts, but many A. poculata do exist in a symbiotic state and receive energy from photosynthesis in addition to predation (Trumbauer et al. 2021). A. poculata allows for the relationship between the amount of zooxanthellae present and other factors related to the coral’s survival to be examined, something that is significantly more challenging in the tropical, obligately symbiotic reef corals.
Symbiotic state varies across the population of A. poculata and fluctuates in individual corals based on various environmental factors like seasonal temperatures, light cycles, and changes in inorganic nutrient availability (Dimond and Carrington 2007, Brown et al. 1999, Fagoonee et al. 1999, Stimson 1997). These symbiotic cycles are also influenced by location, as A. poculata located in temperate northern regions experience shifts from varying ratios of autotrophy and heterotrophy throughout the year to predominantly or exclusively heterotrophy in the winter, although this shift may be less pronounced in the tropical portions of its range (Trumbauer et al. 2021). Individuals are naturally found in both aposymbiotic heterotrophic states and symbiotic mixed hetero- and autotrophic states. Both states are found coexisting in the same environments, but there is some variation due to depth, where shallower corals with more access to the sun are more commonly found in a symbiotic state and corals in deeper waters are more commonly aposymbiotic (Linday et al. 2025). Additionally, symbiotic zooxanthellae are obtained after the coral has settled and not passed on from preceding generations, which means all individuals have an equal opportunity to be symbiotic or aposymbiotic at the outset, excluding factors like depth or latitude (Szmant-Froelich et al. 1980).
Previous research indicates that heterotrophy is generally more significant than autotrophy for the survival of A. poculata, at least for corals in the more northern portions of the range, since individual corals may participate in varying amounts of autotrophy due to the dynamic state of symbiosis (Trumbauer et al. 2021). The added energy acquired from photosynthesis is still beneficial, though, and a higher symbiont density for A. poculata increases the rate of wound healing and provides a significant advantage over aposymbiotic specimens in this field (Burmester et al. 2017). Additionally, corals with access to heterotrophy tend to have a significantly higher symbiont density than corals that are starved (Szmant-Froelich and Pilson 1980). Little research has been done, however, examining the direct relationship between biomass and symbiont density.
This research attempts to better understand the relationship between the biomass of A. poculata and its symbiont density, and how each is affected by the relative contributions of autotrophy and heterotrophy. Since there is a significant relationship between red channel value (RCV) and chlorophyll density, with higher correlation in corals with increased pigmentation, RCV was used as a proxy for chlorophyll density (Dimond and Carrington 2007). Chlorophyll density in turn is generally representative of symbiont density. The red channel value indicates the average value of the intensity of red across pixels in an image in a range from 0 to 255, where higher channel values indicate higher brightness: a channel value of the lowest possible value (0) would appear as black, while a channel value of the highest possible value (255) would appear as white. In this case, only the polyps in each image were factored into the analysis. Therefore, a lower red channel value for a coral indicates a darker color, which in turn indicates a higher symbiont density. Analyzing the polyp color from an image of each coral to extract the RCV provides a non-destructive method of quantifying
symbiont density (Winters et al. 2009, Dimond and Carrington 2007).
The hypothesis for this experiment was that a lower RCV from the coral image, corresponding to a higher symbiont density, would correlate to a higher biomass due to the higher rates of photosynthesis and the resulting increase in energy. Additionally, corals with access to both heterotrophy and autotrophy (i.e. fed symbiotic corals) were hypothesized to have greater biomass as well as a lower RCV, because they would have more abundant access to energy.
Materials and Methods
Experimental design. Corals were collected in Newport, RI in January 2024 via SCUBA. They were kept at 22°C in recirculating 1.7L tanks for 90 days. Corals were divided into eight groups by treatment. Each group consisted of ten corals, five in an aposymbiotic state and five in a symbiotic state. The corals were categorized as aposymbiotic and symbiotic at the beginning of the experiment through a visual judgment against a color scale as in (Burmester et al. 2017, 2018), in which corals with greater than 25% coloration were labeled symbiotic and those with less than 25% coloration were labeled aposymbiotic. The treatments were starved in darkness (0% irradiance), starved in low light (33% irradiance), starved in medium light (66% irradiance), starved in high light (100% irradiance), fed in darkness, fed in low light, fed in medium light, and fed in high light. The fed corals were given brine shrimp at a density of 8 brine shrimp per polyp (genus Artemia) every 5 days. Each coral was assigned a 6-character tag for identification. After 90 days in these conditions, they were photographed and measured.
