International Journal of Aquaculture 2024, Vol.14, No.1 http://www.aquapublisher.com/index.php/ija © 2024 AquaPublisher, registered at the publishing platform that is operated by Sophia Publishing Group, founded in British Columbia of Canada. All Rights Reserved. Publisher Aqua Publisher
International Journal of Aquaculture 2024, Vol.14, No.1 http://www.aquapublisher.com/index.php/ija © 2024 AquaPublisher, registered at the publishing platform that is operated by Sophia Publishing Group, founded in British Columbia of Canada. All Rights Reserved. Publisher Aqua Publisher Editedby Editorial Team of International Journal of Aquaculture Email: edit@ija.aquapublisher.com Website: http://www.aquapublisher.com/index.php/ija Address: 11388 Stevenston Hwy, PO Box 96016, Richmond, V7A 5J5, British Columbia Canada International Journal of Aquaculture (ISSN 1927-5773) is an open access, peer reviewed journal published online by AquaPublisher. The journal publishes all the latest and outstanding research articles, letters and reviews in all working and studying within varied areas of aquaculture, containing the latest developments and techniques for practice in aquaculture; information about the entire area of applied aquaculture, including breeding and genetics, physiology, aquaculture-environment, hatchery design and management, utilization of primary and secondary resources in aquaculture, production and harvest, the biology and culture of aquaculturally important and emerging species, aquaculture product quality and traceability, as well as socio-economics of aquaculture and impacts. All the articles published in International Journal of Aquaculture are Open Access, and are distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. AquaPublisher uses CrossCheck service to identify academic plagiarism through the world’s leading plagiarism prevention tool, iParadigms, and to protect the original authors' copyrights. Aqua Publisher is an international Open Access publisher specializing in the field of marine science and aquaculture registered at the publishing platform that is operated by Sophia Publishing Group (SPG), founded in British Columbia of Canada.
International Journal of Aquaculture (online), 2024, Vol. 14 ISSN 1927-6648 http://aquapublisher.com/index.php/ija © 2024 AquaPublisher, registered at the publishing platform that is operated by Sophia Publishing Group, founded in British Columbia of Canada. All Rights Reserved. Latest Content 2024, Vol. 14, No.1 【Scientific Commentary】 Redefining Intercellular Signaling: Trafficking Mechanism of The Wnt5b-Ror2 Complex in Zebrafish 37-39 Sarah McGrew DOI: 10.5376/ija.2024.14.0005 【Research Article】 Effect of Storage Density and Temperature on the Survival and Fatty Acid Profiles of Cyclopoid Copepods (Thermocyclop sp.) 9-19 Ronald Semyalo DOI: 10.5376/ija.2024.14.0002 Sustainable Oceans: Experiences and Lessons Learned from Implementing Effective Fisheries Management Strategies 20-28 Min Xia, Rudi Mai DOI: 10.5376/ija.2024.14.0003 The Application of Algal Biomarkers in Water Quality Monitoring 29-36 Christina Jin, Yulu Pan DOI: 10.5376/ija.2024.14.0004 【Research Report】 Research on the Correlation Between Fatty Acid Composition of Aquaculture Fish and Human Health 40-50 Fan Wang, Peiming Xu DOI: 10.5376/ija.2024.14.0006 【Review and Progress】 Biochemical Adaptability of the Relationship between Tropical Hard Corals and Photosynthetic Symbiotic Algae under Climate Change 1-8 Xuesong Yang DOI: 10.5376/ija.2024.14.0001
International Journal of Aquaculture, 2024, Vol.14, No.1, 1-8 http://www.aquapublisher.com/index.php/ija 1 Review and Progress Open Access Biochemical Adaptability of the Relationship between Tropical Hard Corals and Photosynthetic Symbiotic Algae under Climate Change Xuesong Yang Zhuji Anhan Biotechnology Co., Ltd., Zhuji, 311800, Zhejiang, China Corresponding email: 2984078657@qq.com International Journal of Aquaculture, 2024, Vol.14, No.1 doi: 10.5376/ija.2024.14.0001 Received: 01 Nov., 2023 Accepted: 10 Dec., 2023 Published: 01 Jan., 2024 Copyright © 2024 Yang, This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Preferred citation for this article: Yang X.S., 2024, Biochemical adaptability of the relationship between tropical hard corals and photosynthetic symbiotic algae under climate change, International Journal of Aquaculture, 14(1): 1-8 (doi: 10.5376/ija.2024.14.0001) Abstract Tropical coral reefs, a vital component of the global marine ecosystem, are currently under threat from climate change factors such as rising temperatures, ocean acidification, and extreme weather events. High temperatures induce coral bleaching, resulting in the loss of their energy supply and an acceleration of metabolic rates, rendering them more vulnerable. Ocean acidification affects the formation of calcium carbonate skeletons in symbiotic algae and decreases photosynthetic efficiency, further exacerbating the risk of damage to the symbiotic algae in high-temperature conditions. Extreme weather events directly cause physical damage to corals and alter marine environments, reducing their chances of survival. This review focuses on the impact of climate change on the biochemical adaptability between tropical hard corals and photosynthetic symbiotic algae, exploring their ecological relationship, the influence of climate change on this relationship, and the adaptive mechanisms. Understanding the adaptive mechanisms between hard corals and symbiotic algae is crucial for developing conservation strategies and management plans to maintain the functionality and biodiversity of coral reef ecosystems. It also aids in ensuring the survival and prosperity of this delicate relationship under the challenges posed by climate change, allowing future generations to continue enjoying the magnificence of tropical coral reefs. Keywords Tropical hard corals; Photosynthetic symbiotic algae; Climate change; Biochemical adaptability; Marine conservation Tropical hard corals are the core components of coral reef ecosystems, providing shelter and ecosystem services to numerous organisms. They are also crucial factors for the biodiversity and productivity of global marine ecosystems. However, hard corals do not exist in isolation, their relationship with symbiotic algae is the ecological foundation of coral reefs. This symbiotic relationship is unique and delicate: corals offer habitat and protection to algae, while algae, through photosynthesis, provide energy and organic matter to corals (He et al., 