Phyllodesmium kabiranum from Palawan, Philippines

Dear Mike,

These milky blue colours are often structural colours rather than pigment colours -that is the difference between the colour of paint on your house and the colours produced by light on an opal or diamond. I am not sure if that is the case here, but in Pteraeolidia ianthina, the darker brown the background colour is, the more noticeable is the blue colouration on the animal. Interestingly, Pteraeolidia is a solar-powered slug, and the darkness of the brown in its body is a measure of the number of the single-celled plants [zooxanthellae] it has in its tissues. For some reason the white surface pigments seem to reflect blue light better if there is a dark layer beneath them. Interestingly. P. kabiranum also harbours photosynthetic zooxanthellae.  Perhaps Ingo Burghardt, or one of the other researchers at present working on these fascinating zooxanthellae symbioses, can 'throw some light' on this.

At the top of your photo there are some similar coloured anemone-like polyps of the xeniid soft coral (Xenia or Heteroxenia), which your Phyllodesmium was probably eating.

Best wishes,
Bill Rudman