Comparative Morphology of Tube Feet Among the Asteroidea: Phylogenetic Implications

Tube-foot morphology has been included among a variety of taxonomiccriteria for the Asteroidea over the past twenty-five years.Other than a few families belonging to the order Paxillosida,which are thought to have pointed, non-suckered tube feet thatare used for digging and burial in soft sediments, the presumptionhas been that asteroids have flat-tipped, suckered tube feet.This has become an accepted model despite the fact that thecomparative morphology of asteroid tube feet has not been considered.In the present study we examine tube-foot morphology of 45 speciesof Asteroidea representing 19 families. Our analysis confirmsthat members of the Luidiidae and Astropectinidae (order Paxillosida)lack suckers on the tips of their pointed tube feet. We demonstratethat there is considerable variation in tube-foot morphologyamong members of the Asteroidea including an entirely new typeof flat-tipped, non-suckered tube foot in species belongingto the order Valvatida. The external morphology of tube feetin species belonging to the order Velatida could not be distinguishedfrom "typical" flat-tipped, suckered tube feet; nonetheless,histological sections revealed a distinctive internal morphology.Finally, we report the first observations of the tube-foot morphologyof representatives of deep-sea asteroids belonging to the ordersNotomyotida and Brisingida, a group that also lacks the typicalflat-tipped, suckered tube-foot morphology. The results of ourstudy demonstrate that the current tube-foot morphology modelneeds to be reconsidered, as there is considerably greater variationthan was previously believed to be the case. Moreover, we concludethat while tube-foot morphologies show consistent similaritieswithin orders, tube-foot morphology is less appropriate as ataxonomic character below this level.