DYNAMIC VARIABILITY IN THE SPECTRAL-POLARIZED LIGHT FIELD
MEASURED OVER SEAGRASS AND SAND HABITATS
Russell, Brandon1; Dierssen, Heidi1, R.C. Zimmerman2
1
Department of Marine Sciences/Geography, University of Connecticut, Groton, CT 06340, USA
Department of Ocean, Earth & Atmospheric Sciences, Old Dominion University, 4600 Elkhorn
Ave., Norfolk VA 23529, USA
Brandon.russell@uconn.edu
2
INTRODUCTION
Biological camouflage has evolved in multiple phyla as an adaptation to visually-orienting predators.
(Hanlon 2009, Claes 2010, Hacker 1991) Underwater, the light field is polarized to a much greater and
more complex degree than in terrestrial environments. The ability to detect and utilize polarized light
for vision is present in a large number of marine animals. (Chiou 2008, Shashar 2000, Cronin 2003,
Mathger 2006) Organisms may change their polar spectral reflectivity to minimize their contrast to
their environment and thereby avoid detection. Here, we present measurements of the dynamic and
complex light field in the nearshore, coastal environment as a foundation for exploring the underlying
camouflage mechanisms of select species of fish found in these habitats. Spectral polarized light and
reflectance measurements were made in waters near Port Aransas, Texas in June 2010. Two habitats
were explored: 1) seagrass meadows, habitat for the dynamically camouflaging pinfish; and 2) sandy
sediment, habitat of the southern flounder. Benthic reflectance spectra from these two environments
are presented, as well as simultaneously recorded polarized measurements. Additionally, polarized
spectra from two local species of seagrass were compared to floating macroalgae (Sargassum sp.).
These field and laboratory measurements will serve as parameters for light-field modeling and
laboratory organismal studies, as well as facilitate interpretation of the dynamic camouflage
mechanisms specific to organisms dwelling in these different habitats.
METHODS
Research was based at the University of Texas, Marine Science Institute in Port Aransas, TX. Work
was conducted between June 14 and 25, 2010 using the R/V Shearwater. Field spectral measurements
presented here were taken at a sheltered location (27° 48.644’N, 97° 7.147’W) in the lee of Mustang
Island, Chorpus Christi Bay on June 18, 2010.
In-situ hyperspectral measurements were made of seagrass canopies and sandy sediment bottoms using
Satlantic HyperOCR radiometers from a Hyperspectral Tethered Spectroradiometer Buoy (HTSRB)
package, measuring Lu at a frequency of1Hz. Sensors were fixed to a hand-held wand for
measurements normal and 45° to the substrate. Interchangeable plastic polarization filters (Alight)
were attached to the fore-optics for polarization measurements. Linear, right circular, and left circular
filters were used. Sensors were held 3cm from the target surface. Polarized measurements were
coupled with simultaneous downwelling and upwelling plane irradiance measured by the Diver
Operated Benthic Bio-Optical Spectrometer (DOBBS), consisting of a HydroRad-4 radiometer system
(HOBI Labs). The two plane irradiance sensors measuring Ed and Eu were mounted to an adjustable
wand and positioned just above the target surface.
In addition to field measurements, locally collected specimens of Thalassia testudinium, Halodule
wrightii, and Sargassum sp. were analyzed in the laboratory for reflectance of both linearly and
circularly polarized light. Linear vertical, linear horizontal, linear 45°, and left and right circular
polarization orientations were examined.
Lab measurements were carried out using an ASD FieldSpec radiometer in a darkened room. The
illumination source was positioned 0.7m above the specimens. Plant samples and 99% spectralon
reference were placed on a matte black surface, under 6cm seawater. The fiber optic probe was held,
submerged, 1cm from the target and at 45° to vertical, 90° from the plane of illumination. Polarization
filters were held perpendicular and in contact with the probe. Spectra were averaged over 5 samples.
PRELIMINARY RESULTS OF MEASUREMENTS MADE IN THE FIELD (Fig. 1-3) AND
LAB (Fig. 4-6)
In-situ DOBBS measurements of T. testudinium canopy and sandy benthos. (Fig. 1)
In-situ HTSRB measurements of T. testudinium canopy and sandy benthos using polarization filters.
(Fig. 2)
Reduction of light intensity through polarization filters over T. testudinium canopy and sandy benthos
using HTSRB. (Fig. 3)
Laboratory FieldSpec measurements of T. testudinium, H. wrightii, and Sargassum sp. Spectra
measured through polarization filters were normalized to filtered measurements of spectralon reference
standard. (Figs. 4, 5, 6)
At the location and time of field measurements, LISST data shows a particulate concentration of
approximately 60micro-liters/liter, most being in the 45-100 micron range, and Kd approximately 4
over the spectral range used here.
