Reviews in Aquaculture (2014) 6, 1–18
doi: 10.1111/raq.12087
Coral aquaculture: applying scientific knowledge to ex situ production s3, Dirk Petersen4 and Ronald Osinga5 Miguel C. Leal1,2, Christine Ferrier-Page 1 2 3 4 5
Department of Biologia & CESAM, University of Aveiro, Aveiro, Portugal Skidaway Institute of Oceanography, University of Georgia, Savannah, GA, USA Scientific Centre of Monaco, Monaco, Monaco SECORE Foundation, Bremen, Germany Aquaculture and Fisheries, Wageningen University, Wageningen, The Netherlands
Correspondence Miguel C. Leal, Departamento de Biologia & CESAM, Universidade de Aveiro, Campus Universit ario de Santiago, 3810-193, Aveiro, Portugal. Email:
[email protected] Received 1 April 2014; accepted 2 October 2014.
Abstract Coral aquaculture is an activity of growing interest due to the degradation of coral reefs worldwide and concomitant growing demand for corals by three industries: marine ornamental trade, pharmaceutical industry and reef restoration. Although captive breeding and propagation of corals is a well-known activity among aquarium hobbyists and public aquariums, the link between coral science and aquaculture is still poorly developed. Research on coral biology has increased in the past decades and resulted in abundant scientific information that is pivotal to further advance coral aquaculture. This review presents a holistic overview of coral aquaculture in relation to coral biology, with particular focus on ex situ aquaculture. Success factors for commercial coral aquaculture are outlined, which include qualitative aspects, such as shape, coloration and natural product content, and quantitative parameters such as growth and volumetric productivity. Manipulation of environmental factors to maximize coral quality and volumetric productivity is thoroughly discussed, and a comprehensive overview of current propagation techniques is provided. Knowledge gaps are pinpointed to indicate directions for future research. Key words: ex situ, growth, reproduction, symbiosis.
Introduction Coral reefs have high economic interest, primarily as a provider of food, natural products and coastal protection, and secondarily as an attraction for tourists from all over the world (Halpern et al. 2012). The highly productive and biodiverse coral reef ecosystems also provide hundreds of target species for the marine ornamental trade (MOT), which has increased over the last decades to a point that it is becoming a threat for reef organisms (Wabnitz et al. 2003; Rhyne et al. 2012a). Corals are among the most exploited species within the MOT (Delbeek 2001; Rhyne et al. 2009). Corals are also needed for reef restoration efforts (Jaap 2000; Young et al. 2012) and to supply the pharmaceutical industry with biological materials for drug discovery and development (Rocha et al. 2011). Consequently, an increased effort to develop ex situ (aquarium based) and in situ (sea based) © 2014 Wiley Publishing Asia Pty Ltd
coral aquaculture has been made (Pomeroy et al. 2006; Osinga et al. 2012). This review focuses on ex situ aquaculture as the possibilities to manipulate culture conditions to maximize coral production are greater than in situ methods. Factors that influence qualitative and quantitative aspects of coral culture and their optimization through the manipulation of key abiotic (e.g. light, water flow) and biotic (e.g. live prey, species interaction) variables are reviewed. Success factors for commercial aquaculture are outlined, with focus on qualitative aspects such as shape, coloration and natural product content, and quantitative parameters such as growth and volumetric productivity. We also discuss methods for sexual and asexual coral propagation. It is important to note that this review focuses on symbiotic stony corals, that is, scleractinian corals harbouring dinoflagellates from genus Symbiodinium in their tissue, as these organisms have high aquaculture potential and are in great 1