Red channel value collection. After 90 days, the corals were photographed. They were placed into bins full of water on top
of an RGB (red, green, blue) color standard, with each coral’s identification tag written in Sharpie next to it. Each coral was photographed through a light ring to ensure good lighting using an Olympus Tough F2.0 camera in underwater mode. Each photo included all 3 colors from the RGB standard, the entire coral, and the entire identification tag. The photos were then sorted into folders by tank and date of collection. The image files were renamed with the tag, date of collection, and day of collection. Using a program written by Dr. Jeff Chabot, each photo was analyzed with MATLAB to gather the color data from the image. Each polyp on each coral was manually selected using the software in order to ensure that color data was collected exclusively from living tissue and the rest of the image was ignored. The software gathered the color data from the image and the “red” value extracted from each one was used as the red channel value.
Biomass collection. To obtain the ash-free dry weight, aluminum tins were first burned in a muffle furnace at 500°C for 4 hours to remove any possible organic detritus. After photographing the corals to obtain the red channel value, the corals were lowered into liquid nitrogen for one minute each to kill them. Each tin was engraved with the beetag number for one of the corals, with two tins per coral, and then the weight of each tin was recorded. Each coral was airbrushed into a plastic bag in order to remove and collect all of the live tissue. The resulting tissue-free skeletons were placed into one of their corresponding tins and put into the oven at 50°C to dry. The live tissue slurry from each coral was put into the coral’s other tin and dried overnight at 50°C. After this, the dried tissue was returned to room temperature and weighed on the analytical balance to obtain the dry weight. Then, the dried tissue samples were placed in the muffle furnace to burn off all organic matter at 500°C for 5.5 hours and left to cool afterward. The samples, now only the ash remnants of any inorganic material in the coral slurry, were weighed and recorded. The weight of the
ash sample was subtracted from the original dry weight to obtain the ash-free dry weight for each sample, or the biomass (in grams) of only the organic tissue for each coral.
The surface area was collected using the method described in (Marsh 1970, Veal et al. 2010). Aluminum foil was first burned at 450°C for five hours to remove any dust or organic matter. The foil was then manually molded to the surface of each coral skeleton, ensuring that all crevices were filled and notching it where necessary in order to fit the surface adequately without overlap. The foil was trimmed to cover only the living layer of coral. Then, it was removed and weighed. This process was repeated three times for each coral piece, and the average weight of the tinfoil in grams for each coral was calculated. The factor for the weight-to-surface-area conversion was obtained by measuring the surface area of 15 flat pieces of tinfoil and weighing them, and then calculating the conversion factor between the weights and surface areas. Using this factor, the coral measurements were converted from the tinfoil’s weight in grams to centimeters squared. The biomass in grams per centimeter squared was then obtained for each coral sample by dividing the biomass in grams by the surface area in centimeters squared.
Statistical analysis. Analyses of variance (ANOVA) were performed using Microsoft Excel across the data set to calculate the variance between treatments for symbiotic corals’ biomass, aposymbiotic corals’ biomass, all corals’ biomass, symbiotic corals’ RCV, aposymbiotic corals’ RCV, and all corals’ RCV. Additionally, multiple T-tests were performed across these smaller data sets to calculate the variance between treatments that were specifically selected for relevance, such as comparing fed in high light to fed in darkness. For biomass, these tests were performed using the natural log of the actual value in order to ensure a normal distribution.
Results
We found no overall significant relationship between biomass and RCV (p > 0.05, df = 79). When we assessed this relationship separately for the specific data sets of aposymbiotic and corals only and symbiotic corals only, there was again no significant relationship between biomass and RCV for symbiotic corals (p > 0.05, df = 39), but the relationship was statistically significant for aposymbiotic corals (p < 0.001, df = 39). For the aposymbiotic corals, a higher biomass was correlated with a higher RCV, which means a less pigmented or “whiter” coral and a lower chlorophyll density. The trend lines indicate that the relationship for the symbiotic corals was closer to that of the entire dataset versus aposymbiotic corals (Fig. 1). Relatively, the symbiotic corals had slightly higher biomass and lower RCV, while the aposymbiotic corals had slightly lower biomass and higher RCV. Examining specifically the average RCV for each treatment, separated into aposymbiotic and symbiotic groups, the average aposymbiotic RCV for each treatment were universally higher than their symbiotic counterparts, with the exception of the starved dark groups, where the opposite was true.