Seagrass and Sand Reflectance
R b (dimensionless)
0.12
Sand
0.1
0.08
0.06
0.04
Seagrass
0.02
0
400
450
500
550
600
650
700
750
Wavelength (nm)
Figure 1. Reflectance of Thalassia canopy and sand measured at 15 cm above seafloor with the
Diver operated benthic spectrometer (DOBBS).
Ambient Spectra, Thalassia and Sand
2.5
2
Seagrass No Filter
(uW/cm^2/nm/sr)
Sand No Filter
1.5
Seagrass Linear
Sand Linear
Seagrass L Circ
Sand L Circ
1
Seagrass R Circ
Sand R Circ
0.5
0
400
450
500
550
600
650
700
750
Wavelength (nm)
Figure 2. Ambient light field at seagrass canopy and sand at 3cm from target with Hyperspectral
Tethered Spectroradiometer Buoy Lu sensor.
Ambient Light Reduction
0.7
0.6
Relative Intensity
0.5
0.4
Seagrass Linear
Sand Linear
Seagrass L Circ
Sand L Circ
Seagrass R Circ
Sand R Circ
0.3
0.2
0.1
0
400
450
500
550
600
650
700
750
Wavelength (nm)
Figure 3. Intensity of ambient light over seagrass canopy and sand through polarization filters,
relative to unfiltered measurements. Light intensity from both is reduced to between 40% and 60%
when passed through any filter.
Thalassia
Reflection Normalized to Reference
0.16
0.14
Relative Reflection
0.12
0.1
No Filter
Vert
Horiz
45 Deg
L Circ
R Circ
0.08
0.06
0.04
0.02
0
450
500
550
600
650
700
Wavelength
Figure 4. Relative reflection of different orientations of polarized light for T. testudinium.
Halodule
Reflection Normalized to Reference
0.16
0.14
Relative Reflection
0.12
0.1
No Filter
Vert
Horiz
45 Deg
L Circ
R Circ
0.08
0.06
0.04
0.02
0
450
500
550
600
650
700
Wavelength
Figure 5. Relative reflection of different orientations of polarized light for H. wrightii.
Sargassum
Reflection Normalized to Reference
2
1.8
1.6
Relative Reflection
1.4
No Filter
Vert
Horiz
45 Deg
L Circ
R Circ
1.2
1
0.8
0.6
0.4
0.2
0
450
500
550
600
650
700
Wavelength
Figure 6. Relative reflectance of different orientations of polarized light for Sargassum sp.
ACKNOWLEDGEMENTS
This work was supported by a grant from the Office of Naval Research Multi-University Research
Initiative. We wish to thank the faculty and staff of UT MSI, as well as our colleagues conducting
simultaneous research who rendered assistance.
REFERENCES
Chiou, TH, Klienlogel, S, Cronin, T, Caldwell, R, Loeffler, B, Siddiqi, A, Goldizen, A, Marshall, J.
2008. Circular polarization vision in a stromatopod crustacean. Curr. Biol. 18, 429-434.
Claes, J, Aksnes, D, Mallefet, J. 2010. Phantom hunter of the fjords: Camouflage by
counterillumination in a shark (Etmopterus spinax). J. Exp. Mar. Biol. Ecol. 388, 28-32.
Cronin, TW, Shashar, N, Caldwell, RL, Marshall, J, Cheroske, AG and Chiou, T. H. 2003. Polarization
vision and its role in biological signaling. Int. Comp. Biol. 43, 549-558.
Hacker, SD, Madin, LP. 1991. Why habitat architecture and color are important to shrimps living in
pelagic Sargassum: use of camouflage and plant-part mimicry. Mar. Ecol. Prog. Ser. 70, 143-155.
Hanlon, RT, Chiao, CC, Mäthger, LM, Barbosa, A, Buresch, KC and Chubb, C. 2009. Cephalopod
dynamic camouflage: bridging the continuum between background matching and disruptive coloration.
Phil. Trans. R. Soc. B 364, 429-437.
Mathger, LM, Hanlon, RT. 2006. Anatomical basis for camouflaged polarized light communication in
squid. Biol. Lett. 2, 494-496.
Shashar, N, Hagan, R, Boal, JG, Hanlon, R. 2000. Cuttlefish use polarization sensitivity in predation
on silvery fish. Vision Res. 40, 71-75.