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demand by the three core industries targeting coral production (Rhyne et al. 2012b; Young et al. 2012; Leal et al. 2013a). Alcyonacean corals (order Alcyonacea, class Anthozoa), usually known as soft corals, are also briefly contemplated in this review. However, gorgonians, which are also known as horny corals and also belong to order Alcyonacea, will not be considered. Coral quality and productivity This section describes quality aspects for corals cultured for different purposes, outlines quantitative principles regarding productivity and provides the background information on coral ecology and physiology that is needed to better understand how coral quality and productivity can be optimized. Quality Coral culture has the primary goal of producing high-quality organisms. However, the definition of coral quality varies upon the targeted industry. Delbeek (2009) defined the perfect coral for the MOT as easy to keep and colourful, with a good appearance and high growth rate. It should be resilient to water quality variations, tolerant to different light intensities, not too expensive and readily available. For instance, Turbinaria and Pocilloporidae species may be good candidates for MOT as they have high growth rates and are resilient. Contrastingly, massive species such as Porites display low growth rates and are therefore less interesting candidates for aquaculture ventures (Dullo 2005). For pharmaceutical purposes (i.e. drug development), the quality of a coral is reflected by the quality and quantity of target metabolites and amount of tissue per coral weight (Leal et al. 2013a). Although the ecological roles of such metabolites and their metabolic pathways are still poorly understood, it is generally acknowledged that these bioactive molecules are important agents against predators and pathogens, as well as agents for interspecific chemical interactions (Lages et al. 2006; Fleury et al. 2008; Shnit-Orland & Kushmaro 2009). Soft corals, such as Sarcophyton sp. and Sinularia sp., have been important model species for marine drug discovery (Koh et al. 2000; Khalesi et al. 2008). While bioactive metabolites with great potential for drug discovery have also been detected in stony corals such as Tubastrea sp. and Cladocora caespitosa (reviewed by Rocha et al. 2011), stony corals have been neglected because of their low tissue-to-skeleton ratio and because all stony coral species are protected. Corals for reef restoration are usually selected based on conservation status of target species (i.e. locally or regionally extinct species) and their ecological functions (i.e. importance for reef complexity and habitat creation). Genetics of transplanted corals is also important to con2
sider and thus guarantee that genetic diversity of the natural population does not decrease. Additionally, adaptation potential of target coral species and specimens should be considered to minimize negative effects of adverse environmental conditions to the transplanted corals. For instance, acroporid corals are keystone species (Carpenter et al. 2008), which makes them appealing for restoration efforts. However, it is likely that most endangered corals are also the most fragile species with less adaptation potential. Another feature that notably affects the quality of a coral for any industry is their vulnerability to diseases and the latent presence of pathogens in the broodstock. Coral aquaculture usually applies high densities of organisms, thus carrying higher risks for pathogen virulence and communicability. The cause of coral diseases, its prevention and treatment are beyond the scope of this review, because these aspects have already been thoroughly reviewed (Borneman 2001; Raymundo et al. 2008; Bourne et al. 2009; Sweet et al. 2012; Sheridan et al. 2013). Productivity Productivity of an aquaculture system can be assessed by accounting volumetric productivity, that is, the amount of product produced per m3 of culture system per unit of time. For coral aquaculture, the amount of product can be expressed either in numbers (coral nubbins and colonies) or in biomass (size/volume) of the target organism. To calculate the volumetric productivity, it is of major importance to have information on both the growth rates and the growth kinetics of the targeted organism. Growth rates largely determine productivity, whereas growth kinetics will determine the optimal size of the organism for costefficient aquaculture. As corals do not form an exception to this general principle, it is important to consider information on their growth rates and growth kinetics to understand how to maximize the coral production. Numerous studies have been conducted to quantify coral growth rates for aquaculture purpose (reviewed by Dullo 2005). Growth has been measured as linear extension (expressed in mm per year or similar) in most of these studies. However, fewer studies have considered the kinetics of coral growth (Bak 1976; Crabbe 2007; Osinga et al. 2011) and only one of these studies (Osinga et al. 2011) related growth rates and kinetics to productivity. Depending on the underlying growth kinetics, growth rates may vary over time. Figure 1 shows the different principles of growth kinetics that may be observed in corals. If growth kinetics is linear (not to be confused with linear extension!), the increase in biomass over time is independent of the size of the organism (Fig. 1a). If growth is exponential, the increase in biomass will increase with increasing organism size (Fig. 1b). Organisms that grow Reviews in Aquaculture (2014) 6, 1–18 © 2014 Wiley Publishing Asia Pty Ltd