For both fed and starved corals, the dark groups had a higher RCV, meaning they were whiter and had less chlorophyll density. Also for both fed and starved corals, the low light symbiotic groups had a lower RCV than any of the other groups, with fed low light symbiotic corals having the lowest RCV of all the groups on average, indicating that they were more pigmented than even the high light corals. In the medium and high light groups, there was little difference between the starved and fed averages in either the aposymbiotic or symbiotic groups. There was more variability in RCV for symbiotic corals than for aposymbiotic corals, which retained a more consistent average across treatments (Fig. 2).
Fig. 1. Relationship between biomass (g/cm2) and red channel value (RCV) after 90 days. The 3 trend lines display the relationship between biomass and RCV for aposymbiotic corals (black), symbiotic corals (orange), and all corals (purple). This relationship is only statistically significant for aposymbiotic corals (p < 0.002); p > 0.05 for symbiotic corals and for combined aposymbiotic and symbiotic corals. Higher RCV indicates a whiter coral and less chlorophyll density, while lower RCV indicates a more pigmented coral and higher chlorophyll density. Each white dot represents an aposymbiotic coral and each orange dot represents a symbiotic coral. For each dot, n = 1.
ANOVAs performed on the red channel values to determine the difference in means between treatments revealed statistical significance in RCV treatments for symbiotic corals (p < 2*10-5, df = 39) and for combined symbiotic and aposymbiotic corals (p < 0.02, df = 79) but not for aposymbiotic corals (p > 0.05, df = 39). T-tests performed on the red channel values to determine the variance between selected pairs of treatments revealed several significant relationships as well: symbiotic starved dark vs. starved high light (p < 0.004, df = 8) revealed that the starved dark corals had a higher RCV than the starved high light corals on average; symbiotic fed dark vs. starved dark (p < 0.05, df = 8) revealed that the starved dark corals had a higher RCV than the fed dark corals on average; and combined aposymbiotic and symbiotic fed dark vs.
fed high light (p < 0.05, df = 18) revealed that the fed dark corals had a higher RCV than the fed high light corals on average. No other pairs of treatments tested displayed compelling relationships.
Fig. 2. Average red channel value (RCV) for each treatment after 90 days, separated by symbiotic state. RCV was extracted from each coral image using MATLAB. White dots indicate a group of coral samples that was categorized as aposymbiotic at the beginning of the experiment and orange dots indicate a symbiotic group. n = 5 for each dot.
There was no compelling difference in biomass between fed and starved coral. However, there was a quasi-linear relationship for fed symbiotic corals, where biomass increased as light level did (Fig. 3). This relationship is not present in either aposymbiotic or starved corals: instead, for both starved and fed corals, aposymbiotic biomass generally remained low and decreased slightly as light level increased.
Fig. 3. Average biomass (g) for each treatment after 90 days, separated by symbiotic state. Biomass was obtained for each coral by dividing its ash-free dry weight (g) by its surface area (cm2). White dots indicate a group of coral samples that was categorized as aposymbiotic at the beginning of the experiment and orange dots indicate a symbiotic group. n = 5 for each dot.
The point with the highest average biomass was symbiotic, fed, and in high light, meaning it had access to both autotrophy and heterotrophy. In both starved and fed corals, aposymbiotic and symbiotic corals had similar biomasses in lower light levels, while the symbiotic corals had much higher biomasses than aposymbiotic corals in higher light levels.
Additionally, ANOVAs performed revealed no significant relationship between the treatments for biomass in symbiotic (p > 0.3, df = 39), aposymbiotic (p > 0.7, df = 39), or combined (p > 0.6, df = 79) corals. T-tests between the average biomass in selected pairs of treatments revealed no significant relationships, although the one performed between symbiotic fed dark corals and symbiotic fed high light corals indicated a marginally insignificant relationship (p < 0.065, df = 8), where fed high light corals had a higher biomass on average.