2022). This mutualistic association supports the prosperity of hard coral reefs but also renders them highly sensitive to environmental changes. Before understanding the impacts of climate change on tropical hard corals and their symbiotic algae, it is essential to recognize the widespread threats posed by climate change to tropical marine ecosystems. Global temperature rise leads to elevated sea temperatures, acidified seawater conditions, and more frequent and intense storm events. This series of climate changes affects coral reef ecosystems, disrupting the stable relationship between hard corals and symbiotic algae, jeopardizing the survival of tropical hard coral reefs. Therefore, the biochemical adaptive mechanisms between hard corals and symbiotic algae are crucial for addressing the challenges of climate change (Petrou et al., 2021). This relationship is not only vital for the survival of hard coral reefs but also has profound effects on the biodiversity and productivity of global marine ecosystems. Current research progress indicates a severe threat to the relationship between hard corals and symbiotic algae due to climate change. High temperatures cause coral bleaching, this phenomenon directly harming their health and depriving them of necessary energy supplies, accelerating metabolism, and increasing vulnerability. Ocean acidification has a direct negative impact on the formation of the calcium carbonate skeleton of symbiotic algae and the efficiency of photosynthesis, further exacerbating the damaged risk of symbiotic algae under high-temperature conditions. Additionally, extreme weather events such as storms cause direct physical damage to hard corals, disrupting the marine environment and reducing their chances of survival.
International Journal of Aquaculture, 2024, Vol.14, No.1, 1-8 http://www.aquapublisher.com/index.php/ija 2 This study explores the biochemical adaptability between tropical hard corals and symbiotic algae, particularly under the ecological conditions of climate change pressure. The research investigates how hard corals and symbiotic algae adapt to the rising temperatures and acidification of seawater, as well as how they adjust photosynthesis and antioxidant defense mechanisms to cope with the constantly changing environment. Furthermore, the study discusses changes in the species of symbiotic algae and metabolic adaptability in hard corals, providing a scientific basis for the healthy development of marine ecological balance. Understanding the biochemical adaptive mechanisms between tropical hard corals and symbiotic algae helps them survive the challenges of climate change. This relationship is crucial for the stability of hard coral reefs and the health of global marine ecosystems. Through in-depth research and global cooperation, can assist this fragile relationship in surviving and thriving under the threats of climate change, allowing marine environments to become more diverse and multidimensional. 1 Basic Ecological Relationship between Hard Corals and Symbiotic Algae Tropical hard coral reefs are renowned as one of the most rich and diverse ecosystems on Earth. The foundation of this ecological marvel lies in the delicate ecological relationship between hard corals and symbiotic algae. These small organisms play a crucial role in maintaining the biodiversity and productivity of tropical marine ecosystems. 1.1 Classification of pearl morphology Hard corals (Scleractinia) are the builders of coral reefs, known for their hard calcareous bones, which are, composed of calcified collagen. This rigid external skeleton provides protection for hard corals, allowing them to survive in the turbulent waters and predation by predators. Despite appearing rigid and immobile, hard corals are vibrant organisms, with generally slow growth rates; most corals only grow a few millimeters to centimeters each year. Hard corals (Figure 1) are widely distributed in shallow marine environments in tropical and subtropical regions globally, particularly in the Pacific, Indian, and Atlantic Oceans. The warm waters of these regions provide favorable conditions for the survival and growth of hard corals. Hard corals usually live in shallow sea water, most species of hard corals live in areas not exceeding 50 meters in depth, although some specialized species can be found in deeper waters. Figure 1 Hard coral
International Journal of Aquaculture, 2024, Vol.14, No.1, 1-8 http://www.aquapublisher.com/index.php/ija 3 1.2 Role of symbiotic algae in hard coral tissues Symbiotic algae, typically belonging to the genus Zooxanthellae, are single-celled algae that closely associate with hard corals. These algae reside within the tissues of hard corals, imparting brown or golden pigments to the coral's transparent tissues, hence often referred to as "brown algae." Photosynthetic symbiotic algae convert sunlight into energy through photosynthesis, and then transport organic carbon and nutrients to the hard coral, providing the necessary resources for growth and survival (Hazraty-Kari et al., 2022). This symbiotic relationship plays a crucial role for hard corals. Symbiotic algae contribute approximately 90% of the energy through photosynthesis, enabling hard corals to grow rapidly and construct their robust skeletons. Additionally, the symbiotic algae provide the necessary organic substances for hard corals, including glucose, amino acids, and fatty acids. Hard corals also offer a suitable habitat for symbiotic algae in this relationship, as well as protection from suspended sediments and predators. 1.3 Mutual dependency between symbiotic algae and corals The relationship between hard corals and symbiotic algae is one of mutual dependency, where they cooperate to maintain the stability of the entire coral reef ecosystem. However, this relationship is delicate and highly sensitive to environmental changes. Hard corals obtain most of their energy from symbiotic algae, but adverse environmental conditions such as elevated sea temperatures, deteriorating water quality, or storms may harm the symbiotic algae. In such cases, hard corals may expel the symbiotic algae, leading to a phenomenon known as coral bleaching. Coral bleaching poses a severe threat to the hard coral ecosystem as it causes the loss of a substantial energy supply for hard corals (Wu et al., 2022). If this phenomenon persists for an extended period, hard corals may eventually die, negatively impacting the entire coral reef ecosystem. This makes the interdependence between hard corals and symbiotic algae more significant and underscores its significance as a core issue in the face of climate change threats to tropical hard coral reefs. The close mutual dependency between hard corals and symbiotic algae is a key factor in the success of tropical coral reef ecosystems. Symbiotic algae provide energy and organic matter to hard corals, enabling their growth and reproduction, while hard corals offer a suitable habitat and protection for symbiotic algae. However, this relationship faces numerous threats, particularly environmental changes resulting from climate change. 