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(a)
Figure 1 Conceptual overview of growth kinetics in relation to linear extension rates and coral morphology. The figures represent the growth of hypothetical corals during three time intervals of equal length. (a) Single branch morphology, linear growth kinetics. The linear extension rate is constant over time, the growth increment (represented by the grey area) is constant over time, and specific growth rate (SGR) decreases over time. (b) Single branch morphology, exponential growth following the first-order kinetics. The linear extension rate increases over time, the growth increment (represented by the grey area) increases over time, and SGR remains constant. (c) Boulder morphology, constant linear extension rate. The growth increment increases over time (exponential growth), but SGR slowly decreases over time. (d) Boulder morphology, exponential growth following the first-order kinetics. The linear extension rate increases over time, the growth increment increases over time, and SGR remains constant. (e) Boulder morphology, linear growth kinetics. The linear extension rate decreases over time, the growth increment is constant over time, and SGR strongly decreases over time.
fully exponentially (first order kinetics, see Osinga et al. 2011) have a constant specific growth rate (SGR, usually expressed as time1), whereas organisms with linear growth imply an SGR that decreases over time. Many corals neither follow first-order kinetics nor follow linear growth. Boulder- and tabular-shaped corals tend to grow exponentially, exhibiting a rather constant linear extension rate and, hence, a slowly decreasing SGR in time (Fig. 1c), as was demonstrated by Schutter et al. (2010) for the bouldershaped coral Galaxea fascicularis. Colony size is an important parameter for ornamental coral culture with respect to its market price, to the pharReviews in Aquaculture (2014) 6, 1–18 © 2014 Wiley Publishing Asia Pty Ltd
(b)
(c)
(d)
(e)
maceutical industry for its tissue volume and concomitant metabolite concentration and to restoration purposes as larger colonies have greater survival chances (Epstein et al. 2001). This makes linear extension rates a useful measure for growth. If linear extension is constant, it is possible to calculate the time needed for an average coral fragment to grow to commercial size accurately from randomly measured linear extension rates, such as those provided in the review by Dullo (2005). However, it is better to base culture design on growth kinetics for two reasons. First, linear extension rates may vary in time (Fig. 1d,e), and therefore, random measurements are not always representative for 3
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the entire lifespan of the coral. Second, the use of growth kinetics allows analysis of the effects of size-dependent growth on culture efficiency. A higher SGR implies a higher percentage of growth per unit of coral size and hence a higher productivity and a lower cost for maintenance. As an example, for corals such as G. fascicularis, which show a decrease in SGR in time, the best strategy is to start with fragments that are as small as possible, that is, very low number of polyps (Shafir et al. 2001). In contrast, branched corals such as Stylophora pistillata and Seriatopora caliendrum tend to grow following the first-order kinetics, in particular during the first 2 years of maturation. Therefore, their SGR is size independent because their absolute growth rate increases with size as there are more branches (i.e. more surface area) growing. This suggests that fragment size is not influencing culture efficiency. When commercial culture is ongoing, two additional aspects should be taken into account: mortality and the need for broodstock materials to initiate the next culture cycle. For these two aspects, we recommend to follow procedures similar to those outlined by Schippers et al. (2012) for designing sponge aquaculture. The equations proposed by Schippers et al. (2012) can be applied as follows to ornamental coral culture: NIF ¼ Y=PF ;
ð1Þ
where NIF is the number of initial fragments to be seeded, Y is the desired harvest (i.e. the number of commercialsized colonies) and PF is the average productivity per fragment. PF can be calculated from Equation 2: PF ¼ ð1 MÞFt Y2 =Y F0 ;