Discussion
The lack of a relationship between biomass and RCV across the corals is unexpected. Using red channel value as a proxy for chlorophyll and symbiont density, the assumption was that the additional symbiont density would lead to more energy intake from photosynthesis and thus more growth and more biomass for the coral. However, symbiont density seems to have no meaningful effect on biomass (Fig. 1). This is not wholly surprising, as symbiont density has a varying impact on other physiological aspects of A. poculata. For example, symbiotic state has a major impact on wound healing, where symbiotic corals experience faster and more complete recovery (Burmester et al. 2017, 2018), but it has little impact on a coral’s microbiome (Sharp et al. 2017). Additionally, there may be little difference in added energy, and therefore biomass, between a coral with high symbiont density and a coral with moderate or low symbiont density (but not aposymbiotic). If true, this would likely be because autotrophy is seemingly less significant for energy than heterotrophy and because the biomass of the zooxanthellae itself is very low and therefore does not meaningfully impact the overall biomass of the holobiont (Trumbauer et al. 2021).
Interestingly, for aposymbiotic corals only, the relationship between biomass and RCV is significant, but in the opposite way than expected. Instead of being inversely proportional, where higher biomass correlates with lower RCV, the higher-biomass corals tend to have higher red channel values (Fig. 1). This means that the higher the biomass of an aposymbiotic corals, the lower the symbiont density. Since this relationship is only present for the corals marked as aposymbiotic, meaning low or zero symbiont densities at the beginning of the experiment, it is possible that this phenomenon is because the acquisition of new zooxanthellae over the 90 days used up energy that otherwise would have contributed
to biomass growth. This would explain why aposymbiotic corals that retained a lower symbiont density tended to have a greater biomass, and why symbiotic corals, which had higher symbiont densities to start with, did not display this relationship. Without biomass data for the corals from day 0 of the experiment to track growth, though, this is difficult to prove or disprove. Alternatively, some species of Symbiodinium have been shown to shift from a mutualistic relationship with the coral to a parasitic one in stressful circumstances such as high temperatures and high light levels (Baker et al. 2018). This shift to a parasitic relationship may have occurred among the starved high light corals, of which both the aposymbiotic and symbiotic groups had a relatively lower biomass than the medium light starved groups (Fig. 3). It is possible the light level was too high and caused the zooxanthellae to begin sequestering nutrients for themselves instead of sharing it with the coral, and since the corals didn’t have access to heterotrophy like the fed corals, they experienced lower growth rates as a result.
The majority of aposymbiotic corals had a higher RCV than the median value of 170 and the majority of symbiotic corals had a lower RCV than the median, but there were still many from each group on the opposite extreme (Fig. 1.). This can be attributed to the fact that the corals were categorized into these groups at the very beginning of this experiment, 90 days before the final image collection; since symbiotic state in A. poculata is dynamic, with high levels of seasonal variation (Trumbauer et al. 2021), many samples vary from their original classification.
Many of the relationships between the various treatments and RCV were unexpected. For both fed and starved corals, the samples in the dark had the highest RCV and thus the lowest symbiont density; this is expected, since they had no access to photosynthesis (Fig. 2). However, the treatments with the lowest average RCV (and thus the highest symbiont density), were the
low light groups. These groups seemed to retain their symbionts despite having less light to photosynthesize than either the medium or high light groups. It is possible that the low light level caused the starved corals to devote more energy towards zooxanthellate growth in order to maximize the little light they were getting, since they were not receiving any energy through heterotrophy. If true, this does not explain why the fed low light corals showed such high levels of symbiont density. There is more variability in RCV for symbiotic corals than for aposymbiotic corals. It may be easier to lose zooxanthellae than it is to attain new ones, which would explain why aposymbiotic corals retained their low symbiont densities from the beginning of the experiment: corals are proven to be capable of expelling their zooxanthellae in order to regulate the symbiont density, but it remains unclear how difficult it is to increase symbiont density (Baghdasarian and Muscatine 2000). This phenomenon likely explains why the starved dark symbiotic group had such a high RCV on average, since it may have expelled most or all of its zooxanthellae to reduce its nutrition needs, in the absence of any energy intake from either photosynthesis or predation (Fig. 2). The lack of a significant visual difference in RCV between fed and starved corals for the medium and high light groups indicates that little of the additional energy and nutrients derived from heterotrophy is used to sustain the zooxanthellae (Fig. 2). Further examination of the interactions between A. poculata and its zooxanthellae would be illuminating, as it is currently unclear how symbiotic state influences many aspects of the coral’s biology.
