Skin colour, colour preferences and retinal structure of pot bellied seahorse, Hippocampus abdominalis

This study investigated the effects of background colour (light and tank colour) on the skin colour changes, colour preferences and retinal structure in the pot ‚ÄövÑvÆ bellied seahorses Hippocampus abdominalis. Most of the seahorses changed their skin colour when held in different coloured tanks (red, yellow, green, blue and white) over 56 days. Exposure to coloured light (56 days), however did not have a significant influence on skin colour. Skin colour was measured every fortnight with the RAL design TM colour charts, where body parts (ventral, dorsal and spots) of fish were compared to the chart and their colour values recorded. In both experiments survival (F = 0.68, df 4, 19; P = 0.617; F = 3.17, df 4, 14; P = 0.063) and growth (F = 0.66, df 4, 19; P = 0.353; F = 1.71, df 4, 14; P = 0.224) was not significantly affected by any of the experimental colours. Measuring the colour preferences of non‚ÄövÑvÆadapted seahorses of different life stages in a free ‚ÄövÑvÆ colour ‚ÄövÑvÆ choice experiment for background colour or lighting colour (test colours: red, yellow, green, blue and white) resulted in a significant preference for a white background (F = 39.89 df 4, 45; P < 0.001) and green, blue and white light (F = 1.82, df 16, 225; P <0.05). Seahorses adapted to different tank (F = 9.01 df 24, 100; P < 0.001) and lighting colours (F = 14.37 df 24, 100; P < 0.001) also preferred white in the background colour preference test, while in the light colour preference test, green was significantly preferred over the other test colours (background adapted fish: F = 5.41 df 24, 100; P < 0.001), (light adapted fish: F = 9.61 df 100, 125; P <0.001). Histological examination of the retinal structure of the colour‚ÄövÑvÆadapted seahorses (56 d) showed that there was an influence of the green adaptation colour on retinal layer thickness and the blue adaptation colour on cone mosaics in both experiments (tank colour, light colour). Seahorses adapted to a green background or light had relatively thinner pigment epithelia and thicker absolute ganglion cell layers than seahorses of the other colours. Adaptation to a blue background or light caused the number of single cones to change in the retinal mosaic where the square units had two or three central single cones compared to other colours with just one central single cone.