Plant Cuttings

This image is part b of Figure 3 from Dakota McCoy et al. (2024) showing how “condensing lenses with truncated caps under the windows focus sunlight” onto the photosynthetic dinoflagellates within the tissue of the heart cockle.

Free-floating – phytoplanktonic (Rebecca Lindsey & Robert Simmon) – microscopic algae within the relatively shallow illuminated upper depths of the ocean are surrounded by the sunlight they require for photosynthesis*. But, what about those algae that are integrated within the tissues of hosting animals, and therefore separated from that sunlight-suffused environment? Take for example the relationship between the heart cockle (Corculum cardissa) and its resident symbiotic** (Aryeh Brusowankin) dinoflagellates*** Symbiodinium corculorum (Mark Farmer et al., 2001)?

In this association the internalised algae [technically known as endosymbiotic zooxanthellae] are not only sequestered within the soft tissues of the mollusc, but are also separated from direct exposure to sunlight because the two shells of the bivalve remain shut (Mary Watson & Philip Signor III, 1986). In this way the algae are presumably denied access to the sun’s energy-providing, life-sustaining radiation and instead imprisoned in a darkened dungeon-like dwelling-place. Yet, the autotrophic (Hilary Costa et al.) algae apparently survive and persist in this location. In this, presumably photosynthetically-prohibitive predicament, how do the algae get sufficient light to photosynthesise?

The answer appears to be that ‘windows’ in the heart cockle’s otherwise opaque shell transmit sufficient sunlight to illuminate the algae within. So much is already known from the work of such investigators as Mary Watson & Philip Signor III (1986) and JG Carter & JA Schneider (1997). Whilst that might seem to be the end of the story, Dakota McCoy et al. (2024) have taken our understanding of this photosynthetically-facilitative phenomenon to another level.

Investigating several species of heart cockle, Corculum cardissa and Corculum sp., and building upon the work of previous investigators, they show that these marvellously manipulative marine molluscs use “biophotonic adaptations to transmit sunlight for photosynthesis” (McCoy et al. (2024).

Specifically, heart cockle windows transmit 11–62% of photosynthetically-active radiation [PAR (McCoy et al. (2024), light of wavelengths between 400 and 700 nm]. Furthermore, microlenses beneath each window condense that light and allowing it to penetrate more deeply into, and become focussed upon****, the symbiont-rich tissue (Dakota McCoy et al. (2024).

Whilst high levels of transmission of PAR is presumably a good thing for the algae, one might also be concerned that harmful wavelengths of sunlight – e.g. UV – might also be piped-down to the zooxanthellae. Investigating this, McCoy et al. (2024) found that only 5–28% of potentially harmful 300–400 nm UV radiation was transmitted via the windows. In other words, the windows screen out UV radiation. This is proposed to be a protective adaptation “to resist DNA damage and reduce bleaching risk [of the algae] from high-energy UV radiation” (McCoy et al. (2024). This is likely to be advantageous in the case of heart cockles because they are found in relatively shallow water (down to approx. 20 m) where amounts of oceanic UV are higher – and therefore more harmful (e.g., RA Kinzie III, 1993; Susan Anderson et al., 2001) – than at greater depths (Esther Fleischmann, 1989; Patrick Neale). This particular UV-benefit is attributed to a combination of at least two factors by McCoy et al. (2024): first, calcium carbonate [CaCO₃] – the main component in the mollusc’s shells – has strong absorption in the UV range, and second, its light-scattering ability “tends to be inversely proportional to wavelength”, i.e. longer wavelengths – such as UV – get scattered more than shorter wavelength PAR, and therefore tend not to reach the algae.

As interesting as these biological properties are, most interest in this scientific article in the media [evidenced by headlines such as “Heart-shaped sea creatures hold the key to faster internet” (Jess Thomson), and “Nature’s first fiber optics could light the way to internet innovation” (Elie Dolgin)] relates to the structure of the material within the light-transmitting windows. Made of aragonite [which, along with calcite (RV Dietrich), is one of the two forms of calcium carbonate found in seashells (David Chandler)], its particular crystalline arrangement here – which is described as “bundled fiber optic cables” by McCoy et al. (2024) – allows it to “transmit more light than many other possible designs” (McCoy et al. (2024). Additionally, this natural fibre optic cable system is capable of “projecting high-resolution images through the window” (McCoy et al. (2024), as for human-engineered fibre optic cables. Although presumably a property that’s not terribly biologically relevant to the symbiosis, revelation of this image-transmitting capacity leads McCoy et al. (2024) – and many of the science media commentators of this work – to wonder if the “heart cockles’ fiber optic cables and microlenses may inspire optical technologies” (Dakota McCoy et al. (2024). We shall see…

For more on this fascinating photosynthetic fibre item, see Willy Van Strien, Alex Wilkins, Ari Daniel, Esther Thole, Jens Thomson, Amit Malewar, Frank Sherwin, RA Smith, Bob Yirka, and Elie Dolgin.