ð2Þ
where M is the mortality factor, which is calculated as (100 percentage survival)/100, Y2 is the desired harvest for the subsequent second production cycle, F0 is the initial fragment size and Ft is the fragment size upon harvesting. For example, supposing that G. fascicularis colonies are seeded as single polyps (F0 = 1) and sold as colonies that have 100 polyps on average (Ft = 100). A coral farm wants to produce 100 colonies of commercial size per culture cycle. Hence, the yield Y is the total number of polyps to be produced, which equals 10 000 polyps. Mortality was found to be 10% (M = 0.1). From these figures, it follows that PF = (1 0.1) 9 100 10 000/10 000 9 1 = 89. Substitution of this value in Equation 1 shows that the number of initial fragments needed for a repeated production of 100 commercially sized colonies of G. fascicularis is at least 10 000/89 = 112.36 (i.e. 113). From these, 11 will be lost before harvesting, 100 can be used for sales, and 1.36 have to be used to prepare new initial fragments for the subsequent culture cycle. 4
In summary, prior to starting up a culture of a particular coral species, we recommend executing a growth experiment on that coral species to assess the time needed for a small coral fragment to grow to commercial size, followed by the analysis of the underlying kinetics. This will provide key information for culture design. Furthermore, mortality rates and broodstock material for the following culture cycles are also key issues to consider. Although all these factors should be taken into account to set up a coral aquaculture, the culture conditions of the production system are also of paramount importance as they will provide the means to maximize coral quality and quantity production. Manipulating coral quality and growth This section provides the biological and physiological background based upon which abiotic and biotic culture conditions can be manipulated to maximize coral quality and growth. The focus is on light, nutrition and water flow, but some other aspects are shortly discussed as well. Light Light is a central factor for symbiotic corals as it supports the photosynthetic endosymbionts inhabiting the coral and, therefore, contributes to maintaining both partners of the symbiosis healthy. Although some corals can be facultatively symbiotic (Piniak 2002), most symbiotic corals need appropriate light to stay in good condition and grow. Light endorses the production of photosynthates by the photosynthetic endosymbionts and consequently their translocation to the cnidarian host (Muscatine et al. 1981; Bachar et al. 2007; Leal et al. 2013b). This process largely contributes to coral calcification and growth (Falkowski et al. 1984; Wijgerde et al. 2012a). Light also affects coral quality-related aspects such as physiological condition, shape, colour and metabolite content (Titlyanov & Titlyanova 2002; Todd 2008; Khalesi et al. 2009). However, the selection of appropriate light conditions for high productivity and high quality is often species-specific and it may be ambiguous because a key feature of symbiotic corals is the adaptation to different light environments (Titlyanov & Titlyanova 2002). Light manipulation in ex situ facilities comprises quantitative (irradiance), qualitative (light spectrum) and technological aspects (types of light sources). Light quantity is usually measured as quantum irradiance falling within the range of photosynthetic active radiation (PAR). Light quality is characterized by its spectrum, which varies with depth and light source (Mass et al. 2010; Rocha et al. 2013a). The main light sources used for coral culture are fluorescent lamps, metal halide lamps and light-emitting diodes (LEDs). Please consider other reviews for a more complete Reviews in Aquaculture (2014) 6, 1–18 © 2014 Wiley Publishing Asia Pty Ltd
