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The light environment of the mesopelagic realm of the ocean changes with both depth and viewer orientation, and this has probably driven the high diversity of visual adaptations found among its inhabitants. The mesopelagic ‘cockeyed’ squids of family Histioteuthidae have unusual eyes, as the left and right eyes are dimorphic in size, shape and sometimes lens pigmentation. This dimorphism may be an adaptation to the two different sources of light in the mesopelagic realm, with the large eye oriented upward to view objects silhouetted against the dim, downwelling sunlight and the small eye oriented slightly downward to view bioluminescent point sources. We used in situ video footage from remotely operated vehicles in the Monterey Submarine Canyon to observe the orientation behaviour of 152 Histioteuthis heteropsis and nine Stigmatoteuthis dofleini. We found evidence for upward orientation in the large eye and slightly downward orientation in the small eye, which was facilitated by a tail-up oblique body orientation. We also found that 65% of adult H. heteropsis (n= 69) had yellow pigmentation in the lens of the larger left eye, which may be used to break the counterillumination camouflage of their prey. Finally, we used visual modelling to show that the visual returns provided by increasing eye size are much higher for an upward-oriented eye than for a downward-oriented eye, which may explain the development of this unique visual strategy. [ABSTRACT FROM AUTHOR]

Many cephalopods can rapidly change their external appearance to produce multiple body patterns. Body patterns are composed of various components, which can include colouration, bioluminescence, skin texture, posture, and locomotion. Shallow water benthic cephalopods are renowned for their diverse and complex body pattern repertoires, which have been attributed to the complexity of their habitat. Comparatively little is known about the body pattern repertoires of open ocean cephalopods. Here we create an ethogram of body patterns for the pelagic squid, Dosidicus gigas. We used video recordings of squid made in situ via remotely operated vehicles (ROV) to identify body pattern components and to determine the occurrence and duration of these components. We identified 29 chromatic, 15 postural and 6 locomotory components for D. gigas, a repertoire rivalling nearshore cephalopods for diversity.We discuss the possible functional roles of the recorded body patterns in the behavioural ecology of this open ocean species. [ABSTRACT FROM AUTHOR]

Examines the ability of the hydromedusa Aequorea victoria to produce coelenterazine, the luciferin used by at least seven marine phyla. Regeneration of extracted apophotoprotein from nonluminous Aequorea victoria; Induction of bioluminescence by feeding; Mechanical stimulation of medusae after direct injection.

At oceanic depths >200 m, there is little ambient sunlight, but bioluminescent organisms provide another light source that can reveal animals to visual predators and prey [ 1–4 ]. Transparency and mirrored surfaces—common camouflage strategies under the diffuse solar illumination of shallower waters—are conspicuous when illuminated by directed bioluminescent sources due to reflection from the body surface [ 5 , 6 ]. Pigmentation allows animals to absorb light from bioluminescent sources, rendering them visually undetectable against the dark background of the deep sea [ 5 ]. We present evidence suggesting pressure to reduce reflected bioluminescence led to the evolution of ultra-black skin (reflectance <0.5%) in 16 species of deep-sea fishes across seven distantly related orders. Histological data suggest this low reflectance is mediated by a continuous layer of densely packed melanosomes in the exterior-most layer of the dermis [ 7 , 8 ] and that this layer lacks the unpigmented gaps between pigment cells found in other darkly colored fishes [ 9–13 ]. Using finite-difference, time-domain modeling and comparisons with melanosomes found in other ectothermic vertebrates [ 11 , 13–21 ], we find the melanosomes making up the layer in these ultra-black species are optimized in size and shape to minimize reflectance. Low reflectance results from melanosomes scattering light within the layer, increasing the optical path length and therefore light absorption by the melanin. By reducing reflectance, ultra-black fish can reduce the sighting distance of visual predators more than 6-fold compared to fish with 2% reflectance. This biological example of efficient light absorption via a simple architecture of strongly absorbing and highly scattering particles may inspire new ultra-black materials. • Reflected bioluminescence can reveal deep-sea animals to predators or prey • At least 16 species of deep-sea fishes have ultra-black skin (<0.5% reflectance) • Fish achieve low reflectance using a continuous layer of melanosomes in the skin • The size and shape of these melanosomes are optimal for reducing reflectance Davis et al. investigate the distribution and production of ultra-black camouflage in deep-sea fishes. These fishes have a continuous layer of melanosomes in the dermis that are optimized in size and shape to allow them to reflect <0.5% of light. Sighting distance models suggest low reflectance reduces predator sighting distance up to 6-fold. [ABSTRACT FROM AUTHOR]