2 Impact of Climate Change on the Relationship between Tropical Hard Corals and Symbiotic Algae Tropical hard coral reefs are among the world's most vulnerable ecosystems, playing a crucial role in the Earth's ecosystems. However, the threats of climate change have had severe impacts on these valuable ecosystems. Among them, the relationship between hard corals and symbiotic algae is particularly fragile, drawing significant attention to the interaction between tropical hard corals and climate change. 2.1 Effects of elevated sea temperatures on corals One of the most significant impacts of climate change is the rise in sea temperatures. Tropical hard corals are highly sensitive to temperature changes and exist within a relatively narrow temperature range. When sea temperatures increase, hard corals face a series of challenges. Firstly, high temperatures can cause damage to the symbiotic algae within the coral, leading to coral bleaching (Pang et al., 2021). Coral bleaching (Figure 2) is a severe phenomenon as it disrupts the stable relationship between corals and symbiotic algae, causing hard corals to lose the majority of their energy supply. Furthermore, elevated temperatures accelerate the metabolic rate of hard corals, increasing their demand for more energy. If hard corals cannot acquire sufficient energy, they may cease growth or even start to dissolve, ultimately leading to the decline of coral reefs. Therefore, the rise in sea temperatures poses a severe threat, not only is it harmful to hard corals themselves, but also has adverse effects on the entire coral reef ecosystem.
International Journal of Aquaculture, 2024, Vol.14, No.1, 1-8 http://www.aquapublisher.com/index.php/ija 4 Figure 2 Bleached coral 2.2 Impact of ocean acidification on symbiotic algae Another issue caused by climate change is the acidification of seawater (Figure 3). The increase in atmospheric carbon dioxide (CO2) concentration leads to a decrease in the concentration of carbonate ions (CO3 2-) in seawater, affecting the ocean's acid-base balance. Ocean acidification has a direct impact on the symbiotic algae of hard corals. Figure 3 Acidified seawater Symbiotic algae rely on CO3 2- in seawater to form their calcium carbonate skeletons, but ocean acidification makes this process more challenging. Additionally, acidified seawater affects the efficiency of photosynthesis in symbiotic algae, reducing their energy supply. This makes symbiotic algae more vulnerable to the high-temperature stress caused by climate change (Zhang et al., 2012). When seawater becomes both warmer and more acidic simultaneously, symbiotic algae are more susceptible to damage, consequently affecting hard corals. 2.3 Impact of storms, sea level rise, and other climate change factors In addition to the rise in sea temperatures and ocean acidification, climate change brings other threats, such as storms, sea level rise, and more frequent extreme weather events caused by climate change. Storms can cause physical damage to hard corals, disrupting their robust external skeletons. Furthermore, sea level rise results in larger waves and impacts on coral reef ecosystems, exacerbating the vulnerability of hard corals. Climate change also triggers more frequent coral bleaching events, leading to the disruption of the relationship between hard corals and symbiotic algae. Additionally, climate change affects water quality, leading to excessive nutrient input and coral reef degradation. This series of climate change factors collectively threatens the ecological relationship between hard corals and symbiotic algae, intensifying the crisis faced by tropical hard coral reefs.
International Journal of Aquaculture, 2024, Vol.14, No.1, 1-8 http://www.aquapublisher.com/index.php/ija 5 Climate change has severe impacts on the ecological relationship between tropical hard corals and symbiotic algae. Factors such as elevated sea temperatures, ocean acidification, storms, and sea level rise pose threats to the survival and reproduction of hard corals, rendering these coral reef ecosystems more fragile. 3 Adaptive Biochemical Mechanisms to Climate Change In the face of environmental changes induced by climate change, the delicate relationship between tropical hard corals and their symbiotic algae becomes even more fragile. However, these organisms are not helpless; they exhibit impressive biochemical adaptive mechanisms to cope with new environmental challenges. 3.1 Photosynthetic adaptation Symbiotic algae are crucial for the survival of hard corals as they provide the necessary energy through photosynthesis. However, the increased temperature and light intensity resulting from climate change can stress symbiotic algae, leading to coral bleaching. To address this challenge, some species of hard corals have demonstrated photosynthetic adaptability. An important adaptive strategy involves certain hard coral species adjusting the type of symbiotic algae to cope with different environmental conditions. Under high-temperature conditions, some hard corals can establish symbiotic relationships with heat-tolerant algae varieties, thus alleviating the pressure on symbiotic algae caused by elevated temperatures (Kawamura et al., 2021). This phenomenon provides additional survival opportunities for hard corals, enabling them to thrive in a broader range of environmental conditions. Hard corals and symbiotic algae (Figure 4) can also regulate the rate of photosynthesis to minimize the damage of photosynthetic products to hard corals. Hard corals can adjust the density and pigment content of symbiotic algae to adapt to different light conditions (López-Londoño et al., 2022). This photosynthetic adaptation mechanism helps hard corals mitigate the risk of symbiotic algae being exposed to excessive sunlight while maintaining a sufficient energy supply. Figure 4 Coral and symbiotic algae 3.2 Changes in symbiotic algae species With the rise in sea temperatures, some hard corals have begun to establish new symbiotic relationships with different types of photosynthetic algae to adapt to higher temperatures. This change in symbiotic algae species is an adaptive mechanism known as "symbiotic algae switching." This process typically involves hard corals forming new symbiotic relationships with heat-tolerant varieties of symbiotic algae, thereby increasing the chances of survival in high-temperature conditions. A research team from California State University developed a global ecological and evolutionary model simulating the response of 1,925 coral reefs to warming and ocean acidification under four climate scenarios (Li
