NEWSLETTER ISSN 1834-4259 NO. 152 SEPTEMBER 2014 Mechanisms, ultrastructure and behavioral flashing in Ctenoides ales: ‘disco clams’ Lindsay Dougherty, University of California, Berkeley, USA Email: [email protected] Dynamic visual displays throughout the animal king- dom are often bright and dramatic. They can be pro- duced through a variety of photic processes including bioluminescence, the use of chromatophores, and struc- tural coloration. Here I describe the mechanism under- lying the striking display of the ‘disco’ or ‘electric’ clam Ctenoides ales (Limidae), the only species of bivalve known to have a behaviorally mediated photic display and whose flashing is so vivid that it has been repeated- ly confused for bioluminescence. The flashing display occurs on the mantle lip, where electron microscopy revealed two distinct tissue sides; one highly scattering side containing dense aggregations of spheres com- posed of silica, and one side containing a strongly ab- sorbing pigment. High-speed video confirmed that the two sides of the mantle lip act in concert to create a vivid broadband reflectance that rapidly alternates with strong absorption in the blue region of the spectrum. Optical modeling suggests that the diameter of the spheres, but not their packing density, is nearly optimal for scattering visible light. This simple mechanism pro- Fig. 1. Ctenoidea ales the ‘disco clam’. duces a remarkable optical effect that may function as a Photo: L. Dougherty. signal. The photonics of structural coloration are of par- microscopy (Fig. 3), elemental analysis (Fig. 4) and par- ticular interest in biomimetics, where nanostructure in- ticle modeling (Fig. 5) has deduced how the photic dis- fluences countless technologies derived from natural play is produced; tissue composed of silica nanospheres design. The use of structural coloration and scattering is rapidly exposed then concealed to create a dynamic by various taxa in the ocean’s euphotic zone is especial- broadband reflectance that is optimized for a light- ly interesting as long wavelengths are absorbed rapidly restricted environment. However, the behavioral pur- with depth, light attenuates with suspended solids, and pose of the flashing display remains unknown. Three available light varies between habitats. Ctenoides ales lives hypotheses are being tested: that the display acts as: (i) a as deep as 50 m underwater and inside small crevices, signal facilitating the recruitment of conspecifics, (ii) a where ambient light is dim and wavelength-restricted. phototaxic prey lure, and/or (iii) a deimatic anti- Despite this, the species evolved a reflective mantle predator display. Research interests center around (i) the edge that emits vivid light, resulting in the common proximate mechanisms that produce the display (how) name ‘disco’ or ‘electric’ clam. Preliminary research in and (ii) the ultimate behavioral purpose of the flashing spectrometry (Fig. 2), high speed video, electron display (why). 1 (Continued on page 3) Society information President Rachel Przeslawski Vice President Kirsten Benkendorff Membership fees 2014 Treasurer Don Colgan Includes Molluscan Research (published four times per Secretary Carmel McDougall year) the MSA Newsletter and discounted registration at Membership Secretary Kathleen Hayes the Molluscs 2015 conference. Public Relations Officer Caitlin Woods Journal Editor Winston Ponder Ordinary members (Aust., Asia, w. 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Email: [email protected] and Jonathan Parkyn, Email: [email protected] This publication is not deemed to be valid for Deadline for articles for the next issue of the Newslet- taxonomic purposes (See article 8.2 in the International ter: 19 December 2014. Code of Zoological Nomenclature 4th Edition.) MSA website http://www.malsocaus.org http://www.facebook.com/ groups/Malsocaus Abdopus aculeatus brooding eggs. Photo: R. Caldwell. 2 (Continued from page 1) tions, including stable isotope analysis of silica origins and optical research into the clams’ visual abilities. In addition to the field work on behavior, a col- laboration investigating the optical capabilities of the species has been established with researchers at the University of Wisconsin and the University of Mary- land. Transmission electron microscopical analysis of the eyes, and molecular testing for the expression of opsins will be conducted. The visual abilities of the clam are important when considering potential commu- nication with conspecifics. Optical biomimetics focuses on structurally- based coloration produced by photonic nanostructures. Research in this area has broad applications including anti-reflective lenses, solar panel surfacing, polarization and angular anti-counterfeiting devices, paints, coatings, tuneable lasers and cell culturing for nanostructures. Behavioral uses of structural colours are diverse, includ- ing species and sex recognition, mate choice, ornamen- tation, aposematic coloration, and orientation, school- Fig. 2. Spectrometry on mantle and lip tissue. Top: C. ales ing and flocking behavior. Structural colours have also and microscope photo of tissue (inset) showing points of measure- been proposed to result in non-communicative func- ment for spectrometry. Bottom: Percent Reflectance for points of tions, including thermoregulation, friction reduction in measurement. burrowing organisms, water repellency, structural strengthening, photoprotection and vision enhance- Behavioral observations and ecological analysis in 2013 ment. There is a wide diversity of organismal light use provided a solid context within which to conduct follow in the euphotic zone of the ocean, ranging from circu- -up experiments in the field in 2014. Behavioral obser- larly polarized light signals in stomatopods, which led to vations showed that organisms lived in clumped situa- the commercial development of quarter-wave retarder tions, which may result from conspecific recruitment. plates, to the use of reflective proteins by Tridacna giant Predatory encounters were never observed, although clams to optimize the photosynthesis of symbiotic al- valves with obvious whelk or octopus predation were gae. common. The study sites, population densities, opera- Expected outcomes of this research include tional setup plans and data analyses were cemented after insight into the behavioral function of the photic dis- exploratory dives last summer. Additionally, the 2013 play as well as comprehension of the molecular and summer field season resulted in several new collabora- evolutionary position and radiation of C. ales. Fig. 3. Transmission Electron Microscopy species comparison. (a) TEM of C. ales inner mantle fold marginal edge showing electron-dense spheres (inset) in the white ventral side, and a lack thereof in the red dorsal side. (b) TEM of congener C. scaber lacks any similar electron-dense spheres. 3 Acknowledgments The authors thank the Lizard Island Research Station, J. Auchterlonie, R. Templin and J. Drennan at the Center for Microscopy and Microanalysis at the University of Queensland, M. Zelman of Surface Optics Corporation (San Diego, CA, USA), D. Elias for High Speed Video assistance and R. Zalpuri of the Electron Microscopy Lab, both of the University of California Berkeley. This work was supported by the University of California Mu- seum of Paleontology Palmer Fund, the NSF East Asia and Pacific Summer Institutes (EAPSI) Award, the Australian Academy of Science, the Professional Asso- ciation of Diving International (PADI) Foundation Award, the Animal Behavior Society Student Research Grant, the Conchologists of America Grant and the Lerner Gray Memorial Fund from the American Muse- um of Natural History. Field work in Indonesia and Australia was con- ducted during the summer of 2013. Work in Australia was supported by the NSF EAPSI in collaboration with the Australian Academy of Science and the Australian Museum. Above is a description of lab and field work results obtained from my funding. Reference Dougherty, L.F., Johnsen S., Caldwell R.L. & Marshall, N.J. (2014). A dynamic broadband reflector built from microscopic silica spheres in the ‘disco’ clam Ctenoides ales. J. R. Soc. Interface 11: 20140407. http:// dx.doi.org/10.1098/rsif.2014.0407 Fig. 4. Energy Dispersive X-Ray Spectroscopy (EDS). EDS elemental analysis shows the composition of the reflective spheres. Blue (Silicon) and red (Oxygen) combine to form the purple, amorphous silica spheres (SiO2), while green (carbon) composes the underlying tissue. Both the outer shells (a) and the cores (b) of the spheres are composed