Below is the Glaucidae section of our Field Guide.
Some members of this family have the ability to farm their food stores. They do this by eating hydroids rich in single celled algae called Zooxanthellae. This algae is then “farmed” within the Cerata (tentacle type sacks located along their dorsum).
From this farm they extract the sugars produced by the algea as their food source. Once a fully functional farm is in place, it is said that some species may never need to feed by mouth again becasue the Zooxanthellae are able to multiply once stored.
Notice that the Cerata sacs of these animals are quite long, allowing light to assist the algal growth. Interestingly the blue coloured Pteraeolidia ianthina has not yet established its Zooxanthellae farm, whereas the green colouration of the same species indicates a fully functional farming system.
Please click on the thumbnail image to view an enlargement.