Iridocytes Mediate Photonic Cooperation Between Giant Clams (Tridacninae) and Their Photosynthetic Symbionts

Rossbach, Susann; Subedi, Ram Chandra; Ng, Tien Khee; Ooi, Boon S.; Duarte, Carlos M.

doi:10.3389/fmars.2020.00465

Cited by 28 publications

(13 citation statements)

References 92 publications

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“…They also back-reflect light of non-productive wavelengths, contributing in part to the distinctive coloration and pattern of the outer mantle of individual giant clams (Holt et al, 2014;Ghoshal et al, 2016). Iridocytes can also protect the symbionts against harmful short wavelength UV radiation by absorbing it and re-emitting radiation of longer wavelengths that are conducive for photosynthesis (Rossbach et al, 2020). Importantly, an iridocyte is a part of the host-mediated carbon concentration mechanism (CCM) that augments the supply of C i to the photosynthesizing symbionts during illumination (Ip et al, 2018).…”

Section: Giant Clams Harbor Extracellular Symbionts In a Tubular Systemmentioning

confidence: 99%

Light-Dependent Phenomena and Related Molecular Mechanisms in Giant Clam-Dinoflagellate Associations: A Review

Chew

2021

Front. Mar. Sci.

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Giant clams can grow to large sizes despite living in oligotrophic waters of the tropical Indo-Pacific as they maintain a mutualistic relationship with symbiotic dinoflagellates (zooxanthellae) and receive photosynthate from them. The phototrophic dinoflagellates live extracellularly inside a tubular system located mainly in the colorful outer mantle and have no access to the ambient seawater. Hence, the clam host needs to absorb exogenous inorganic carbon (Ci), nitrogen (N) and phosphorus (P), and supply them to the symbionts. As photosynthesizing symbionts need more nutrients in light than in the dark, the uptake rates of these exogenous nutrients by the host must increase during illumination, implying that the host’s transporters involved need to be regulated by some kind of light-responsive mechanisms. Furthermore, the growth and development of the host can also be augmented by light, because of the photosynthate donated by the photosynthesizing symbionts. Consequently, giant clams display many light-dependent phenomena related to phototrophy, antioxidative defense, biomineralization, as well as absorption of exogenous Ci, N, and P. These phenomena may involve collaborations among enzymes and transporters in several organs of the host, whereby the gene and protein expression levels of these biocatalysts are up- or down-regulated during illumination. This review aims to examine the molecular mechanisms of light-dependent physiological phenomena that occur in intact giant clam-dinoflagellate associations, and to highlight the differences between giant clams and scleractinian corals in those regards. As the population of giant clams in nature are dwindling due to climate change and anthropogenic activities, a good understanding of their light-dependent processes may generate new ideas to improve their growth and survival under rapidly changing environmental conditions.

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Section: Giant Clams Harbor Extracellular Symbionts In a Tubular Systemmentioning

confidence: 99%

Light-Dependent Phenomena and Related Molecular Mechanisms in Giant Clam-Dinoflagellate Associations: A Review

Chew

2021

Front. Mar. Sci.

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“…Iridophores are aggregates of iridocytes that can deflect light of relevant wavelength to the symbiotic dinoflagellates to promote photosynthesis [8]. They can also absorb harmful UV radiation [9] during insolation. Photosynthesizing dinoflagellates can donate as much as 95% of photosynthates to the clam host to fulfil its energy and nutrition requirements [10], and to support high growth rate with light-enhanced shell formation [11,12].…”

Section: Introductionmentioning

confidence: 99%

The colorful mantle of the giant clam Tridacna squamosa expresses a homolog of electrogenic sodium: Bicarbonate cotransporter 2 that mediates the supply of inorganic carbon to photosynthesizing symbionts