The quasi-linear relationship between light and biomass for fed symbiotic corals, where samples who received higher levels of illumination had a higher biomass on average, indicates that the interaction between heterotrophy and autotrophy is more beneficial for coral growth than either energy source on its own (Fig. 3). However, the lack of a prominent difference between the biomass
of fed and starved corals seemingly indicates that light (autotrophy) is more significant for coral growth than food (heterotrophy), which contradicts previous research indicating that heterotrophy is the predominant source of nutrition for A. poculata in Rhode Island regardless of symbiotic state (Szmant-Froelich and Pilson 1980, Dimond and Carrington 2007). It is possible that the food provided to the samples, brine shrimp in the genus Artemia, was not A. poculata’s preferred food source, and as such they did not engage with it like they would have with their natural prey. Previous studies that determined the dominance of heterotrophy over autotrophy involved wild corals (Dimond and Carrington 2007), which have a much more varied diet, so it is likely that the decreased variety of prey items contributed to the decreased impact.
The lack of any statistically significant relationships between any of the treatments for biomass, with just one pair (symbiotic fed dark and symbiotic fed high light) being marginally significant, also points to the possibility that 90 days was not a wide enough time frame to observe the more long-term effects living in these treatments might cause for the samples. The samples were collected in January and the experiment ran from February to May. A. poculata experiences the least amount of growth of each year in these cold winter and early spring months when they enter quiescence in New England, the northern portion of its range, which is where this experiment took place and where these samples were collected from (Dimond and Carrington 2007, Trumbauer et al. 2021, Grace 2017). During quiescence, corals may cease active feeding and growth due to reduced nutrient availability and induced by lower temperatures, as New England waters become much colder and relatively oligotrophic in the winter months compared to the plankton blooms of the summer (Grace 2017, Burmester et al. 2018). While this phenomenon is largely regulated by temperature and nutrient availability, it is
highly likely that the corals experienced a decrease in growth even in the climate-controlled lab at least at the beginning of the experiment due to being collected during a period of quiescence, whether or not the decreased growth continued until May. This possibility is especially likely for corals in the dark and low light treatment groups, which experienced light conditions closer to the reduced sunlight of the winter and early spring months.
This experiment was important because there is little knowledge on the relationship between symbiont density and biomass for A. poculata, but further research is necessary to determine the interactions of symbiont density with its other biological factors. For example, similar experiments to this one over longer periods of time, measuring rates of heterotrophic feeding of in situ food sources in A. poculata samples of varying symbiotic states, and an examination of energy exchange between the coral and its symbionts would all help to shed more light on the interactions between the two organisms.
References
Aichelman, H. E., Zimmerman, R. C., & Barshis, D. J. (2019). Adaptive signatures in thermal performance of the temperate coral Astrangia poculata. Journal of Experimental Biology, 222(5), jeb189225.
https://doi.org/10.1242/jeb.189225
Baghdasarian, G., & Muscatine, L. (2000). Preferential expulsion of dividing algal cells as a mechanism for regulating algal-cnidarian symbiosis. The Biological Bulletin, 199(3), 278–286.
https://doi.org/10.2307/1543184
Baker, D. M., Freeman, C. J., Wong, J. C. Y., Fogel, M. L., & Knowlton, N. (2018). Climate change promotes parasitism in a coral symbiosis. The ISME Journal, 12(3), 921–930. https://doi.org/10.1038/s41396-018-0046-8
Brown, B., Dunne, R., Ambarsari, I., Tissier, M., & Satapoomin, U. (1999). Seasonal fluctuations in environmental factors and variations in symbiotic algae and chlorophyll pigments in four Indo-Pacific coral species. Marine Ecology-Progress Series - MAR ECOLPROGR SER, 191, 53–69.
https://doi.org/10.3354/meps191053
Burmester, E. M., Breef‐Pilz, A., Lawrence, N. F., Kaufman, L., Finnerty, J. R., & Rotjan, R. D. (2018). The impact of autotrophic versus heterotrophic nutritional pathways on colony health and wound recovery in corals. Ecology and Evolution, 8(22), 10805–10816.
