Lilies of the Sea

Crinoid or Sea Lily from American Samoa (photograph by Cristi Richards)

by Benjamin L. Richards

Today the benthic team recovered three Autonomous Reef Monitoring Structures (ARMS) which have been attached to the seafloor in Fagatele Bay National Marine Sanctuary, American Samoa, for the past two years.  This smallest and most remote of all the National Marine Sanctuaries is also the only true tropical reef in the National Marine Sanctuary Program.  Fagatele Bay, on the southwestern coast of the island of Tutuila is a small eroded volcanic crater which provides shelter for a wide variety of organisms that thrive in its protected waters.

After locating the dive site, we slipped over the side of the boat into crystal clear waters and descended to a sea floor covered in coral. We located the three ARMS easily and, after installing a new set of ARMS and a set of calcification plates which will be used to investigate the impacts of ocean acidification, we removed the old ARMS and brought them to the surface.

After returning to the ship, we spent several hours disassembling the ARMS and sorting through all the various creatures who had made it their home.  The biodiversity was amazing. We found a host of crabs, snails, shrimps and a myriad of other tiny and amazing creatures.  We also found our first crinoid.

Crinoids, or sea lilies, are echinoderms (relatives of sea stars and sea urchins) and have lived in the tropical oceans since at least the Ordovician period (~450 million years ago).  Like sea stars and urchins, most crinoids are free swimming and feed by filtering small particles from the passing water with their feathery arms.  Once the food is trapped by a sticky mucus on the tube feet, it is moved towards the mouth at the center of the body.  It has been found that crinoids living in environments with a relatively low abundance of plankton have longer arms than those living in plankton-rich waters.  This is presumably to increase the surface area where food can be trapped.

Finding a such a beautiful and delicate creature in the ARMS was exciting for members of the ARMS team as well as for those who stopped by the lab to glimpse the latest arrivals from the reef.  The diversity of cryptic invertebrates being found is an exciting testimony to how much more there is to learn about reef ecosystems.  As part of the Census of Coral Reef Ecosystems (CReefs) project of the Census of Marine Life, CRED is collaborating with international partners to deploy ARMS on coral reefs around the globe to establish biodiversity baselines and monitor changes over time.