International Journal of Aquaculture, 2024, Vol.14, No.1, 1-8 http://www.aquapublisher.com/index.php/ija 6 and Yi, 2021). The model incorporates two competing coral species that respond to future ocean warming and acidification through either the symbiotic algae switching mechanism (replacing existing algae types with more stress-tolerant types) or the symbiotic algae evolution mechanism. The model indicates that the switching mechanism is more effective, and the greatest impact on coral reef degradation is ocean warming rather than acidification. However, the ultimate outcome on a global scale will depend on the impact of warming and specific adaptive mechanisms. While the model provides a simplified representation of coral ecology and evolution, the research results enhance our understanding of coral adaptability. This understanding can help guide coral reef conservation efforts and suggest future research directions. 3.3 Antioxidative defense mechanisms Under conditions of high temperature and high light intensity, photosynthetic algae produce excessive oxidative substances, such as oxygen free radicals. These oxidative substances can damage the cells and tissues of hard corals, leading to coral bleaching. To counteract this stress, hard corals have developed a series of antioxidative defense mechanisms. Oxygen molecules possess strong oxidizing properties and are the main source of free radicals generated within an organism. If free radicals cannot be effectively cleared, they can cause damage to cells and tissues, leading to aging and even death of the organism. Therefore, antioxidants are crucial for organisms, as they can eliminate free radicals within the body, thereby protecting the cells and tissues of corals. Corals can obtain antioxidants through various pathways. On one hand, corals can self-synthesize antioxidants such as vitamins C and E to enhance their own antioxidative capacity. Additionally, corals can utilize exogenous antioxidants from marine microorganisms and seaweed in the ocean to enhance their antioxidant capacity. These microorganisms and seaweed contain carotenoids, polyphenols, flavonoids, and other antioxidative components, effectively clear free radicals within corals, providing protective effects. Hard corals can produce antioxidative substances, such as superoxide dismutase and catalase, to neutralize oxygen free radicals (Kramer et al., 2022). Hard corals can also adjust the rate of photosynthesis in symbiotic algae to reduce the risk of excessive production of oxygen free radicals. These antioxidative defense mechanisms help hard corals alleviate oxidative stress caused by climate change, thereby increasing their chances of survival. 3.4 Metabolic adaptations of hard corals Hard corals also exhibit metabolic adaptations to cope with different environmental conditions. When water temperature rises, the metabolic rate of corals increases, leading to faster growth. However, prolonged exposure to high temperatures can exert significant stress on corals, potentially leading to their death. To adapt to high-temperature environments, corals produce heat shock proteins, which protect the cellular structure and functions of corals from damage caused by elevated temperatures. Under high-temperature conditions, some hard corals can adjust their metabolic rates to reduce oxygen and symbiotic algae's demands (Zhang et al., 2021). This helps hard corals survive in high-temperature and low-oxygen environments, enhancing their resilience to the impacts of climate change. The biochemical adaptability mechanisms between hard corals and symbiotic algae enable them to seek new strategies to maintain their survival and reproduction in the face of climate change challenges. However, although these adaptive mechanisms can help hard corals cope with challenges in the short term, in the long term, they still face significant threats. Therefore, conservation and management measures remain crucial to ensure the ongoing existence of the relationship between hard corals and symbiotic algae, maintaining the ecological balance of coral reef ecosystems. 4 Summary and Outlook In nature, tropical hard coral reefs play an indispensable role in sustaining biodiversity and productivity in the global marine ecosystem. However, environmental changes triggered by climate change, especially elevated
International Journal of Aquaculture, 2024, Vol.14, No.1, 1-8 http://www.aquapublisher.com/index.php/ija 7 temperatures, ocean acidification, and extreme weather events, pose a severe threat to the ecological relationship between tropical hard corals and symbiotic algae. High temperatures pose a serious threat to the relationship between hard corals and symbiotic algae. Elevated sea temperatures lead to coral bleaching, indicating the loss of the primary energy source provided by symbiotic algae. High temperatures also accelerate the metabolic rate of hard corals, increasing their energy demands and making them more vulnerable to the pressures of climate change. Additionally, ocean acidification directly impacts symbiotic algae, reducing their ability to form calcium carbonate skeletons and affecting the efficiency of photosynthesis (Scucchia et al., 2021). This exacerbates the risk of symbiotic algae being overexposed to high temperatures, further highlighting the mutual dependence between hard corals and symbiotic algae. Furthermore, extreme weather events such as storms and sea-level rise directly threaten hard corals, causing physical damage and reducing their chances of survival. This study, through exploring the biochemical adaptive mechanisms of this relationship, reveals the survival strategies of hard corals and symbiotic algae in responding to the challenges of climate change. This is not only the importance of understanding these