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analysis on light properties and light sources (Delbeek & Sprung 1994; Fossa & Nilsen 1996; Osinga et al. 2008). Most research on the effect of light on symbiotic corals has been focused on quantity, rather than on quality or technology, although the latter aspects may be of equal importance for aquaculture and aquarium hobbyists. Only few studies evaluated the energy efficiency of different light sources (e.g. Wijgerde et al. 2012b; Rocha et al. 2013a; Wijgerde & Laterveer 2013). Comparative evaluation of different light spectra and technologies is also of interest to aquarists trying to boost coral colours, but these aspects have up till now hardly been investigated by scientists. Light quantity Up to a certain limit, an increase in light quantity will enhance photosynthetic rates, which induces an increase in calcification rates (Houlbreque et al. 2004; Moya et al. 2006). Heterotrophic feeding also increases the photosynthetic capacity of corals, particularly chlorophyll content and photosynthetic rates (Houlbreque et al. 2004). Water flow also acts synergistically with light and feeding to increase coral growth (Schutter et al. 2011; Wijgerde et al. 2012c). However, relatively high light levels (e.g. >600 lmol quanta m2 s1) may trigger coral bleaching and concomitantly decrease its growth or even induce mortality (Hoegh-Guldberg & Smith 1989; Osinga et al. 2008). Variations in photosynthetic rates associated with light changes are usually short-term adaptations, whereas medium- and long-term adaptations are observed through coral morphology, as corals will change their growth pattern according to the ambient light conditions (Titlyanov & Titlyanova 2002). A specimen growing under low light will try to expose more horizontal surface to the incoming light and will thus develop a more flattened shape than a specimen of the same species growing under high irradiance. Light variations also induce changes in colour. In general, corals may increase the total amount of chlorophyll present in the photosynthetic endosymbionts living in their polyps when moved to low light (Dubinsky et al. 1984; Rocha et al. 2013b). Due to chlorophyll properties, this change will intensify the brownish colour of the coral surface. Such changes occur on a medium timescale, as it takes several weeks to observe such differences (Titlyanov & Titlyanova 2002). This effect will only occur when light levels are low, yet high enough to support the photosynthetic apparatus of Symbiodinium (Rocha et al. 2013c). However, these changes will not occur if photon capture efficiency is already close to 100%, as there is no need to further intensify pigmentation with lower light. Changing a coral from low light to high light reduces the intensity of the brown coloration, as corals reduce pigmentation to prevent oxidative stress (Lesser & Shick 1989). These changes are usually observed when light levels Reviews in Aquaculture (2014) 6, 1–18 © 2014 Wiley Publishing Asia Pty Ltd
increase beyond 200 lmol quanta m2 s1 (personal observation). In addition to changes in chlorophyll content, coral colour can change due to changes in the levels of other pigments and proteins present in the symbionts and/ or in the coral tissue (Leal et al. 2014a). Such proteins, commonly referred to as fluorescent proteins, are usually defined by their excitation and emission spectra and display very different and attractive colours. A study by D’Angelo et al. (2008) showed that colonies illuminated with low light intensities appeared brownish, whereas those growing under moderate light displayed distinctive colorations, such as blue, green and red. The increase in pigmentation of moderate light-treated specimens was striking when the fluorescent proteins in coral’s tissue were excited by blue or green light. It has been suggested that these proteins have a photoprotective role, as well as other host pigments that are responsible for the intense bluish, green or reddish hues (Salih et al. 2000; D’Angelo et al. 2008). Coral exposure to high light intensity produced a more intense green fluorescence in Acropora nobilis and increased pigmentation in a pink morph of Pocillopora damicornis (Takabayashi & Hoegh-Guldberg 1995; D’Angelo et al. 2008). Besides responding to light fluctuations, fluorescent proteins can also be produced in response to other types of stress, such as increased temperature, breakage, fish bites, predation and disease (Bandaranayake 2006; Matz et al. 2006; Palmer et al. 2009). Quantum irradiance may also affect secondary metabolism in corals. To our knowledge, there is only one study that assessed the effect of light intensity on metabolite production. Maximum concentrations of flexibilide, a metabolite with diverse pharmaceutical applications produced by the soft coral Sinularia flexibilis (Khalesi et al. 2008), were observed at irradiances of 400 lmol quanta m2 s1 (Khalesi et al. 2009). These results are likely associated with a chemical response of the coral to light stress-induced adaptive bleaching, as S. flexibilis also shows higher flexibilide concentrations 1 month after bleaching (Michalek-Wagner & Bowden 2000). However, the same study reported contrasting results for other secondary metabolites produced by S. flexibilis (sinulariolide) and by Lobophytum compactum (isolbophytolide). Although bleaching is the loss of the intracellular endosymbionts and is usually associated with light and/or thermal stress, the synthesis of terpenoid secondary metabolites in both latter species is associated with the host and not with the symbionts (Michalek-Wagner et al. 2001). Hence, the mechanism by which light intensity influences the production of these metabolites remains unknown. Light quality and source Light quality is primarily defined by light source, as different sources can produce distinct light spectra. Variable light 5