https://doi.org/10.1002/ece3.4531
Burmester, E. M., Finnerty, J. R., Kaufman, L., & Rotjan, R. D. (2017). Temperature and symbiosis affect lesion recovery in experimentally wounded, facultatively symbiotic temperate corals. https://doi.org/10.3354/meps12114
Dimond, J., & Carrington, E. (2007). Temporal variation in the symbiosis and growth of the temperate scleractinian coral Astrangia poculata. Marine Ecology-Progress Series - MAR ECOL-PROGR SER, 348, 161–172. https://doi.org/10.3354/meps07050
Fagoonee, I., Wilson, H. B., Hassell, M. P., & Turner, J. R. (1999). The Dynamics of Zooxanthellae Populations: A Long-Term Study in the Field. Science, 283(5403), 843–845. https://doi.org/10.1126/science.283.5403.843
Grace, S. (2017). Winter Quiescence, Growth Rate, and the Release from Competition in the Temperate Scleractinian Coral Astrangia poculata (Ellis & Solander 1786). Northeastern Naturalist, 24, B119–B134. https://doi.org/10.1656/045.024.s715
Hoegh-Guldberg, O. (1999). Climate change, coral bleaching and the future of the world’s coral reefs. Marine and Freshwater Research. https://doi.org/10.1071/MF99078
Lindsay, T., Dunster, W., & Prada, C. (2025). Macroalgae and Light Availability Modulate the Distribution of the Temperate Coral Astrangia poculata. Marine Ecology, 46(1), e70001. https://doi.org/10.1111/maec.70001
Marsh, J. A. (1970). Primary Productivity of Reef-Building Calcareous Red Algae. Ecology, 51(2), 255–263. https://doi.org/10.2307/1933661
Sharp, K. H., Pratte, Z. A., Kerwin, A. H., Rotjan, R. D., & Stewart, F. J. (2017). Season, but not symbiont state, drives microbiome structure in the temperate coral Astrangia poculata. Microbiome, 5(1), 120. https://doi.org/10.1186/s40168-017-0329-8
Stimson, J. (1997). The annual cycle of density of zooxanthellae in the tissues of field and laboratory-held Pocillopora damicornis (Linnaeus). Journal of Experimental Marine Biology and Ecology, 214(1), 35–48. https://doi.org/10.1016/S0022-0981(96)02753-0
Szmant-Froelich, A., & Pilson, M. E. Q. (1980). The effects of feeding frequency and symbiosis with zooxanthellae on the biochemical composition of Astrangia Danae Milne Edwards & Haime 1849. Journal of Experimental Marine Biology and Ecology, 48(1), 85–97.
https://doi.org/10.1016/0022-0981(80)90009-X
Szmant-Froelich, A., Yevich, P., & Pilson, M. (1980).
Gametogenesis and Early Development of the Temperate Coral Astrangia danae (Anthozoa: Scleractinia).
Biological Bulletin, 158. https://doi.org/10.2307/1540935
Trumbauer, W., Grace, S. P., & Rodrigues, L. J. (2021).
Physiological seasonality in the symbiont and host of the northern star coral, Astrangia poculata. Coral Reefs, 40(4), 1155–1166. https://doi.org/10.1007/s00338-02102119-5
Veal, C. J., Holmes, G., Nunez, M., Hoegh-Guldberg, O., & Osborn, J. (2010). A comparative study of methods for surface area and three-dimensional shape measurement of coral skeletons. Limnology and Oceanography: Methods, 8(5), 241–253. https://doi.org/10.4319/lom.2010.8.241
Weil, J., Trudel, M., Tucker, S., Brodeur, R. D., & Juanes, F. (2019). Percent ash‐free dry weight as a robust method to estimate energy density across taxa. Ecology and Evolution, 9(23), 13244–13254. https://doi.org/10.1002/ece3.5775
Winters, G., Holzman, R., Blekhman, A., Beer, S., & Loya, Y. (2009). Photographic assessment of coral chlorophyll contents: Implications for ecophysiological studies and coral monitoring. Journal of Experimental Marine Biology and Ecology, 380(1–2), 25–35.
https://doi.org/10.1016/j.jembe.2009.09.004