adaptive mechanisms, but also the need to explore future research directions to maintain the stability of this critical ecosystem. However, despite these adaptive mechanisms helping hard corals and symbiotic algae cope with challenges in the short term, they still face significant threats. Therefore, conservation and management measures remain crucial. Reducing greenhouse gas emissions, improving marine quality, and establishing protected areas are essential measures to maintain the relationship between hard corals and symbiotic algae. This requires coordinated efforts from the international community to address the threats of climate change to tropical hard coral reefs. In-depth research on symbiotic algae transitions, understanding how different species of hard corals implement this strategy to adapt to climate change, and the impact of these new relationships on coral reef ecosystems will provide valuable insights for future conservation strategies. Additionally, further research into the antioxidant defense mechanisms of hard corals, understanding how to better mitigate oxidative stress caused by climate change, is a key direction for future studies. Genetic research can also provide more insights into the climate adaptability of hard corals and symbiotic algae, offering potential pathways for possible genetic engineering methods to enhance their chances of survival. Simultaneously, more effective establishment and management of protected areas to reduce anthropogenic pressures and provide secure habitats will be crucial for the future health of hard coral reefs, ensuring that future generations can continue to marvel at the magnificent beauty of hard coral reefs. Conflict of Interest Disclosure The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest. 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International Journal of Aquaculture, 2024, Vol.14, No.1, 1-8 http://www.aquapublisher.com/index.php/ija 8 Petrou K., Nunn B.L., Padula M.P., Miller D.J., and Nielsen D.A., 2021, Broad scale proteomic analysis of heat-destabilised symbiosis in the hard coral Acropora millepora, Scientific reports, 11(1): 19061. https://doi.org/10.1038/s41598-021-98548-x Scucchia F., Malik A., Zaslansky P., Putnam H.M., and Mass T., 2021, Combined responses of primary coral polyps and their algal endosymbionts to decreasing seawater pH, Proceedings of the Royal Society B, 288(1953): 20210328. https://doi.org/10.1098/rspb.2021.0328 Wu K., Yang F., and Xu Y., 2022, Coral reef bleaching monitoring based on multitime Landsat-8 remote sensing image series, Dizhi Keji Tongbao (Bulletin of Geological Science and Technology), 41(5): 181-189. https://doi.org/10.5846/stxb201011011560 Zhang C.L., Huang H., Huang L.M., and Liu S., 2012, Research progress on the effects of ocean acidification on coral reef ecosystems, Shengtai Xuebao (Acta Ecologica Sinica), 32(5): 1606-1615. Zhang H.Y., Zhao M.X., Zhong Y., Lu L., Liu G.H., Yang H.Q., and Yan H.Q., 2021, Seasonal monitoring of photosynthesis characteristics of scleractinian corals in the Northern South China Sea, Haiyang Dizhi Qianyan (Marine Geology Frontiers), 37(6): 84-91.
International Journal of Aquaculture, 2024, Vol.14, No.1, 9-19 http://www.aquapublisher.com/index.php/ija 9 Research Article Open Access The Effect of Density and Temperature on Survival and Fatty Acid Profiles of Copepods (Thermocyclopsp.) Gerald Kwikiriza 1,2,3, Ronald Semyalo 3 , Gladys Bwanika 3, Alex Barekye 2, Justus Kwetegyeka 4, Andrew Arinaitwe Izaara 5, IvanAbaho6 1 Institute of Integrative Nature Conservation Research, Department of Integrative Biology and Biodiversity Research, University of Natural Resources and Life Sciences, Gregor Mendel Str., 331080, Vienna, Austria 2 Kachwekano Zonal Agricultural Research and Development Institute, National Agricultural Research Organization (NARO), P. O. Box 421, Kabale, Uganda 3 Department of Zoology, Entomology and Fisheries Sciences, College of Natural Sciences, Makerere University, P. O Box 7062, Kampala, Uganda 4 Department of Chemistry, Faculty of Science, Kyambogo University, P.O. Box 1, Kyambogo, Uganda 5 Mukono Zonal Agricultural Research and Development Institute, National Agricultural Research Organization (NARO), P.O. Box 164, Mukono, Uganda 6 Bulindi Zonal Agricultural Research and Development Institute, National Agricultural Research Organization (NARO), P.O Box 101, Hoima, Uganda Corresponding author: ronald.semyalo@mak.ac.ug International Journal of Aquaculture, 2024, Vol.14, No.1 doi: 10.5376/ija.2024.14.0002 Received: 18 Sep., 2023 Accepted: 30 Oct., 2023 Published: 10 Jan., 2024 Copyright © 2024 Kwikiriza et al., This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Preferred citation for this article: Kwikiriza G., Semyalo R., Bwanika G., Barekye A., Kwetegyeka J., Izaara A.A., and Abaho I., 2024, The effect of density and temperature on survival and fatty acid profiles of copepods (Thermocyclop sp.), International Journal of Aquaculture, 14(1): 9-19 (doi: 10.5376/ija.2024.14.0002) Abstract Cyclopoid copepods (Thermocyclopsp.) have the nutritional attributes of an ideal diet for fish larvae. However, long-term production and availability of copepods for feeding larval fish in hatcheries remain a challenge. The present study investigated the effect of density and temperature on survival and fatty acid profiles of Thermocyclop sp. at densities: 1 000, 3 000, and 5 000 individuals/L and temperatures: 4 °C, 8 °C, and 12 °C. A log-rank test showed a significant difference between the percentage survival of Thermocyclop sp. at 12 °C and 4 °C (P<0.001) and a significantly higher survival at 1 000 than at 5 000 individuals/L (P<0.001). Generally, saturated fatty acids (SFAs) were dominant compared to monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs). For essential fatty acids, no significant differences were observed between storage temperatures on the 7th and 14th days of the experiment. The results demonstrated that at least a 50% survival rate is obtained when these organisms are stored at 12 °C with a density of 1 000 individuals/L for 14 days, with no significant changes in fatty acid profiles. Further studies are necessary to determine the effect of increased storage conditions, perhaps with aeration, on storage time. Keywords Thermocyclopsp.; Density; Temperature; Survival; Fatty acids Over the last decades, climate change, illegal, unregulated, and unreported (IUU) fishing, and pollution have led to the depletion of fisheries resources (Nakiyende