Such a feelgood piece of work should definitely ‘warm the cockles of one’s heart’…

* Although, as they slowly sink under gravity (Sedeer Al-Showk; Antonello Provenzale), they will eventually fall below the depth at which photosynthesis is possible, the euphotic zone. Consigned to the Stygian gloom of the ocean depths, where no sunlight penetrates, those that are not consumed during their descent will become energy-rich organic matter. This material, part of what is known as ‘marine snow’ (Liv Ward), helps to fuel the ecosystem at the bottom of the ocean (Liv Ward). [Ed. – Should this information cause you now to wonder how such sinking-susceptible phytoplankton actually manage to persist in the oceans, have a look at Jef Huisman et al. (2002).]

** Whilst a mutually-beneficial symbiotic (Regina Bailey) arrangement is presumed for the heart cockle and its resident algae, there’s no evidence in McCoy et al. (2024)’s paper that the algae are photosynthetic or that any of their photosynthetic products are passed to the mollusc. Indeed, the only observation I’ve seen so far with a bearing on nutrition in this particular ‘multi-organism association’ seems to suggest that algae may be digested by the mollusc (Siro Kawaguti, 1950), so this could actually be a case of cultivation of and ‘grazing upon’ algae by the mollusc. And the mollusc’s manipulation of light may merely be a mechanism ‘to fatten up’ the algae for later consumption…

It is, however, likely that the features examined by McCoy et al. (2024)’s study promote greater photosynthetic capacity by the alga to the benefit of both species. But, as the authors themselves recognise, “Just because a natural material has certain optical properties does not mean that those optical properties serve a biological purpose to the organism” (p. 9 in McCoy et al., 2024). To establish the link between structure and function one needs to ‘join up the dots‘ to relate the optical discovery to a functioning photosynthetic capability – which must await further work.

*** It is also a dinoflagellate that is the symbiotic partner within the coral polyp.

**** As a Botanist, it is most heartening to note that, in support of the light-focusing role of heart-cockle shell windows and lenses, McCoy et al. (2024) draw upon parallels from the plant world with such work as RA Bone et al. (1985) on ‘epidermal cells functioning as lenses in leaves of tropical rain-forest shade plants’, and Craig Brodersen & Thomas Vogelmann (2007) considering the question ‘Do epidermal lens cells facilitate the absorptance of diffuse light?’

REFERENCES

Susan Anderson et al., 2001. Indicators of UV exposure in corals and their relevance to global climate change and coral bleaching. Human and Ecological Risk Assessment: An International Journal 7(5): 1271–1282; https://doi.org/10.1080/20018091094998

RA Bone et al., 1985. Epidermal cells functioning as lenses in leaves of tropical rain-forest shade plants. Applied Optics 24: 1408-1412; https://doi.org/10.1364/AO.24.001408

Craig R Brodersen & Thomas C Vogelmann, 2007. Do epidermal lens cells facilitate the absorptance of diffuse light? American Journal of Botany 94: 1061–1066; https://doi.org/10.3732/ajb.94.7.1061

JG Carter & JA Schneider, 1997. Condensing lenses and shell microstructure in Corculum (Mollusca: Bivalvia). Journal of Paleontology 71(1): 56-61; doi:10.1017/S0022336000038956

Mark Farmer et al., 2001. Morphology of the symbiosis between Corculum cardissa (Mollusca: Bivalvia) and Symbiodinium corculorum (Dinophyceae). Biol. Bull. 200: 336–343; https://doi.org/10.2307/1543514

Esther M Fleischmann, 1989. The measurement and penetration of ultraviolet radiation into tropical marine water. Limnology and Oceanography 34(8): 1623-1629; https://doi.org/10.4319/lo.1989.34.8.1623

Jef Huisman et al., 2002. How Do Sinking Phytoplankton Species Manage to Persist? The American Naturalist 159(3): 245-254; https://doi.org/10.1086/338511

Siro Kawaguti, 1950. Observations on the heart shell, Corculum cardissa (L.) and its associated zooxanthellae. Pacific Science 4: 43–49.

RA Kinzie III, 1993. Effects of ambient levels of solar ultraviolet radiation on zooxanthellae and photosynthesis of the reef coral Montipora verrucosa. Marine Biology 116: 319–327; https://doi.org/10.1007/BF00350022

Dakota E McCoy et al., 2024. Heart cockle shells transmit sunlight to photosymbiotic algae using bundled fiber optic cables and condensing lenses. Nat Commun 15: 9445; https://doi.org/10.1038/s41467-024-53110-x

Mary E Watson & Philip W Signor III, 1986. How a clam builds windows: shell microstructure in Corculum (Bivalvia: Cardiidae). Veliger 28(4): 348–355; https://biostor.org/reference/129196