Example 6b
All images copyright protected, © Doug Segar and Elaine Stamman Segar.
This image is an enlargement of the section of the sea pen just below the open polyps near its tip. We have also turned the photograph upside down. Yes, the other animal is a commensal crab. This species is probably a porcelain crab, Porcellanella picta. You can see that it is almost pure white, which makes it quite visible against the background in this photograph. However, the sea pen polyps are not extended, as they would normally be whenever the sea pen is extended above the seafloor. Imagine how difficult it would be to see this crab when hidden away among the flower-like white polyps of a fully extended sea pen like the one shown below. The porcelain crab gains an extra measure of protection through its association with the sea pen. When there are dangerous predators around (often throughout the daylight hours), the sea pen withdraws, taking the crab with it to a safe haven under the surface of the sand.
Look carefully at this photograph and you can see that this porcelain crab has a fine web-like structure at the end of two of its feeding appendages (maxillipeds), as do other porcelain crab species. This structure is for filter feeding. The crab waves the web-like structure back and forth, collecting plankton and organic particles, which it then transfers to its mouth. This feeding practice is perfectly matched to that of its host, also a filter feeder. Porcelain crabs are versatile since they can also feed directly using their claws.
Click the photo to see another photo of a sea pen.