et al., 2023). For example, overfishing results in the removal of a large number of fish from the population. This leads to a decline in the overall size and abundance of the targeted fish species, making it difficult for the population to sustain itself (Kwikiriza et al., 2023a). Presently, there is an increased global investment in aquaculture, seen as an alternative for enhancing fish production to meet the demand-supply gap arising from the continual decline in capture fisheries (Abaho et al., 2016; FAO, 2022). By 2021, the leading African countries contributing to aquaculture production were Egypt (72.20%), followed by Nigeria (12.60%), and Uganda (6.33%) (FAO, 2022). Even though global aquaculture continues to increase, the African contribution remains low, estimated at 2.7% (Adeleke et al., 2020; FAO, 2022), and this is attributed to many factors, majorly; lack of access to quality and affordable feeds (Izaara et al., 2020) and poor-quality seed among others (Kwikiriza et al., 2023b). Poor-quality fish seed results in lower survival rates in fish farms, thus increasing financial losses for farmers who invest in fingerling production. To boost aquaculture, there is a need to invest in raising quality fish seed, and one way to achieve this is by enhancing larval nutrition. Larval nutrition poses a major bottleneck in aquaculture hatcheries worldwide, impeding the full commercialization of most domesticated fish species (Abate et al., 2016). Developing larvae are very small, fragile, and have poorly developed physiological systems, which limits their ability to efficiently utilize formulated feeds, resulting in poor growth and survival rates (Kimmerling et al., 2018). Presently, aquaculture production relies on live food organisms like Brine shrimp (Artemia sp.), rotifers, and copepods to meet the nutritional requirements of these small larvae (Olivotto et al., 2010; Kimmerling et al., 2018). The availability of live starter organisms like Brine
International Journal of Aquaculture, 2024, Vol.14, No.1, 9-19 http://www.aquapublisher.com/index.php/ija 10 shrimp, copepods, and rotifers is thus vital for the successful fry production of different fish species in aquaculture (Abaho et al., 2016). Amongst the live starter feeds, copepods are known to be nutritionally superior as they contain higher levels of Docosahexaenoic acid (DHA) and a protein content of 44%~52% with a suitable amino acid profile (Radhakrishnan et al., 2020). Additionally, the copepodites and adults have digestive enzymes required in the early life stages of fish and crustacean larvae (Alejos et al., 2022). The size suitability of nauplii and early copepodite stages of copepods are easily utilized by small-sized larval fish compared to Brine shrimp and rotifers during larval fish nutrition (Chepkwemoi et al., 2013). Copepods classes include Calanoid, Harpacticoid, and Cyclopoid copepods. Among these classes, Cyclopoid copepods are advantageous in different ways; they are easy to culture and can be maintained in higher densities compared to Calanoids (Park et al., 2021). Also, the presence of the paired egg sacs attached to the female genital segment means that higher production of Cyclopoid copepods is achieved compared to Calanoids (Mironova and Pasternak, 2017). The short development times of 4-5 days to maturation for copepods make them ideal to use in the mass culture and subsequent feeding of the fish larvae (Chepkwemoi et al., 2013). Therefore, all these advantages make the cyclopoids better copepods for larval nutrition. In the family Cyclopidae, Thermocyclop sp. is dominant. The species is distinguishable from other copepods by the first antennae, which are shorter than the combined length of the head and thorax, along with the uniramous second antennae (Chepkwemoi et al., 2013). Thermocyclops sp has a wide distribution in freshwater systems including lakes, rivers, and marshes (Jaime et al., 2021). The species is predominantly pelagic thrivings in littoral zones characterized by dense stands of immersed macrophytes. The Thermocyclop sp. also tolerate salinities of up to 7.2% and pH, ranging from 5.9 to 8.4. Ecologically, the species plays a pivotal role as the primary link connecting phytoplankton's primary production to higher predators, including shrimps and juvenile fish (Abaho et al., 2016). Besides, these organisms can elongate essential fatty acids to produce polyunsaturated fatty acids (DHA and EPA), which are required for the physiological functioning of fish (Chepkwemoi et al., 2013). These unique qualities provide a competitive advantage for Thermocyclop sp. as an ideal food source in larval fish culture. Although copepods present a competitive advantage as crucial live starter feeds in aquaculture, their appropriate storage conditions have not been thoroughly explored (Chepkwemoi et al., 2013; Abaho et al., 2016; Beingana et al., 2016; Izaara et al.; 2020). For example, sustaining their availability for use in hatcheries is still challenged by inadequate information on ideal storage temperatures and densities in the Ugandan aquaculture industry. Therefore, the present study explored the effects of storage conditions (temperature and density) on the survival and fatty acid profiles of Thermocyclop sp. It was hypothesized that the manipulation of the storage density and temperature of Cyclopoid copepods (Thermocyclop sp.) results in variations in survival rates and fatty acid profiles of the copepods. The successful storage and packaging of live Thermocyclop sp. will enhance their accessibility and utilization in fish hatcheries as alternatives to the presently commonly used Artemia in Uganda. The results from this study will directly impact larval fish nutrition by providing insights into how storage conditions influence the fatty acid profiles of copepods. This information can be translated into practical recommendations for ensuring that larval fish receive the best possible nutrition during their critical early stages. Subsequently, interventions will bridge the supply gap to live starter feeds thus contributing to the growth of aquaculture in Uganda. 1Results 1.1 Survival rates Generally, there was a gradual decrease in the survival rate of Thermocyclop sp. at different temperatures with time. The percentage survival rates of Thermocyclop sp. were significantly higher at 12 °C than at 4 °C (Figure 1) (2 = 9.9, df = 2, P= 0.007). Percentage survival after 14 days was 55.7 ± 0.9% (12 °C), 44.0 ± 1.5% (8 °C), and 30.7±1.2% (4 °C).
