PHOSPHORESCENT SEA PEN
A sea pen is not an animal; it is a colony of many individuals. Phosphorescent sea pens most clearly demonstrate the co-ordination of communal living when pulses of bioluminescence and fluorescence run over them—a Mexican wave of light. Phosphorescent sea pens are formed by different types of polyps, which are soft bodied animals similar to miniature sea anemones. A sea pen’s upright central stalk that reaches up into the sea and also anchors down into mud or silt is formed by an oozoid polyp. Autozooid polyps fused together form branch like structures that stick out from the central stalk. Their tentacles are visible and work to catch food as it drifts past in moving seawater. Siphonzooid polyps have reduced tentacles and serve to draw in water, thereby maintaining the turgid structure of the sea pen.