Seahorses are often celebrated for their unusual shape, but another remarkable feature tends to receive less attention: their ability to alter body colour in response to their surroundings. For animals that spend much of their lives anchored to seagrasses, corals, and other coastal structures, blending into the background can be an important part of staying unnoticed. A new study published in Reviews in Fish Biology and Fisheries takes a closer look at how this process unfolds and what may be happening behind the scenes.
Using the lined seahorse (Hippocampus erectus), the authors exposed individuals to different background environments and monitored their appearance over time. Rather than changing colour within seconds or minutes, as seen in some marine animals, the seahorses gradually shifted from dark coloration to yellow, red, or grey forms over several days. Most colour transitions occurred within the first five days, after which the response approached a plateau.
To better understand the biological basis of these changes, the researchers combined behavioural observations with microscopy, comparative genomics, and gene expression analyses. Their results suggest that colour shifts are associated less with the production of new pigment and more with changes in the distribution and organisation of existing pigment granules within pigment cells. Dark individuals showed densely distributed melanophores, whereas lighter colour morphs exhibited reduced pigment coverage and more aggregated pigment granules.
Visual input
The study also explored the possible role of vision. In a separate experiment, seahorses whose eyes were covered did not show detectable colour changes during the 72-hour observation period. The authors are careful to note the limitations of this experiment and do not interpret it as definitive proof. Nevertheless, when considered alongside the genomic and transcriptomic data, the findings are consistent with a possible contribution of visual input to background-associated colour change.
One of the most interesting aspects of the work is its broader evolutionary perspective. By comparing seahorses with colour-changing reptiles and amphibians, the researchers identified shared genetic signatures linked to visual perception, neural signalling, cytoskeletal remodelling, and pigment transport. Although the mechanisms differ among groups, the study suggests that similar biological pathways may repeatedly contribute to adaptive colour modulation across vertebrates.
Beyond the evolutionary questions, the findings may also be relevant for aquaculture and the ornamental fish sector. The authors propose that carefully designed visual environments, including substrate and background colour, could help influence body coloration in captive seahorses while working with their natural behavioural responses. At the same time, the study offers a reminder that camouflage is not simply a matter of colour. It may depend on an intricate interaction between perception, physiology, and the environment itself.