International Journal of Aquaculture, 2024, Vol.14, No.1, 9-19 http://www.aquapublisher.com/index.php/ija 11 Percentage survival rates were also significantly higher at the lower packaging densities (1 000/L) than at higher packaging densities (5 000/L) (Figure 2) (2 = 8, df = 2, P = 0.02). Percentage survival after 14 days was 67.9 ± 1.0% (1 000/L), 50.2±0.6% (3 000/L) and 29.3 % (5 000/L). Figure 1 Percentage survival of Thermocyclopsp. with time at different temperatures Figure 2 Percentage survival of Thermocyclopsp. with time at different storage densities 1.2 Fatty acid composition Generally, the fatty acid (FA) composition was dominated by saturated fatty acids (SFAs) followed by monounsaturated fatty acids (MUFAs) while Polyunsaturated fatty acids (PUFAs) were the least dominant (Table 1). At 4 °C, SFAs increased from the 7th day (59.00%) to the 14th day (65.66%), while a decrease in MUFAs was observed from the 7th (32.99%) to the 14th day (25.8%) of the experiment. At 12 °C, total SFAs were highest on the 14th day (61.08%) compared to MUFAs and PUFAs at 33.31 ± 6.71% and 5.61 ± 1.62% respectively (Table 1). Amongst the SFAs at 4 °C, Palmitic acid (C16:0) and Stearic acid (18:0) were dominating. The highest
International Journal of Aquaculture, 2024, Vol.14, No.1, 9-19 http://www.aquapublisher.com/index.php/ija 12 composition of MUFAs was observed at 8 °C on the 14th day (34.71 ± 8.96) with Palmitoleic acid (16:1n7), Oleic acid (18:1n9), and vaccenic acid (18:1n7) dominating. Table 1 Relative proportions of fatty acids (% of total FAs; mean ± SE, n=3) of the Thermocyclop sp. were stored at varying temperatures: 4, 8, and 12 °C after the 7th and 14th day of the experiment Fatty Acids 4 °C 8oC 12oC Day7 Day14 Day7 Day 14 Day7 Day14 14:0 2.43±0.32 2.07±0.69 1.84±0.40 1.22±0.52 3.80±1.32 2.39±1.15 16:0 38.48±0.89 41.01±4.18 26.39±0.56 35.26±3.54 37.45±3.75 39.16±2.75 18:0 15.49±0.57 20.31±2.89 9.31±0.15 14.72±1.18 12.23±0.43 18.49±2.78 20:0 1.41±0.11 nd 0.61±0.14 nd 0.28±0.05 nd 22:00 0.47±0.13 0.95±0.18 nd 0.71±0.13 0.29±0.04 1.04±0.07 24:0 0.73±0.14 1.33±0.23 nd nd nd nd Total SFAs 59.00±2.16 65.66±8.18 38.15±1.25 51.92±5.37 54.04±5.58 61.08±6.76 14:1n5 2.11±0.75 2.98±1.91 0.74±0.24 0.91±0.27 0.88±0.17 1.81±1.27 16:1n7 8.20±0.09 5.61±1.20 9.72±0.24 7.18±1.75 9.40±1.47 6.79±1.67 18:1n9 5.07±0.56 5.35±0.28 11.45±0.71 7.44±2.22 5.65±0.58 5.18±0.81 18:1n7 4.94±0.24 6.48±0.93 7.63±0.00 6.84±2.15 5.35±0.24 6.12±1.11 20:1n9 0.67±0.07 nd 0.13±0.03 2.13±0.29 1.71±0.10 nd 22:1n9 0.41±0.14 0.73±0.06 nd 0.82±0.10 nd 1.39±0.12 24:1n9 1.60±0.21 2.92±0.72 nd 1.04±0.24 0.83±0.12 1.90±0.54 Total MUFAs 32.99±3.13 25.89±5.43 33.54±2.02 34.71±8.96 32.99±4.02 33.31±6.71 18:2n6 1.73±0.22 1.66±0.10 11.19±0.71 3.36±0.63 2.40±0.65 1.53±0.45 18:3n3 1.28±0.23 1.59±0.11 6.58±0.42 2.96±1.12 2.32±0.17 0.52±0.09 20:2n6 0.75±0.42 0.71±0.05 0.62±0.07 0.81±0.25 2.49±0.36 0.97±0.31 20:3n3 1.60±0.15 nd 2.53±0.28 0.76±0.07 nd 1.76±0.15 20:4n6 1.32±0.28 2.60±0.33 nd 2.40±0.15 3.43±0.61 nd 20:5n3 0.61±0.07 1.01±0.01 4.54±0.53 1.35±0.29 1.25±0.37 0.83±0.61 22:6n3 0.71±0.12 0.88±0.16 3.48±0.21 1.73±0.71 1.08±0.15 nd Total PUFAs 8.01±1.48 8.45±0.76 28.95±2.21 13.38±3.21 12.97±2.31 5.61±1.62 Notes: Not detected (nd), Saturated fatty acids (SFAs), Monounsaturated fatty acids (MUFAs), and Polyunsaturated fatty acids (PUFAs) The total PUFAs were highest on the 7th day at 8 °C (28.95 ± 2.21%). Among the total FAs, Linoleic acid (18:2n6) and Linolenic acid (18:3n3) were dominating. Notably, Arachidonic acid (20:4n6) and Docosahexaenoic acid (22:6n3) were not detected at 12 °C on the 14th day. Amongst the essential fatty acids: Arachidonic acid (AA; 20:4n6), Linoleic acid (LA;18:2n6), Linolenic acid (LNA;18:3n3), Eicosapentaenoic acid (EPA; 20:5n3) and Docosahexaenoic acid (DHA; 22:6n3), no significant difference was observed across the storage temperatures (P = 0.81) on the 7th and 14th day (P= 0.87). Additionally, there was no significant interaction effect of temperature and day on fatty acid composition (P=0.88). 2 Discussion Successful viable storage and packaging of live Thermocyclop sp. is an essential requirement for their utilization as alternatives to the presently used Artemia in fish hatcheries. Live feeds provide a more nutritionally rich food and one that can be utilized by small-mouthed fishes/ larvae. However, use of the live feeds is limited by many challenges in defining appropriate cultural and storage conditions. Therefore, the present study investigated the effect of storage conditions on the survival and fatty acids of Cyclopoid-copepods as a starter diet for the African catfish larvae. 2.1 Effects of storage temperatures onThermocyclop sp. Generally, there was a gradual decrease in the survival of Thermocyclop sp. with time at different temperatures. This decrease could be due to starvation since these organisms were not fed throughout the experimental period (Koussoroplis et al., 2014; Hansen et al., 2020). Starvation of organisms lowers the amount of energy required to
