The behavior and the morphology of sea lilies with shortened stalks: implications on the evolution of feather stars.
Extant crinoids can be divided into two groups, stalked sea lilies and stalkless feather stars. Feather stars are considered to have evolved from stalked ancestors by losing most of the stalk, but other differences are present between the two groups. The unsegmented centrodorsal, long and curved cirri near the crown, small calyx, and the ability to swim are all feather star features not found in the sea lilies. To figure out which of the above features evolved directly correlating with loss of the stalk in feather stars, we cut off the stalk from the sea lily Metacrinus rotundus and kept them alive in an aquarium. The specimens with shortened stalks were able to stand and crawl with their arms without the support of their stalks, but swimming was not observed for any of the animals. Morphologically, neither fusion of the remaining segments nor the reduction of the size of the calyx were observed, but the cirri became long and curved near the crown. Therefore, the extant sea lilies possess a potential to adapt to incidents of stalk loss. Specimens autotomizing most of their stalks were observed, suggesting that the potential is actually employed in nature. This mechanism linking the reduction of the stalk and the changes in the morphology of cirri may have played an important role in the evolution of the feather stars, if the stalked ancestors of feather stars also possessed this potential. Experimental zoological approaches as this study may provide new insights to the questions of evolution.
Nakano
Hiroaki
H
Laboratory of Innovational Biology, Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Kashiwa, Chiba, Japan.
Hibino
Taku
T
Hara
Yuko
Y
Oji
Tatsuo
T
Amemiya
Shonan
S
D000818:Animals; D001522:Behavior, Animal; D005075:Biological Evolution; D007448:Invertebrates / Q000033:anatomy & histology* / Q000502:physiology*