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spectrum is also observed in nature, particularly with increasing depth, where corals generally respond to such changes by modifying the quantity and/or quality of photoprotective pigments and fluorescent proteins (Titlyanov & Titlyanova 2002). Corals from shallow water tend to be lighter in colour, and most have UV-absorbing pigments that tint them with colours such as purple, pink, blue and green (Delbeek & Sprung 1994; Osinga et al. 2008). This adaptive response is not immediate, and placing corals that were adapted to low light, or had been temporarily lightstarved, into a location with high irradiances can induce a light shock, which may ultimately damage the coral beyond its capacity to repair itself (Warner et al. 1999). D’Angelo et al. (2008) showed that the molecular response of Acropora pulchra and A. millepora after 8 h of light stimulation was still lower than a control group of coral exposed to blue light during 4 weeks. This relatively slow increase in the genetic expression of corals suggests that the accumulation of pigments is a medium-/long-term adaptive process. The production of fluorescent proteins simultaneously provides the coral with intense colours. Although in these situations coral health will likely not be optimal, because the coral is under stress, it will display colours that are more attractive to humans, thus improving its quality for the MOT. This approach may be combined with coral stress diagnostic tools (Kenkel et al. 2014) to avoid reaching a point of no return. Another study investigated the individual and combined effect of blue and red light in coral photophysiology (Wijgerde et al. 2014). Results of the latter study are in agreement with previous studies (Kinzie et al. 1984; Wang et al. 2008), demonstrating that blue light (either narrow bandwidth or as part of a full spectrum) is essential to the growth of corals and both in hospite and ex hospite Symbiodinium. As previously mentioned, light quality can also be changed through the use of different light sources. Each type of lighting has its own advantages and disadvantages. They all vary in purchase and maintenance cost, light spectrum, longevity, efficiency and power (Rocha et al. 2013a). Metal halide lamps and fluorescent lighting have been the two most commonly used types of aquarium lighting (Osinga et al. 2008), particularly the first, because they closely resemble the sunlight spectrum. However, for aquaculture purposes, they are not the most energy-efficient light source. Fluorescent lights, as well as LEDs, are more efficient in terms of conversion of energy into light and in addition allow for more flexibility with respect to manipulation of the light quality provided to the corals. Further, LED light sources provide a larger array of light colours available to culture corals (Wijgerde et al. 2014). A recent study investigated the effect of using light-emitting plasma (LEP) or LED light sources, as well as different irradiances, on the growth of the stony symbiotic coral Galaxea fascicu6
laris (Wijgerde et al. 2012b). Higher growth rates were observed for corals reared using LEP for two of the tested higher irradiances (125–150 and 275–325 lE m2 s1). Another study by Rocha et al. (2013a) compared the effect of T5 fluorescent lamps, LEP and LED on the photobiology, growth and protein concentration of two commercially important scleractinian corals (Acropora formosa and Stylophora pistillata). After 5 months of exposure to the different light treatments, results showed that LED is superior as the blue light spectrum of LED promoted high growth rates for both coral species and decreased the energetic cost of the culture system. However, protein content, which is usually measured as a proxy for tissue growth, was similar between light treatments for each coral species. Nutrition While asymbiotic corals entirely rely on heterotrophic sources (particulate and dissolved organic matter), symbiotic corals are mixotrophs as they