Surveying Cuttlefish Camouflage Behaviour - SA
- The AUV Sirius surveying cuttlefish spawning grounds at Whyalla, South Australia (credit: Roger Hanlon, Marine Biological Loboratory).
In collaboration with Roger Hanlon from the Marine Biological Laboratory in Woods Hole, USA, the AUV Sirius was used to document nocturnal camouflage behaviour in cuttlefish at a well known spawning site in Whyalla, South Australia. The AUV's ability to fly at low altitude during day and night while closely following a desired survey pattern provided improved data collection compared to divers and previous work with a small ROV. Over the course of the week long expedition, the AUV Sirius was deployed on 38 dives at three sites in the survey area and collected tens of thousands of stereo images. Of these, nearly a thousand were seen to contain cuttlefish during post cruise analysis, with a large proportion showing evidence of camouflage. The distribution of images containing cuttlefish suggest that the animal concentrations were substantially higher closer in to shore in shallow waters, where the flat rocky substrate occurs; females lay their eggs on the underside of these rocks. Results demonstrate the strengths of using an AUV for surveying near-shore benthic habitats of ecological interest, with a particular emphasis on the ability to operate during both day and night time operations.
At night, previous studies have shown that the cuttlefish typically descend to the bottom and blend in with their surrounds. Three particular sites were targeted during the deployments described here. The figure below shows details of the AUV dives, with the white lines showing the estimated vehicle path during each dive. The markers designate the position at which an image captured by the AUV was seen to contain cuttlefish during post cruise analysis by a human expert. The distribution of images containing cuttlefish suggest that the animal concentrations were substantially higher closer in to shore in shallow waters. This supports anecdotal evidence provided by SCUBA divers working in the area. SCUBA diving activities were, however, predominantly restricted to daytime operations as night diving can be difficult to manage even in these relatively sheltered conditions.
- The survey tracks showing the dive locations relative to the Whyalla peninsulla. The inset shows the location of the study site at the mouth of the Spencer Gulf near Whyalla in South Australia.
Sample images showing cuttlefish during the day and night are shown below. As can be seen, both day and night images show evidence of camouflage. Of 931 images showing cuttlefish during these dives, 771 were camouflaged. Since one aim of the study was to see if cuttlefish have color-coordinated camouflage both night and day, we strived to get comparable numbers of camouflage images (even though we know that more animals are camouflaged at night, and more are signaling in the day during sexual selection behaviors). Of the 771 camouflaged images, 435 were day and 336 were night. Initial results suggest that color matching at night may not be as good as during daytime. We note here that the daytime AUV trials do not accurately reflect the number (or proportion) of displaying cuttlefish, which is very high, because the display aggregations are very concentrated in small areas (and thus missed by the AUV) while camouflaged animals are more evenly distributed.
- Sample images of cuttlefish in various states of camouflage as captured by the AUV. (a) and (b) show examples of images captured during daytime dives while (c) and (d) were from night time dives. These are a small subset of the 951 images in which cuttlefish were identified. Note that these images have been downsampled from the original 1380 x 1024 images captured by the AUV.
Details of a 3D reconstruction from one of the dives can be seen below. Here we see examples of 3D stereo meshes blended and texture mapped using imagery captured by the vehicle's stereo imaging system, illustrating the increasing detail with which the meshes can be viewed. These meshes aggregate hundreds or thousands of texture mapped 3D stereo meshes, allowing the scientist to zoom out to examine the habitats at a broad scale or zoom in to examine particular features in detail.
- A 3D photo reconstruction of one survey showing cuttlefish locations. (a) An overview of parts of the survey legs. Each leg of this dive was 100m long with a spacing of approximately 10m between legs. The vehicle flew at an altitude of 1.5m resulting in an image footprint of approximately 1.2m x 1.0m (b) Two parallel tracklines with cuttlefish locations marked with red arrows. These are difficult to see at this resolution but the user can zoom in to inspect the cuttlefish. (c) A close-up view of one of the study animals against a portion of trackline. This segment represents an area of 1.0m x 7.0m, integrating numerous overlapping images projected onto a 3D mesh generated using the stereo imaging system.