International Journal of Aquaculture, 2024, Vol.14, No.1, 9-19 http://www.aquapublisher.com/index.php/ija 13 keep them active, and this results in death (Hansen et al., 2020). Starvation in copepods results in more than 50% loss in body weight and a decrease in Polyunsaturated Fatty Acids, leading to death (Koussoroplis et al., 2014). However, Eudiaptomus gracilis and Calanus sp. are able to regulate the fatty composition of their cell membranes to slow down metabolism and prolong their survival with time (Titocci and Fink, 2022) and this aligns with the current findings where more than 50% survival was observed after the 10th day of the experiment for all temperatures. The significantly higher survival rate of the copepods at 12 °C and 8 °C could be attributed to the relatively higher temperatures that increase the enzymatic activity of the organisms (Koussoroplis et al., 2014; Bai and Wang, 2020). Additionally, moderate temperatures (6 °C~15 °C) increase the survival of Thermocyclop sp. since the energy reserve depletion related to oxygen consumption is decreased (Werbrouck et al., 2017). This survival pattern has also been seen in adult Calanoid sp. (Devreker et al., 2009) with higher survival at 10 °C compared to lower temperatures of 3 °C. This suggests that the survival of copepods rises with temperature until reaching a maximum threshold, beyond which survival decreases with further temperature increases (Van Dinh et al., 2019). Thermocyclop sp. stored at 4 °C in the current study experienced higher mortalities than other temperatures. The decreased survival rate of Thermocyclop sp. at 4 °C may be associated with the impairment of their enzymatic activity, leading to mortality (Payne and Rippingale, 2001; Werbrouck et al., 2017). Temperature can directly affect the activity of enzymes by changing their physical structure and thereby changing catalytic efficiency (Cailleaud et al., 2007; Svetlichny et al., 2022). Moreover, various studies have found that different species of estuarine copepods reduce their metabolism to a minimum to sustain physiological activity in low temperatures and conserve energy in high temperatures (Koussoroplis et al., 2014; Werbrouck et al., 2017). These observations are consistent with the results obtained in the current study. Studies by Frisch and Santer (2004) observed higher mortality of Cyclops strenuus at 5 °C, and this is in line with the current findings where higher mortality was recorded at 4 °C. Therefore, the results of the present study demonstrate that the Thermocyclop sp. can be collected daily from the tank culture units. Copepods can be stored at 8 °C or 12 °C, and more than 50% of the copepods would be available after 10 days post-harvesting for use as a live starter feed for fish larvae. 2.2 Effect of different densities on survival of Thermocyclop sp. The density of the copepod population can significantly impact their growth, survival, development, and fecundity (Frisch and Santer, 2004). The percentage survival of Thermocyclop sp. increased with a decrease in packaging densities. The lowest survival of the Thermocyclop sp. at 5000 individuals/L could be attributed to density-dependent mortality. High copepod packaging densities result in many types of stressors including limited food resources, oxygen depletion, accumulation of metabolic products, and physical interaction with other individuals (Jepsen et al., 2015; Rajkumar and Rahman, 2016; Punnarak et al., 2017). Therefore, the accumulation of such higher metabolic wastes and low oxygen levels could have resulted in higher mortalities at 5000 individuals/L. Densities ranging from 50 to 1000 mature Calanoid Acartia tonsa /L have shown little or no negative effects on the Cyclopoid copepods (Jepsen et al., 2015). Higher density of adult copepods up to 6000 individuals/L has shown negative effects like higher mortalities as well as cannibalism for each other (Drillet et al., 2015; Franco et al., 2017). Different studies show that several omnivorous copepods become carnivores when phytoplankton concentration decreases in the culture medium (Drillet et al., 2015; Punnarak et al., 2017; Franco et al., 2017; Heneghan et al., 2023). In the current study, the copepods were starved throughout the experiment, which may have elicited cannibalism thus reducing the number of Thermocyclop sp. at higher densities. Similar rates of survival (67%) at 1000 individuals/L were reported for Acartia tonsa (Drillet et al., 2015). This confirms that it is possible to store copepods at 1,000 Individuals/L while keeping low mortality for 14 days without feeding and achieving a more than 50% survival rate. 2.3 Impacts of temperature and density on fatty acid profiles of Thermocyclopsp. The general decrease of FA content with time in the present study could be due to the degradation of the cell organelles of the Thermocyclop sp. (Mäkinen et al., 2017; Werbrouck et al., 2017). In degradation processes, these
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