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Biology

Cuttlefish camouflage: A new method for studying the masters of disguise

Goodwin, E., & Tublitz, N. (2013). Video analyses of chromatophore activity in the European cuttlefish, Sepia officinalis.  Journal of Experimental Marine Biology and Ecology, 447, 156-159.  DOI

How do cuttlefish camouflage?

Cuttlefish, like other cephalopods, have chromatophores.  Chromatophores are pigmented organs controlled by muscles that expand and contract to change the size of each cell.  More specifically, chromatophores are neuromuscular organs that respond to electric impulses, much like our brains.

A group of researchers, including Dr. Roger Hanlon from the Marine Biological Labs in Woods Hole, Massachusetts, created a video to demonstrate the neuromuscular function of squid chromatophores in response to music played from an iPod (Wardill et al. 2012).  The video also links to a TED talk demonstrating the same concept applied to a cockroach leg.

Together, these cells create fantastic patterns to mimic a variety of backgrounds.  For more information and examples of cuttlefish camouflage, check out this 2008 video interview of Dr. Hanlon (New York Times).  As Dr. Hanlon indicates in the interview, there is a lot we don’t know about cephalopod camouflage.  Furthermore, the techniques to answer many research questions have not been developed.

About the project

The authors, Goodwin and Tublitz, tested a new method for measuring chromatophore activity in European cuttlefish. This method would help develop answers to questions about how chromatophores work and why chromatophores of different color behave differently.  Researchers developed computer software to analyze images of chromatophores and compared computer results to techniques used in prior studies

Methods

The researchers received cuttlefish eggs from The Smithsonian Aquarium (Washington, DC) and raised them to adulthood.  They proceeded to extract samples of fin tissue from six of the cuttlefish, without killing them.  To induce changes in chromatophore shapes and sizes, each tissue sample was washed with artificial sea water for 1.5 minutes, followed by a 5-minute application of a neuropeptide solution, and another 3-minute artificial sea water wash.  Throughout this, images were collected at 40 frames per second.

Pictures of the tissues were then assessed in image analysis software.  Chromatophores were first manually separated into color categories (black, red-brown, and yellow).  Red-brown and black were ultimately grouped together as “red-black” because the colors could not be distinguished from one another.

A comparison of red-black and yellow chromatophores, using pixel data, detected differences between the two color categories accurately by setting thresholds for red, green, and black color intensity in each category.  These threshold values were then used in developing the new image analysis program.

The program was created to track chromatophores over several images by a process called segmentation (see Figure 1). Using this segmentation process, the computer can isolate each chromatophore in an image to assess its color category (red-black or yellow), and size.  The program would then track the changes in that chromatophore over consecutive images to record changes in size.

Figure 1, Goodwin and Tublitz 2013

Figure 1. A schematic of the image segmentation process. An image before color intensity thresholding (Panel 1) is processed and a polygon template is drawn around chromatophore A (second row, left). It serves as a region of interest for the duration of the analysis, resulting in a two-toned template as seen in panel 3. After a polygon mask is drawn, color thresholds specific to the color type of chromatophore A are applied to the frame producing the binary image (Panel 2). White pixels within Panel 2 are assigned the value ‘1’ while black pixels are assigned ‘0’. The program then calculates the area of white in Panel 4 which is equal to the area of chromatophore A in Panel 1.

 

Results

Comparisons between area measurements taken using both the new image analysis program and previous methods revealed no significant differences.  Therefore the new program was accurate.

The new program also used fin tissue images to track chromatophores by color category and to calculate changes in area as the tissues were exposed to neuropeptides and artificial sea water.  Tests indicated that both the red-black and yellow chromatophores increased in area when exposed to the neuropeptide and contracted back to original size when washed with the artificial sea water solution (see Figure 2).

 

Figure 2, Goodwin and Tublitz 2013

Figure 2. Chromatophore area assessed by image analysis software program during neuropeptide exposure. Red-black chromatophore (a, b) and yellow chromatophore (c,d) response over time.

 

Significance

The authors’ new image analysis software technique to measure chromatophores allows users to measure changes in chromatophore sizes on living cuttlefish and other cephalopods.  Previous invasive methods required unit conversions to reach area measurements.  The new method is not perfect.  Area measurements have tradeoffs between accuracy and variability depending on the color intensity thresholds set for red-black and yellow color categories.

This method is particularly useful because the program assesses several chromatophores of different colors, from the same images.  We can use this to answer questions such as whether chromatophores respond to neurotransmitters differently by color.  Previous techniques were not able to accurately assess yellow chromatophores.

 

Discussion

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  1. […] use of chromatophore cells, organisms can change their color or skin pattern to match their current background (or for signaling). Octopus skin has light-sensing capabilities […]

  2. […] http://oceanbites.org/cuttlefish-camouflage-a-new-method-for-studying-the-masters-of-disguise/ […]

    UST Animal Behavior blog - April 24, 2014

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