Date of Completion
camouflage, Sargassum, hyperspectral imaging, Portunus sayi, Planes minutus, coral bleaching, Symbiodinium, coral reef
Field of Study
Doctor of Philosophy
Hyperspectral imaging (HSI) represents a powerful tool for measuring both the spatial and spectral components of a target. It has a wide variety of uses in marine science, but has previously been restricted to either large spatial scales or laboratory studies. Here, a portable imager is used in field research on biological camouflage and coral/dinoflagellate symbiosis.
Mats of the pelagic macroalgae Sargassum represent a complex environment for the study of marine camouflage at the air-sea interface, where endemic organisms have convergently evolved similar colors and patterns. Using HSI, spectral camouflage of two crab species (Portunus sayi and Planes minutus) was assessed. Crabs matched Sargassum reflectance across blue and green wavelengths (400-550 nm) and diverged at longer wavelengths, with maximum discrepancy in the far-red (i.e., 675 nm) due to Chlorophyll a absorption in Sargassum. Predator visual modeling showed that both species have effective color matching against blue/green sensitive dichromat fish, but are discernible to red sensitive, tetrachromat birds. The two species showed opposing trends in background matching with relation to body size. Held in a naturalistic light regime, P. sayi displayed a distinct diel cycle of dark/pale color change not observed under constant illumination. Individuals changed color in response to monochromatic black, grey, and white backgrounds, as integrated reflectance (ΣR) of crabs generally followed background albedo. Dynamic color change in this species may play a photoprotective role, with possible use in enhancing cryptic color matching.
The imaging technology and methodology utilized in studying camouflage in the Sargassum environment was then applied to a different area of optical marine science. For stony corals, reflectance is driven by the pigments of photosynthetic endosymbionts. The warm inshore bays and cooler offshore reefs of Palau share a variety of coral species with differing symbiotic dinoflagellates (genus: Symbiodinium). Hyperspectral imagery revealed that coral integrated reflectance (ΣR 400 – 700 nm) had an inverse correlation to symbiont cell density. As hypothesized, coral colonies from offshore (Clade C symbionts) showed greater bleaching response to experimental heating than inshore counterparts with thermally resistant S. trenchii. Although no unique reflectance features were found to distinguish symbiont species, differences related to symbiont density could prove useful in field and remote sensing studies.
This dissertation demonstrates the suitability of portable hyperspectral imaging for a variety of field studies in marine science. This includes a unique at-sea use of HSI to study animal camouflage.
Russell, Brandon J., "Application of a Portable Hyperspectral Imaging System to Field Studies in Animal Camouflage and Coral Reef Symbiosis" (2016). Doctoral Dissertations. 1093.