rely on hetero- and autotrophy (synthesis of organic molecules from inorganic ones). The animal host is heterotroph, ingesting a wide range of particles, while the symbionts are autotrophs, through their photosynthetic activity. Photosynthates are, however, very rich in carbon, but often deficient in other elemental molecules, such as nitrogen and phosphorus, which are essential for growth (reviewed by Houlbreque & Ferrier-Pages 2009). Thus, both nutrition modes are important for coral fitness. Organic nutrients Depending on its form, organic matter is usually categorized in dissolved and particulate matter. The latter can be divided into detrital or living particulate matter. Dissolved organic matter includes, among others, sugars, free amino acids and urea (Grover et al. 2006, 2008). Detrital organic matter is mainly in the form of marine snow, or sediment particles, which are deposited and can be resuspended near the corals (Anthony 1999). Finally, live particulate matter contains all sorts of planktonic organisms, including pico-, nano- and mesozooplankton (Houlbreque & Ferrier-Pages 2009). However, due to physical constraints or to their feeding selectivity, not all coral species can ingest all ranges of particles (Sorokin 1991; Sebens et al. 1996; Leal et al. 2014b). It is important to note that there is no ideal prey to nourish corals, as feeding preferences vary with coral species and prey capture rates. In addition, digestion is species specific (Leal et al. 2014c). Fed corals have a twofold faster organic matrix synthesis, calcification, protein and lipid content than starved corals (Houlbreque & Ferrier-Pages 2009). Skeletal growth increases in fed corals, as was verified for Stylophora pistillata, Turbinaria reniformis, Pocillopora damicornis, SeriatoReviews in Aquaculture (2014) 6, 1–18 © 2014 Wiley Publishing Asia Pty Ltd
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pora caliendrum and Galaxea fascicularis (Ferrier-Pages et al. 2003; Treignier et al. 2009; Wijgerde et al. 2012a). Heterotrophic feeding also improves coral quality as it contributes to maximize resilience to stress and potential for recovery (Grottoli et al. 2006). This is important to increase the tolerance of corals to changes in water quality and light variation that often occur in the MOT, as well as to improve the adaptation potential of corals that are cultured for reef restoration purposes. Furthermore, the increase in the tissue-to-skeleton ratio promoted by coral feeding (Ferrier-Pages et al. 2003; Houlbreque et al. 2003) is critical for coral cultures targeting the pharmaceutical industry (Leal et al. 2014d). Heterotrophy provides the coral with important nutrients (mainly nitrogen, carbon and phosphorus) in an appropriate biological ratio, which is not expected to disturb the nutrient balance inside the coral. However, feeding frequency may alter this balance, particularly if the nutrient load in the surrounding water decreases water quality and interferes with the natural inorganic nutrient concentrations. Forsman et al. (2011), for example, observed a significantly slower growth of M. capitata and P. damicornis, when feeding doses increased too much and deteriorated water quality. Trace elements and amino acids present in the coral’s diet may also contribute to colour manipulation (Balling et al. 2008). Although, to our knowledge, the manipulation of coral colour through nutrition has never been studied, Borneman (2001) suggests that some pigments, such as carotenoids related to vitamin A, may be dietary and produce red, orange and some yellow orange coloration. Inorganic nutrients Inorganic nutrients, in particular the macronutrients phosphorous and nitrogen, are needed as building blocks for biomass production. Only symbiotic corals can take up inorganic nutrients, as this is a process mediated by the symbionts. Both internal and external nitrogen sources are available to corals. Indeed, urea and ammonia, two forms of nitrogen that are by-products of coral host metabolism, are not excreted and released by the symbiotic association, but are reused by the symbionts and transformed into photosynthates (Muscatine & D’Elia 1978). The uptake of sea water inorganic nitrogen is also mediated by the symbionts, although the coral host has adapted to transport these molecules through its membranes (Grover et al. 2002; Godinot et al. 2009). The availability in nitrate (NO 3 ) and ammo), which are the two main forms of sea water nium (NHþ 4 inorganic nitrogen, is critical for coral growth and is usually a limiting nutrient in oligotrophic coral reef environments (concentrations