Boo

Chew

2021

PLoS ONE

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Giant clams live in symbiosis with phototrophic dinoflagellates, which reside extracellularly inside zooxanthellal tubules located mainly in the colourful and extensible outer mantle. As symbiotic dinoflagellates have no access to the ambient seawater, they need to obtain inorganic carbon (Ci) from the host for photosynthesis during illumination. The outer mantle has a host-mediated and light-dependent carbon-concentrating mechanism to augment the supply of Ci to the symbionts during illumination. Iridocytes can increase the secretion of H+ through vacuolar H+-ATPase to dehydrate HCO3− present in the hemolymph to CO2. CO2 can permeate the basolateral membrane of the epithelial cells of the zooxanthellal tubules, and rehydrated back to HCO3− in the cytoplasm catalysed by carbonic anhydrase 2. This study aimed to elucidate the molecular mechanism involved in the transport of HCO3− across the apical membrane of these epithelial cells into the luminal fluid surrounding the symbionts. We had obtained the complete cDNA coding sequence of a homolog of electrogenic Na+-HCO3− cotransporter 2 (NBCe2-like gene) from the outer mantle of the fluted giant clam, Tridacna squamosa. NBCe2-like gene comprised 3,399 bp, encoding a protein of 1,132 amino acids of 127.3 kDa. NBCe2-like protein had an apical localization in the epithelial cells of zooxanthellal tubules, denoting that it could transport HCO3− between the epithelial cells and the luminal fluid. Furthermore, illumination augmented the transcript level and protein abundance of NBCe2-like gene/NBCe2-like protein in the outer mantle, indicating that it could mediate the increased transport of HCO3− into the luminal fluid to support photosynthesis in the symbionts.

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“…Recently, scientists have discovered that the mantle color of giant clams is the result of structural color, which is aroused by a display of different iridocytes according to the RGB law display (Ozog, 2009;Todd et al, 2009;Holt et al, 2014;Rossbach et al, 2020;Long et al, 2021). During the photosymbiotic process of giant clams, the forward scattering of iridocytes illuminates the internal zooxanthellae deeper inside the mantle, and the backward reflection produces the unique color of the giant clam mantle (Ghoshal et al, 2016).…”

Section: Introductionmentioning

confidence: 99%

“…These platelets form a crystal lattice, which produces maximum light interference at wavelengths of about 400 nm or slightly higher than 400 nm. Interference may extend the reflected light into the blue and green parts of the visible spectrum (Griffiths et al, 1992;Neo et al, 2015;Rossbach et al, 2020). Iridocytes can be divided into four types (red, yellow, green, and blue) and can be expressed as one, two, or more types in a single individual (Ghoshal et al, 2016).…”

Section: Introductionmentioning

confidence: 99%

Genetic Recombination of the Mantle Color Pattern of Two Boring Giant Clam (Tridacna crocea) Strains

Wang

Zhou

et al. 2021

Front. Mar. Sci.

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According to the RGB law display, the polymorphism of the giant clam mantle color pattern is through four iridocytes. The boring giant clam (Tridacna crocea) exhibits diverse mantle colors, including blue, green, purple, gold, and orange. In order to evaluate the genetic laws driving these mantle color patterns, a complete diallel cross between two color strains [blue strain (only blue iridocyte) and the yellow-green strain (yellow and green iridocytes)] was performed. Using a single-to-single mating system, two intra-strain crosses (BB and YY) and two reciprocal inter-strain crosses (BY and YB) were produced in triplicates. Higher fertilization rate and hatching rate were observed in all experimental groups, suggesting that there was no sperm–egg recognition barrier between the two strains. In the grow-out stage, the size of the reciprocal hybrids was larger than that of the two pure strains with a degree of heterosis. In addition, compared with the two pure strains, the hybrids have higher larval metamorphosis rate and higher survival rate. At 1 year of age, the mantle color pattern of pure strains showed 100% stable inheritance, while the reciprocal hybrids exhibited colorful patterns (a combination of blue, yellow, and green), suggesting that there was a genetic recombination of the mantle colors during the stable expression period. These results provide a theoretical basis for the formation of the mantle color of giant clam and its genetic segregation law, as well as provide guidance for genetic breeding of giant clams.

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Iridocytes Mediate Photonic Cooperation Between Giant Clams (Tridacninae) and Their Photosynthetic Symbionts

Cited by 28 publications

References 92 publications

Light-Dependent Phenomena and Related Molecular Mechanisms in Giant Clam-Dinoflagellate Associations: A Review

Light-Dependent Phenomena and Related Molecular Mechanisms in Giant Clam-Dinoflagellate Associations: A Review

The colorful mantle of the giant clam Tridacna squamosa expresses a homolog of electrogenic sodium: Bicarbonate cotransporter 2 that mediates the supply of inorganic carbon to photosynthesizing symbionts

Genetic Recombination of the Mantle Color Pattern of Two Boring Giant Clam (Tridacna crocea) Strains

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